Why are Multimeters 10A and not 20A? - Page 1
Forum section: electronics / testgear

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 06:52:27 am
content: A viewer asked this, and it's probably an interesting question.Why is the "defacto" standard multimeter current range usually 10A and not say, 20A?They used a 16A breaker as a practical example of usability.Yes, many will go 20A or something for X seconds, and fuses won't just blow at 10A or 11A or whatever.And yep, there is internal shunt heating, contact ratings and all sorts of things that play into this.But why are most meters seemingly designed around 10A and not 20A for example?Would have made sense in the days of early 3 1/2 digit 1999 count meters.I don't think I've ever really thought about it...

author: LazyJack
author_rank: Frequent Contributor
timestamp: June 10, 2019, 06:59:09 am
content: I would guess it has something to do with the dissipation of the current shunt. On a usual 2000 count it would be 200mV x 20 A = 4 W. Seems to be pretty much in the confined space inside a handheld instrument.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:00:12 am
content: Quote from: LazyJack on June 10, 2019, 06:59:09 amSure, but you design around that by halving your shunt value. These are specially manufactured 4 terminal wire shunts.So why 10A instead of 20A when almost all digital meters started out as 2000 count?Sure, the spec will change if you do that, but 10A isn't the greatest spec anyway, so why not 5A and get even better spec? Or 2A and use your full 2000 resolution?

author: Fungus
author_rank: Super Contributor
timestamp: June 10, 2019, 07:06:28 am
content: Probably more to do with the leads than the meter. 20A leads would be much chunkier.

author: MosherIV
author_rank: Super Contributor
timestamp: June 10, 2019, 07:07:12 am
content: Shunt linearity?When current shunts get hot, their resistance changes. It can be compensated for but it is not a simple linear equation. So it makes sense to try and limit the heating of the shunt rather than try and compensate for the heating.That is my guess.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:10:08 am
content: Quote from: Fungus on June 10, 2019, 07:06:28 amWhy would you let that dictate your meter design decisions? Sell pricer 20A probes to those that need them.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:10:56 am
content: Quote from: MosherIV on June 10, 2019, 07:07:12 amHalve the shunt value, it heats up the same.

author: maginnovision
author_rank: Super Contributor
timestamp: June 10, 2019, 07:11:08 am
content: Wouldn't you just hang a clamp meter to monitor current of an entire breaker circuit? I also don't know what 20A affords me over 10A beyond that. I've got 3 different clamps for different currents personally. Are there any real safety restrictions? Eventually the shunt needs to be shielded if you're looking at a couple hundred volts and 20A. Self heating is gonna be worse and the typical solution of smaller value shunt could be more susceptible to noise since you need to amp the signal more. Considering there are 20A meters though my real guess is tradition.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:12:55 am
content: Quote from: maginnovision on June 10, 2019, 07:11:08 amSo why not 5A instead?The point I'm trying to make here is why was 10A settled upon as a kind of defacto standards here?If it's because it's a nice round value, and then everyone copied it, then so be it, but that's the devil in the question...QuoteLikely, butwhy....Who started it? Who copied it first and why etc etc

author: Fungus
author_rank: Super Contributor
timestamp: June 10, 2019, 07:17:56 am
content: Quote from: EEVblog on June 10, 2019, 07:12:55 amIt's a nice round number.Quote from: EEVblog on June 10, 2019, 07:12:55 amProbably comes from older analog meters, which usually went up to 10.

author: Performa01
author_rank: Super Contributor
timestamp: June 10, 2019, 07:24:33 am
content: Back in the good old days, analog multimeters usually didn’t have current ranges beyond a couple amperes. This changed when affordable digital multimeters emerged in the eighties of last century. For instance, we got 20A ranges in the most popular Metex 36XX and 46XX meters (which were subsequently sold for many years also under various other brand names), but they were unfused. The internal shunt was nothing more than a piece of copper wire, several millimeters in diameter.This has all changed at the end of last century, when it became fashionable to have the high-Amp socket fused. So it probably has something to do with the fuse and maybe the new safety requirements could not be met for currents that high.EDIT: Apart from that, I for one don't care, as I hold the view that high currents are a task for external shunts or clamps anyway.

author: Berni
author_rank: Super Contributor
timestamp: June 10, 2019, 07:30:14 am
content: Hmm good question. There are some 20A meters out there but most are 10A.I think its just going for a nice round number since most meters are also 1000V these days even tho the 1000V mark doesn't really have any special significance. Why not just 500V? its still enough for mains. Or if meters are commonly 2000 count why not have them go to 2000V?Tho i do agree that if you want to measure above 10A in mains then using a current clamp is a better idea anyway. Such currents would cause a good deal arcing on probes.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:40:24 am
content: Quote from: Berni on June 10, 2019, 07:30:14 am>10A is pretty common on low voltage DC supplies and products. Meters aren't just for AC.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:42:33 am
content: Quote from: Performa01 on June 10, 2019, 07:24:33 amYep, now we are getting somewhere were I think the real answer(s) lies.Everyone copied the early Flukes for example, in specs in features, and many other ways. So I'm half expecting the answer to be "it's because Fluke used 10A first".

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:46:01 am
content: The Fluke 8020A and 8060A were only 2A, so they seemed to be following the "1999 full resolution" thing with those.But then the first Fluke 77 had 10A instead of 20A... what was the limitation or reason there?

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 07:47:51 am
content: Quote from: Fungus on June 10, 2019, 07:17:56 amI don't recall a tradition with analog meters having 10A?

author: Someone
author_rank: Super Contributor
timestamp: June 10, 2019, 07:53:16 am
content: AVO, 12A top range:https://en.wikipedia.org/wiki/AvometerNo explicit explanation may be possible to find when it goes back that far. Fluke stuck at 10A but the 8000 series which started DMMs as a product segment were only 2A requiring an (optional) external shunt for 10A continuous, 20A peak just as their current models. The 10A figure probably entered some large contract specification and has just sat there, a competitor hasn't convinced the customer for the important need to increase this specification so its obviously of niche use.

author: Berni
author_rank: Super Contributor
timestamp: June 10, 2019, 07:56:31 am
content: Quote from: EEVblog on June 10, 2019, 07:40:24 amThat was mostly in response to the reasoning of 16A breakers being common (In the EU they are indeed everywhere).But DMMs usually have way too much burden voltage to be used on a 5V 20A supply rail anyway. Maybe we need a high current model of the µCurrent ?

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 10, 2019, 08:10:56 am
content: Quote from: EEVblog on June 10, 2019, 07:47:51 amNo, but they had 3 and 10 at full scale for voltage, so it makes sense to use the same for Amps.

author: maginnovision
author_rank: Super Contributor
timestamp: June 10, 2019, 08:14:17 am
content: Thinking about automotive stuff, breaking the circuit isn't usually useful. You don't want to need to take connectors apart and have all sorts of pigtails. You just throw a clamp on the right wire and read it on your meter. That's for any motor really, battery monitoring you can't break the circuit because you could be changing something by doing that, starters, battery charge rate. Even when I was at an electronics manufacturing company when doing repairs you don't always know what's there. Schematics and service manuals just aren't around. Wave soldering machines, lead cutters, washers, electric forklifts... Even 20A you don't want to risk a fuse for nothing and with the added effort of manually passing current through the meter. I know there are edge cases but it really seems to me to be a feature looking for a niche.

author: Fungus
author_rank: Super Contributor
timestamp: June 10, 2019, 08:17:32 am
content: Quote from: EEVblog on June 10, 2019, 07:47:51 amNot 10A, but the scales generally went to "10".I'm just guessing though. There might be some really popular old meter that went to 10 and everybody simply copied it.

author: Terry01
author_rank: Frequent Contributor
timestamp: June 10, 2019, 08:40:02 am
content: You could ask Brymen and UEI why both your meters are 10A and see what they come back with just out of curiosity Dave. There may be an answer none of us have thought of or it may just be tradition like you say. It will for sure be interesting to find out.

author: Fungus
author_rank: Super Contributor
timestamp: June 10, 2019, 08:48:39 am
content: 6000 counts is quite common, too. Why aren't 6000 count meters rated to 60A?

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 08:49:54 am
content: I've called on resident Fluke gray beard forum user drtaylor who designed the 8060, see if he has any insight.https://theamphour.com/180-an-interview-with-dave-taylor-multi-talented-meter-maker/

author: Muttley Snickers
author_rank: Unknown
timestamp: June 10, 2019, 09:01:14 am
content: Back in the early days most of my weekend work involved automotive and marine fitouts so the higher 20 amp current range on my Metex M-3650CR was handy for testing things like headlights, spotlights, winches, relays and a stack of other stuff. Nowadays I use clamp meters for those high current applications but back then there wasn't much to choose from nor the availability that we have today.

author: golden_labels
author_rank: Super Contributor
timestamp: June 10, 2019, 09:35:05 am
content: Current carrying capacity of traces on the PCB? You could make them 2× wider, but that takes a lot of valuable space. Alternatively you can double copper thickness, but that increases PCB cost — possibly beyond what is seen as balanced by consumer’s demand for 20A handheld meters. This could be feasible for more expensive DMMs, but engineers may be simply following “what everyone else has”. C’mon, how many innovations have you seen among DMM features, compared to the huge number of models designed? Autoranging? Bargraph? Faster response? Feature-wise they are all just copies of each other.Dave, you were rolling out your own meter: recall, why you haven’t thought about making it 20A.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 10, 2019, 09:37:06 am
content: I don't know if that explains it, but most VOMs had a 50µA movement (the traditional20k­­Ω/V) and the range doubler was a common feature, which took it to the 10A magic number in the upper 5A scale.Several of the modern meters mention the support to 20A for "X" seconds, despite indicating only 10A nominal on their input.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 10, 2019, 10:07:37 am
content: Quote from: Fungus on June 10, 2019, 08:17:32 amAVO "model 8"!

author: tautech
author_rank: Super Contributor
timestamp: June 10, 2019, 10:26:54 am
content: Quote from: vk6zgo on June 10, 2019, 10:07:37 amMy Model 7X Panclimatic has 10A range, Model 8 Mk5 also is 10A and my 1980 Japanese Kaise SK-160 is 12A (AC or DC) using a dedicated jack......all analog meters.

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 10:32:24 am
content: Quote from: golden_labels on June 10, 2019, 09:35:05 amI think the original spec was 10A continuous (because that's what's expected) and "whatever else you can get intermittently".High current was simply not something I personally cared about much.And it uses the same (standard) fuses as the BM235

author: joeqsmith
author_rank: Super Contributor
timestamp: June 10, 2019, 10:36:50 am
content: Fluke offered an option of an internal shunt on my first digital meter (8000A) that was for 10A. Or you could run up to 20A for 1 minute. At 10A, the burden was 500mV plus the drop of the leads. There was no fuse. The meter would normally be fuses at 2A. I suspect they used 2A and 20A because the meter could read up to 1999 counts. Makes the math nice and simple.I have an old analog meter that was made in 1974, roughly around the age of the 8000A. While the meter does have a fuse, oddly enough it was not there to protect the current input.Now why they made that first Fluke of mine only support 1999 counts may be a good question. Cost?

author: EEVblog
author_rank: Unknown
timestamp: June 10, 2019, 11:07:56 am
content: Quote from: joeqsmith on June 10, 2019, 10:36:50 amI think I asked Dave Taylor that one in the podcast, but can't remember...

author: Fungus
author_rank: Super Contributor
timestamp: June 10, 2019, 11:21:50 am
content: Quote from: joeqsmith on June 10, 2019, 10:36:50 amBenford's law:https://en.wikipedia.org/wiki/Benford%27s_law

author: coppice
author_rank: Super Contributor
timestamp: June 10, 2019, 11:31:59 am
content: Dave has at least one piece of equipment that uses 4mm shrouded sockets for 20A continuous rating - his Voltech PM300 power meter. However, for 20A continuous operation other good quality power meters, like Yokogawa, use more substantial terminals. I'm not sure 4mm sockets are the greatest option for continuous use at more than a few amps, especially as they wear and get dirty over time.Multimeter makers like simple instruction manuals. "This has a 20A amp range, but not if you use it continuously with those nice flexy test leads we supplied." might be a rather clumsy was to present the product's capabilities.

author: coppice
author_rank: Super Contributor
timestamp: June 10, 2019, 11:37:08 am
content: Quote from: joeqsmith on June 10, 2019, 10:36:50 amDigital meters going to 1999 rather than 999 goes back to the very earliest meters. The top digit was little more than an overflow bit, with people expecting to use the meter as a 999 count device, without the annoyance that readings hovering the high 900s would keep hard clipping on peaks, yet changing range would leave them with only a 2 digit reading.

author: tooki
author_rank: Super Contributor
timestamp: June 10, 2019, 12:26:30 pm
content: Quote from: EEVblog on June 10, 2019, 06:52:27 amWell, the explanation I’ve seen somewhere (I don’t recall where, though) is very simply that in most circuits of more than a few amps, it’s easier to simply use a current clamp. And that also avoids the issues of needing adequately beefy test leads (and a reliable way to insert them into the circuit).As for the meter hardware itself, the impression I get is that, fuses aside, most can actually handle 20A for a little while. Many el cheapo meters with unfused A ranges do indeed state 20A, either outright, or as “10A — 20A 30secs max” or similar, indicating that heat dissipation might be an issue.

author: Performa01
author_rank: Super Contributor
timestamp: June 10, 2019, 12:35:02 pm
content: Quote from: coppice on June 10, 2019, 11:31:59 amA-brand 4mm (banana) connectors are rated from 16 to at least 70A continuous current. 32A is standard for 2.5mm² lab cables for instance.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 10, 2019, 12:36:29 pm
content: The power meter I use has clamps. The connectors are basically what you would have with a modern handheld.Non linear systems had a 99999 count meter in 1962. It would cost roughly $33,000 today.After looking at shunts out of several meters I scrapped, I made an attempt to improve the UT61e's current shunt Watt loading so it could handle 20A for an extended time with no changes to the gain. The connectors were changed out along with the fuse and beefed up copper over the traces..

author: rsjsouza
author_rank: Super Contributor
timestamp: June 10, 2019, 01:23:08 pm
content: Quote from: coppice on June 10, 2019, 11:37:08 amThat or something more sinister:

author: DDunfield
author_rank: Regular Contributor
timestamp: June 10, 2019, 01:35:12 pm
content: Quote from: EEVblog on June 10, 2019, 07:10:08 amPerhaps because people would use the 20A range without buying the necessary probes, which leads to failures, support issues etc. You could add stuff to detect when the correct probes were fitted, but that would add cost and complexity.Maybe it has something to do with the available ADC range over an acceptable burden voltage.Assuming you halved the shunt to keep dissipation the same as has been suggested, the burden voltage at 20A would be the same as it was at 10A but you would need double the ADC resolution as it would now be full scale instead of half scale.Or maybe it's just historical - my oldest analog meter has an 10A range.DaveAgreed it would make more sense if all ranges were the same except for orders of magnitude.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 10, 2019, 02:34:47 pm
content: A 1980 TI app note for a 1999 count meter. IC can read 2V. For the 2A range, they use a 1ohm shunt at 4W.http://www.ti.com/lit/an/snoa592c/snoa592c.pdf

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 10, 2019, 03:30:14 pm
content: Quote from: coppice on June 10, 2019, 11:37:08 amThe very first DMM I used was a 999 count, red 7-segment LED display. Made by B&K, but can’t recall the model number.

author: HKJ
author_rank: Super Contributor
timestamp: June 10, 2019, 03:32:13 pm
content: In most meters the voltage drop across the current shunt matches the mV ranges, this means the 10A or 20A shunt has to be 0.01ohm or there would have to be an extra OpAmp in the meter (A few meter do have a x2 OpAmp, but not many).This also defines the power loss in the shunt, it will be 1 watt for 10A and 4 watt for 20A, the last requires a very large shunt! In addition to that add power loss in connection and fuse and it is just before the meter needs a fan.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 10, 2019, 06:17:47 pm
content: 1. Historically, not sure.2. Presently, I suspect somewhere in IEC 61010, IEC 60990, or some such standard one can interpret the requirements to result in an answer of 10A. Notice that a typical fuse used in Fluke meters:https://www.digikey.com/products/en?keywords=DMM-B-11AIs rated to 11A, "This goes to 11..."Unfortunately the UL file for it, E19180, does not seem to list this series:http://productspec.ul.com/document.php?id=JDYX.E19180The implication is that to go from a rated* current of 10A to 20A, one crosses the threshold of requirements for insulation thicknesses, surge current capability, thermal protection, and other such material and construction related requirements.* What it is sold to do, not what it is capable of based on some subset of performance characteristics.

author: CDaniel
author_rank: Frequent Contributor
timestamp: June 10, 2019, 08:37:01 pm
content: In the first place it is not correct to cut a 20A trace ( or even a 10A ) , wire , etc to insert the multimeter shunt + probes resistance + some contact resistance , so what is the point ? Who use this and for what ? Maybe when you are a beginer in electronics . Using a clampmeter is a far more inteligent way to test a power supply , transformer ,or any kind of circuits with wires .

author: windsmurf
author_rank: Frequent Contributor
timestamp: June 10, 2019, 09:14:19 pm
content: Quote from: EEVblog on June 10, 2019, 07:10:08 amI think it has to do with the market.Sell pricier 20A probes to those who need them? So no 20A probes included with my new 20A meter? Users would probably expect that. Also, there's liability... if only 10A probes are included with a 20A meter, and it ended in a fire because the user was measuring 20A with the included probes...You could design a 20A meter, but why? Probably not going to lead to increased sales... just increased production cost.People expect 10A measurement capability, because all others have it. 10A rated probes are standard and low-cost.If you need to measure high current directly, there's already specialty equipment for that... a high cost/low volume product.

author: EEVblog
author_rank: Unknown
timestamp: June 11, 2019, 12:20:35 am
content: Quote from: windsmurf on June 10, 2019, 09:14:19 pmYep, you'd have to either supply no probes or 20A rated ones to match the fuse.If you can see a market for 20A then certainly go for it, but the customer will always need those non-standard fuses and leads to make it safe.QuoteWhich is never really crossed my mind on the 121GW. The lower current ranges were more important, and I wanted to use the same fuses as the BM235 if possible.

author: EEVblog
author_rank: Unknown
timestamp: June 11, 2019, 12:32:33 am
content: Quote from: CDaniel on June 10, 2019, 08:37:01 pmMany circuit and projects are powered from bench supplies with banana plugs for development and testing. Common as mud to use the 10A range of a meter in series to get accurate and higher resolution current readings you can't get with a clamp meter.Clamp meters are usually for higher current and have the appropriate higher ranges, e.g. 50A minimum range.You can get smaller more specialised clamps that do the 4/20mA loops, but generally a clamp designed for say 1A or 10A aren't common.The multimeter is what you use in the several amp class.BTW, bench meters usually don't include a 10A range, only a couple of amps.

author: EEVblog
author_rank: Unknown
timestamp: June 11, 2019, 01:20:08 am
content: Quote from: DDunfield on June 10, 2019, 01:35:12 pmMy Simpson is 5A, and my Triplett is 12A

author: Electro Detective
author_rank: Super Contributor
timestamp: June 11, 2019, 01:32:13 am
content: Fluke 8060A handheld and 8050 bench style have easy access 2 amp current fuses, usually cheap fast blow glass M205,or sometimes slightly larger UK style BS1362 2A ceramic.AND an internal expen$ive 3 amp HRC fuse, just in case the 2 amp gets blown and 'field' replaced with a piece of folded apple pie foil, galavanised nail,or course thread screw for extra 'T' performance ()i.e. that jerry rig fail will award Jerry Riggs a non functioning meter in current mode,and cost $20 for a new 3 amp HRC and 10 pack of 2 amp glass fuses = lesson learned, going cheap on protective measures can be very pricey,especially if the meter gets zapped beyond the owner's repair skill set and or parts availabilityfwiw even decent current clamp meters need a bit of playing with to find the AC jaw sweet spots, for accurate results close to what a break in circuit reading gives.With low level DC, you also have to play the gravity/rotation and constant 'Zero Out' game tooStill, clamps are better for most 'good enough' tasks, rather than chopping wires, unscrewing stuff, taking mental notes, risk botchups/spark shows,remembering to restore meter lead connections and switch/range, trusting the state of grubby, stretched and neglected leads,and all the other good stuff

author: tautech
author_rank: Super Contributor
timestamp: June 11, 2019, 01:36:01 am
content: Quote from: EEVblog on June 11, 2019, 12:32:33 amYou'd be thinking DC amps right ?Sure there's a selection of cheapish clamp meters available these days however they're somewhat limited when it comes to even low frequency switching supply development.Then you need a scope current probe, something host hobbyists aren't prepared to spring hundreds or even thousands of dollars for.QuoteYet some modern ones do.

author: DDunfield
author_rank: Regular Contributor
timestamp: June 11, 2019, 02:18:21 am
content: Quote from: EEVblog on June 11, 2019, 01:20:08 amLike anything, specific samples can be all over the map, but I've seen a lot of meters with 10A ranges go by over the years (I'd guess more often than other higher-current ranges, but I've not been keeping track). It's not just analogs either, I still have a Fluke 8010 bench DMM (late 70's I think) with a 10A range.Probably a combination of cost, safety and "match the one we are trying to be like".FWIW, another data point: I have a Micronta 22-168A which is actually a Metex M3640 in street clothes ... and it has a 20A range, normal sized leads and banana plugs.Uses a separate banana jack for the 20A which is common as you don't want to be putting that much current through a PCB switch. It has the notation "MAX 30 sec EACH 15 min".Has two fuses, a 2A and a 20A. The 2A seems high as the next lowest range below 20A is 200ma.I keep it because it has an actual RS-232 serial port - very easy to talk to.

author: TiN
author_rank: Super Contributor
timestamp: June 11, 2019, 04:07:55 am
content: More interesting question is why 8508A is 19.99900 (and no, NOT 20A, it cannot measure 20A, nor 20V).Fancy-pancy 8588A is also 10A and 30A.

author: VK5RC
author_rank: Unknown
timestamp: June 11, 2019, 05:09:00 am
content: I would think that the 'real reason' is because we generally have 10 fingers - ie we mostly use a decimal system - so current measurement tended to get stuck there. The cabling/plug-socket issue and power dissipation also tended to stifle any change.Voltage on the other hand, has has to contend with batteries often in the 13 V area, and line voltages 120 or 240V level so ranges were adopted to say 130% of intended use.A theory only

author: windsmurf
author_rank: Frequent Contributor
timestamp: June 11, 2019, 08:01:35 am
content: Quote from: EEVblog on June 11, 2019, 12:20:35 amI just found the meter I was surmising about...It has 20A input, but they supply 10A rated probes... I don't see 20A rated probes available through them.https://www.bkprecision.com/products/multimeters/390A-test-bench-dmm-with-protective-rubberized-case-and-usb-interface.htmlI'm guessing if its 14 gauge wire or better, its probably ok... just seems like a lawsuit waiting to happen. Do we congratulate BK Precision for having cojones?https://forum.nachi.org/t/14-gauge-on-a-20-amp-breaker/72408

author: Fungus
author_rank: Super Contributor
timestamp: June 11, 2019, 09:54:22 am
content: Quote from: windsmurf on June 11, 2019, 08:01:35 amTha manual says:So printing "20A" on the front of the meter is a bit of a lie.(many meters will read up to 20A for a few seconds even though they only say "10A" on the front)

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2019, 11:55:27 am
content: The Vici VC99 I bought showed the leads rated for 20A. All of the other leads I looked at were rated for 10A. I ran a test where I swept the current to 20A, measuring the lead resistance, power dissipation and such. I would expect the 20A leads would far out perform all the other leads but as usual, it was not the case.Sweeping the Vici leadshttps://youtu.be/fQowDZstguw?t=935Showing the test resultshttps://youtu.be/fQowDZstguw?t=1802

author: macboy
author_rank: Super Contributor
timestamp: June 11, 2019, 01:21:20 pm
content: Actually, you would need 1/4 the shunt resistance because of course, P=I2R.I suspect it really has something to do with the really "dumb" nature of old (and modern cheap) multimeters. The classic dual slope ADCs (e.g. ICL7106) produce a display based on counting the duration of the run-down phase of the dual slope conversion. This meant that you couldn't just take an ADC reading and scale it in software to fit an arbitrary shunt resistance. The shunt needed to produce the same (typically) 200 mV full scale that is used for all other ranges. A 0.01 Ohm shunt would produce the necessary 100 mV at 10 A for a 1000 count display, and dissipate 1 W of power. At 20 A, it would dissipate 4 W which is probably not sustainable for very long. If a 0.0025 Ohm shunt was used, then an amplifier would be needed to produce the needed 1000 counts at 10A input. Alternately, the reference voltage could be reduced for this range only, but that is probably just as difficult. Either solution would also introduce an additional source of error and/or noise.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 11, 2019, 01:34:21 pm
content: McBoy: The most logical answer so far....Having said this, how about the meters that read 4000 or even 6000 counts?The options would be to read 4 or 6 amps respectively, which is low. Or go all the way to 40 or 60 amps, with humongous power dissipation.To play it safe, they spec it at only 10 amps continuous, which in my opinion, is a reasonable value for a live current measurement. Higher than that, one enters clamp meter territory.

author: CDaniel
author_rank: Frequent Contributor
timestamp: June 11, 2019, 03:02:53 pm
content: Every ADC has a minimum input sensitivity for full resolution , you can't use a lower reference and extrapolate the result for full scale , sorry this is not the way an ADC works , cheap or expensive .But the leads resistance in a multimeter is much higher than the shunt so it is useless to be made lower ... this is not a panelmeter just made to measure and not to be handheld .Even the fuse resistance is higher than the shunt .A panelmeter could be made easily to measure hundreds of amps like a clamp meter , with a very low shunt resistance and virtually no losses .

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2019, 03:51:42 pm
content: For the ICL7106 and similar lowering the reference somewhat is possible. However the more usual way would be to have an extra amplifier for the very low voltages at a high current shunt. So a little like the µC current.For just 3.5 or 4.5 digit resolution a smaller shunt is not such a big deal - it may even save money (or give better performance), as the smaller shunt would produce less heat.However there is still the heat from the fuse - HRC fuses tend to have a relatively high resistance and there is not very much one can do about this. Even at 10 A the fuse likely produces more heat than the shunt.In addition with now 10 A as the more common maximum current, HRC fuses for some 11 or 13 A are easier to get (and likely cheaper) than 20+x A.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 11, 2019, 04:03:48 pm
content: Quote from: macboy on June 11, 2019, 01:21:20 pmIn the 1970's a precision 0.01Ω shunt could be purchased or made of thick wire (although with a problem of space), thus power would be quite irrelevant for the shunt's durability.A case could be made that larger power would unacceptably increase the inside temperature, however the 8020A and its family all the way through the 8060A had a 0.1Ω shunt,thus yielding the same power dissipation of 4W @ 2A.Actually, 0.4W. \$P=R \times I^2 \Rightarrow P=0.1 \times 2^2 \Rightarrow P=0.4W\$Fun factor: our ancient ICE 680R dissipated 16W! (0,64Ω @ 5A)I don't know when the first 7106-based handhelds started having 10A inputs, but it seems to be as arbitrary as any other choice.

author: CDaniel
author_rank: Frequent Contributor
timestamp: June 11, 2019, 04:59:10 pm
content: Losses are just a part of the problem , the high series resistance ( leads + fuse + shunt ) will influence the circuit to be measured , so 20 Amps are anyway possible in a high voltage , "high" resistance circuit like mains or similar . For 12V circuits or lower the equivalent resistance for 20A is too low and the meter will read inaccurate values .This is certainly a reason for not branding a multimeter for 20A . Usually cheap non brands multimeters were branded so high .

author: Fungus
author_rank: Super Contributor
timestamp: June 11, 2019, 05:06:27 pm
content: Quote from: rsjsouza on June 11, 2019, 04:03:48 pm...and a drop of 3.2V in your circuit.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 05:07:58 pm
content: My Tektronix DMM916 and Beckman RMS225 both use 15 amp fuses. The documentation for the DMM916 explicitly says 20 amps for 30 seconds. The documentation for the RMS225 just says 10 amps but I know it measures up to 20 amps and indicates overload above that.So my conclusion is that the 10 amp limit comes from the metric SI specification for current in units of multimeters. One multimeter is 10 amps.Historically, going back to the 70s and early digital multimeters, the input range of the 3-1/2 digit ADC could be changed between 2 volts and 200 millivolts by dividing the reference by 10 and this was commonly done to extend the sensitivity by one range. (1) That neatly explains the 10 amp range if you assume the meter would be designed to handle 20 amps (19.99 amps) for a limited time. Designing for less would require adding a separate overload detection circuit. Why bother?As a practical matter, the limit comes from self heating under worst case conditions for the 10 amp circuit path including the shunt, connections, and conductors. They have to pick a specification to design to and that is it.I am including a photograph of the main board to my Beckman RMS225 below for laughs. Note that in both cases, those fuses use the correct gauge for a copper fusible link of the proper rating. I had it apart a couple days ago to change the battery and was considering putting proper fuses in since it seems I have finally outgrown blowing up my multimeter fuses after 20 years. Check the date codes. It works like a champ and is my favorite low end meter although it was hardly low end at the time or even now.(1) This is literally how early 3-1/2 digit meters provided a 200 millivolt input range. The integrated integrating ADCs used were specifically designed to operate correctly under these conditions albeit with extra calibration and you can find it in their specifications and application notes. I am not sure what the discrete designs at the time did but it would be easy enough to check.

author: drtaylor
author_rank: Regular Contributor
timestamp: June 11, 2019, 07:11:25 pm
content: As a DMM designer, it usually came down to available components. There was a 4 terminal shunt manufactured by Ohmite (I think) in the early 80s that was quite accurate and compact. It was rated 0.01ohms and 10A. Yes you could run 20A through it for a short time. This particular shunt was used by lots of DMM manufacturers. But I believe the main reason to stop at 10A is ratings of typical test leads, and the fact that most standard banana plugs were not well designed for high current (typ 10A to 15A max). I, for one, would not run >2A through most of my test lead sets. I have 7mm Abbatron banana plugs and jacks that are rated for 25A. A much better solution than the typical 4mm banana setupSafety issues: Some of the early units with 10A shunts were completely unfused and used a separate banana jack for that range only. As more safety standards came into play, getting away with an unfused input was no longer deemed acceptable. Fuses rated for >600V at 20A are huge and difficult to fit into a smaller DMM.Then there's the burden voltage problem: 20A fuses still have far more resistance than you can get in a good 4T shunt. Add the small gage wires typical of most off the shelf test leads, makes even a 10A range burdensome (pun intended). The resulting burden voltage of a fused shunt pretty much makes it useless for low voltage circuitry. Only mains current testing could tolerate it.Heat: A .01ohm shunt at 20A dissipates 4W. However a properly rated fuse would dissipate more. You do not want heat sources like this in a handheld. Some larger DMMs could tolerate it. Using smaller values is of course possible, but to maintain high accuracy with sustained current, there are not many very low value shunts that perform well. There are some exotic material shunts available today, so it could be done. I once worked out a relay protection circuit that induced lower burden voltage, but the price was prohibitive, and it wouldn't fit in a small DMM. I believe the thinking at the time was the industry was better served by external probes, either coil types or hall effect types.The proper way to measure DC high current at low voltages is to use an very low resistance (≤1milliohm) external shunt, and use the most sensitive range of the DMM as a 4 term sense. Then the diameter of the DMM leads doesn't matter.So in conclusion, like most engineering problems, the 10A range limitation was a combination of trade-off factors, not the least of which was component availabilty. In the days of trimmer adjusted calibrations, you had to use a multiple of 1 to get the proper full scale output for the A/D converter. Some early high current shunts were designed to be higher value than needed, and then were trimmed by actually soldering a sense wire in the proper place for the output needed. Now with software calibration, much more flexibility can be had. But burden voltage and plug/jack and wire ratings are still the main reason why you do not find high current inputs on most DMMs.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 11, 2019, 07:19:43 pm
content: ...straight from the designer's mouth. Thank you David!

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2019, 07:35:39 pm
content: QuoteTo be fair to BK, the 30 second limitation is stamped on the case below the 20A jack. My 2831E bench DMM however, has a 20A/20 sec limitation and there's no mention of it on the front panel.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2019, 07:55:00 pm
content: Quote from: drtaylor on June 11, 2019, 07:11:25 pmI think you have to go back farther in the chain of causality.Ohmite *had* to make the shunt 0.01 ohms because the only choice for the most common ADC input range of 200 millivolts was 0.01 ohms. 0.10 ohms to support a 2 volt input range is too high in both power dissipation and burden voltage and that was the only other choice.QuoteThe original dual banana jacks happily support 20 amps with suitable gauge leads and I have sets like that to use at high currents (1) but I agree, common lead sets are marginal even at 10 amps and who knows what any individual is using. Modern shrouded banana jacks and plugs are even less robust.(1) Actually, I just make them as needed. I never use commonly available test leads for high current measurements for the reasons you give.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2019, 11:01:04 pm
content: Quote from: Kleinstein on June 11, 2019, 03:51:42 pmFor the most part, agree. The 20A fuse I used in the UT61E was a little lower than the shunt. Custom made shunts:https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/msg1174713/#msg1174713

author: windsmurf
author_rank: Frequent Contributor
timestamp: June 12, 2019, 06:41:32 am
content: Quote from: Fungus on June 11, 2019, 09:54:22 amThank you Fungus! Its interesting they use 20A fuses but only tell you 30 seconds max... I wonder if the fuse will really blow at 20A?Even the dial says 20A on the selector...

author: EEVblog
author_rank: Unknown
timestamp: June 12, 2019, 07:01:04 am
content: Quote from: windsmurf on June 12, 2019, 06:41:32 amNo, it won't. A fuse rated for 20A will in theory that forever to blow at 20A, it needs more.

author: EEVblog
author_rank: Unknown
timestamp: June 12, 2019, 07:15:49 am
content: Quote from: drtaylor on June 11, 2019, 07:11:25 pmEpic 100th post!

author: Cnoob
author_rank: Regular Contributor
timestamp: June 12, 2019, 08:55:52 am
content: You can pick up cheap current shunts off ebay, I have a 75mV @ 30Amp one useful to use PC psu.s

author: joeqsmith
author_rank: Super Contributor
timestamp: June 12, 2019, 11:56:29 am
content: Testing some HRC fuses at 10A and measuring their resistance. At 11:30, you can see the fuse I used in the UT61E.https://youtu.be/RRhFYERTSxY?list=PLZSS2ajxhiQDrk4o1Y45auwK7LomjnNBU&t=394Temperatures inside the meter reached 160C during these tests. It would have gone higher had I let it run. I changed the shunts design to improve the Watt loading but the heat would still be a problem. If I were to lower the shunts resistance, as mentioned the fuse would still be a problem.Fun experiment but didn't seem very practical.

author: e0ne199
author_rank: Regular Contributor
timestamp: June 12, 2019, 02:06:53 pm
content: probably it has something to do with safety...anyway, who is dare enough to measure live AC or DC current with current more than 10A using probes?especially AC current, one wrong move and you are dead...because what is the most dangerous thing about electricity is its current...

author: windsmurf
author_rank: Frequent Contributor
timestamp: June 12, 2019, 03:12:14 pm
content: Quote from: e0ne199 on June 12, 2019, 02:06:53 pmOne wrong move at 100 milliamps and you're dead too, according to this:From:https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.htmlI like how they indicate death between .1 and .2 Amps only, as if above that you won't die... you just stop breathing

author: windsmurf
author_rank: Frequent Contributor
timestamp: June 12, 2019, 03:23:06 pm
content: Quote from: EEVblog on June 12, 2019, 07:01:04 amSo I guess the 30 second limit they write about in the manual is because the shunt gets too hot or the probe leads getshot... like Fungus said, its like they took a 10A meter and just put a 20A fuse in it, just so they can claim its a 20A multimeter... so I guess none of those meter safety certifications cover this kind of thing... ?I also thought BK Precision was supposed to be a reputable brand.

author: rancor
author_rank: Contributor
timestamp: June 12, 2019, 04:50:59 pm
content: Quote from: windsmurf on June 12, 2019, 03:12:14 pmFrom the source you linked that seem to be true. I can't find other sources about why that is the case.Quote

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2019, 07:20:09 pm
content: Quote from: e0ne199 on June 12, 2019, 02:06:53 pmI have occasionally and it is one reason I much prefer the terminal arrangement on multimeters where every terminal is equally distant from common so standard 3/4" dual banana plugs may be used.But I do not use standard test probes. Instead, I make a custom test lead using appropriately sized wire and a 3/4" dual banana jack and just wire the multimeter into the circuit temporarily which could be days or weeks.

author: max_torque
author_rank: Super Contributor
timestamp: June 12, 2019, 07:37:44 pm
content: May i suggest an altogether less exciting reason? Simply the fact that the need for more than 10A DC current measurement is simply unusual and hence rare.Most people use multimeters for bench top electronics (needing a couple of amps max) to fault find appliance wiring, and to do some basic automotive investigations. Just like most people don't have a bench top power supply rated for more than 10amps, most people simply don't need to measure more than 10 amps! Due to resistive heating and I2R losses, most products and applications will where-ever possible, run at a higher voltage and lower current for any given power requirement!The exception is of course passenger cars, stuck at 12v nominal, but even here, 10 A is 120watts (more like 150w when the engine is running at system voltage is 14.6v) which is a significant amount of power, and really only exceeded by big current devices like the starter motor or the alternator etc.I think over the years i've been doing electronics, both hobby based on the bench, i've either needed less than 10A, or waaaayy more (like >100A), meaning a 20A multimeter is completely pointless...........

author: BillB
author_rank: Unknown
timestamp: June 12, 2019, 07:40:16 pm
content: Quote from: rancor on June 12, 2019, 04:50:59 pmMaybe its the smoldering, smoke and smell of burnt flesh that cause the onlooker to react more quickly and seriously thus initiating resuscitation and revival? Below that (.1-.2A), the onlooker just kicks at the person, perhaps pokes them with a sharp stick, and ridicules them for being a wuss?

author: Fungus
author_rank: Super Contributor
timestamp: June 12, 2019, 07:53:43 pm
content: I think we should also ask why they don't measure up to 2000 Volts. Why stop at less?

author: tautech
author_rank: Super Contributor
timestamp: June 12, 2019, 08:27:27 pm
content: Quote from: Fungus on June 12, 2019, 07:53:43 pmExactly.....why not even 3KV ?If you repair old CRO's this is a necessary requirement.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2019, 08:41:52 pm
content: Quote from: max_torque on June 12, 2019, 07:37:44 pmIf you have ever used a multimeter with a 2 amp range for long enough which used to be common, then you will identify another reason for the 10 amp range which operates up to 20 amps or more intermittently.It is very easy to blow the 2 amp fuse with a nominally less than 1 amp power circuit measurement due to things like capacitve inrush currents. I regularly use the 10 amp range even when I know I will be measuring currents within the nominal capacity of the next lower current range simply so that I do not blow out the lower current fuse. Conveniently, 10 amp ranges usually or at least often deliver 1 milliamp resolution.Quote from: tautech on June 12, 2019, 08:27:27 pmBack when CRT repair was more common, many portable meters did support up to 3000 volts so they could directly measure the cathode voltage.

author: tautech
author_rank: Super Contributor
timestamp: June 12, 2019, 08:59:09 pm
content: Quote from: David Hess on June 12, 2019, 08:41:52 pmSure for analog meters but newer DMM's 3KV capability is pretty rare.My 1980 Kaise and mid 70's AVO 8 Mk5 both have 3KV ranges.....albeit via dedicated sockets.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 12, 2019, 09:11:29 pm
content: Quote from: tautech on June 12, 2019, 08:27:27 pmYes... As I said before, it seems to be as arbitrary as any other choice. At least for voltage, <1kV is arbitrarily classified as "low voltage" for distribution systems.Regarding current, thedesignerof the venerable Fluke 8060A already mentioned the decision process taken at the time. 'nuff said (at least for me).Quote from: tautech on June 12, 2019, 08:59:09 pmPuny digital circuits and their fancy-schmancy sensitive inputs... Our ICE 680R went to 2kV with the range doubler button and no dedicated inputs.

author: windsmurf
author_rank: Frequent Contributor
timestamp: June 12, 2019, 09:12:07 pm
content: Quote from: BillB on June 12, 2019, 07:40:16 pmYes... and check the circuit to see if the victim received more than .2 amps... if so, begin resuscitation and call the paramedics. If they received between .1 and .2 amps, then don't bother with resuscitation attemps; don't call an ambulance; immediately call the morgue.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2019, 09:16:11 pm
content: Quote from: tautech on June 12, 2019, 08:59:09 pmI suspect that by the time that digital multimeters became commonly available, they figured this feature was not such a good idea and could be left to safer high voltage probes or more specialize instruments. There was also a trend to lower voltage testing in general because of the shift in technology from vacuum tubes to solid state circuits.I have actually had this issue come up a few times since working to restore analog oscilloscopes. It would be very handy to have a meter with 3 kilovolt capability built in but I make due with a 15 kilovolt high voltage probe.

author: tautech
author_rank: Super Contributor
timestamp: June 12, 2019, 09:27:26 pm
content: Quote from: David Hess on June 12, 2019, 09:16:11 pmAppropriate addition above ^As we wish for more and more capability from an instrument something always suffers and where a heap of capability is squeezed into ahandheldit's little surprise the KV ranges have been dropped.The AVO's of old while not really a handheld were very portable more so than a bench meter and while new standards insist everything must have some CAT rating, the AVO's in particular were subjected to a 7KV flashover test.Their original leads are of excellent quality but difficult to find in good condition these days.

author: EEVblog
author_rank: Unknown
timestamp: June 13, 2019, 05:02:25 am
content: Quote from: windsmurf on June 12, 2019, 03:23:06 pmYep, a 20A fuse in a non-continuous 20A rated meter seems completely dodgy.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 13, 2019, 05:27:04 am
content: Unless you can still find some of those fuses that blow in 30 seconds at 20A...

author: Electro Detective
author_rank: Super Contributor
timestamp: June 13, 2019, 10:25:42 am
content: Quote from: shakalnokturn on June 13, 2019, 05:27:04 amEven if you find them, you won't buy themThey cost as much as a multimeterAnything over 8 amps best to buy an AC/DC clamp meter or clamp attachmentand optional CDs >>> Highway To Hell and Back In BlackThe meter shunt, PCB and leads will thank you for it

author: golden_labels
author_rank: Super Contributor
timestamp: June 13, 2019, 10:49:12 am
content: Someone has earlier mentioned wires and banana jacks as the possible cause of meters being limited to 10A.I’ve checked, how much can some non-specialized wires handle. The burden voltage across asinglewire at 5A(1)(4):Original UT70A 116cm, needle: 441mVOriginal UT70A 20cm, alligator: 338mVShenzhen Market 99cm, needle(2): 640mVDT830 66cm, ROTFL quality(2)(3): 653mVUniT probes are perhaps not the top quality, but decent enough and I doubt you can get extremely better for kit probes. So even with that we have nearly 1V drop. Extrapolating to 20A: it would be nearly 2V and over 15W in heat. So I guess the hypothesis, that this is a factor, makes sense: a multimeter would have to be supplied with probes designed specifically to handle 20A.A quick check on Digikey reveals that indeed banana jacks are generally sold in the 5–15A range.For entertainment alone I’ve checked, how Shenzhen Market alligator wires deal with current. Those which you may hunt down for $0.10/10pcs and treat as expendable. The result is in the attachment.____(1) I have no meter to measure 10A or a power supply good enough to provide exactly that value.(2) Added only for completeness, because I had them around.(3) So bad that the needles have fallen off before the first use.(4) All measurements at 5A±3%, 60s for letting the current stabilize, 10 samples mean with ±0.5%,1d accuracy.

author: ptricks
author_rank: Frequent Contributor
timestamp: June 13, 2019, 11:49:19 am
content: It is a very good question. I'm guessing that economics comes into it somewhere, the ,we could do more amps but what are people most likely to need for the cost to us, mentality. 10A is a number that isn't too high and not too low, it is probably what I would have picked.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 15, 2019, 10:52:08 am
content: Quote from: golden_labels on June 13, 2019, 10:49:12 amAs per your picture aboveShenzhen Market alligator wires deal with current badly because OneHungTooLow either crimps the wire poorly,or the solder does not flow on to the alligator/croc properly, hence the high resistance hot spot/BBQi.e. the solder is crap, so is the wire, the alligator has an oily film on it, so has the wire,the'will solder for food'flip flop and shorts wearing tech is wielding a OHL soldering iron with not enough heat,the iron is dirty and or knackered.. the list goes on and on and on...Cheap car jumper cables aren't much better, and give little to no love trying to pull 200 amps on a cold night to power up a flat car battery.The rich local dirt belly importers that make their money flogging such rubbish, crap they would never use themselves, need a good public tar and featheringanda rodgering by a gorillawell, I won't say it here

author: Berni
author_rank: Super Contributor
timestamp: June 15, 2019, 01:45:23 pm
content: Yep cheap crocodile clips are always horrible. Be careful!The wire they use is tiny and its always crimped into the crocodile with the copper strands bent back against the insulation and shoved in with the insulation included. This sort of crimp joint is high resistance and very unreliable.Looking around enough you can get properly made ones where the crimp only holds the insulation while the wire is soldered to the crocodile with a nice well flowed out ball of solder. One such that i have found are the SparkFun branded crocodile clips(https://www.sparkfun.com/products/12978). The wires handle a few amps just fine too (Despite looking so thin, most of it is copper inside).Because of this i developed a test routine that i run my crocodile cables trough. They are put across a PSU set to 3A and then having there cables janked and wiggled while watching that the voltage drop stays constant, then the 3A is left to flow trough them for a few minutes. If they keep consistent drop and get slightly warm they pass, if not they are banned from the crocodile clip storage area.Tho sometimes clips are decent, but just the wire is crap You can fix those by just ripping the wire out the back and properly attaching your own nice thick flexible silicone wire, worth the few minutes of work to get crocodile wires that are superior that almost anything you find in a store.

author: madires
author_rank: Super Contributor
timestamp: June 15, 2019, 02:52:37 pm
content: One of my DMMs does 20A for 10s and requires a pause of 15min before the next measurement (protected by a 10A fuse). I think the main reason for a 10A range is cost vs. market value. A DMM properly designed for 20A would be larger and more expensive while not many users would need the 20A range. Why adding an expensive feature to a "mass market" product? For high current measurements I'd use a clamp meter (no burden voltage) or an external current shunt for more precision.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 15, 2019, 04:42:48 pm
content: Or some sort of magnetoresistive sensor. I have a few made by LEM (the blue device mounted to the side).

author: joeqsmith
author_rank: Super Contributor
timestamp: June 15, 2019, 08:41:59 pm
content: Weston 6000 from 1976 showing an intermittent 15A for 15 seconds. Max, of 20A with the 0.01 ohm shunt.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 15, 2019, 11:29:07 pm
content: Math and Metallurgy aside, it can't be doing a 10A shunt trapped in a sealed plastic box/multimeter any favors by approaching and or going beyond it's limits for a period of time,especially with repetitive measurements.i.e. there must be some expansion and contraction stuff going on that may affect the accuracy of the final meter reading.The only way to find out for sure is to have two brand new identical model meters (no, I'm not going there about which brand)hey anything decent, maybe with a wishful thinking Cal cert tooand ensure they both read the same with stepped amperages from 1 to 10, taking notes along the way and any differences if any.Then subject one of them to it's documented limits, let it cool down, and a few more repetitive measurements.Labeland store both meters in the same place for say 24 or 48 hours so they are at the same room temperature,and do those initial amperages from 1 to 10 again and compare notesI'm betting there has to be some difference, regardless of whether they be expensive multimeters, middle of the range, or cheapie wheel chocsOr are the shunts 'broken in' already to account for this, which I doubt because it may add 1 cent to production costs

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2019, 02:06:21 pm
content: QuoteQuoteCould you define the number of steps and the dwell time.QuoteAre you suggesting the meters be cycled using the higher limits? Cool down would normally be part of it. How many cycles makes up a few? 5, 5000? I suggest it be automated or the human error may bias your data.Quotefollow but I doubt you need to wait that long. The meters normally can't resolve down very far.QuoteI agree about having the control and glad you included it. Your not just a spit shinning audio buff.The choice of meters may be a bit more important. They would need to be sensitive enough to pick up these changes you are attempting to find. You also need to pick one where it documents the higher current condition. You may also want a meter that can log, just to make documenting the results easier depending on how you plan to cycle it. Some of the meters use higher quality shunts. The Watt loading varies a fair amount as well. Personally, I would try to find something with a small surface area, small meter (case wise), with the most aggressive specs. Basically bias the test in your favor. Maybe then repeat the test using a higher class meter and see how much it varies.How much change are you expecting?

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 16, 2019, 04:56:52 pm
content: You somehow have to connect to that 20A source safely, neither do i have any test clamps rated for 20A to 4mm jacks, so even if the meter would support it, the leads are supplied by user mostly - nor would i want to take special considerations for inserting them into the meter, so all inrush current and such will likely happen when connecting.Over a certain power the switch should be somehow assisted to achieve a fast and low-wear switching process.The clamp ammeter is the better, safer choice for that.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2019, 05:02:51 pm
content: Good point. Electro Detective, are you wanting to test the meter, or the meter + leads?

author: HKJ
author_rank: Super Contributor
timestamp: June 16, 2019, 05:14:56 pm
content: Test leads for 10A is not a problem, I tested some here with banana to alligator clips:https://lygte-info.dk/review/Equipment%20DANIU%201M%204mm%20Banana%20Plug%20to%20Alligator%20Clip%20UK.htmlMost interesting is this photo probably, it is after 15 minutes at 10A:

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2019, 05:56:54 pm
content: Quote from: HKJ on June 16, 2019, 05:14:56 pmAbsolute statements are seldom correct. The following suggests otherwise:https://www.youtube.com/watch?v=fQowDZstguw&feature=youtu.be&t=1230

author: HKJ
author_rank: Super Contributor
timestamp: June 16, 2019, 06:00:09 pm
content: Quote from: joeqsmith on June 16, 2019, 05:56:54 pmI did not say you could use any random test lead for 10A, but you can get some that can handle it.And if you look at my thermophoto you will see they handle it nicely, there is no risk of smoke.

author: CDaniel
author_rank: Frequent Contributor
timestamp: June 16, 2019, 06:05:55 pm
content: We discussed earlier that even if you accept the losses in cables , shunt , fuse , the low voltage circuits will be greatly influenced by this series resistance , so it is useless to have 20A , 10A let's say is a somewhat reasonable compromise for high-ish voltages . That's why usually crappy no name multimeters were the first to be branded for 20A .

author: joeqsmith
author_rank: Super Contributor
timestamp: June 16, 2019, 06:10:04 pm
content: Quote from: HKJ on June 16, 2019, 06:00:09 pmTest leads for 200A are not a problem. I've tested a few and they handle it nicely. So, yes, I am sure we can buy/make what ever is required. But not all leads marked for 10A will handle it.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 16, 2019, 11:04:31 pm
content: IMHO the test leads should be oversized so as to reduce/negate any effects I touched on earlier about comparing two meters shunts, with one meter getting a good burn.Once confirmed there is some or no shunt performance hit, any leads and their performance with 10A > 20A can be done any timeand what limitations if any notedi.e. start with the meter first to know what the deal is on extremes, before hooking up to test the ampacity of other gearand perhaps copping fluctuating readings

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2019, 01:29:15 am
content: Quote from: Electro Detective on June 16, 2019, 11:04:31 pmSee:https://www.eevblog.com/forum/testgear/why-are-multimeters-10a-and-not-20a/msg2488905/#msg2488905If you are looking for a major error, it's possible to run the test using a single meter. Just get a baseline at some temperature. Run what ever profile you want and repeat the test at that same temperature.Another option if you are only concerned with how the shunt changes, is only test the shunt. Everything I have done with the DMM shunts has been destructive, but I believe there are a couple of junk meters sitting in the to be recycled bin yet (that are missing some pots).

author: tooki
author_rank: Super Contributor
timestamp: June 17, 2019, 04:32:20 am
content: Quote from: joeqsmith on June 16, 2019, 05:56:54 pmI think HJK’s point whooshed right over you. He said that  leads that can carry 10A is not a problem. (This is true: it’s easy to do.) He did not say that 10A is not a problem foralltest leads!Indeed his reply was a response to earlier posts making blanket statements that banana plugs are only good for a few amps. (That statement is patently false.) So lecturing him about absolutes is absurd, and you’re coming back arguing essentially the opposite, that there are no truths, either:The existence of falsely labeled leads does not change the truth that it is eminently possible to make 10+ amp banana test leads. The fakes don’t suddenly make the real deal vanish...

author: Electro Detective
author_rank: Super Contributor
timestamp: June 17, 2019, 05:37:21 am
content: Would a multimeter shunt get the same/more/less hammering from 10A of DC, or AC?or crazy waveform from a crap UPS, or a programmable waveform PSU ?i.e. leads/cables aside, will the shunt get hotter/same temp faster/or amps is amps, regardless of alternating or direct currentfwiw I've never had the need to current test DC past 4 or 5 amps via a multimeter,but have done 2400 watts/10A AC briefly, to compare the readings to my clamp metersand ensure they are doing the job properly plus find any jaw sweet spots (for want of a better description)to reduce any error factor, and if required rig up some foam thingie or whatever to keep the wire where it needs to be in the jaw.Anything over 5 amps DC or AC is clamp meter city for me = less thinking, less drama, less sparks,no barbequed leads to patch up or toss..BTW is anyone clued to what's actually going on with multimeters sporting AC TRMS vs non TRMS when measuring suspect to distorted AC currents?Are the readings more or less akin to voltage mode readings, but with reduced AC bandwidth for AC currrent ?all the above assumes no oscilloscopes and current probes in da house,and 50/60 hz to keep it simple..for now

author: Fungus
author_rank: Super Contributor
timestamp: June 17, 2019, 08:55:21 am
content: Quote from: Electro Detective on June 17, 2019, 05:37:21 amAmps is amps. Number of electrons going past per second.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2019, 09:53:53 am
content: Quote from: tooki on June 17, 2019, 04:32:20 amActually, I quoted their post. "Making" was not used and actually they show a purchased part. I am not sure what their post was in response to as they did not provide a link. If you read the comments, I doubt there is a question on if there are leads available that can handle 10A.My second response, is every bit as accurate as HKJ's original post. I make no claims that there are not leads that can't handle 200A nor do I state all leads can handle 200A. I also don't specifically define test leads. Lots of missing details, on purpose. It's the lack of details in their original comment that cause me to interpret it as a blanket statement.Hope that helps clear things up for you.

author: HKJ
author_rank: Super Contributor
timestamp: June 17, 2019, 10:20:34 am
content: Quote from: joeqsmith on June 17, 2019, 09:53:53 amMy post was in response to all the discussion about 10A/20A probes and shows that you can easilybuya cable that can handle 10A and I show a real test of it (Melting a cable may be fun, but do not show what it can handle).

author: joeqsmith
author_rank: Super Contributor
timestamp: June 17, 2019, 11:46:08 pm
content: Quote from: HKJ on June 17, 2019, 10:20:34 amI suspect most of the leads I tested could handle the 10A they are rated for. I attempted to swept them all to 20A for a far longer period than the meters call for. I would step them from 2A to 20A in increments of 2A, holding at each level for 30 seconds. Or 5 minutes total. For the most part, they held up pretty well to this level of testing.10A with 15 minutes of dwell time at room temperatures is not much of a test. It really doesn't show a whole lot but its better than nothing. I test to failure to get some idea how much headroom we have.

author: AG6QR
author_rank: Frequent Contributor
timestamp: June 18, 2019, 01:57:56 am
content: Quote from: Electro Detective on June 17, 2019, 05:37:21 amOne definition of the TRMS value of an arbitrary waveform is: the steady-state DC value of current that would produce the same heating as the arbitrary waveform, when both are applied to identical resistances.In other words, if their TRMS values are the same, they will both heat up a resistor the same.

author: HKJ
author_rank: Super Contributor
timestamp: June 18, 2019, 04:50:55 am
content: Quote from: joeqsmith on June 17, 2019, 11:46:08 pmA thermophoto shows headroom very well.

author: bugi
author_rank: Regular Contributor
timestamp: June 18, 2019, 10:12:55 am
content: Quote from: AG6QR on June 18, 2019, 01:57:56 amSort of. If a short time peak current heats a small volume, there might not be enough time to spread the heat and the "averaging" fails. Can be seen easily e.g. with semiconductors where peak/pulse current ratings can be higher than the continuous ratings, yet the max pulse averaged can be a fraction of continuous rating. Discrete resistors are certainly more robust, but they too have limits with peaks/pulses.At high frequencies (or very thick power cables) the skin effect would also come to play, but I think anyone working with those already knows what they are doing, and is likely unrelated to this 10A vs. 20A case.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 18, 2019, 04:09:00 pm
content: Quote from: HKJ on June 18, 2019, 04:50:55 amCare to explain how?

author: HKJ
author_rank: Super Contributor
timestamp: June 18, 2019, 04:58:11 pm
content: Quote from: joeqsmith on June 18, 2019, 04:09:00 pmThe wire only gets about 15°C hotter from the 10A, there is no bad hotspots. Doubling the power would mean about 30°C temperature rise and that would not be dangerous for the wire either.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 18, 2019, 06:53:29 pm
content: Quote from: HKJ on June 18, 2019, 04:58:11 pmFrom 10 to 20A the power is quadrupled.Thus, going from 10A to 20A you are talking about an increase of 60°C over the base temperature which, depending on the insulation and the room temperature, can melt it.Interesting info:https://www.physicsforums.com/threads/current-temperature-equation.364495/#post-2503388https://en.wikipedia.org/wiki/Heat_transfer_coefficient

author: HKJ
author_rank: Super Contributor
timestamp: June 18, 2019, 07:08:30 pm
content: Quote from: rsjsouza on June 18, 2019, 06:53:29 pmYes, I know and therefor I would not recommend that test lead for 20A. I might survive depending on the plastic used.

author: tooki
author_rank: Super Contributor
timestamp: June 18, 2019, 09:10:18 pm
content: Quote from: joeqsmith on June 17, 2019, 09:53:53 amYou quoted their postbut you didn't understand it.As for why I said "making" and not "buying", it's because a) if you make it yourself, you can get a >10A-capable cable, and b) a purchased cable must also have been made (at some point), and therefore it's also possible for a cable made by someone other than yourself to be capable of >10A.Quote from: joeqsmith on June 17, 2019, 09:53:53 amBut several comments in the prior discussion DID express doubt. That's literally the whole point here.Quote from: joeqsmith on June 17, 2019, 09:53:53 amIt wasn't worded as a blanket statement. It was simply saying "10A leads do exist", not "all leads that exist are 10A". His statement isn't in perfectly worded native English, but it's eminently clear that he was saying the former, not the latter. Hence why it's silly of you to pontificate about blanket statements, when indeed he was refuting prior blanket statements...Quote from: joeqsmith on June 17, 2019, 09:53:53 amThe whole point of me replying to you is to clear up YOUR confusion. (But I've clearly failed.) I was not, and am not, confused.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 18, 2019, 11:33:40 pm
content: Quote from: tooki on June 18, 2019, 09:10:18 pmI thought the point was to answer Dave's questionQuoteQuote from: tooki on June 18, 2019, 09:10:18 pmI explained why in interpreted the statement as I did. Your continued ramblings suggest you are still confused.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 18, 2019, 11:45:25 pm
content: Quote from: HKJ on June 18, 2019, 07:08:30 pmNo doubt your picture would look nice hanging on the wall but I can pretty much touch the leads and tell if they are getting too hot. Personally, I would rather have the data for how much voltage they drop, their resistance and such at various points. I can also get an idea when they start to thermally run away and if the insulation will catch fire.I had thought about running a pull test on them. I still have the second probe out of these tests and may do something with them in the future.

author: tooki
author_rank: Super Contributor
timestamp: June 19, 2019, 03:59:15 am
content: Quote from: joeqsmith on June 18, 2019, 11:33:40 pmThat you think of them as "ramblings" despite having clearly explained, cogent replies proves to me that you aren't even attempting to read it, you've skimmed it at best and gotten a poor understanding of it, as you did with HJK's original comment and his later clarification, which confirmed MY interpretation of his original comment. If you can't understand plain English, I can't help you, dude. But don't you dare callme"confused" just because YOU can't understand things.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 19, 2019, 06:40:33 am
content: Quote from: HKJ on June 18, 2019, 04:50:55 amOf course, but don´t forget that these leads are for being handled by hand, so even if their insulation withstands 70°C, they would not be cool enough to be touched. This might be a problem for all considerations regarding ratings, as long as these ratings are electrical ratings they might not apply for handheld devices and their safety of use.I am sure this drives requirements up by a good amount and would require much stiffer leads, harder to use in practical cases.Last thing you want is touching something hot in an electrical cabinet while measuring 20A and throw it somewhere else.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 19, 2019, 11:37:43 am
content: Quote from: tooki on June 19, 2019, 03:59:15 amI understand things very well, but I dare say you just appear confused. You shouldn't expect me to waste my time reading your long-winded ramblings. Your efforts will be wasted on me.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 19, 2019, 11:48:56 am
content: Quote from: SparkyFX on June 19, 2019, 06:40:33 amAs part of my test, I applied a 15KV peak transient across them and monitored to see if they would breakdown. I don't remember any of them having a problem. With your statement, I was thinking that if the lead were pinched while running the thermal test it may cause the insulation to to fail.It seems there were parts of the safety standard that talks about the leads. I'll have a look and see what sort of testing they require.

author: tooki
author_rank: Super Contributor
timestamp: June 19, 2019, 12:28:25 pm
content: Quote from: joeqsmith on June 19, 2019, 11:37:43 amNo, you’ve demonstrated vividly and repeatedly that you do NOT understand things, never mind “very well”. If I appear confused to you, it’s becauseyouare not understanding the words I write.Quote from: joeqsmith on June 19, 2019, 11:37:43 amMy original reply to you was short and concise. The replies only got longer in trying to spell things out for you, since you did not (and still do not) understand.Quote from: joeqsmith on June 19, 2019, 11:37:43 amThat much has become evident. Your conviction in remaining ignorant is impressive.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 19, 2019, 01:56:37 pm
content: Quote from: tooki on June 19, 2019, 12:28:25 pmThat fact you acknowledge that you understand you are wasting your time, yet continue to do so, is what I find impressive about you. I don't mind the banter but I doubt others feel this way as it really isn't productive. So before the admins step in, I will refrain from responding and let you have the last word.

author: ggchab
author_rank: Frequent Contributor
timestamp: June 19, 2019, 02:33:44 pm
content: Sometimes I'm (still) surprised to see how smart people can behave like little boys in a playground

author: tszaboo
author_rank: Super Contributor
timestamp: June 19, 2019, 03:32:38 pm
content: Also worth noting, that it is surprisingly difficult to get current shunts in some ranges. In the lower ranges there are shunts made out of sheet metal, stamped, welded etc. In the higher ranges, there are the thin and thick film resistors. In between, there is a gap, where there is no (cheap) technology to make shunts.I was searching for shunts in the 0,1-1 Ohm range, few ppm tempco, precision resistor.I contacted manufacturers from both sides, the sheet metal gets " too flimsy" if you want higher resistance, the film cannot get that thick. Or they just tell you to place them in parallel. I ended up with a custom order, laser trimmed, and overloaded the production line of the supplier with a "too big order".You have similar problems, when you have a design and they ask you for "just a little bit more current". Like a relay board with 20A relays instead of 10. A PCB mounted 20A relay looks completely different than the 10A types, and with the change, the entire board needs a redesign.

author: Fungus
author_rank: Super Contributor
timestamp: June 19, 2019, 03:39:26 pm
content: Quote from: NANDBlog on June 19, 2019, 03:32:38 pmI use these for 0.1:https://www.ebay.com/itm/152249838861

author: bugi
author_rank: Regular Contributor
timestamp: June 19, 2019, 04:38:31 pm
content: Quote from: Fungus on June 19, 2019, 03:39:26 pmI don't think that one is "precision" (has 1%) or has "few ppm tempco" (not told, and I can't find specs). The examples of that type of power resistors that I've seen have much higher tempcos, like 20 at best, much worse typical, and 1% tolerances at best, hardly "precision". In such normal power resistor category there are some choices in that 0.1-1ohm range.

author: tszaboo
author_rank: Super Contributor
timestamp: June 19, 2019, 04:42:50 pm
content: Quote from: bugi on June 19, 2019, 04:38:31 pmDid I say, I needed thousands of it for a board with maximum height of 12mm?Anyway it was years ago. And there is nothing precision about those wirewound resistors. You dont want it in a multimeter for example.

author: Fungus
author_rank: Super Contributor
timestamp: June 19, 2019, 04:52:16 pm
content: Quote from: bugi on June 19, 2019, 04:38:31 pmYeah, I know, but as noted 0.1Ohm with a lot of watts is hard to find. Even those max out at 10A.I just pulled one out of the drawer and did a 4-wire measurement on it. It was about 0.5% tolerance.

author: Fungus
author_rank: Super Contributor
timestamp: June 19, 2019, 04:54:42 pm
content: Quote from: NANDBlog on June 19, 2019, 04:42:50 pmNope.Quote from: NANDBlog on June 19, 2019, 04:42:50 pmThe spec was "shunts".

author: joeqsmith
author_rank: Super Contributor
timestamp: June 24, 2019, 01:09:08 am
content: QuoteQuoteQuoteQuoteQuote from: Electro Detective on June 15, 2019, 11:29:07 pmIt's too bad you never responded. Are you no longer interested? I was thinking, I could use a precision shunt and my bench meter for the reference and then test using single meters without a second control. You never mention what change you are looking for but I doubt if there was a detectable change that it would show up with the lower resolution meters.Looking at the higher resolution meters I have, the 121GW and TPI 194II make no mention of being able to over drive them. The Fluke 87V, 196 and CEM DT9939 do not specify cool down times. The Gossen Ultra has a fairly detailed requirements but is limited to 16A. Many of the manuals cover the ambient temperature requirements but I don't see that as being a problem.The leaves the following meters:Brymen BM869s & BM839, 20A 30sec, 300sec coolOWON B41T, 20A 10sec, 900sec coolUNI-T UT181A, 20A 30sec, 600sec coolAll four meters can resolve down to 1mA. There are some advantages to using the BM869s. I have two of them so you could have your control. It also offers the fastest cycle time. Hard to say how many cycles it would need to run for you to be satisfied. If I arbitrarily ran 1000, we could have the test ran in five days, including your 24 hour cool down. I would also use the leads supplied with the meters.Plan would be to just run all three in series, step the current from 1 to 10A in 1A steps and hold it for say 30seconds at each step. The five minutes they are on for should not pose any problem. One meter would then be cycled and then I would repeat the step test after the 24 hour cool down.

author: bitseeker
author_rank: Super Contributor
timestamp: June 24, 2019, 06:20:13 am
content: To add to the historical examples:The 1969 Fluke 8100A has a base ~10000 counts with 20% overranging (e.g., 1.1999 max on 1V range). Although it doesn't have built-in current measurement capability, its companion model A90 external current shunt has the following ranges: 100 µA, 1 mA, 10 mA, 100 mA, 1 A, and ... 10 A, which fit the 8100A (each range outputs 100 mV to the DMM).I don't have a manual for the A90, but the 10 A range is in red. I assume that means no 20% overrange on that shunt.

author: David Hess
author_rank: Super Contributor
timestamp: June 24, 2019, 01:50:57 pm
content: It looks like the current economical state of the art is 1, 3, and 5 watt 5% 20ppm/C wire shunts of 10, 5, and 3 milliohms for less than $2:.https://www.mouser.com/Passive-Components/Resistors/Current-Sense-Resistors/_/N-7fjcfZ1yzvvqx?P=1z0wsz3Z1z0wszxZ1z0wui7Z1yzu2eoZ1yzv6m0Z1yzv09oZ1yzv3s6Z1z0z7l5&Rl=7fjcfZerdlZ1yu1nq5Z1z0vjvqSGTThat leaves plenty of margin for an inexpensive 20 amp design and higher is feasible with a 5 or 3 milliohm shunt depending on the desired accuracy; 20ppm/C into 0.5% is 250C.Running shunts in parallel for greater current or accuracy would be an interesting exercise. Accuracy is limited by the temperature rise of the shunt so a more powerful shunt with lower thermal resistance yields better accuracy.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 04:14:13 pm
content: The shunt is the easy part. With a 3 mOhms shunt it would still be some 60 mV and 1.2 W of power dissipation. So even the 5 W shunts would run rather hot and could produce quite some self heating error, both from the TC and thermal EMF effects. The range could still be acceptable, though lower accuracy beyond some 10 A.The more tricky parts are likely: availability of a suitable 20-25 A HRC fuse. This could especially be a problem if in some countries a higher breaking current is required for a higher nominal current. Fuse size could also be a problem for a portable instrument.The fuse could also pose a serious heat problem - even at 10 A the fuse could produce some 4-8 W of heat, that is hard to remove from a plastic case. At 20 A one would expect about twice the heat. Even with a 10 A fuse I would ideally like to have a temperature sensor to warn of the meter gets too hot inside from prolonged use at high temperature (or possible other issues).The normal 4 mm sockets also could reach it's limit at more than 15 A - at least requiring good connectors and possibly more trouble from overheating connectors if poor probes are used.Given the problems with high current, it would probably be easier to have an extra clamp meter or clamp attachment.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 25, 2019, 12:04:10 am
content: Showing the basic setup. Using the Probe Master leads I normally use. I am running a bit over the 20A max for 30 seconds on-time and 5 minutes off time.It's been running for some time now and as you would expect, it does not appear to be changing.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 26, 2019, 12:09:44 am
content: I let the Brymen BM869s cycle for over a day. I then let it set at 10A for an hour and then allowed it to cycle some more. About a day and a half total run time.During this time, the Brymen never appeared to drift from the Fluke 189 with the shunt. There was some change in the Brymen's case temperature but hardly worth mentioning. I didn't record it.I did not allow the meter to cool but rather just attached it back in series with the other BM869s. I had ran a check at 5.00A to keep that last digit. If there was a shift, it's below what the meter can detect.Of course this doesn't mean that other meters would not shift. I would try to find something with the smallest shunt rated for the highest duty cycle.