Potentially Dangerous Multimeter... what do you think? - Page 1
Forum section: electronics / testgear

author: sambonator
author_rank: Regular Contributor
timestamp: January 31, 2019, 03:12:22 am
content: So I bought a "Commercial Electric" Home Depot exclusive digital multimeter, model MS8301B. Everything checked out ok except the mA and uA settings.It seems to have a flaw which I think is potentially dangerous. The two meters in the pictures are hooked up in series, powered by a bench supply powering an electromagnet.What do you think? The pictures tell the whole story, I think... (note the dial setting vs meter readings).

author: joeqsmith
author_rank: Super Contributor
timestamp: January 31, 2019, 03:29:58 am
content: Move the leads from the Amps to the mA when you change to the mA range.

author: KaneTW
author_rank: Frequent Contributor
timestamp: January 31, 2019, 03:34:07 am
content: The left connector is for amps. The right is for mA and uA.

author: Brumby
author_rank: Unknown
timestamp: January 31, 2019, 04:08:53 am
content: Quote from: sambonator on January 31, 2019, 03:12:22 amYes, the pictures do tell the whole story.You have said "note the dial setting vs meter readings" to which I add my voice to the above comments, which is to say:note the sockets you are using.Don't feel too bad, though. There would be very few of us who haven't tripped ourselves up on something as trivial - but we try and keep those moments within the privacy of our own workbench.

author: Fiveohfour
author_rank: Contributor
timestamp: January 31, 2019, 04:53:48 am
content: Quote from: Brumby on January 31, 2019, 04:08:53 amRandom question: Who makes the meters with the MS prefix originally? Mastech?Sent from my iPhone using Tapatalk

author: sambonator
author_rank: Regular Contributor
timestamp: January 31, 2019, 04:55:35 am
content: Thanks guys! (or maybe that's presumptuous... should it be Thanks folks?)! Yes I noticed my error as soon as I posted it. I thought I deleted it before anyone responded but apparently the delete failed.Anyhow, its strange that it gives the incorrect readings... the Aneng Q1 rebrand that's in the pics don't behave that way whether you plug into the A or the mA/uA input... is this a common thing with many multimeters?Thank you!Sam

author: Fiveohfour
author_rank: Contributor
timestamp: January 31, 2019, 04:58:53 am
content: Quote from: sambonator on January 31, 2019, 04:55:35 amIt looks like the Aneng only uses the second jack for uA and shares A/mA on the other. That’s my take at leastSent from my iPhone using Tapatalk

author: sambonator
author_rank: Regular Contributor
timestamp: January 31, 2019, 05:21:17 am
content: Quote from: Fiveohfour on January 31, 2019, 04:53:48 amYes it looks very much like the Mastech MS8301 they have on their website. This must be a version they make exclusively for Home Depot under the Commercial Electric brand.Quote from: Fiveohfour on January 31, 2019, 04:58:53 amYes... but its just strange that the this MS8301B is able to read milliamps from the wrong input terminal for the dial setting, and give a reading that is spot on except by a factor of 100 (or 10,000 when switched to uA)...

author: Fiveohfour
author_rank: Contributor
timestamp: January 31, 2019, 06:35:51 am
content: Quote from: sambonator on January 31, 2019, 05:21:17 amThe switch tells it where to put the decimal, you’re reading in this case at least is within the defined ranges, you’re just filtering that reading the wrong translator effectively. I can’t speak to normal behavior, but it stands to reason if you have a meter with a resolution x places after the decimal in amps, then measuring anything up to that point while in amps will simply show the behavior you experienced.Maybe I’m missing something though. I could see some meters making a point of designing it in such a way so that anything reading of microamps with a 0 before the decimal triggers an out of range error to help clue you in.Thanks, I’m trying to figure out which brands are the originals so I can know to ignore the 39 clone brands of the “original clone manufacturer” lmaoSent from my iPhone using Tapatalk

author: w4tch0
author_rank: Newbie
timestamp: January 31, 2019, 02:04:06 pm
content: I have aTecpel DMM-8088"automotive multimeter" at home that someone donated to me and it behaves in a similar way. Putting the leads in the 20Amp range input and setting the DMM dial to the mA range input (<400mA) will show a reading that is an order of magnitude lower (or maybe two orders of magnitude, I don't remember anymore) than the actual current.Thought it's maybe a result of the abuse this meter had to experience in its earlier life as the Meter had both fuses blown when I got it.

author: m98
author_rank: Frequent Contributor
timestamp: January 31, 2019, 02:25:25 pm
content: This is indeed quite strange, maybe the SoC has only one current measurement input?

author: glarsson
author_rank: Frequent Contributor
timestamp: January 31, 2019, 02:54:23 pm
content: Not strange. The current does not pass through the chip. It passes through a shunt resistor and the voltage over the resistor is measured by the chip. The reason for current through either input being "visible" is the common design of many multimeters.Imagine two resistors in series, one 0.1 ohm and one 99.9 ohm. The 0.1 ohm resistor is connected between COM and mA. The 99.9 ohm between mA and A. The chip measures the voltage over both resistors. If mA is used the the effective shunt resistor is 100 ohm. If A is used then the effective shunt resistor is 0.1 ohm. In this case the 99.9 ohm resistor will be in series with the chip input, but that doesn't matter as its impedance is much higher than 99.9 ohms. More expensive multimeters solve this problem by sensing what input is used and beep if selected function and input doesn't match, or physically block the inputs by sliding in a shutter over the non-matching input.

author: rsjsouza
author_rank: Super Contributor
timestamp: January 31, 2019, 09:32:39 pm
content: Quote from: glarsson on January 31, 2019, 02:54:23 pmOr in the case of the el-cheapo Mastech MAS830L (M830B-class), the rotary switch simply disconnects the other input from the jacks, giving zero uA or mA if the probe is on the A jack.

author: glarsson
author_rank: Frequent Contributor
timestamp: January 31, 2019, 09:44:36 pm
content: Quote from: rsjsouza on January 31, 2019, 09:32:39 pmBut does it do the opposite, i.e. disconnect the A jack? 10A through the rotary switch?

author: sambonator
author_rank: Regular Contributor
timestamp: January 31, 2019, 09:50:48 pm
content: Quote from: glarsson on January 31, 2019, 02:54:23 pmThank you Glarsson.Last time I touched a multimeter was many years ago, a cheap Radio Shack meter I used as a teen.I recently took an interest in electronics again after watching a bunch of EEVBLOG and related videos on Youtube. I decided to start building up equipment for my own bench with a few pieces of equipment, and begin at first with lower cost equipment, hoping to upgrade later if I decide to stick to this hobby. For meters, I purchased the Aneng rebrand and most recently, the CE MS8301B in the pics.As you saw above, I was fooled by the terminal layout. Intuitively I moved the leads out of the Voltage slot, to the Amps slot to measure current. They even marked the indicators yellow to match the yellow labeled slot. Nice! So I thought... I even got readings in A, mA, and uA settings... must be working!But no... mA, uA readings were off by 100x and 10,000x. I thought this must be a bug... and potentially a dangerous one.So I decided to post here, only to discover my mistake after carefully examining the small print on the input markings... ? What, I measure Volts and amps (mA, uA) from the same input?So begain this thread.And now, after input from all the folks smarter than me above, I have more questions/concerns.1. Is it this a common design, to have the Voltage input shared with the amps input (mA, uA in this case). I'd imagine you'd easily blow the 400mA fuse if you're measuring voltage and change the dial to the A.2. Do you think the yellow A, uA and mA markings that match the yellow Amps input misleading? Surely misled me, and I'd imagine many others as well, especially Home Depot DIY shoppers.3. I'm thinking this is a safety flaw. I think this is potentially dangerous, especially for amateur home DIYers trying to fix electrical things, and getting fooled into thinking something is outputting 10mA when in fact its 1A, or worse if you're in the uA setting. What do you think? Or is this normally accepted and a non-issue in the electrical/electronics test equipment industry?Thank you!Sam

author: glarsson
author_rank: Frequent Contributor
timestamp: January 31, 2019, 10:09:31 pm
content: It is common to have one input jack for all low current signals (V, ohm, mA,...) and switch the signal around using the rotary switch. It isn't a good idea to switch high currents (e.g. 10A) through the rotary switch, so it gets its own input jack. The majority of handheld multimeters today use either four input jacks (A, mA/uA, COM, V/ohm/F/...) or three input jacks (like yours).I agree that the color coding is confusing.I don't know about dangerous. If can be argued that one should know the instruments before connecting them in a potentially dangerous situation. Even when correctly marked you can create a dangerous situation by connecting the 10A input jacks across something that can't survive the short circuit..

author: Fungus
author_rank: Super Contributor
timestamp: January 31, 2019, 10:15:02 pm
content: Quote from: glarsson on January 31, 2019, 09:44:36 pmLet's hope not.Fun fact: You can check the state of the 10A on meters by switching to continuity mode and poking the red probe into the 10A hole. No need to take the meter apart.(and onsomemeters you can check the mA fuse, too, eg. my Fluke 27).

author: tsman
author_rank: Frequent Contributor
timestamp: January 31, 2019, 10:24:36 pm
content: Quote from: sambonator on January 31, 2019, 09:50:48 pmYes. This is extremely common. People measure the current of something then decide to measure the voltage of something but forget to change the mode. The low shunt resistance means they're effectively shorting out whatever they're testing. If it is a crap meter then you're in trouble as it may not be fused or has the wrong type of fuse that isn't designed to handle high fault currents.Quote from: sambonator on January 31, 2019, 09:50:48 pmYes. Poor colour choice by Home Depot.Quote from: sambonator on January 31, 2019, 09:50:48 pmI wouldn't call it a safely flaw. You still see that there is current flowing. A bigger problem would be if it didn't read voltages correctly.Always check that you've got the probes in the correct sockets and that it is in the correct mode.

author: rsjsouza
author_rank: Super Contributor
timestamp: January 31, 2019, 10:28:04 pm
content: Quote from: glarsson on January 31, 2019, 09:44:36 pmSorry, I guess I didn't write this well. What I meant is that "(...) the rotary switch simply disconnects the A/D input from the jacks (...)"The load is still connected and current is flowing through the beefy 10A shunt, but the meter does not give false readings. IMO this is a more sensible approach.BTW both UT-61E and UT-136C give weird readings just like the ones on the OP. The UT-61E halves the measurement when the probe is connected to the A jack and the switch is in the mA/µA positions (0.130A becomes 65.0mA or 65.0µA).

author: tsman
author_rank: Frequent Contributor
timestamp: January 31, 2019, 10:33:44 pm
content: Quote from: rsjsouza on January 31, 2019, 10:28:04 pmWouldn't a 0 reading be worse than a wrong reading? You'd need to do the socket detection to do it properly.

author: sambonator
author_rank: Regular Contributor
timestamp: January 31, 2019, 10:58:14 pm
content: Quote from: tsman on January 31, 2019, 10:33:44 pmTo me, a 0 reading would make me double-check the meter, setup, probe, etc., so in a sense seems safer...

author: Fiveohfour
author_rank: Contributor
timestamp: January 31, 2019, 10:58:18 pm
content: Quote from: sambonatorSam, no need to fall on your sword and allow yourself to conclude everyone of us who replied yesterday explaining it is “smarter” than you. Intelligence is not defined as a thorough and up to date understanding of electrical metrology devices.That said I think you’re simultaneously being a little too hard on yourself and not hard enough on yourself. When it comes down to it, there is no “flaw” here except a flawed understanding driven by some assumptions on your part and a failure to thoroughly familiarize yourself with the device and how it functions, or even how the single function you wanted to use...well, functions.Is it an understandable error? Heck yea, but that doesn’t make it any less of a PEBCAK situation through and through.I mean all the answers are literally written right there for anyone who’s taking the measurement of electricity as seriously as we all should even if we often don’t.That you felt like it was acceptably safe to use a device you’re not familiar with by glancing to make sure there’s some yellow on all the areas you’re using is surely a much greater safety “flaw” then the design of this unit, of which there’s no actual flaw, at least not yet, I know the world is heading in the direction of trying to make user error somehow be considered a “design flaw”, and sure in many cases human nature needs to be anticipated and corrected for to prevent catastrophe, there’s no such potential here though luckilyAnd just tbc again, give yourself a break. I tried to measure current through a 12v Car battery 5 seconds after opening the first multimeter I’d ever held in my hands and did so by attaching it to the proper terminals, selecting 10A, and connecting red to red and black to black, because hey it’s gotta work that way right, what’s the worst that can happen, I’ve got a 10A fuse between me/my $5 meter, and the ~1,000amps the battery could dump out on demand, so safety precautions have been fully implemented.A moment later the negative probe launched itself off of the wire it had been physically attatched to a moment before, and I had a sudden suspicion reading the instructions were in order.Point being, we all have momentary lapses of judgement/common sense etc. at some pointSent from my iPhone using Tapatalk

author: Fiveohfour
author_rank: Contributor
timestamp: January 31, 2019, 11:15:44 pm
content: Quote from: rsjsouza on January 31, 2019, 10:28:04 pmRight and the UT61E has a separate Jack for mA/uA right next to the 10A fused jack, so it’s not weird at that point is it?We know the devices in question aren’t auto ranging, therefore it relies on the selection we make to correctly route the current/interpret the values and display them accordinglyAnd we lastly know that there’s a Jack for the high current mode, and a Jack for the lower range, so it would seem logical that selecting a range and using anything but the terminal intended to be matched up w/that mode will have some effect on the resulting values, as budget device makers aren’t in the business of using more components than needed/added complexity for no reason at all.The best part of this though is if you lose one of those ranges somehow & the other still works, you can keep using it so long as you keep a calculator handySent from my iPhone using Tapatalk

author: Fungus
author_rank: Super Contributor
timestamp: February 01, 2019, 01:01:52 am
content: Quote from: sambonator on January 31, 2019, 10:58:14 pmYep. It should say "OL", not 0.00.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 01, 2019, 01:49:19 am
content: Quote from: sambonator on January 31, 2019, 09:50:48 pmI have looked at several meters where they share inputs like yours. The worst ones will fuse all of the other functions with the current. So you have your meter in the mA and blow a fuse, the entire meter is dead. Not as common but I run into them. I've looked at a lot of low end meters and use one still for my garage work. I wouldn't use one in CAT III even though many claim they are fine for that environment but they have their place.

author: Fiveohfour
author_rank: Contributor
timestamp: February 01, 2019, 03:05:26 am
content: Quote from: joeqsmith on February 01, 2019, 01:49:19 amYea this design in itself is perfectly safe/usable etc so long as it’s designed correctly. One of my most enjoyable things to do is watch BigClive continue to find lower bars for sketchy/unsafe products and watch then as he spends 3 minutes to sketch out a PCB that’s better in every way and includes the added bonus of no longer being deadly if you touch it while chargingThe deadly Dalek that sent mains power through the USB port’s metal facade, and all because of a little LED light that served no purpose was connected in a way that made the whole unit unsafe ...that was one of my favoritesSent from my iPhone using Tapatalk

author: rsjsouza
author_rank: Super Contributor
timestamp: February 01, 2019, 03:10:56 am
content: Quote from: Fiveohfour on January 31, 2019, 11:15:44 pmI am not entirely sure what you mean here. My post is not pointing a finger to the fact the user is making a mistake, but instead what the meter does when it happens. Both Uni-Ts show an erroneous measure just like the OP, with a special distortion on the values showed by the UT-61E.Quote from: Fiveohfour on January 31, 2019, 11:15:44 pmAnd yet, the cheapest of them all does a much more elegant solution in my opinion.

author: rsjsouza
author_rank: Super Contributor
timestamp: February 01, 2019, 03:42:03 pm
content: And the Fluke 27/FM (the ancient grey/brown one) also disconnects the ADC input, just like the Mastech MAS830L