Multimeter - safe construction? - Page 1
Forum section: electronics / testgear

author: TheCynicalCris
author_rank: Newbie
timestamp: August 23, 2015, 11:16:29 pm
content: Hi everybody,I've found this multimeter in a hardware store for around 10USD (believe me it wasn't the cheapest model). Anyway, I'm kinda worried about this "UNFUSED" label and amps on the same port as volts. Is it actually safe?Thanks =).

author: nidlaX
author_rank: Frequent Contributor
timestamp: August 23, 2015, 11:27:54 pm
content: What kind of work do you plan to use the multimeter for? (voltage and current, potential for spikes, etc.)

author: ketil b
author_rank: Regular Contributor
timestamp: August 23, 2015, 11:32:29 pm
content: I do like how the COM port look like they have just used a marker to make it black

author: TheCynicalCris
author_rank: Newbie
timestamp: August 23, 2015, 11:33:45 pm
content: I plan on using it mainly during computer repairs, so DC voltage, amps but also for some general purposes as well.

author: Muxr
author_rank: Super Contributor
timestamp: August 24, 2015, 12:07:45 am
content: At least it says it's unfused. A lot of cheap DMMs don't even tell you that. I would definitely go for a fused meter if I were you. Otherwise you're running a risk of injury.

author: Muttley Snickers
author_rank: Unknown
timestamp: August 24, 2015, 02:22:47 am
content: I have about 20 handhelds and every one of them has the amps jack on the left so for that reason and the fact that computers run on mains and you may have to check IEC leads and power supplies probably best to leave it alone as others have said.The meter pictured below has been in my work trucks for over twenty years and was the first decent meter I ever owned, it has served me very well but in all honesty it shouldn't be there as I work on mains and machinery and it seems to be the meter I reach for most often because it happens to fit nicely in my toolbox, It's a similar size to a Fluke 87 with a wide low profile unlike all of my other meters, 117, 179, 187, 189, U1272a and heaps of others.I have more than enough other meters that are better suited to do the various work that I am involved with so there is no excuse for me to use it any longer, Fluke 87V or a bigger toolbox.Metex M-3650CR the new three strike rule applies, sorry my good friend....1. 20 amp current with no fuse and a 2 amp M205 fuse on other current ranges....2. Pc interface with no opto's and the worst RS 232 connection you could imagine....3. Transistor tester, what the hell was I thinking but back then everything had them....MuttleyEdit: Added Picture 3, Well there you go it looks like either of these will fit in the toolbox, I didn't think they would but the height is about the same. The U1272a is a bit tight and the 117 is a bit sloppy, the 179 might be just right, anyway the Metex is not going back onboard.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: August 24, 2015, 03:25:14 am
content: Quote from: TheCynicalCris on August 23, 2015, 11:16:29 pmQuote from: TheCynicalCris on August 23, 2015, 11:33:45 pmIt is "safe" as long as you do no let more than 10 A pass though your multimeter.Notice that the mA selection is bright orange; this is to prevent you from selecting this position by error. All the safety features of this multimeter rely on the operator not making any mistakeIf you bought this model because it is the best you can afford, just be careful when you are using it.

author: Fungus
author_rank: Super Contributor
timestamp: August 24, 2015, 04:05:10 am
content: Quote from: TheCynicalCris on August 23, 2015, 11:16:29 pmHey, at least they're being honest! Most cheapo meters just lie.A short circuit situation without a fuse the probe wires will probably melt and burn you. Is that worth saving $5 for?There's also the problem of "confidence". How confident are you that the reading on a $10 meter will be correct?Me? I wouldn't buy a multimeter in a 'hardware store'.Quote from: TheCynicalCris on August 23, 2015, 11:16:29 pmThat depends on how you define 'safe'.It probably won't hurt you if you always use it 100% perfectly. The problem is that humans aren't 100% perfect.'Safety' is there for when you get the dial position wrong (which youwill) or make a bad assumption about what you're connecting it to (especially risky for AC mains measurements - mains isn't always a perfect 230V sine wave).

author: Lightages
author_rank: Unknown
timestamp: August 24, 2015, 04:46:22 am
content: IMHO, a non-standard input jack layout, common mA on the volts input, and no fuse on the 10A jack is just asking for a mistake to ruin some device or blow up the multimeter. I would toss it in the bin and look for something not designed to fail.

author: TheCynicalCris
author_rank: Newbie
timestamp: August 24, 2015, 10:11:46 am
content: Quote from: Fungus on August 24, 2015, 04:05:10 amWell, as you said at least they're being honest. In the manual they're claiming that it's about 0,8% accurate on DC, 2% on AC, 2% in amps and 1% on ohms and it seems to be about right.

author: TheCynicalCris
author_rank: Newbie
timestamp: August 24, 2015, 10:13:43 am
content: Quote from: SkyMaster on August 24, 2015, 03:25:14 amYea, unfortunately it was the best that I could afford for now, maybe later I'll ask you guys about something better. Anyways, as a first year student of computer studies I don't have too much money to spend =).

author: tautech
author_rank: Super Contributor
timestamp: August 24, 2015, 10:21:25 am
content: Quote from: TheCynicalCris on August 23, 2015, 11:16:29 pmUnfused relates to the 10A current mode.For ten bucks just use it but beware of these limitations, don't put yourself at any undue risk.We all started with cheapo DMM's and we lived through it by being cautious, you should too.Get yourself a cheap Fluke for stuff that might bite.

author: Lightages
author_rank: Unknown
timestamp: August 24, 2015, 06:36:50 pm
content: My point was that this meter could cost more money in damage because of its unconventional design and layout. All it takes is a lapse of concentration and you either have a dead and unfused short across whatever, or you move the selector too mA with the leads across a circuit and maybe blow some parts. It could cost more than the meter in the end even if the meters is not damaged. Why any manufacturer thinks it a sane idea to use common jacks for current and voltage is beyond me, Brymen included with the BM27!

author: Neilm
author_rank: Super Contributor
timestamp: August 24, 2015, 08:52:08 pm
content: My first thought is that it is not safe and also not compliant with IEC61010. However, as it says it is unfused, I am wondering if there is some get out clause in the multi-meter section. I had thought that a meter rated for 600V had to be tested on a supply with the leads misconnected into the amps range, assuming that the leads fit.I will have to have a read of the standard at work tomorrow

author: Rick Law
author_rank: Super Contributor
timestamp: August 24, 2015, 09:08:57 pm
content: Quote from: TheCynicalCris on August 23, 2015, 11:33:45 pmQuote from: TheCynicalCris on August 24, 2015, 10:11:46 amOn these DMM, if you are planning on measuring current from the PSU... Be aware, you may have a hard time not because of accuracy but because of the burden voltage.Many of it use a 0.1ohm current sense resistor. I had one hooked up to test PSU for 5V at 8A to 12A (8 disk drives each pulling .9A to 1.2A). The voltage drop just mess things up. The damn drives won't even spin up.At 10A and with a 0.1ohm current sense, mathematically, the 5V is now merely 4V.I ended up using 3x0.1ohm or 4x0.1ohm in parallel.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2015, 09:52:45 pm
content: Usually these cheap multimeters are made for a 200 mV burden voltage at the shunt. If there is a fuse, you loose something extra. So no fuse tends to be good for a low burden. Alone for cost reason no 100 mOhms resistor for the 20 A range - this would be way to much power. The 10 Ohms version is only 1/10 the material.If you are lucky shunts can be even smaller with more amplification instead of power rating. At high currents, lower burden voltages (e.g. 50 mV) is not that uncomon.The trouble are poorly made 4000 count DMMs - they sometimes had a 400 mV + fuse burden at full range. The other poor category are not so good autoranging DMMs. Here one shunt may be used for 3 ranges - so high dropo at the upper limit and possible drift at the low end.

author: saturation
author_rank: Super Contributor
timestamp: August 25, 2015, 03:14:50 pm
content: The simplest thing is insure it has a safety mark used in your country or the EU, for example, the VDE or TUV marks. You can check its online database to insure its not counterfeit.https://www.vde.com/en/institute/onlineservice/vde-approved-products/pages/Detail-Search.aspxBeyond that, you have to know electrical and electronics fairly well to decipher if the construction if 'safe' by inspecting the internal construction.In the past 5+ years there are increasing a class of DMMs with unfused high amp functions that have a PTC resettable fuse in series with the sense resistor of the amp circuit. They allow users to read current for so many seconds before the PTC trips and causes the current output or the meter to read error or overload. They have to be designed well because the only protection to the user is the PTC and the PCB traces [ which can be designed to fail as a backup to the PTC]. If its safety rated, the agency has tested that the PTC and/or the traces fail safely at overload. If not permanently damaged, the instructions will say to wait until the meter has cooled down to reactivate the amps or even the whole DMM again.PTC are not precision nor stable relative to the shunt resistor, so the accuracy of the amp range is affected by the PTC. The absolute PTC resistance and trip point are ambient temperature dependent. PTC have limited voltage withstand capability. At the maximum working voltage, and the PTC is in the high impedance state and the probes remain connected, it will prematurely fail the PTC. PTCs 'wears' out, so the amps function has a 'limited' run time if used routinely rather than used as a safety device.

author: Fungus
author_rank: Super Contributor
timestamp: August 25, 2015, 03:53:55 pm
content: I'm not sure why you'd ever need to measure amps for PC repair work.The only things you really need are volts and continuity.

author: rvalente
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:28:37 pm
content: Quote from: Fungus on August 25, 2015, 03:53:55 pmWell you can measure the +5V or +12V lines of a HDD and compare with the labels, but I must say this is very very rare.