overloading a multimeters amp range - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: April 18, 2010, 04:49:22 pm
content: something has bugged me for ages. The amp range of a digktal multimeter puts the current through a very low value resistor and senses the voltage drop on it. therefore for a temporary overload that does not cause dangerous heat levels the meter should handle it right ? I mean basically the small voltage drop will be amplified by the circuitry to no more than the meters on power supply voltage so apart from going out of range there should be no adverse effects right ? of course extreme overloading may well burn out the high current carrying path but otherwise its no where near as harmful as putting 1500 V into a multimeter made for 600 v max ?not that I intend overusing my amps range but the thought occured to me and may be the reason that real cheap meters don't have the high amperage fuse (and stupidly rely on the users competence)

author: lhc
author_rank: Contributor
timestamp: April 18, 2010, 08:51:42 pm
content: There should be a fuse, I don't know why cheap chinese multimeters don't have fuses on 10A ranges. If user connects it to mains or car battery etc. in parallel (high A range is effectively shorted) it can effectively damage the instalation or device being tested. There is no excuse for not including a fuse on this range. People make mistakes, devices can also have some shortages inside. Testing equipment should be safe for user and tested devices.

author: Simon
author_rank: Unknown
timestamp: April 18, 2010, 09:00:54 pm
content: True but that does not answer my question.Indeed if i were to put my 10 range straight onto a battery I would probably draw 100-500 amps and make smoke very quikly, but what if i put 15 A through a 10 A range ?

author: mkissin
author_rank: Regular Contributor
timestamp: April 18, 2010, 09:09:42 pm
content: If you read the spec sheet, you'll see that many meters say they can measure a 20A current on the 10A input, but only for 30s or less, and after taking the measurement you must allow the meter to cool for longer than the current was applied for.Obviously you can't take the measurement for too long anyway, as you'll blow the fuse eventually.Also bear in mind that as the shunt heats its resistance will change, making your measurement less accurate. For very high currents, hall-effect clamp meters are typically a better option.

author: mkissin
author_rank: Regular Contributor
timestamp: April 18, 2010, 09:15:13 pm
content: Actually, I just had a look, and the Fluke 80 series meters aren't even rated to handle their maximum 10A current indefinately. They are rated for 10A for less than 20 minutes, after which they require a 5 minute cooling period*. There is no substuitute for knowing your test equipment inside out.*From the Fluke 80 series multimeter detailed specifications PDF, page 2, note 4.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 18, 2010, 09:45:08 pm
content: People you had confuse me for good ..First of , the multimeter , its a multi - meter ..For specific tasks we use a Panel Meter ... " One "Volts ranges .. 12 - 50 - 300 - 1000VAmperes , you name it ... car electricians use 0-60Amps ones , and I had never ever see an single Amperes meter to come fused ..So ... if some one requires an specific meter , he gets the proper parts and makes one device that it will comply with his needs , add fuses , add switches , add LED on it , or what ever you like.Or make this simple design..

author: EEVblog
author_rank: Unknown
timestamp: April 18, 2010, 10:16:34 pm
content: Quote from: Simon on April 18, 2010, 09:00:54 pmThat is quite common, and many (usually good) meters are speced for that. They might have a nominal 10A range and say "15A for 30 seconds, with 10 minutes cool-down"Dave.

author: Simon
author_rank: Unknown
timestamp: April 19, 2010, 06:40:05 am
content: I was mainly asking as i plan to use my 20 A meter to carry up to 20 A and possibly put my 10A meter in parallel to make it up to 30 amps just wondering what sort of temporary overload they could take although my mate has an analog amp guage for running tests

author: Polossatik
author_rank: Frequent Contributor
timestamp: May 03, 2010, 02:42:45 pm
content: Quote from: Simon on April 19, 2010, 06:40:05 amWould it not be better to simply use a nice big external shunt and measure the voltage over that then try to // 2 multimeters ?electricity has a tendency to take the lowest resistance path... which might be your 10amp meter..

author: Simon
author_rank: Unknown
timestamp: May 03, 2010, 03:23:32 pm
content: true and I would also need to check my shunt system, I have a MAX4172 chip board with a 1 m-ohm shunt that can take up to 50 amps for it's wattage rating but I'd want to compare it to a meter, perhaps on the 20 A one only but then I hardly trust that meter

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 03, 2010, 08:06:58 pm
content: Quote from: Simon on May 03, 2010, 03:23:32 pmIt depends if it is an MetraWatt build at 80sHonestly all panel meters are well made this days , even the cheapys .They are not shock resistant , thats why there is always the doubt about them , of remaining stable and accurate if they misused.Still there is a modern way to check if the Ampere meter are calibrated , all that we need,are one modern power supply with current limiter.Just set it to 5 or 10A output, and just short the banana plugs by entering the Ampere meter in the circuit, for few minutes at list .My Kenwood PD-3510 can do max Amperes output for 24/24 365 , its has lots of fans on it.Still any average Power supply can do the max output with no problems for at list 10 minutes,plenty of time to check or calibrate one Ampere meter ..

author: Pyr0Beast
author_rank: Contributor
timestamp: May 06, 2010, 10:55:04 am
content: Cheap chinese multimeters rely on resistance of the measuring leads as fuses.Overloading does not harm multimeters much (if any). I've pushed mine with 20Amp range to 30 or so. Didn't hurt it.

author: kc1980
author_rank: Regular Contributor
timestamp: May 06, 2010, 09:36:05 pm
content: Quote from: mkissin on April 18, 2010, 09:09:42 pmThough, if your talking about a current transformer (CT) type sensor, they are only good for AC. And, for very high currents or transients, the core can become saturated. I still think the best idea is to use a current shunt.http://eolsurplus.com/images/UnknownCurrentShunt200A.jpg

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 06, 2010, 09:55:51 pm
content: Quote from: Pyr0Beast on May 06, 2010, 10:55:04 amFused input or Not ?And the word "Mine" have no value to the many, brand and model could help us to understand what you have there.My crystal ball are currently on strike .

author: mkissin
author_rank: Regular Contributor
timestamp: May 06, 2010, 10:43:33 pm
content: Quote from: kc1980 on May 06, 2010, 09:36:05 pmCT and Hall-Effect probes are quite different (Hall-effect probes will happily measure DC currents), but I agree, you do need to be careful about saturating the core in both types of probe, and of heating effects if you excede their ratings for any length of time.As long as you can break the circuit, a shunt is probably the easiest solution.

author: Pyr0Beast
author_rank: Contributor
timestamp: May 12, 2010, 04:42:23 am
content: QuoteNot fused.You'd blow the fuse the moment you've loaded that poor 2A input with 10AIt has only a thick wire, cut in the middle (or more places).And that can heat to much more than silicon can handle with no problems (except for measuring error), since there is nothing to be broken/burned.Manuals say 20A for 20s max.QuoteMetrix as the brand. Something in the range of 50€. More than enough for probing around, but the price tells you everythingI don't need ultimate precision for my work. To check if there are 3.3 and 5V present, even with 0.5V offset, that is more than enough. However, those are my needs and needs of others differ greatly.But these meters get a bit 'shitty' over time. Dial sometimes doesn't make proper contact and you get all sorts of weird measurements then. But turn it up and down and it is good again. Perhaps some dust/dirt/metal shavings have accumulated on it. At least you know it when that happens.