Idea need how to measure "high" DC current with dmm - Page 1
Forum section: electronics / projects

author: Chriss
author_rank: Frequent Contributor
timestamp: October 30, 2019, 05:18:33 pm
content: Hi!I have a problem some time when I repair electronics around vehicle and I thinking to make a simple device to measure "high" current with my average dam.Here is an example what is happening and why I need this:Let's say I have problem in a circuit where a 25A fuse is fitted.I found the fuse is blown but don't see any short but when I replace the fuse the circuit is fine some time ( hours maybe a day...) and then the fuse blows again.I'm not sure is the fuse maybe the problem or something other in the circuit. Probably something in the circuit.And now, I wish to measure the current on that 25A fuse with my ordinary dmm which can handle max 10A.What would be the best solution for me but if it is possible a DIY project.Thanks for any idea.My best regards.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 30, 2019, 05:46:21 pm
content: By far the best way to do this is with a clamp meter. Some clamp meters are AC only, others can do DC as well. You want the latter.If you want a "DIY" project, the simplest way is an ammeter shunt that can handle >25 amps and won't unnecessarily burden your circuit. Then you just measure with a DMM in millivolt mode. For instance a resistance of 1 milliohm will give you a voltage of 25 mV and will dissipate about 0.5 watt. You won't have tremendous resolution on most meters but it will be enough to see if you are close to the fuse rating.For a really rough reading, measure across the fuse itself. The resistance of the fuse is non-linear (rising with temperature), but the fuse datasheet should have hot and cold resistance or maximum voltage drop at rated current. This should give you a good idea of whether you are close to the rating.

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 07:05:10 pm
content: what i do is use a 55watt headlamp bulb in place of the fuse,when the circuit is in the fault condition it just lights the bulb,when its lit you can then wiggle wireing looms to look for chafes etc and unplug things to see if the fault condition alters,its good as it saves blowing fuses and lets you see when the issue happens!,its a method ive used for 20+ years as a mechanic,its never failed me yet.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 30, 2019, 08:43:11 pm
content: That stuff with the light bulb I also use when I do repairing on the bench.What I need is exactly what ejeffrey wroteQuoteThis is what for I'm searching for.Sometime I have fuses like 30A or 45A etc. in cars and I really don't know how close the current consumption is compared to the fuse rating.What do you think about this gadgets from Aliexpress:https://www.aliexpress.com/item/1786770305.html?spm=a2g0o.detail.1000060.3.7c4d1a2erG1oc1&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.146401.0&scm_id=1007.13339.146401.0&scm-url=1007.13339.146401.0&pvid=8161eced-495d-46c8-9d71-217515e2c754Or this one:https://www.aliexpress.com/item/4000058981609.html?spm=a2g0o.productlist.0.0.47812540F0Z7jN&algo_pvid=cbfb7e80-1f76-46ca-b28b-5df1cdc3cb39&algo_expid=cbfb7e80-1f76-46ca-b28b-5df1cdc3cb39-25&btsid=a5362224-ece3-47a7-b421-dbae43132a1c&ws_ab_test=searchweb0_0,searchweb201602_2,searchweb201603_55Actually I could mV measure with my scope, I have one and that would be fine for me too...

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 09:22:51 pm
content: how about a shunt from one of the cheap led amp and voltmeters from ebay,the 50amp shunted ones should do the job,https://www.ebay.co.uk/itm/Digital-LED-100V-Voltmeter-10A-50A-100A-Ammeter-Panel-mount-volt-amp-meter/292661184098?hash=item4423f74662:m:m20lue267b7i5yJ3YWW36eA

author: ejeffrey
author_rank: Super Contributor
timestamp: October 30, 2019, 10:09:40 pm
content: Those should work. You can also use the shunt with your own DMM which might have better resolution than the cheap panel meter. The big downside of shunts is that you have to insert them inline with the circuit. That's why clamp meters are so nice, you don't have to mess with the wiring.The hall effect ammeter you linked to should also work fine, although it appears to be a single piece pickup coil which is not as convenient as a clamp meter. It is a lot cheaper.Don't measure a high side current shunt (including a fuse) with an oscilloscope unless you have a differential probe or a floating scope.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 30, 2019, 10:22:06 pm
content: I use the UNI-T UT210E just for this. At $30 it has paid for itself a few times. The switch has started to go bad on the one I use most often but other than that, I really like the clamp.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 31, 2019, 12:49:53 am
content: +1 for the UT210E. I even managed to measure 200A with it (it is rated for 100A), by using a "calibrated piece of wood" to increase the air gap in the clamp! Worked a treat to check starter current, proved the starter was bad, and got the job done.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 31, 2019, 12:38:38 pm
content: I must order something cos I need it some time but very rare and just in vehicle repairing.Nothing to deal with high side current...But I have to figure out quickly something how to check now on a car does the 25A fuse blow slowly cos the circuit current consumption is really near the fuse rating or something other is going on...

author: SilverSolder
author_rank: Super Contributor
timestamp: October 31, 2019, 03:07:48 pm
content: Clamp meters are great because they work on both high and low sides.You'll find it useful inside the house too, you can measure currents going to various circuits in the house from the breaker box...The accuracy is actually surprisingly good, I checked the Uni-T with my lab power supply and found it to be close to bang on...