Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

I have an electrolyzer that works with car battery (12 volts DC) and draws current around 15 amps. I have a digital multimeter that can measure 10 amps at most. So how can I measure the current? Maybe I should use a resistance to measure its voltage but I can't think of a resistance that would tolerate 15 amps.

share|improve this question

2 Answers 2

up vote 2 down vote accepted

I'd get a precision 0.01 ohm power resistor and measure the voltage across that. At 20 amps you'd only have to dissipate 4 watts at the resistor.

Alternately, you could find yourself a Hall Effect sensor. The resistor is simpler though.

Power resistors usually look like this: enter image description here
Or this:
enter image description here

share|improve this answer
    
This ^, or a current transformer that outputs a proportionally lower current within your device's range. –  sherrellbc Aug 6 at 14:27
    
@sherrellbc I'm not terribly familiar with current transformers but if they work anything like voltage transformers, they won't work for DC measurement will they? –  horta Aug 6 at 14:33
    
I didn't know about power resistors, seems a good solution thanks. –  Kamtal Aug 6 at 14:37
1  
@horta, you're right. I commented in haste. –  sherrellbc Aug 6 at 20:32

The easiest way (other than buying a DC-capable clamp-on meter) is to use a shunt with Kelvin connections and your multimeter (on a voltage range).

The one shown provides 75mV output with 50A flowing, so you can measure the voltage on the 200mV range of your meter. Take the reading in mV and multiply by 2/3 to get amperes. Chinese shunts are available for a few dollars each.

The power connections go to the LARGE (outside) screws. The meter connections go to the SMALL (inside) screws. This is called a Kelvin connection (named after William Thomson), and it means the connection and wire resistance and variations of those resistances do not appreciably affect the reading. Consider the schematic below:

www.ebay.com/itm/NEW-FL-2-75mV-External-Shunt-Resistor-for-DC-50A-Current-Ampere-Meter-Ammeter-/331271243265?pt=BI_Circuit_Breakers_Transformers&hash=item4d214e0601

schematic

simulate this circuit – Schematic created using CircuitLab

The exact values of R2-R5 do not affect the current reading hardly at all, provided the meter is high impedance. So even if the screw is a bit tighter or looser, or the wire going to the connections heats up a bit and changes resistance, the reading will stay steady.

http://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin#mediaviewer/File:Lord_Kelvin_photograph.jpg

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.