# Best Way To Temporarily Measure DC Amps?



## PeterH (Mar 20, 2009)

Hi there,

What is the best way to measure DC amps during a build or troublshooting? I bought a POC clamp on meter made down the road from where my CALB cells made... meaning somewhere in China... 

I'm not very happy with it so I need a better, more reliable and accurate way to measure current while I'm building or prototyping a system. I need something that will measure up to 600 amps but accurately measure much less.

Any recommendations?

Thanks,
Peter H.


----------



## major (Apr 4, 2008)

PeterH said:


> What is the best way to measure DC amps during a build or troublshooting? I bought a POC clamp on meter made down the road from where my CALB cells made... meaning somewhere in China...
> 
> I'm not very happy with it so I need a better, more reliable and accurate way to measure current while I'm building or prototyping a system. I need something that will measure up to 600 amps but accurately measure much less.
> 
> Any recommendations?


Hi PeterH,

You can't go wrong with Fluke. But.....$$$. I found this a few years ago and it's worked well. Now have one at home and one at the lab. 

http://www.mcmelectronics.com/product/72-7228 

Got mine on sale for less. Check the 400A model on sale now for $49. 

http://www.mcmelectronics.com/produ...commendation&utm_source=prod&utm_term=72-7226

Regards,

major


----------



## frodus (Apr 12, 2008)

I got my 600A AC/DC clamp multimeter from Sears and like it a lot. Not sure how accurate it is up high (seems pretty accurate), but down low, under 20A, it matched my bench meter.

They don't sell it anymore, but they sell a better one that goes to 800A AC/DC
http://www.sears.com/shc/s/p_10153_12605_03473756000P?prdNo=15


----------



## EVfun (Mar 14, 2010)

A decent DMM (digital multi-meter) with a good millivolt DC readout and a 500 amp shunt. Even my old Fluke 70-3 can accurately read down to 5 amps with that.


----------



## PeterH (Mar 20, 2009)

Ok, thanks everyone... I like Fluke meters. I bought the Fluke 289 DMM and nearly broke the bank... but I'd like to think I've learned my lesson about going cheap on tools.

I happen to have a 500 Ohm shunt laying around so that would work for higher currents. For anything less than 10 amps I can use the Fluke 289. I've never tried measuring current with the 289... only had it for a few weeks. I just looked up in the online manual what I needed to try it tomorrow. 

Still like the ease of using the clamp on so I'll check out those other suggestions. I'm just wondering how accurate the clamp on meters really are. 

Thanks,
Pete


----------



## otp57 (Feb 7, 2012)

PeterH said:


> Ok, thanks everyone... I like Fluke meters. I bought the Fluke 289 DMM and nearly broke the bank... but I'd like to think I've learned my lesson about going cheap on tools.
> 
> I happen to have a 500 Ohm shunt laying around so that would work for higher currents. For anything less than 10 amps I can use the Fluke 289. I've never tried measuring current with the 289... only had it for a few weeks. I just looked up in the online manual what I needed to try it tomorrow.
> 
> ...


500 Ohm shunt is that super large resistor for an EV or am I missing something?


----------



## PeterH (Mar 20, 2009)

Actually, I'm glad you asked... because I need to review this subject a bit to be sure I understand it.

Since I'm not an electircal engineer nor do I play one on TV, I might be wrong... if so, I hope someone will step up and correct anything I have wrong.

So with that said, my answer to your question is: not really in the sense of "big" meaning "high" resistance... but yes in the sense of physically big as far as "normal" resisters go. My 500 amp shunt looks like this:










Physically, the shunt I have is about 2 and a half inches from end to end.

As I understand it, a shunt in the context of a DC electrical circuit is a device of a known, precise resistance that is in series with the current flow. Meaning all of the current flows through it. The voltage drop across that precise resistance is read by a volt meter that is calibrated to read millivolts. The millivolt potiential that appears between the two ends of the shunt directly corresponds to the current flow through the shunt.

My Fluke meter says my 500 AMP shunt measures exactly 0.10 ohms. It is also stamped on the side indicating that 50 MV will appear across the shunt when 500 amps is flowing throught the shunt.

A shunt like this can only be used in a DC circuit. This thread started when I was having trouble getting an accurate measurement of an AC circuit where I can't use a shunt to help accurately measure current flow. Now that I look back, I just realized that I asked about measuring DC instead of AC current... wow... I shouldn't post things like this that late at night. Sorry about the confusion.

Hope this helps.

Pete


----------



## EVfun (Mar 14, 2010)

That looks like a regular 500 amp shunt. It could pass up to 1000 amps briefly without harm (accuracy drops over 150F.) It can measure DC current or AC current, but can you accurately measure the small AC voltage? Many DMMs do a lot better job of measuring low DC voltage. 

The resistance you see with your DMM is mostly the resistance within it and the test leads. The shunt is 0.1 milliohms, just 1/1000th of what you measured.


----------



## PeterH (Mar 20, 2009)

EVfun, thanks for adding to this.

Interesting. I'm glad I posted my understanding of the situation. Obviously I was not completely on target.

I just confirmed your point as best I can by measuring resistance just in the leads by connecting positive to negative and got the same .1 ohms I get when I measure the resistance of the shunt. So clearly the internal resistance of the shunt is way below my capability to measure accurately with a hand held DMM.

I didn't know that I can use this shunt in an AC circuit either.

So in an AC circuit, I can just measure the AC millivolts across the shunt to read the amperage?

I plan to use the Fluke to measure the current in the AC circuit tonight, by using the meter itself in series with the load. The manual indicates I can do that safely up to 10 amps of AC current. I'll go back and re-read that again to be sure... please let me know if you see me heading towards a shocking evening!

Thanks,
Pete


----------



## otp57 (Feb 7, 2012)

EVfun said:


> That looks like a regular 500 amp shunt. It could pass up to 1000 amps briefly without harm (accuracy drops over 150F.) It can measure DC current or AC current, but can you accurately measure the small AC voltage? Many DMMs do a lot better job of measuring low DC voltage.
> 
> The resistance you see with your DMM is mostly the resistance within it and the test leads. The shunt is 0.1 milliohms, just 1/1000th of what you measured.


 *A shunt should be as low as a resistance as possable to avoid power loss in the form of heat in a power circuit. 500 ohm is a bit high but for testing I guss it can work*.


----------



## Sunking (Aug 10, 2009)

otp57 said:


> *A shunt should be as low as a resistance as possable to avoid power loss in the form of heat in a power circuit. 500 ohm is a bit high but for testing I guss it can work*.


Let's see here 1 amp through 500 om's requires 500 volts and 500 watts. Hope that shunt is oil cooled.

Most every shunt is designed to work on 50 mv full scale. Does not matter if it is AC or DC. Common Shunt values are:

10 Amp : .005 ohms, .5 watts
20 amp : .0025, 1 watt
50 amp : .001 ohms, 2.5 watts
100 Amp : .0005 Ohms, 5 watts

From there you can work out the math for any shunt. To read directly with a DMM you need a good accurate Milii-Volt scale. Most good Fluke meters have a 200 mv scale.


----------

