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> Measuring 30-40 Amps With Multimeter, How to ?
Areal Person
Posted: July 15, 2008 08:31 pm
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Hi,

I have basic multimeter that allows the measurement of amps
It has a 10A setting on the multimeter dial.

QUESTIONS:

Is 10 amps the max it will measure ?

I need to measure up to 30-40 amps on my hydrogen fuel cell project.

Will I burn my multimeter out measuring that many amps ?

I see others measuring 30+ amps ?

And also, is this the correct way to hook it up ? I never done it before

1) Connect the RED Probe to the 10A Meter Port
Connect the BLACK Probe to the COM Meter Port

2) Set the dial to 10A

3) Connect the POS voltage source (Hot) directly to the RED meter probe

4) Connect the BLACK probe to what will be the POS source of the project

5) Connect the NEG (GROUND) to the NEG source of the project

The multimeter should now display amps

Is this correct ?

Thanks,
-Areal
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tekwiz
Posted: July 15, 2008 08:39 pm
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Yeah, that's correct. If your meter is digital, the probe polarity doesn't really matter...the meter will read "negative" amps if they are reversed. To measure more current that your meter will show, you must employ a voltage reading across a low value shunt resistor. There is no limit to the amount of current you can measure this way, other than obtaining a low enough value resistor. cool.gif


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CGFxColONeill
Posted: July 15, 2008 08:43 pm
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tek beat me to it
and I was at least partially wrong lol


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PassTheSalt
Posted: July 15, 2008 09:01 pm
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Is your DMM fused at 10A?

If so, you'll blow a fuse trying to use it as an ammeter for something over 12-13 amps. I'd do the shunt resistance. It's a bit more work, but, IMO, it's probably the most accurate way to get 'er done.

If the 10A is just a decimal place designataor, I wouldn't worry about it too much.
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kellys_eye
Posted: July 15, 2008 09:06 pm
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Yeah, the key word is "shunt".

Typically, commercial shunts are rated at (say) 75mV (for a given full-scale current measurement) and this link:

http://cgi.ebay.co.uk/SHUNT-for-DC-200A-75...p3286.m14.l1318

shows a 200A shunt. When connected in circuit you would measure 75mV across it IF 200A was flowing through it. Obviously it scales downwards (i.e. 7.5mV at 20A etc).

Shunts aren't expensive but home made ones aren't easy to calibrate. You could make one from a length of heater wire (element) if you could measure the low resistance accurately. It doesn't matter what resistance you end up with other than it must be able to dissipate the power developed across it. Simple Ohms Law will tell you what current is flowing.

Your digital multimeter has a shunt in it but these are only limited in use (check your manual as it may say you can only meaasure the full 10A for a few seconds... not permanently).

NOTE: with a shunt in place your meter will need to be able to read LOW volts accurately. As I mentioned, you're looking at 75mV for most shunts.



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CWB
Posted: July 15, 2008 09:12 pm
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if the meter says it's good for 10A then that is it . try to pass any more current than that and the shunt in the meter aint gonna like it .
like tek said ... use a low value resistor in series with the load and measure the voltage drop across it . the trick is knowing the value of the resistor accurately .

i've seen some shunt resistors precision measured down to .1 % ... close enough for the work i did .

ps ... ammeters are always connected in series with the load .
else wise it is like trying to measure the resistance of an ac outlet .

/me is still pissed about a friend that used my meter to do that


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johansen
Posted: July 15, 2008 09:25 pm
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It this is a 3.5 digit meter then it has a .1 or .01 ohm shunt inside. some are fused, most $9.99 ones aren't.

every meter i've bought will measure up to 19.99 amps
EDIT: and get quite warm doing it biggrin.gif
EDIT: what CWB said
I suspect some of the higher end meters that have a 3999 count display will read up to 39.99 amps, but every expensive meter has a fuse at 10/20 amps to protect the 18 gauge test leads and pcb traces.



I thought 50mv was the standard? shock.gif

Although this is true; "home made ones aren't easy to calibrate" I've always had a bigger problem with temperature rise changing the resistance.


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Areal Person
Posted: July 15, 2008 11:45 pm
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Thanks for the advice,

I can only find regular multi-meters that are 10A max.

I also cannot afford an expensive one. Iím thinking about looking for
a junk meter, or ordering a basic one.

Do all amp meters hook up the same way? Like the way I described?

Sometimes itís tuff finding the simple/basic parts you need. We have a Loweís
here where I live, and itís stocked so poorly that itís depressing.

Thanks again!
-Areal blink.gif

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johansen
Posted: July 16, 2008 12:14 am
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just build a shunt out of 1 foot of 10 gauge wire and that is .001 ohm

i doubt you will find anything better than a -60-0-60 amp car alternator charging meter at an auto parts store.


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Areal Person
Posted: July 16, 2008 07:05 pm
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Ok, I found a video that uses a hacksaw blade as a shunt

HereÖ
http://www.youtube.com/watch?v=uVlCqK_Or0g

But I donít understand how he has hooked this up ?

Is the second voltmeter always required? or is it only used one time for calibration ?

Is it so simple that itís confusing me ? Could someone please explain exactly?
how the wiring goes ? I would be grateful to understand how this is wired up
and used to measure the higher amp readings.

Is this correct, or did I get it wrongÖ

1) Connect the RED Probe to the 10A Meter Port
Connect the BLACK Probe to the COM Meter Port

2) Set the dial to 10A

3) Connect the POS voltage source (Hot) directly to the RED meter probe

4) Connect the BLACK probe to one end of the SHUNT and the other end of the
SHUNT to the POS terminal of the project

5) Connect the NEG (GROUND) to the NEG source of the project

The multimeter should now display amps

Thanks,
-Areal
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tekwiz
Posted: July 16, 2008 07:14 pm
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Nope. When using a shunt, the meter must be set to measure volts, not amps. The shunt is connected in series with the load, & the meter(set on DCV) is connected in parallel with the shunt. I wouldn't use steel or heater wire for a shunt, because it's resistance changes too much with temperature. Any length of heavy copper wire will do as a shunt resistor. You can find the exact resistance from a wire table. cool.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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Areal Person
Posted: July 16, 2008 07:22 pm
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Thanks !

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