Proper usage of a shunt

[Painless]

To make sure I understand it right:

If I want to use a voltmeter to measure amps, connect the test leads to either end of a foot long piece of 10 gauge wire (or whatever gives 1 ohm of resistance). If the volts read 20, then that is 20 amps.

Correct?

I want to make some more exact amp measurements for my next series of experiments, an analog meter just isn't accurate enough.

[H2OPWR]

To make sure I understand it right:

If I want to use a voltmeter to measure amps, connect the test leads to either end of a foot long piece of 10 gauge wire (or whatever gives 1 ohm of resistance). If the volts read 20, then that is 20 amps.

Correct?

I want to make some more exact amp measurements for my next series of experiments, an analog meter just isn't accurate enough.

Russ, I am not sure exactly how to do it but that does not sound right or I am not understanding. I do know that there is an way to tell based on voltage drop from one end of the shunt to the other end. The more amps through a known resistance the more voltage will drop over the shunt because of the resistance.

[ridelong]

Painless,
A shunt will convert current to voltage through the formula E = I*R.

For example, if you use a .01 ohm shunt, and measure the voltage across it,
at 20 amps the voltage would be E = 20*.01 = .2 volts. If you set the voltmeter to the 200 mv. scale, 200 mv would be 20 amps.

You want to use very small resistance values so there isn't a lot of heating and therefore power loss.

Power loss is expressed as P = I(squared)*R.
At 1 ohm, the power loss at 20 amps is 20*20*1 = 400 watts.
At .01 ohm the power loss at 20 amps is 20*20*.01 = 4 watts.

You can use a heavy ga. wire for a cheap shunt. I buy .01 ohm 1% accuracy 25 watt power resistors from Mouser Electronics for about 4 bucks.

Russ

[mustangpro1]

how do you hook of a stunt i get how you get the restance cut the wire until you get 1 ohm of restance but were do you hook each end of the wire

[Painless]

You basically do this:

Battery+ -> Shunt -> Cell -> Ground

In other words, the shunt wire should be in series with the positive feed to the cell.

[cully]

heres a quick drawing of how it is connected

http://img.photobucket.com/albums/v159/cully/hho/shunt.jpg

you measure the voltage across the shunt at point (V) with a millivolt meter usually up to 500mV depending on the shunt resistance used

check out here (http://www.uoguelph.ca/~antoon/gadgets/shunts/shunts.html) for more info

If a shunt resistor overheats it can permanently change the resistance of the shunt. so make sure the copper your using is rated for the current going through it