Amps and mV

[gsalmon4]

I have just verified some measuring devices from 0mA upto 180amps. I got taut how to do this and part of the phase I dont understand i.e on the higher ranges (i.e above 1amp test phases) I had to switch the Agilent desktop multimeter to mV, which had a coil of 20 wraps. So if you set the Agilent multimeter to 9,000mv this equates to 180.000.00mA (180Amps). How is this so? Why does setting the multimeter in mV alter the result you get to Amps?

In what way do Amps have a relation to mV?

Many thanks to all replies

 

[crutschow]

It's ohms law, V=IR. The current is going through a shunt (series) resistance and the meter is measuring the voltage across this resistance. For 9mV to equal 180Amps the resistance is R=V/I = 9mV/180A = 50µΩ (a very small resistance).

 

[Hero999]

You can get hall effect sensors or current transformers for this kind of thing.

 

[Sceadwian]

Your meter probably has an internal shunt for low currents. The external shunt is used for high power applications which is calculated from the amp and mv rating of the shunt. Many shunts are rated at 0-20mv or 0-100mv full scale. Meaning if you measure a differential voltage of 10mv on a 100amp 20mv full scale shunt you're measuring 50 amps.

 

[Hero999]

Just a couple of things you also need to be aware of:

You need a true sine wave meter to measure the true current if the load is non-linear.

If you want to measure power consumption, you need a power meter if the load isn't purely resistive.

 

[gsalmon4]

I used a shunt for the higher values over 1 amp. for the lower values i used a 50ohm resister in series. The shunt was an external one plugged into the multimeter.

I think I need to get used to the ohm law I think as I should understand it for electronics.

How can you work out the current or resistance if you only know the Voltage? Or visa versa how would you work out the voltage if you only know the current?

Thanks for replies.

 

[Nigel Goodwin]

I used a shunt for the higher values over 1 amp. for the lower values i used a 50ohm resister in series. The shunt was an external one plugged into the multimeter.

I think I need to get used to the ohm law I think as I should understand it for electronics.

You certainly should, it's the first thing you should learn - you will find you need it with everything electronic you do.

How can you work out the current or resistance if you only know the Voltage? Or visa versa how would you work out the voltage if you only know the current?

You always need to know two - it's simple algebra "V= I x R" - you can find any third if you know the other two.

If you only have one, you have to find someway to find a second before you can find the third.