current meter

[ctesla75]

Hi All,
I have been trying to mae a circuit which i could use to measure the current accross a shunt . I md a circuit using an lm3914, but i a not having any luck,the lm3914 is powered by the transformer of the unit i am testing(regulated and rectified),if my positive and negative come from the transformer , i take one pin to one side of the shunt,should i have an opamp going to both sides of shunt and then to the lm3914,anyb help would be appreciated ,as i am at a dead end.
ctesla75

 

[MikeMl]

You need to use an **broken link removed**with high common-mode rejection" to convert the differential voltage drop across the shunt to a ground-referenced voltage proportional to the current. Another way is to use a "high side current monitor" IC like the ZXCT1009.

 

[MrAl]

Hello,


You can not measure the current across the shunt, you measure the voltage across the shunt. Also, that voltage is usually very very small like 50mv or even lower for a full 50 amps flowing through it so you need to amplify that voltage before you can measure it with a device like an LM3914 or similar.
What you do is use an op amp like Mike said and a few resistors and that gets you up to enough voltage to measure with the LM chip.