I'm working on a piece of bench test equipment (for testing device gain), which seems to be working, but I need to put the final touches on it.

Suppose I have a wire carrying a small 50Hz AC current superimposed on a DC current. Say, 0.5mAac on 10mAdc. I want to detect when the AC current falls to zero. Is there a simple way to do this, ideally using passive components.

At the moment I simply have a 22R resistor across which I'm viewing the AC voltage signal on a scope, which isn't portable!

I was thinking perhaps of a small transformer with a bi-colour LED on the secondary. The LEDs should flicker while there is AC, but not when the AC has been nulled. I don't have a transformer to try this yet, so does anyone have any other ideas?

 

Is the AC current always 0.5mA?

 

Is the AC current always 0.5mA?

No it varies depending on the device being tested. Essentially, the device gets a 50Hz signal and you adjust a component bridge until the AC current falls to zero- then you know the gain of the device. AC current can be as high as 5mA, but I need to be ble to detect it down to, say, 100-200uA.

 

Use a current transformer with the appropriate turns ratio as to light the LEDs in your schematic when the current is just 100uA. IF the current is less the LEDs will be off.

 

Does it really work like that? I mean, the current transformer with primary current of 100uA can light a LED at the secondary. Doesn't the voltage at the primary must be large for that to happen?

 

Does it really work like that? I mean, the current transformer with primary current of 100uA can light a LED at the secondary. Doesn't the voltage at the primary must be large for that to happen?

Yes and yes.

Assume the LED takes 10mA with a 2.0V drop.

Turn ratio of CT = 10mA / 0.1mA = 100. Primary is say 300 turns and secondary = 3 turns.

Operating voltage of CT primary = Vsec * turn ratio = 2 * 100 = 200V.