# How to measure oscillating circuit current draw

Thread Starter

#### sparkie78

Joined Nov 5, 2016
21
Is there any way to accurately measure the current draw of an oscillating DC circuit? I have a circuit that oscillates at 6 kHz for .6 seconds and repeats every 1.3 seconds and is supposed to draw 250mA. Using a digital multimeter to read current, the numbers are all over the place. I assume this is due to the current turning on and off at such a high rate and the much slower speed of the digital display. When I press the MAX button, it shows a number that I didn't see during direct reading. Would this be the actual current draw? Can a digital meter read a current that only lasts for about 166 microseconds?

#### Ian0

Joined Aug 7, 2020
8,947
Can a digital meter read a current that only lasts for about 166 microseconds?
No - use an oscilloscope.
Put a 1 ohm resistor in series and measure the voltage across it. It should be 250mV

#### Dodgydave

Joined Jun 22, 2012
11,152
Can you measure the current and keep the oscillator going all the time say for 2secs..?

Thread Starter

#### sparkie78

Joined Nov 5, 2016
21
No - use an oscilloscope.
Put a 1 ohm resistor in series and measure the voltage across it. It should be 250mV
I'm sorry, I don't understand. Put the resistor in series with what?

Thread Starter

#### sparkie78

Joined Nov 5, 2016
21
I'm sorry, I don't understand. Put the resistor in series with what?
Oh wait. do you mean in series with the load? Why should I read 250mV?

Thread Starter

#### sparkie78

Joined Nov 5, 2016
21
Can you measure the current and keep the oscillator going all the time say for 2secs..?
Oh wait. do you mean in series with the load? Why should I read 250mV?
Oh wait. do you mean in series with the load? Why should I read 250mV?
Geez, duh. I understand. That's pretty clever. Thank you.

#### Ian0

Joined Aug 7, 2020
8,947
1 ohm in series with the supply. 250mA x 1 ohm = 250mV
It depends exactly what you mean by "should draw 250mA". If you mean that the oscillator and its load should draw 250mA from the supply.
On the other hand, if you mean that the load should draw 250mA from the oscillator, then the resistor should be in series with the load. But then, the current may be AC?

Thread Starter

#### sparkie78

Joined Nov 5, 2016
21
No, what I got is a rectifier connect to the mains wiring. The rectifier output has two branches. The first, what I call the 'control' branch, is regulated down to 5 volts and is run to 3 IC's which produce the oscillation and timing I require with the output connected to the base of a darlington transistor. The other branch, the 'load', simply comes from the rectifier output to a 400 ohm resistor that is connected to the darlington collector and the emitter connected to ground. So the oscillator is only controlling the transistor and is not part of the load. The 250mA is actually just an approximate target. So I could put the 1 ohm resistor between the resistor and collector or even the emitter and the ground to get the voltage reading?

#### Ian0

Joined Aug 7, 2020
8,947
Sounds a bit complicated - how about a circuit diagram?

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