measuring the current

Hi,
In my circuit the current is changing with the time. I'm going to measure the current. I used a shunt(a 1ohmresistor in serries with the circuit) but the changes are too small to be calculated by the osciloscope. The Ic shouldn't draw more than 200 mA and I'm worried about that.
1) How can I measure the current?
2) Should the maximum current or the average current be less than 200mA?

Fahime said:

Hi,
In my circuit the current is changing with the time. I'm going to measure the current. I used a shunt(a 1ohmresistor in serries with the circuit) but the changes are too small to be calculated by the osciloscope. The Ic shouldn't draw more than 200 mA and I'm worried about that.
1) How can I measure the current?
Use a milliAmp meter is series with pcb's +V supply.
2) Should the maximum current or the average current be less than 200mA?
The current flowing into the pcb circuitry, depends upon what components you have in the circuit

Click to expand...

Please post a circuit diagram.

ericgibbs said:

Please post a circuit diagram.

Click to expand...

Dear Eric,
I've already had a multimeter in series with the V+ before, but I can't read the multimeter due to fast changes of current. What can I do then?

ericgibbs said:

Please post a circuit diagram.

Click to expand...

Dear Eric,
I've already had a multimeter in series with the V+ before, but I can't read the multimeter due to fast changes of current. What can I do then?

Fahime said:

Dear Eric,
I've already had a multimeter in series with the V+ before, but I can't read the multimeter due to fast changes of current. What can I do then?

Click to expand...

If you want to detect changes in the current, the common way is to use a low value resistor
[ able to carry the current you expect] and place it in series in the ground 0V line.
Between the DC power supply 0V and the 0V to the pcb under test. Usually a 1R0 is OK.

Disconnect ALL other ground connections to the pcb, so that the pcb current dosn't sneak past the 1R0.

Connect your scope across the 1R0, with the 0V/Common on the scope probe on the end of the 1R0 connected to the PSU
[as described above] and the scope probe to the end of the 1R0 that goes to the pcb on test.

Set you scope Y amp/div to say to 100mV/cm DC input, the scope will display the total current flow proportional to voltage.
ie: 1R0 * 0.2A= 200mV, two divisions on the scope.

Now switch the scope to AC input, set the Y amp to 10mV/cm AC, so now you will see the 'variations' in the current .
Example 1R0 * .01A= 10mV, one division on the scope.

Got the idea?