Voltage/Current Meas. what am I doing wrong here?

The Schematic shows part of a circuit that I am working on. I need to vary the current through the diodes using the series resistor R and take measurements. I have tried this in two ways:

a- using a scope current probe and
b- using two scope voltage probes to measure the voltage drop across the resistor and divide by resistance

The problem is the measurements from these methods do not match:

a- The current probe measures 350mA on the pulse see Capture-1 attached
b- Ch1 measures 2.23v and Ch2 10mv(see Captures 2,3 attached), with R = 2.5 ohm, the current should be I = 2.2/2.5 = 880 mA on the pulse

Why are these two results different?Am I doing something wrong here?

Thank You

Ch1 and ch2 voltage measurements seems to be wrong.
Can be due to wrong connections.

one , did you connect the transistor base and emitter properly?

the waveform show the channels as : Base and Emitter , which is correct? (across R-2.5ohm)

Why the markings of "base" and "emitter" on your voltage plots ? Please check this & revert.

srizbf said:

Ch1 and ch2 voltage measurements seems to be wrong.
Can be due to wrong connections.

one , did you connect the transistor base and emitter properly?

the waveform show the channels as : Base and Emitter , which is correct? (across R-2.5ohm)

Click to expand...

Sorry Guys, I took the measurement in a hurry and forgot to change the labels they belong to a previous measurement. Ch1, 2 are connected to the points that I marked in the schematic

Does your current probe measure peak value or RMS, or trying to calculate that?

Hi,

* please confirm that your current probe is able to measure DC currents. (Current probe setup and scope setup)

**********

* Each of the two voltage probes measure the voltage with respect to the GND clip. Therefore you need to connect both GND clips to the same reference point. I recommend to use your circuit GND.

* Both channels need to be set in "DC" mode.

* I recommend to select the same channel_gain for both channels.

* Then use "math" function "Ch2-Ch1"

Klaus

Gorgon said:

Does your current probe measure peak value or RMS, or trying to calculate that?

Click to expand...

I believe it's set to measure peak but I'll double check

- - - Updated - - -

KlausST said:

Hi,

* please confirm that your current probe is able to measure DC currents. (Current probe setup and scope setup)

**********

* Each of the two voltage probes measure the voltage with respect to the GND clip. Therefore you need to connect both GND clips to the same reference point. I recommend to use your circuit GND.

* Both channels need to be set in "DC" mode.

* I recommend to select the same channel_gain for both channels.

* Then use "math" function "Ch2-Ch1"

Klaus

Click to expand...

- Both GND clips are tied to the circuit/supply GND.

- Both channel are set to 1M ohm and DC coupling

- I believe the gains are set to 'Auto' but I'll double check

- The math function will give the same result as my manual calculation but I'll try it anyway

Hi,

- I believe the gains are set to 'Auto' but I'll double check

Click to expand...

Ch1 shows 1V/div
Ch2 shows 20mV/div

I expect the ON voltage of Ch1 to be at least 1.2V, but it shows only millivolts..

Klaus

I figured out the problem guys, the probe connected to ch2 was faulty. It was only picking up a small fraction of the signal. Measuring with a good probe both measurements give same results. Thank you all for your suggestions.