Constant current pulse generator

[slashguitar]

Hi,
I decided to build a small ESR meter similar to Bob Parkers(full documentation here http://members.ozemail.com.au/~bobpar/k7214.pdf )
I don't understand how the constant current pulses are generated ( Q3-R6 ,Q4-R8 and Q5-R10 on the top left of the schematic ) .
The way I see it the current through the DUT will be (5V-Vcesat)/(R6+ Rdut)  so it varies with Vcesat and load resistance....please correct me if I'm wrong
Should I use opamps for the constant current pulses?

Thanks

[Jay_Diddy_B]

Hi,

There is no real current sources in the design.

However, the circuit approximates to a current source.

One of the transistors is driven into saturation, there is very small Collector - Emitter voltage. There is also a very small voltage across the capacitor under test.
So the voltage across the resistor is approximately equal to the supply voltage. We have approximately constant voltage across  the resistor, therefore approximately constant current.

Jay_Diddy_B