Current Sensing circuit

[brian_mk]

I have been looking at an OCXO 'oven ready' current sensing circuit. See attached schematic.
The basic idea is that when the OCXO reaches temp, the supply current drops.
The circuit senses the drop in current and turns on a LED. Simple.
I want to do something similar.

However, I'm struggling to understand what limits the current through Q5 when Q6 turns on and the LED switches off.
It looks as if when Q5 turns on, it virtually shorts the power rail through the Q6 base-emitter junction.
I expected a current limiting resistor beween Q5 collector and Q6 base.

I tried creating an LTSpice model to investigate. It confirms there is no large current through Q5 when it turns on but I don't understand why.
What am I missing?

[fourtytwo42]

What you are missing is the part played by R21/R22 these introduce a schmitt action (hysteresis) but also control the collector current of Q5 by starving Q5 of base current once Q6 starts to turn on. It is somewhat dependent on the current gain of the transistors used and indeed you could add a small resistor to Q5's collector to provide a certain limit (say 100R).

Oops my mistake 

[brian_mk]

I was aware that R22 provides hysteresis.
However, when Q6 turns on, R22 pulls the voltage on Q5 base lower so Q5 base emitter voltage and Q5 base current increases.
That will increase Q5 collector current rather than decrease it as you suggest.

I am starting to think that Q5 collector current must be limited by R21.
I agree that will depend on the gain of Q5.
It means that Q5 never turns fully on and does not go into saturation.

If I remove R22 from my LTSpice simulation, it removes the hysteresis (as you would expect) but has little effect on the current through Q5 when Q6 turns on.



 

[magic]

That's correct, R22 resistance is much too high to have much effect.

Q5 collector current is simply β times R21 current, and the latter depends on R18 voltage hopefully not being too large.

[ledtester]

If I remove R22 from my LTSpice simulation, it removes the hysteresis (as you would expect) but has little effect on the current through Q5 when Q6 turns on.

Can you share your .asc file?

(Unfortunately the forum doesn't allow .asc attachments so just change the suffix to .txt or put it in a zip file and attach the zip.)