I've built up my kit - and it is not working
Having got through two batteries I've connected it to my lab power supply. It draws about 10mA. The voltage at the end of the diode chain is a healthy 106V.
The voltage at the transistor across C2 is 35V. The Voltage across the 50 ohms is 0.33mV which corresponds to 6.6 microAmps. The voltage drop across the 10M R1A is about 71V
so the current through this is approx 7.1 microAmps.
This means, I guess, that Q1 is leaking too much current to reach avalanche at 40V?
The voltage across R5 is 252mV where as it perhaps should be 106V*20k/(10M+20k) =209mV. In the circuit diagram R5 is only 10k whereas on the board layout
and as supplied in the kit it is 20k?
Any suggestions as to what Voltages and currents I should be seeing would be most welcome. Is 7 microAmps very leaky? (I've no idea.)
Should I be connecting a 10M R1? (No R1 came in the kit, I thought R1A replaced it?)
If I added a 10M as R1 then the voltage drop would be halved and the Voltage across Q1 would rise to about 53V potentially (though presumably it would
do what it is supposed to do and breakdown at 40V).
It's nearly midnight so I'm off to bed but any help would be much appreciated.
EDIT (following morning) :
I've added a 10M as R1 and it is now working!
At least it is avalanching at a rate of just under 40kHz or every 25 microsecs. Q1 looks like around 5M when it is not avalanching, so the circuit is
two 10M in parallel then a 2pF in parallel with 5M so it is not quite as simple as just RC for the time constant - the effective R is 12.5M - I suppose the last bit of charging is stretched out. I've just looked at it with a probe but will properly terminate it and get some plots when I get a bit of time later on today.
The current draw is still about 10mA so most of this is probably the current of the led (around 8mA or so) - so if you want to save battery life disconnect the led.