Now the alien translator wrote the PMTs I am using needed 500 V. But deciphering the documents that came with the PMTs revealed those need 1750 V.
The avid reader of this newsgroup will have noticed I made a nice 500 V design, stabilised, and all that, tested too, used little power, see thread little 'audio transformer'...
So could I 'push up' that design to say 2 kV? (seems a safe value if it is OK at 2 kV it should work at 1750 V).
So I tried a few things yesterday, I could get reliably to 1kV with the existing design, as published, but at about 1200 V the transformer flashed over, and I had to wind a new one. Insulated it better, but now there was a much bigger problem. Those alien PMTs need a voltage divider that cannot have a higher Ri than 1.1 MOhm... Now 1.1 MOhm at 500 V is 250 mW. But 1.1 MOhm at 2 kV = 4 W! the design was to be able to run an battery - for a reasonable amount of time, say a 12 hour day -.
Shattered dreams, went to sleep, to 'sleep on it'. This morning after waking up it occurred to me, after thinking about zeners, transistors with resistors, and all that sort of stuff to make all the voltages and replace the resistor divider for the PMT, and get power consumption down, to use a multi-winding transformer. Such a transformer would have a winding for each anode of the PMT, and a diode, and a capacitor, and all that in series. I was faced with a bigger core (for space, and insulation), and a multi section (10 actually) coil former, all needed to be specially made, isolation problems at the transformer connections.. 2 kV is not nothing.
So that sort of looked like the end of this design adventure. Now I gave up, and put the problem back into the subconscious area of the brain to have it work out the impossible. Engineering is after all making new things from the things you HAVE. I did not have such a transformer, and the prospects of getting one at a reasonable price seemed too dim. Call it lateral thinking (I think somebody did), brain did a scan of what we DID have, and showed me a picture in my mind of all those nice 33 uH coils I got for almost nothing from ebay (a bag full, bought them for switchers). I had used things like those before with a few extra turns of wire added for feedback in this project:
PMT ... . ... . \ / k . ----------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a1. ---------------------- . . |-------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a2. ---------------------- . . |--------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a3. ---------------------- . . |-------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a4. ---------------------- . . |--------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a5. ---------------------- . . |-------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a6. ---------------------- . . |--------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a7.---------------------- . . |--------|>|-------- . . | )| ------- +5V . . === )|( . . | )| --------------A . a8 ---------------------- . . |------------------------------------- 10 M --------- voltage sense = current limiter reference to PIC comparator . . |--------|>|-------- | . . | )| ------- +5V [ ] . --- . === )|( | ....|.... | )| ------------- A /// | ------------------- | anode | | /// ---| ---| |------- PWM from PIC |---| | IRLZ34A |------------- current sense to PIC comparator [ ] 1 Ohm | ///
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