Quote:
Originally Posted by Argus25
Where you can with a supply like this you are always better , if you are able, to avoid using the voltage multiplier on its output as it substantially raises the supply's output impedance and requires more parts. If you can it works better simply to find a transformer with a suitable turn's ratio.
this is likely to be my only option for a 1.2V supply. Getting the circuit working at 3V with a high enough output has been tricky. I eventually found that the miniature audio transformers I was trying could cope at 9V supply but wouldnt get above 230V from 3V. Swapping to what I now know to be a common mode choke from a SMPSU has allowed me to get a little over 400V from it with a 3V supply
What this means of course if you double the supply voltage V, you double your peak output voltage.
Indeed, as the above experiments have shown!
So when making a flyback supply, a good way to regulate it, if you want it regulated, is to control the length of time the transistor is switched on for per cycle, so at the end of each cycle you have controlled the peak primary current I. This is where a 555 can come in handy or many types of pwm IC too.
PWM is one of the techniques ive tried, using a 555 and the common mode choke, from 9V. This works but the regulation is a little loose, varying around +/- 10V at 430V out. This isnt actually too bad for a G-M tube, so long as the nominal voltage is set for the middle of the G-M plateau
So there are a number of configurations that work & are fun. An all discrete type is good of you are starting with a low level supply voltage like 3V.
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This is my goal, ultimately a single 1.2V cell powering a cute little 2cm long alpha tube, and all in a box small enough for a shirt pocket! I think a true transformer might be the only way to go here. I believe the transformers from disposable flash cameras can be used, if I can find one!
The circuit as shown, with the transformer replaced by the common mode choke and the BC327/BC337, didnt like running from 3V with a 100uF cap for the pulse timing. With a neon bulb as part of the regulator chain, it could be seen visibly to flash! A bit of experimentation showed that the highest output voltage was had with a 4u7 capacitor. As you say, timing the pulses so as to reach the peak current in the inductor but not go over.
When I started this project, the aim was simply to generate 400V to drive the G-M tube, and all my attention was on the indication/counting/metering circuits. Im actually now getting quite interested in the power supply side! Its proven to be quite a learning curve.