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Registered Member #863
Joined: Tue Jun 26 2007, 09:29AM
Location:
Posts: 2
Hi,
I've got what I believe is an old x-ray power supply that uses 2 CW Multipliers generating +/-35kV for a total of 70kV. I'll be sinking it in oil and using it to drive a small dental x-ray tube (which wasn't from the same equipment as the PSU).
PSU
My initial rough schematic produced by following the board layout
A redrawn schematic
The filament only needs a low voltage at a high current, but it also needs to be connected to the negative side of the HV supply. If the supply was 70kV-0V then it seems as though this shouldn't be a problem, but as my supply is -35kV I was wondering if I would then need to insulate the low voltage filament supply, or will it be okay as the HV is floating?
Also, I was unsure about how best to drive the transformers given their layout. I've tried driving them as shown in the 3rd image using an IRFP250/555 type circuit and that seems to work (I've only driven the outputs to +/-2kV so far), but I'm not sure if they way I've hooked it up is the best?
Lastly, I was going to connect an analogue mA meter between the grounds of the multipliers to measure the tube current. What sort of protection will it need, if any?
I understand the risks of the x-ray tube and will be shielding it with lead sheet/concrete blocks and driving it from +25m away.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
In your sketch the multipliers are correct, in your redrawn schematic the -ve (red) multiplier is drawn incorrectly.
The hv capacitors look like polystyrene types, very good for insulation, low loss etc BUT you need to be careful which oil you use as some can degrade polystyrene.
I'd join the centre-taps of the transformers together.
Not sure why two transformer secondary windings are required
Registered Member #863
Joined: Tue Jun 26 2007, 09:29AM
Location:
Posts: 2
Thanks Sulaiman, good catch with the schematic. I've redrawn it:
Thanks for the warning about the oil too. I wasn't aware that some types might damage the caps. I'll be sure to check that.
EDIT: Actually, looking at my hand drawn schematic again, the centre taps of the two transformers do actually seem to be connected like you suggested doing. I think my simplified diagram in the top left corner may just be wrong.
There is probably something to be said for using a battery as a floating supply in these circumstances. Work out a resistor that limits the current you need without high voltage first. I would think the mA meter should be in a faraday cage inside a thick plastic box, with a capacitor and a big choke for low pass operation. I don't think those would need to be EHT rated.
You will want to under run the filement a long way I think. Most dental tubes are only designed to be run at rated power for a few seconds at a time.
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X-ray multiplier PSU +/-35kV
