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Registered Member #57520
Joined: Thu Oct 15 2015, 11:21PM
Location:
Posts: 15
Hi again, 4hv users. I need help with a 70KV power supply I will build soon. The problem is that the cathode goes to the heated filament of the tube so x-rays are made. If I do this, I will have to build a small isolation transformer for the filament that runs with 2.5-3V. I will also have to isolate the Cockroft Walton multiplier and tube anode with epoxy, see diagram:
(Note: the multiplier cathode goes from the beginning of the multiplier of course, I have drawn it wrong, corrected in next diagram)
(Note: that resistor between filament and grid is required so emision won't be very high)
But I asked for advice to the tube manufacturer and they said that if I ground the cathode, I will only have to isolate the multiplier and anode. This would be great because it means that I would be able to run the filament with any DC source, isolated or not, because the filament is connected to the cathode and this would be at 0V. See picture where I have added grounds.
I have experience in HV and I have already built a X-Ray machine, but this "grounding method" is new to me, I don't even know if those grounds are well placed. So 4HV users that are more experienced, please give me any advice you have.
Registered Member #57520
Joined: Thu Oct 15 2015, 11:21PM
Location:
Posts: 15
teravolt wrote ...
have you thought about using a small marx to give pulsed ex-rays
Yes but I'm not looking for pulsed x-rays. Btw the tube is a CEI OX/70-G4 and I will use it at about 120W, I don't expect the cooling to be a problem here.
I want to build a C-W multiplier like CEI has: but I'm not sure how I have to connect grounds, I mean, that square wave is coming from arduino, is it safe to connect arduino, primary coil and C-W multiplier grounds all together? This is the way that CEI uses, and it is the best since you don't need insulation on cathode's side because it's grounded and at 0 volts.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
What Sulaiman was talking about was having to float your anode cooling system at high voltage. This can be more complicated than floating a filament. How do you plan to cool your anode?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Rechargeable batteries are a very simple, practical, and reliable, way of implementing a floating heater or filament supply.
You would in any case want to start the heaters in a standby mode before applying EHT to avoid damaging the tube, so you can simply switch your batteries into the heater with a little local switch - with battery and switch close to the tube - before you start the High Voltage supply.
If you want to control the heater current as a means of controlling the tube current, you can just use a high wattage variable resistor, one of the big ceramic ones, or an old fashioned rheostat.
Since most tubes need filament supplies between 1V - 4V you can also make this up out of combinations of rechargeable cells, for example:
1.2V - NiCd 2V - Pb-acid 2.4V - 2*NiCd 3.2V NiCd + Pb-acid 3.6V 3*NiCd or Li 4V - 2*Pb-acid
Take care you choose cells with sufficient Ah capacity, as your tube current will start to fall with the filament voltage, so the anode voltage will rise (unless regulated) with a risk of flash over - also if the filament voltage falls, causing the anode voltage to rise, the tube will produce fewer but harder rays. If the filament voltage fails completely, the tube may go into field emission mode, losing focus, and emitting x-rays in unpredictable directions.
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Help with a HV source for X-Rays
