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question about rectification of high voltage transformer

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Vlad

Thu Jul 09 2015, 03:34PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
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Posts: 65

I recently rectified a small (50 KV) x-ray transformer to DC. Since the transformer was not center tapped and filament tied to ground, I used the same circuit as here:

I thought this would now give me a single, positive high voltage DC and ground, but when I touched the ground side, I got a shock. My plan was to use a separate DC supply to drive the filament of a tube, with both the discrete DC supply and x-ray supply tied to the same ground, but now I'm not so sure.

My question is that I'm wondering why there's floating voltage on what should be the ground side of the rectifier?

Thank you.

GrantX

Thu Jul 09 2015, 03:42PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

It will only be at ground voltage if you have physically connected the terminal to mains earth, or a metal stake in the ground. Since you have a floating supply (the HV secondary coil is isolated I presume) you can connect either the positive or negative terminal of your bridge rectifier to ground, allowing the polarity to be switched.

Touching the live HV secondary would have been an unpleasant surprise!

johnf

Thu Jul 09 2015, 07:47PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
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Posts: 284

Vlad
If you physically ground your 0V pin to earth then your supply will not give you a shock. I'm not surprised you got a tingle as your high voltage output will float to some arbitary value

Wolfram

Thu Jul 09 2015, 09:37PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

As the others say, one end of your rectified output needs to be grounded, ideally the negative side if you want to run your x-ray tube with a grounded cathode. Before you do that, it's important to check that it is safe for the transformer. Does the transformer have a filament winding? Does it have any current measurement terminals? Do both secondary wires measure infinite resistance to the core? A picture is helpful if you have one.

Vlad

Fri Jul 10 2015, 11:33AM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
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Yes, the transformer does have a filament winding, but I won't be using it as I plan on running the transformer at maybe half capacity and I felt that the filament voltage offered wouldn't be enough. Yes, I'll be able to measure the current as there were terminals to add a meter. Resistance measurements didn't reveal anything useful except in the case of the filament winding. Measurements mostly read infinite even with a 10M input meter and in fact the only way I was able to figure out which side of the filament to use for the rectifier negative side was to take voltage measurements with the transformer powered up with a very low input voltage. Sorry, no pictures as the transformer was rehoused in its new junction box home. Originally, there were four connections with filament tied directly to ground so not adjustable. The original tube in the unit was a Eureka 2-OID-A-W, and I have been unable to find any data on it. I assumed this transformer was something in the 50-60 KV range being half the size of 90 KV heads.

Wolfram

Fri Jul 10 2015, 12:46PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

The transformer is most likely center tapped in practice. These transformers usually consist of two secondaries, with the inner ends connected to the core (which is grounded). When there is a current measurement point, this is usually the inner end of one of the secondaries.

It is very important to make sure that there is no possibility of having high voltage between the inner end of either secondary and the core, and also between the core and the primary. This usually damages the transformer immediately.

You canâ€™t use this style of transformer with a bridge rectifier, unless the rectifier output is floating. If you ground the negative output of the rectifier, the center tap of the secondary would float to a high voltage and arc to the core, or if itâ€™s already connected to the core, the core would arc to the primary. If you grounded the center tap and the rectifier output, you would create a short circuit.
The easiest way around this is to make a two diode full wave rectifier like I suggested in my the other thread. That allows you to run with the center tap grounded. You will only get half of the end-to-end voltage of the transformer, but this should be perfect for a mammography tube. Make sure to ground the current measurement terminal to the core of the transformer.

Some 50 kV x-ray heads were made, but 70 kV ones are far more common. The amount of insulation you need is not linear with the voltage, so a 70 kV head can be much smaller than a 90 kV one. The tube is likely rated for 70 â€“ 80 kV, around 8 mA for a few seconds, and with a focal spot of around 1 mm. This is very typical for dental x-ray tubes.

Here's my guess of the original x-ray head setup, and how I would connect it for a mammography tube.

Vlad

Fri Jul 10 2015, 01:44PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
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Vlad

Fri Jul 10 2015, 02:18PM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
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Vlad

Sat Jul 11 2015, 08:50AM

Registered Member #9711 Joined: Sat Jan 19 2013, 03:27PM
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After testing and thought, I think I'm going to abandon this. I had hoped to rectify to DC and use the full winding at low voltage to drive the tube, but just having half a winding and not very good re-oiling of the transformer doesn't look good for even 20 KVDC (which means I'd have to adjust to 40 KV and my re-oiled system wouldn't handle it). So..... starting a new thread with questions about a supply I already have and did use in the past but there are difficulites I'm running into now with the current set up I want to implement.

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