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cascaded OBITs for higher voltage?

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Vlad

Sat Jul 25 2015, 09:01PM

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

While tinkering with my power supply in the other thread, I also found a bunch of OBITs I've had in storage. I was tempted to try the following instructions in order to get higher voltage from them, and then convert to DC:

(The information on cascading them is toward the bottom of the page)

I have some questions about this: 1) Has anyone here done it and what were your results? 2) Is each transformer winding only handling 5 KV at a time and is this why there isn't internal arcover and other issues?

This is the only website I've ever come across with this info, which makes me wonder whether or not it's possible, and, presumably, I would think the same sort of configuration could be done with midpoint grounded NSTs, correct?

Wolfram

Sun Jul 26 2015, 07:49AM

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

It's a clever method. It should be safe and robust, but the power will be very limited to less than what a single OBIT can supply, so I guess this is why the method isn't more popular. Most people in the HV community want to make long arcs, so limited power does not appeal to many. If the power of a single OBIT is enough for your application, it should be fine.

Do you want this for powering an X-ray tube? Didn't you have a transformer?

Dr. Slack

Sun Jul 26 2015, 08:21AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It's the centre-tapped-ness of the secondary that allows this. You can pull the exact same trick with NSTs, most have a centre-tap, though not with MOTs. You can however connect a pair of MOTs to make a centre-tapped pair, if you pay attention to phasing, and then cascade those.

Three things to bear in mind ...

a) as the man says, the cases are hot, so insulate and don't touch them for safety and correct working
b) the cases, and the redundant mains connections, will start spraying corona as you go much above 10kV, so no pointy connections
c) as the transformers are current limited, that could be exacerbated by the cascade in different ways depending on where in the construction the magnetic leakage path is. If it's 'on the primary', and the secondary windings are well coupled, then it's as good are you are going to get. Unfortunately the manufacturer may have taken advantage of the expected isolation between the two ends of the secondary and made them discrete windings (I certainly would if I was trying to get cheap and effective insulation between the two outputs) and put a leakage path per secondary, that would really kill the impedance of the cascade. A MOT pair cascade will fall foul of this unless you pull the shunts out.

Vlad

Sun Jul 26 2015, 03:07PM

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

Wolfram wrote ...

Do you want this for powering an X-ray tube? Didn't you have a transformer?

I was going to try it as a standby (or even primary) PS for the tube at some point if the flyback based supply I currently use fails. Originally there was the transformer, yes, and I asked you about rectifying it some weeks back, but being only able to use 1/2 winding meant I'd have to put too much input power into it creating an arcover risk since only part of the transformer was still in oil.

Wolfram

Sun Jul 26 2015, 05:49PM

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

Vlad wrote ...

Wolfram wrote ...

Do you want this for powering an X-ray tube? Didn't you have a transformer?

My suggestion was to full-wave rectify it using two diodes. That way, you get to utilize the full secondary winding. The voltage you get out is half of the secondary peak voltage, and the current you get is sqrt(2) times the rating of the secondary (for the same amount of heating in the secondary). It's a very nice compromise that works well in practice. How much power do you need? A dental X-ray transformer will likely be able to supply more power than any number of OBITs cascaded in this way.

I gave up replying to your original thread as I got confused by you deleting the content of posts left and right.

If only half of the transformer is in oil, I think adding more oil is easier than trying to make a new supply.

Vlad

Sun Jul 26 2015, 07:20PM

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

Wolfram wrote ...

My suggestion was to full-wave rectify it using two diodes. That way, you get to utilize the full secondary winding. The voltage you get out is half of the secondary peak voltage, and the current you get is sqrt(2) times the rating of the secondary (for the same amount of heating in the secondary). It's a very nice compromise that works well in practice. How much power do you need? A dental X-ray transformer will likely be able to supply more power than any number of OBITs cascaded in this way.

I still have the x-ray transformer in its new environment with the container half filled with oil. It sits on the same shelf as the current flyback supply I'm going to use. I was having a lot of difficulty figuring out the correct wiring for the transformer (unable to find the center tap for starters if it even had an access point), so gave up on it for now, plus I don't have a local source for the transformer oil it used. Mineral oil is readily available from the farm store, but I was hesitant to try adding this to the existing transformer oil.

kilovolt

Wed Jul 29 2015, 01:16PM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

Dr. Slack:

You can however connect a pair of MOTs to make a centre-tapped pair, if you pay attention to phasing, and then cascade those.

That doesn't work, because the high voltage windings of a pair of transformers are not on the same core. The voltage will only be transformed if both coils are on the same core like on a center tapped transformer.

Regards kilovolt

Dr. Slack

Wed Jul 29 2015, 01:37PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

kilovolt wrote ...

Dr. Slack:

You can however connect a pair of MOTs to make a centre-tapped pair, if you pay attention to phasing, and then cascade those.

I didn't spell out that you have to connect the primaries, because as you point out, it obviously doesn't work if you don't. That 'puts them on the same core'. The comment about phasing should be suffiencient of a hint, it is redundant if you only connect the cases together and use the secondaries, so it must refer to the primaries being connected as well.

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