Where does one get a low to medium power HV xfmr with a floating secodary? / High Voltage / Forums

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Where does one get a low to medium power HV xfmr with a floating secodary?

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jpsmith123

Sun Mar 16 2008, 07:24PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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I guess if you want to make a floating HV power supply (say 25 kv to 50 kv, 0.5 to 1 kw) you have to build your own transformer? Or is there something-of-the-kind reasonably available somewhere?

Dr. Dark Current

Sun Mar 16 2008, 07:28PM

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How much "floating" do you want? Usually this is limited by the innermost windings arcing to the core. If no additional isolation is put there I would imagine the break down occur at a few kV of pri-sec. offset.

jpsmith123

Sun Mar 16 2008, 09:21PM

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Well generally speaking, what I mean by "floating" is the ability of either side of the winding to be grounded with the other side at full rated voltage.

Sulaiman

Mon Mar 17 2008, 05:09PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
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If one end of the secondary is 'earthed' (e.g. TV/Monitor flyback)
and you want to reverse the output voltage
then reverse the output rectifier and reverse the primary.

jpsmith123

Mon Mar 17 2008, 09:18PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Well, basically what I'm after is a general purpose lab bench type HV power supply. As I envision it, it'll be a transformer followed by a full-wave doubler, able to drive a grounded load at either polarity. Having to use a xfmr with a grounded secondary would be a PITA and not as generally useful, IMO.

Electroholic

Mon Mar 17 2008, 10:08PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

ground is kinda an arbitrary thing.
if it is an isolated transformer, you can tie it to "ground" however you want(or not at all), as long as you have enough isolation/insulation so that it will not arc over.

Kolas

Tue Mar 18 2008, 03:23AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
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The normal convention is to make the transformer with one side of the secondary "grounded" to the core, especially in 50-60hz transformers. this is because the inner most layer of the secondary is going to be very close to the core or the primary. the primary is usually not very isolated from the core and has a very low voltage across it.
you typically will not find mass produced transformers with an isolated secondary, because it is deemed unsafe. it is quite possible to full wave double a single secondary transformer who's secondary is grounded to the core of the transformer. you will end up with a B+ and a B-. B- is actually B amplitude below 0V

did that help?
Kolas

Tonic

Tue Mar 18 2008, 10:42AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
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Kolas wrote ...

The normal convention is to make the transformer with one side of the secondary "grounded" to the core, especially in 50-60hz transformers. this is because the inner most layer of the secondary is going to be very close to the core or the primary. the primary is usually not very isolated from the core and has a very low voltage across it.
you typically will not find mass produced transformers with an isolated secondary, because it is deemed unsafe. it is quite possible to full wave double a single secondary transformer who's secondary is grounded to the core of the transformer. you will end up with a B+ and a B-. B- is actually B amplitude below 0V

The secondary lead is the one that is most close to core, right? So the potentials between them is equals, thus preventing arcing.

The main question is, does connecting this secondary lead and core to ground change something? Or is connecting secondary lead to core is enough?

Evilren

Tue Mar 18 2008, 11:52AM

Registered Member #1392 Joined: Sun Mar 16 2008, 05:00AM
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The closest thing I can think of would be at that voltage level that you might consider as a standard item it a small utility transformer. Probably a little large for what you need but they really inexpensive used. If you look around you may even find one as small as 5Kva, although at the higher secondary voltages the small ones are not that common.

jpsmith123

Tue Mar 18 2008, 01:29PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Thanks for all the opinions and ideas, everyone.

I thought I remembered seeing some commercial HV power supplies (e.g., Spellman) with the feature of easily reversible output polarity, but I just looked again, and it seems that feature is only available at relatively low voltages.

In any case, rather than try to wind my own coil (which, in my experience is always a PITA), I think I'll forget the idea of easily reversible output polarity, and just try to use two ignition coils or two flyback coils together driving a full wave multiplier, set up for, at first, negative output.

I have a few questions about ignition coils (HEI coils in particular), but I suppose I should start another thread to discuss it.

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