Transformer question

[Prune]

On an old microwave oven transformer I have the high voltage secondary has one lead going into a diode, and the other one is soldered to the transformer core. Can I disconnect this so I can use a bridge rectifier?

BTW, I'm very well aware of safety issues, lest someone should feel in a patronizing mood in replying.

[Sch3mat1c]

Probably not, the insulation isn't very great against the core. If your circuit is isolated (no), you could.

Can use two in series to get a CT'd plus and minus with FWB's though. Then you have the 2kV or so difference in DC which may or may not be isolatable.

Tim

[Prune]

If your circuit is isolated (no), you could.

I'm not sure what you mean by whether my circuit is isolated. Do you mean from earth ground?

There is no way for me to get another transformer like this, as I found it on eBay. I paid more for shipping than for product -- it is a good deal larger than any other MOT I've ever seen, which is why I got it.
RMS under 1 A load is 2100 V (or approximately so; I had to use a resistor divider as my multimeter doesn't go that high). I was hoping to get close to 2 kV DC filtered...

BTW, I'm not sure whether to leave the 3V heater winding on or remove it. Three turns right on top of the HV winding. I don't know how to figure out how much current it can supply; could I feed into a full wave doubler to use for 6.3 V tube filaments?

[Prune]

Oops, I just realized that I can wind my own filament winding.

I read on another board a post by someone that left the secondary connection to the core and still used a FWB. Does the core need to be grounded?

[Sch3mat1c]

Hmm, that is indeed a monstrous transformer! Where'd you find loads for something like that? (125W light bulbs?)

If you use a FWB, the core (and the rest of the winding) will be at a DC potential of half B+. This can be seen easier by imagining what a center tap does. In addition to the bias, it will be at AC potential due to swinging full Vout. The result is that the minimum peak voltage (with respect to circuit ground, after the FWB) reached is about .8V and the maximum is .8V less than +V, simply because those very voltages are derived from those very sine wave peaks, minus a bit of drop.

As a result, you end up with full ACV placed across either the secondary-to-core or primary-to-core insulation.

If you ground the core, you get ACV swinging on the power supply instead. Think: ground loop up the...erm... words allowed here cannot describe the place it goes to.

What I mean by isolation is: if you can't have a power transformer isolating, you can have input and output transformers rated for the isolation instead. I drew up (and will eventually build) such an isolated line operated design.

In terms of DC isolation instead, just use coupling caps rated for the DCV. Ground the CT so you only have to worry about ripple on the negative rail.

Tim

[Prune]

Last time I saw a microwave open I was left with the impression that the core was grounded to the chassis, and presumably earth. This schematic at least shows the first one:
http://stefanbinder.privat.t-online.de/pics/uw9.jpg

I don't need isolation. As I'm feeding a glow discharge, I just care about relative potentials between the electrodes. I'm trying to figure out a way to get about 2 kV (filtered for ripple; 300 mA will do). Is this possible with this transformer, and how?

BTW, there seems to be contradictory advice on the Internet whether the magnetic shunts should be left in or knocked out. As I don't need the full current capability of the transformer, current limiting should be a good thing. Are there any possible cons to leaving them in?

Also someone ground down the welding of the core sheets (runs cooler without it with less losses in the core) and rebuilt them, covering with laquer, adding more winding insulation, all without disturbing the windings. If there's no suggestion of how I can achieve what I said in the first paragraph, I'll go this extreme route, with the risk of wrecking the beast.

[Sch3mat1c]

Hm rebuilt? That's neat... Can you contact the guy and see if it's rated?

Ok, since you only need potential, a FWB will work. But how do you interface that to your signal source?

At that level you can leave the shunts alone. Without, you'll have a bit better coupling and as such more voltage, better regulation and unlimited current (like a normal transformer). Hm, since you want a variable current output, that might be a good thing. You decide.

Tim (is eventually going to use a similar, though 800V 15A, transformer for a "small" induction furnace)

[Prune]

So should I ground the core or not in a FWB arrangement?? I opened up my microwaves and the transformers are all earthed, both core and one lead of the secondaries. The other lead goes into a 1 uF capacitor with diode to earth on the other end of the cap -- half-wave multiplier biasing the heater coil.

The coils are very solidly packed with resin-impregnated cardboard or something like that. I don't think they'll budge if I take apart the core. If I find that isolation is a probem, I'll try to rebuild it. What would be a good replacement isolation?

For interfacing to the audio signal, I'm going to replace the ballast resistor with a tube that will be controlled by the audio signal. I don't mind using AC coupling if needed -- I already have a capacitor in my source's signal path.

[Sch3mat1c]

Yeah sure, ground the core. That way if it does arc over, you have the safety ground handling fault current like it ought to, rather than tracing up the primary wiring, or whatever's connected to the chassis, etc...

In ovens it's both convienient and safe to ground everything.

Tim

[Prune]

What's the best approach for filtering the rectified DC? In Power Supply Designer, a CRC with 200 uF total gives a good result, but that capacitance at that voltage is still very expensive. Is there another arrangement that will be cheaper and not lower voltage too much? And for capacitors, what's the cheapest way to achieving this? I was thinking of series/parallel array of lower voltage caps, say 450 V electrolytics.