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.
BTW, I'm very well aware of safety issues, lest someone should feel in a patronizing mood in replying.
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
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
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?
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?
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?
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
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
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.
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.
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)
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)
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.
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.
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
In ovens it's both convienient and safe to ground everything.
Tim
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.
Ya, series is the deal.
I once e-bayed a surplus board of some 16 x 780uF 450V and 8 x 270uF 450V computer grade caps... plus a tonne of equalizing resistors, and a few chunky diodes. My guess was a three-phase power supply at around 2.5 and 5kVDC... 😀
I'd do CLC with the inductor on the ground side, so it can mount to the chassis without needing absurd voltage ratings. Capacitors will have a bit of AC hum on them so metal cans chassis-mounted are no good. But most of them have voltage vs. chassis anyway.
Tim
I once e-bayed a surplus board of some 16 x 780uF 450V and 8 x 270uF 450V computer grade caps... plus a tonne of equalizing resistors, and a few chunky diodes. My guess was a three-phase power supply at around 2.5 and 5kVDC... 😀
I'd do CLC with the inductor on the ground side, so it can mount to the chassis without needing absurd voltage ratings. Capacitors will have a bit of AC hum on them so metal cans chassis-mounted are no good. But most of them have voltage vs. chassis anyway.
Tim
Are equalizing resistors necessary across series capacitors, and what are appropriate values?
I just bought from eBay 25 pieces of 330 uF rated at 450 V for $46. Couldn't find anything cheaper. How many would I need to put in series to avoid frying them? And what filtering configuration would be optimal for a load from 200 to 400 mA? You mentioned a choke, but power chokes I found for sale are all extremely expensive, $100 and up.
I just bought from eBay 25 pieces of 330 uF rated at 450 V for $46. Couldn't find anything cheaper. How many would I need to put in series to avoid frying them? And what filtering configuration would be optimal for a load from 200 to 400 mA? You mentioned a choke, but power chokes I found for sale are all extremely expensive, $100 and up.
Well with so much power flowing, of course it's not going to be cheap. You said some 2kW???
Stack as many caps as you need. Figure 350-400V per cap, that allows surge overhead to be within ratings. You can go rated (450V/cap) but they might not live quite as long. (Who knows...)
Tim
Stack as many caps as you need. Figure 350-400V per cap, that allows surge overhead to be within ratings. You can go rated (450V/cap) but they might not live quite as long. (Who knows...)
Tim
Not 2 kW. I said 2 kV at 200 to 400 mA, so 400 to 700 W.
What's the best configuration to use, given the capacitance I have? LC, CLC, CRC, or CLCRC, and how do I best distribute the capacitance to these Cs? I tried PSU Designer, but it and LTSpice give me different results. I'd wind my own chokes if I knew where to get appropriate cores and wire...
What's the best configuration to use, given the capacitance I have? LC, CLC, CRC, or CLCRC, and how do I best distribute the capacitance to these Cs? I tried PSU Designer, but it and LTSpice give me different results. I'd wind my own chokes if I knew where to get appropriate cores and wire...
Anyone?
BTW I just found 40 uF at 3 kV on eBay for only $12!
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&category=4662&item=3831827129&rd=1&ssPageName=WDVW
I got four of these. So now I've another 160 uF.
BTW I just found 40 uF at 3 kV on eBay for only $12!
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&category=4662&item=3831827129&rd=1&ssPageName=WDVW
I got four of these. So now I've another 160 uF.
Ouch, big tin can. 😀 720J total, eh?...
Depending on how much ripple can be tolerated, I'd take... 1 of those caps at the rectifier would give about 28Vrms of hum. That's all of 1%. Add on a 2H 300mA choke and another cap or two and you'll get about .6V (0.03%) hum.
Tim
Depending on how much ripple can be tolerated, I'd take... 1 of those caps at the rectifier would give about 28Vrms of hum. That's all of 1%. Add on a 2H 300mA choke and another cap or two and you'll get about .6V (0.03%) hum.
Tim
I haven't gotten the can caps yet, but even with just the 165 uF of the multi-mini-cap in a simple C filter, no CLC or CRC, I'm getting a very quiet glow discharge at 100 mA. Haven't tried more current yet as my ballast resistors overheat.
WARNING!
The following images shows a very dangerous setup and should not be attempted by any sane person. I was two meters from the thing with a dry chemical extinguisher besides me when I powered it up. The capacitors have equalizing resistors about ten times the leakage ratings, and wiring is with 5 kV insulation, except the jumper cables.
The discharge started on its own because I had a very thin wire between the electrodes that burned off. I had my finger on the power bar button. Two cathodes to one anode, about 50 mA each. Yes, this is a glow discharge and not an arc, though it's hard to tell from the poor quality photo. I also tried using a microhollow discharge as the cathode, but my copper/mica sandwitch broke before I could take a picture. That resulted in larger and more stable discharges. Even larger ones were achieved by forcing air through the hollow with an aquarium pump. After about five to ten seconds the ballast resistors overheat and melt the solder.
WARNING! The following images shows a very dangerous setup and should not be attempted by any sane person. I was two meters from the thing with a dry chemical extinguisher besides me when I powered it up. The capacitors have equalizing resistors about ten times the leakage ratings, and wiring is with 5 kV insulation, except the jumper cables.
An externally hosted image should be here but it was not working when we last tested it.
The discharge started on its own because I had a very thin wire between the electrodes that burned off. I had my finger on the power bar button. Two cathodes to one anode, about 50 mA each. Yes, this is a glow discharge and not an arc, though it's hard to tell from the poor quality photo. I also tried using a microhollow discharge as the cathode, but my copper/mica sandwitch broke before I could take a picture. That resulted in larger and more stable discharges. Even larger ones were achieved by forcing air through the hollow with an aquarium pump. After about five to ten seconds the ballast resistors overheat and melt the solder.
An externally hosted image should be here but it was not working when we last tested it.
What resistance is that? Did it make any sound (hissing, buzz, etc)? Did you measure the voltage across the discharge?
Tim
Tim
There is no noise that I can hear, so it's low enough to be drowned out by the transformer buzzing (I still haven't rebuilt the core as the welding proved too tough to grind out by hand). Occasionally the discharge would loose stability (probably due to horizontal placement and air drafts) and momentarily break down into an arc, extinguish as resistors heat and transformer magnetic shunts kick in, and restart as capacitor voltage rises again ; this caused a few crackling noises. It was stable with the MHCD cathode, however, and vertical placement also improved stability (and size). In the final setup I may have about five centimeter discharges.
Ballasts are 25K for each of the two chains. My measurements are not very good since the voltmeter doesn't like the high absolute voltage, and resistors heat a lot and that must change the resistance a good deal between short-circuit and discharge mode, but it seems very roughly that resistors drop on the order of 2/3 of the voltage, and the discharge the other 1/3 (an arc would drop very little voltage). In any case, since I'll be using an array of microhollow cathode discharges as the cathodes instead of this simple electrode configuration, this measurement doesn't really mean much.
Sch3mat1c, what would make a good power resistor of about 300 ohms for a CRC? A few incandecent bulbs in series? The problem with bulbs is that their resistance is very low when cold. Especially when I knock out the transformer's magnetic shunts, that would make the circuit breaker trip at power on (already the lights dim). I guess I need some sort of soft start.
I was thinking of using a CRC before a CLC so that the choke would see little ripple and I could use a lower voltage choke, such as one of the Hammonds.
BTW, how can I figure out an appropriate power factor correcting capacitor across the primary without a scope? Most people use 100 to 200 uF.
Ballasts are 25K for each of the two chains. My measurements are not very good since the voltmeter doesn't like the high absolute voltage, and resistors heat a lot and that must change the resistance a good deal between short-circuit and discharge mode, but it seems very roughly that resistors drop on the order of 2/3 of the voltage, and the discharge the other 1/3 (an arc would drop very little voltage). In any case, since I'll be using an array of microhollow cathode discharges as the cathodes instead of this simple electrode configuration, this measurement doesn't really mean much.
Sch3mat1c, what would make a good power resistor of about 300 ohms for a CRC? A few incandecent bulbs in series? The problem with bulbs is that their resistance is very low when cold. Especially when I knock out the transformer's magnetic shunts, that would make the circuit breaker trip at power on (already the lights dim). I guess I need some sort of soft start.
I was thinking of using a CRC before a CLC so that the choke would see little ripple and I could use a lower voltage choke, such as one of the Hammonds.
BTW, how can I figure out an appropriate power factor correcting capacitor across the primary without a scope? Most people use 100 to 200 uF.
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