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Will this two transformer doubler circuit work?

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Ash Small

Sat Aug 02 2014, 07:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've come up with the circuit below, using two MOT's and diodes and capacitors from a microwave oven.

The idea is to generate ~10kV to drive an SGTC, using DC resonant charging (see my thread in the TC section).

I realise I could just connect the secondaries in series and feed the combined output into a doubler, but I want to use standard microwave oven diodes and capacitors, so I'm trying to feed the output from each MOT to a separate doubler circuit, and series the output from those.

Can anyone see any problems with the basic design?

EDIT: The idea is to get +/-5kv. I think the secondaries in MOT's are tied to ground anyway.

GrantX

Sun Aug 03 2014, 01:48AM

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

I've made the exact same circuit before (sans the final filtering caps on the output) and it worked quite well. I had the centre tap of the MOTs and doublers connected to earth. Seemed to work fine, however the microwave oven diodes were running quite hot. Choke coils in series with the output helped with diode temperatures a bit and also helped stabilise the arc when having some short-circuit fun. A "Terry filter" with big wire-wound resistors would probably be best for Tesla coil duty, for the sake of the poor diodes.

Ash Small

Sun Aug 03 2014, 02:07AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks Grant, I wasn't sure if I'd got anything backwards.

I don't want to use a Terry filter, I want to use DC resonant charging, which equates to chokes on the output.

I might use a string of electrolytics in place of the output caps, to help with smoothing.

BTW, anyone who wants to try this, remember it can kill you three times over, but it is a cheap HV supply, if you have a couple of old microwave ovens.

EDIT: I'll post more details in my SGTC thread in the Tesla Coil section, especially regarding the grounding of the centre tap.

kimbomba

Sun Aug 03 2014, 04:44AM

Registered Member #3854 Joined: Fri Apr 29 2011, 03:45AM
Location: Mexico
Posts: 95

How much voltage is each secondary seeing?

Steve Conner

Sun Aug 03 2014, 08:49AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, it should work fine. Just be careful with the phasing of the transformers. The circuit will work with either phasing, but you'll get less ripple on the output when the transformers are in phase. The reason is that the positive and negative halves of the circuit get charged alternately so the combined output has ripple at twice the line frequency.

If you think about it a bit more, given that the secondaries share a common ground and are in phase, they must put out almost the same voltages. So you could parallel them, which gets you back to my quadrupler circuit driven by two MOTs in parallel.

However, using a separate series capacitor for each MOT probably helps with current sharing, especially if the two transformers aren't quite the same.

Random idea: You could add a switch to configure the MOT primaries in series, this would give half the output voltage to allow testing at reduced power.

Ash Small

Sun Aug 03 2014, 11:14AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

kimbomba wrote ...

How much voltage is each secondary seeing?

Well, each secondary is, from what I've read, 2kV RMS, which theoretically gives ~2.8kV peak, but in reality it's apparently less than this, depending on load, I think (maybe due to the shunts and associated leakage inductance, I suppose), so each secondary is at ~2.5kV.

I think this is all that either secondary will 'see'.

Steve Conner wrote ...

Yeah, it should work fine. Just be careful with the phasing of the transformers. The circuit will work with either phasing, but you'll get less ripple on the output when the transformers are in phase. The reason is that the positive and negative halves of the circuit get charged alternately so the combined output has ripple at twice the line frequency.

This is one of the things that was giving me a headache last night. I think the way I've drawn it they are out of phase, and I get ripple at line voltage. Maybe I should simulate it with LTSpice?

Steve Conner wrote ...

If you think about it a bit more, given that the secondaries share a common ground and are in phase, they must put out almost the same voltages. So you could parallel them, which gets you back to my quadrupler circuit driven by two MOTs in parallel.

However, using a separate series capacitor for each MOT probably helps with current sharing, especially if the two transformers aren't quite the same.

I think I want to avoid the quadrupler if possible. The basic idea was to use standard microwave oven diodes and capacitors rather than series diodes and series/parallel arrays of capacitors (keep it cheap and simple). I'll probably use strings of electrolytics on the output though, for better smoothing.

Steve Conner wrote ...

Random idea: You could add a switch to configure the MOT primaries in series, this would give half the output voltage to allow testing at reduced power.

Yep. If I allow for different primary configurations I can have them either in series, anti-series (if that's the correct terminology), parallel or anti-parallel. That way I can ensure I get ripple at twice line frequency, and have reduced voltage for testing. If it will work in any configuration I suppose I don't need to simulate it, just swap it around until I get the desired output

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