Microwave oven transformer as PSU for audio gear

Discussion in 'General Electronics Chat' started by prometei, Jun 28, 2014.

  1. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    greets!

    I have a microwave oven transformer that I want to use for building a PSU for my audio gear. With 230 VAC on the input it outputs 22/23 VAC.

    I want to connect the output to a 35A 1000A rectifier (+smoothing caps) (something like this: http://www.taydaelectronics.com/diodes/single-phase-bridge-rectifier-mic-35a-1000v.html) then split the DC output into a few lines and feed them to switching step down converters based on the LM2596 (http://www.ebay.com/itm/gib-Buck-Co...047?pt=LH_DefaultDomain_0&hash=item27e02dd2ef) after which I want to use 7812 and 7809 linear regulators (in combination with smoothing caps) for the individual (or combined) outputs to the audio devices (guitar effect pedals, synths, etc...).

    The total power consumption of the audio gear does not go over 50W.
    What do you guys think? What problems might there be?

    thanks for your help
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    As long as the transformer gives out 2amps at least should be ok.
     
  3. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    i.e. 2A x 23VAC (rms) = ca. 50W ? So if I'll want to increase the load to 100W the MoT will have to handle 4A on the output without getting too hot?
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    ideally yes
     
  5. PackratKing

    Well-Known Member

    Jul 13, 2008
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    You don't mean the big trans. for the magnetron...:eek:

    The secondary output, by input of 230 volts from that beast would certainly not be 23 VAC.... more on the order of 2 - 4 Kv -- Unless you are intending to back-feed it...

    Either that, or I missed a major point somewhere...
     
    Last edited: Jun 30, 2014
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Risky :eek:. One end of the HV winding is connected to the transformer core on ones I've seen; so you'd have a possibly live core instead of a mains-isolated one.
     
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  7. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    Yes, some of them have that, one of mine also, but I will use one that has the windings isolated.

    Yep, I'll use it reversed.
     
  8. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    on a related note:

    I also want to add a soft-start circuit, but I want to use a standard light bulb or few in parallel, instead of the high wattage resistors or thermistor (connected to the relay). However I'm not sure what wattage bulbs are should use?
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Filament bulbs have a PTC characteristic - that is to say when the filament is cold, it has a much lower resistance than at normal running temperature.

    A typical value for filament bulb inrush current is about 12x normal running, when I used filament bulbs - I arranged a soft start to make them last longer.

    Although I've never seen it used in front of a mains transformer - the usual method with a lot of SMPSU mains inputs is a NTC inrush limiting thermistor.

    The method I see most often in front of mains transformers is a hefty power resistor in series with the primary, there's usually a time delayed relay to short the resistor once everything has settled down.
     
    Last edited: Jun 28, 2014
  10. inwo

    Well-Known Member

    Nov 7, 2013
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    You won't have to worry about that.;)
     
  11. takao21203

    Distinguished Member

    Apr 28, 2012
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    it's a bad idea these transformers are not designed for power supply application. You can remove the winding from the core but you never get certification for that. So you are limited to use it for yourself at home.

    Who not use a laptop power supply? If you need higher voltage, use a 12v/35v converter module, they can be used outside their specification.
     
  12. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    Not worry about which wattage to choose or about the inrush current (i.e. the MoT can handle it)?

    Yep, and most tutorials and circuits I found on the net are from that category.

    Halogen bulbs cool down gradually, they could be heated up (i.e. switched on), then switched in the circuit with the transformer and then switched out, but this would require two timers (555) and two relays. I guess it would be easier to just find some high wattage resistors :)
     
  13. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    I don't want to remove any windings, just use the ones it came with originally.
     
  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    I mean to remove the wire from the core :)

    Dont know how many Watts it can crank out, as the HV secondary is not designed for mains voltage. But 50 Watts should be possible.

    It's just a transformer, after all.
     
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    I meant, that will be the least of your problems.
    Exciting a leakage inductance transformer at 1/10 the design voltage will not result in a huge surge.
     
  16. tcmtech

    Well-Known Member

    Nov 4, 2013
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    This all sounds like gross over design and over kill to power a at best 50 watt audio amp setup.

    Especially since line powered SMPS units that put out a well regulated clean and properly isolated 12 VDC at power levels well above what you need can be bought for or salvaged from other devices for next to nothing.
     
  17. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    Gross over-weight I'd say, since the thing ways like 10 kilos :)


    I do have a PC PSU laying around that has a 12V 12A output line, but I thought a diy PSU would be an interesting project.

    As far as PC SMPS's go I've read that the output lines are unfiltered and are noisy for audio purposes, plus they use a common ground which is a no-no in my case. The one I have looks like a low cost unit. I have not tested it in an audio setup yet, but I assume that it most likely noisy.

    In any case I want to have a clean enough and powerful enough PSU that I won't have to worry in the future as I add new equipment to my setup.

    I also have some audio devices that specify higher than 12V input voltages (ac and dc). And I'm also thinking of modifying some of the audio devices that connect straight to the mains and feed them DC power bypassing their internal transformers, assuming that the voltage from my DIY PSU is appropriate. I reckon that feeding DC to the rectifiers in the devices wont be a problem.
     
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Since the secondary would normally handle ~2kV I see no problem with feeding it 250V (provided the secondary is isolated from the core). As for power, most MOTs are designed for supplying ~850W (albeit for brief periods) so a 50W load should be a doddle.
     
  19. inwo

    Well-Known Member

    Nov 7, 2013
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    Test it before putting any work into it.

    I come up with only about 10 watts useful output.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What about the effect of the magnetic shunt, if fitted?
    Max.
     
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