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Thread: Microwave oven power supply

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  1. #1
    Join Date
    Jul 2004
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    Anyone ever use a power supply from a microwave oven to run a linear amp?

    I am starting work with one of my Elmers on an amp project and he has most of the stuff for the project except for the power supply. One of the things we are hoping to do is use inexpensive or salvaged parts to keep the cost as low as possible. Microwave ovens are cheap or free and can be found everywhere.

    He remembers reading about using microwave oven power supplies for such applications awhile back, but we have been unable to find any information in our search.

    Have any of you guys used such a power supply in an amplifier?



    [URL="http://ragchewnet.forumotion.com/"]http://ragchewnet.forumotion.com[/URL] - A ham forum minus the politics. Talk about what brings us together, not what drives us apart.

  2. #2
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    I remember seeing something... yes! Google is our friend!


    "A pair of microwave transformers offers a very convenient means of producing a high voltage supply at minimal cost. Your local rubbish tip/recycling centre ought to have a good supply of dead microwave ovens. Secondary voltages seem to range from 1500V to 2200V. Technically, they are known as saturable reactors. They are designed to regulate their output voltage through . . . "
    http://www.qsl.net/vk4wss/linear.htm

    Note modification of the transformers.
    (Added in edit): This is shown in the picture at
    http://www.radars.me.uk/The_PSU.htm

    Observe all cautions about HV.

    Also see
    http://oak.cats.ohiou.edu/~postr/bapix/813amp.htm
    and
    http://users.adelphia.net/~alexmm/qro/qro.htm

    Good luck and BE CAREFUL.

    Cortland
    KA5S




  3. #3
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    We must have been putting the wrong words into Google.

    Thanks!
    [URL="http://ragchewnet.forumotion.com/"]http://ragchewnet.forumotion.com[/URL] - A ham forum minus the politics. Talk about what brings us together, not what drives us apart.

  4. #4
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    I designed microwave ovens for 30 years. This question came up locally a few days ago. The transformer & circuit as used is designed to TRY to provide a constant power to the magnetron, which is a large Zener diode, while compensating for line voltage changes. Does a FAIR job #of this.

    These transformers operate just into primary saturation. Should not be over volted.

    The main problem is that for economy the low end of the HV winding is grounded to the core. This should not be removed as the general insulation between the low end and the core is zilch.

    I have not tried this, but a matched pair with the secs in series and the "tap" grounded should work as a two-diode full-wave supply. Or for low-power use, consider using one transformer, grounded, as a single-diode half-wave supply.

    These transformers have a magnetic shunt between the primary and secy pies. If it does not work well, try knocking the shunt out with a rod and hammer. I don't know much about the characteristics w/o the shunt.

    As stated, your friendly local scrap yard may be a cheap source, but if you use two transformers, be sure they are the same. You may also be able to use the HV diode from the oven. If not, I have a bunch.

    Let us all know how this works out.

    TOM K8ERV #Montrose Colo




  5. #5
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    Hi Mike and readers,

    I wouldn't waste my time modifying such a transformer and end up with possible safety issues.

    Like the man said, it's a saturable reactor (split core) designed for current limiting since it works into a dead short. The circuit is a voltage doubler with the maggie as one of two diodes, the other is a small encapsulated silicon stack with one lead grounded and mounted near the transformer.

    The bottom line is because it's output voltage is half of that supplied to the maggie, is current limiting and has one secondary leg grounded internally I use mine as a door stop. No kidding, small but heavy it works well for that purpose.
    I don't suffer from insanity, I enjoy every minute of it.
    73 de Warren KB2VXA

  6. #6

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    There have been all sorts of linear power supplies made using microwave transformers. Some people have gone so far as to raise the transformer above ground so that a full-wave bridge can be used. However, this is VERY dangerous.

    A simpler method is use a full-wave (2-diode with 2 transformers) rectifier and leave the transformer secondaries grounded. You will have to make sure that the secondaries are in the proper "phase". To change the phase just reverse the connections to the primary.

    Glen, K9STH

  7. #7
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    Quote Originally Posted by [b
    Quote[/b] (kb2vxa @ April 26 2005,07:07)]I use mine as a door stop..
    You must have higher winds than I do!!

    TOM K8ERV Montrose Colo

  8. #8

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    Quote Originally Posted by [b
    Quote[/b] (al2n @ April 25 2005,20:06)]Anyone ever use a power supply from a microwave oven to run a linear amp?

    I am starting work with one of my Elmers on an amp project and he has most of the stuff for the project except for the power supply. #One of the things we are hoping to do is use inexpensive or salvaged parts to keep the cost as low as possible. #Microwave ovens are cheap or free and can be found everywhere.

    He remembers reading about using microwave oven power supplies for such applications awhile back, but we have been unable to find any information in our search.

    Have any of you guys used such a power supply in an amplifier?
    I don't see why anyone would want to.

    Microwave ovens, at least the ones built in the past 20+ years, are extraordinarily "low end" devices built of the cheapest possible components that won't ordinarily fail in the application.

    The transformers are usually saturated thus the only "regulation" in the system (once saturation is achieved, current can't increase and voltage is reasonably stable), and there isn't any real filtering; the ~2.5 GHz signal is ripple modulated by 60 Hz -- a lot -- and it doesn't matter.

    I wouldn't use one of the transformers, and I probably wouldn't use the rectifier(s), either, since the ones used in uW ovens cannot handle any inrush current at all -- they don't need to, since in the application, there isn't any. They can handle high inverse voltage, higher than normally needed for any sort of amateur amplifier, but not a lot of current. uW oven rectifiers traditionally cost less than a dollar.

    For relative pocket change, I can find "real" power supply components, including filter capacitors (which are sadly lacking in uW ovens) and rectifiers that can handle surge current, and higher average current. My "high voltage transformer shelf" in the garage is pretty filled with great surplus finds, all of which were picked up locally, so there was no shipping cost.

    If you haven't a local electronics surplus outlet in Anchorage, you may have to settle paying freight charges from the lower 48, but UPS Ground or Parcel Post is pretty cheap, even for a 40-lb transformer.

    Apex Electronics here in L.A. has aisles full of high voltage power transformers...mostly by the pound.

    WB2WIK/6
    A government which robs Peter to pay Paul can always depend on the support of Paul.

    -- George Bernard Shaw

  9. #9

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    What kind of regulated voltage are you looking for? A microwave PS is high wattage, but little regulation or filtering. Cooking food doesn't require a pure signal. I was thinking that televisions or computer monitors using CRT's have pretty high voltage power supplies and most likely good regulation and filtering considering it's a sensitive electronic device.

  10. #10

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    Quote Originally Posted by [b
    Quote[/b] (KF4ZHL @ April 26 2005,15:01)]What kind of regulated voltage are you looking for? A microwave PS is high wattage, but little regulation or filtering. Cooking food doesn't require a pure signal. I was thinking that televisions or computer monitors using CRT's have pretty high voltage power supplies and most likely good regulation and filtering considering it's a sensitive electronic device.
    What???

    A CRT requires high voltage for the anode only. It's very, very low current and low power, and generated at a high frequency, not 60 Hz.

    There's nothing in a monitor to use for a high voltage amateur amplifier power supply...not even the power cord or the on-off switch.

    WB2WIK/6
    A government which robs Peter to pay Paul can always depend on the support of Paul.

    -- George Bernard Shaw

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