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Microwave oven power supply

Discussion in 'General Technical Questions and Answers' started by AL2N, Apr 26, 2005.

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  1. AL2N

    AL2N Ham Member QRZ Page

    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?
     
  2. KA5S

    KA5S Subscriber QRZ Page

    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. AL2N

    AL2N Ham Member QRZ Page

    We must have been putting the wrong words into Google.

    Thanks!
     
  4. K8ERV

    K8ERV Ham Member QRZ Page

    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. KB2VXA

    KB2VXA Ham Member QRZ Page

    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.
     
  6. K9STH

    K9STH Ham Member QRZ Page

    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. K8ERV

    K8ERV Ham Member QRZ Page

    You must have higher winds than I do!!

    TOM K8ERV Montrose Colo
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    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
     
  9. KF4ZHL

    KF4ZHL Ham Member QRZ Page

    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. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    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
     
  11. KB2VXA

    KB2VXA Ham Member QRZ Page

    Hi guys,

    The two phased transformer guy still doesn't understand the power supply is a voltage doubler with the maggie as one of the diodes. Sure, you can phase two and still have the CT as ground but you'll only have half the voltage.

    Then that comment about the CRT second anode supply, fine for Tesla coil experiments but little else.

    I'll stick with the door stop and it can get VERY windy here at the Jersey Shore.
     
  12. K8ERV

    K8ERV Ham Member QRZ Page

    You will have half the voltage of BOTH xfors, but the full voltage of a single xfor. Two 1800 volters should act much like a 3600 ct. I think he wanted the 1800 in the first place, that he should get. Yes, I understood the circuit long before you ever heard of it--[​IMG]

    TOM K8ERV Montrose Colo
     
  13. K9STH

    K9STH Ham Member QRZ Page

    Who said anything about using the voltage doubler circuit from the microwave oven?

    Like Tom says, using two transformers (WITHOUT the rectifiers from the oven) is the same as a center tapped transformer with twice the voltage ratings. Those have been used for over a century with full-wave rectification. You know, the center tap goes to ground, the anode of a diode goes to each of the transformer leads (one diode connected to each lead), the cathode leads of the diodes are then connected in parallel, and the high voltage comes from this junction. That is if you want positive voltage. If, for some reason you want negative voltage then you reverse the diodes.

    The "unloaded" voltage should be at least 1.414 times the voltage across one of the transformers. This will "drop" as the current requirements increase.

    Although not as "good" as a "regular" plate transformer, there have been quite a number of linear power supplies made using microwave oven transformers. I don't know about a linear running the full legal limit, but for a linear in the 500 to 750 watt output range (like the Collins 30L-1, Heath Warrior or SB-200) using a pair of 572B tubes or four 8llA tubes, single 3-500Z, etc., they are a "cheap" alternative to a much more expensive "normal" type of transformer.

    If a pair of identical microwave oven transformers can be obtained for a VERY cheap price then I definitely do not see any reason that they cannot be used for a homebrew linear. Sure, a "normal" plate transformer should be better. But, if the microwave transformers work then who is to argue with success.

    Glen, K9STH
     
  14. AL2N

    AL2N Ham Member QRZ Page

    We are looking to build a single 3-500Z tube amp with target output of 600 watts.

    My main goal for this project is to learn both about amplifiers as well as building a project from scratch. A full KW is not what I desire at this time, 600 watts and the chance to learn is good enough for me.

    All my reading about tube amps has peaked my interest in building with tubes in general. After I get some learning and practical experience under my belt I may try and build some simple cw tranceivers with tubes. They look like a lot of fun!
     
  15. KA9VQF

    KA9VQF Ham Member QRZ Page

    Go for it!!

    Even before I got my novice ticket or had any real training in electronics at all, I was experimenting with tube type equipment. Reading what I could find in small town, backside of the country, Wisconsin.

    Yes, I built simple CW transmitters using tubes and cookbook projects from ARRL handbooks and other written documents I managed to find from time to time. Mainly out of cast off ‘junk’, literally, {we had an open dump at the time anyone could ‘cruise’ through and ‘pick’ it} you may not believe what folks would throw out sometimes. Its really no different these days you just don’t have the opportunity to ‘pick’ it anymore.

    While I still may not fully understand how many of the circuits worked, most of my projects did, at least for a short time. I was lucky enough to have a mostly tolerant/experienced Elmer in town. Most of the time he would encourage me to build things and didn’t seem to mind the occasional ‘room full of smoke’ that I would build.

    There were a few projects that he talked me out of even trying to build, but generally he even helped find a lot of the things I needed to make them ’fly’.

    ‘In the day’ tubes still ruled. Now they are a lot like dinosaurs, getting harder and more expensive to find. It is better when you do your homework and have a real understanding of what is expected from the project.

    I guess it’s like any other hobby. The more time and money you invest in it the better the end result. Some folks just suck at whatever they try to do. No matter how much they spend, buying the finest components available, diligent studying, even using good working methods and still turn out failure after failure.

    The next guy may excel at it {or at least achieve acceptable results} and never really know what he is doing right. Using junk, improper methods, and home rigging of all kinds {I remember this from when I was trying to go fast with my ‘69 Mustang.}
     
  16. K8ERV

    K8ERV Ham Member QRZ Page

    Now I realize why several say not to use the oven diodes. The present diodes are very small sticks, and MIGHT not be heavy enuff for ham use. The early diodes (I have a BUNCH) were much larger and were heat-sinked to a chassis, and would definitely be ok for ham use.

    Sure wish I could attach a pix to a reply. I can send pix to an email address.

    TOM K8ERV Montrose Colo
     
  17. KF4ZHL

    KF4ZHL Ham Member QRZ Page

    :blush: Doh!

    Perhaps I should talk less and listen more here.
     
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