Using Microwave diodes ?

Discussion in 'Amateur Radio Amplifiers' started by K7GFR, Jul 27, 2020.

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  1. K7GFR

    K7GFR Ham Member QRZ Page

    Hi All, Building a 4 kv bridge power supply for an amp project. I used to use K2aw , Silicone Alley HV diodes in years past ,but they are gone.
    Can I use microwave diodes as long as I double them up for better current handling ?
    I actulyy have , and getting a large volt drop on output when I load supply. Don't know if they are the problem?I am also using an old NON hypersil xfrmr. 20uf filter cap.
    Tnx , 73
    Glenn
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Do you mean "microwave oven diodes?"

    Those are usually very high voltage but at pretty low current, e.g., 12kV to 15kV at 0.5 to 0.75A, and they have a very large Vf (forward voltage drop) because there's lots of chips in series in those (typically 12 to18), inside each one. They also have limited surge current ratings, typ. 50A 1/2-cycle (8.33 mS). For most amateur power amplifiers I've built I'd want branch ratings about 2x normal (expected) peak voltage, using ~3A rated rectifiers capable of 150-300A surge current each.

    They're appropriate for the intended application but not so much for higher current use with much smoothing capacitance.
     
  3. K8ERV

    K8ERV Ham Member QRZ Page

    Wik: you know more about oven diodes than I do and I design them-------

    TOM K8ERV Montrose Colo
     
  4. N9LCD

    N9LCD Ham Member QRZ Page

    Microwave diodes????

    I trust that you're not referring to the UHF / SHF mixer diodes. They're small signal diodes with an estimated life of 100 hours in radar sets!
     
    KB1MM likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Design them? The diodes or the ovens?

    I worked with all the uWave oven manufacturers very early on, when not many people owned a uWave oven (c. 1977) and engineers everywhere were trying to figure out why the conventional rectifiers they were familiar with were all failing; they kept using higher and higher "PIV" rated devices, but they still failed.

    We had to demonstrate to them it wasn't the reverse voltage that made them fail, it was the avalanche energy.

    "Ooohhhhhhh."

    That was all pretty much resolved in the early 1980s as I recall.
     
    KB1MM likes this.
  6. K8ERV

    K8ERV Ham Member QRZ Page

    Designed the ovens and specked the diodes. The (new) controlled avalanche design did solve the problem.

    I don't remember you but that was a long time ago. Our (Tappan/Raytheon) oven was introduced in the summer of 1955.

    Now I read that more homes have ovens than TV sets, but not sure I believe that. Never thot I would see one for $100 but now you can't give them away at yard sales.

    TOM K8ERV Montrose Colo
     
  7. KN1M

    KN1M Ham Member QRZ Page

    The first one I bought was in 1976 or '77 and cost $1000. The way they advertised them they would do all of your cooking for you. Now people realize their limitations. I also sold commercial microwaves (you guess the brand) to restaurants and for commercial use. Very HD, very expensive.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wasn't around then. But was around in 1977, as an FAE for Semtech Corporation and we "found" controlled avalanche rectifiers of much lower "PIV" rating solved the rectifier failure problems experienced by almost all the manufacturers. I worked with Magic Chef, et al. The avalanche diodes used a different junction design, we called it "Supertaxial" and it was diffused quite differently from the designs the uWave oven folks had been using previously.
     
  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    While those diodes may not be optimum for your power supply, they may not be the cause of the excessive voltage drop that you are seeing.
    Personally, I would just use a string of good quality rectifier diodes like 1N5408 .
    Maybe the cause of the voltage drop is the type of transformer you are using. If it is an older unit designed for choke input filter after the rectifier, and you are using it with a capacitor input filter, that could explain what you are seeing. What are the specifications for the transformer ?
     

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