Dummy Load Construction

Discussion in 'Homebrew and Kit Projects' started by KC4ART, Nov 17, 2008.

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  1. KC4ART

    KC4ART Ham Member QRZ Page

    I have worked out all the math, and have set on components and values.

    What I am not sure about are how to orient the resistors in the oil. I am thinking if I mount the plates vertical, with the resistors horizonal, then i will get better cooling. Also, where in the oil should they be? My thinking is if I mount them in the lower part of the can, then the convection currents should pull the cooler oil down to the resistors.

    Also, use strips of copper to connect the resistor bank to the SO-239 instead of wire.

    Anything I havent thought of?

    TNX for your help.
  2. K9FV

    K9FV Ham Member QRZ Page

    Well, I don't know what you have or have not thought of.

    What freq do you plan to cover with the dummy load? 1.8 to 30 mhz? thru 200 mhz?

    What power range do you require? 50 watts? 100 watts? 1,000 watts?

    What size can to hold oil are you considering?

    Good luck - I've built a couple of dummy loads in the QRP range.

    73 de Ken H.
  3. KC4ART

    KC4ART Ham Member QRZ Page

    I am useing 20 1k resistors (metel oxide) rated at 10 watts each. I am wanting about 1.5kw handleing capacity, at least for a few minutes. It will all be mounted inside a 1 gallon paint can, and immersed in either transformer oil (if I can get some) or mineral oil. I know the transformer oil will give me a higher rateing.

    Also, anyone have a source for the diode to use for the power metering? Im ordering the resistors from digi-key, but they dont have the diode. Mouser has then, but I dont want to pay thier shipping ($10) for a 6 cent part. Also, how much power will this diode handle?


    Last edited: Nov 17, 2008
  4. PA5COR

    PA5COR Ham Member QRZ Page

  5. K9FV

    K9FV Ham Member QRZ Page

    What diode are you looking at for the dummy load? Most any general purpose diode will work, but I'm told the 1N34A is a tiny bit better...

    Is this your design? OR one from a book or the web?

  6. KC4ART

    KC4ART Ham Member QRZ Page

    The diode that im looking for was a BAV21. I will look for that other one, and see what I can find.

    That website is one that I have looked at. The circuit is pretty much the same. Im just wondering if you turned the reciever bank 90 degrees, if it would cool better, due to better convection. Seems to me that the oil around the resistors would be trapped there, and would not move as well as it should.

  7. VK2TIL

    VK2TIL Ham Member QRZ Page

    I don't think that turning the resistor bank 90 degrees would make much difference, if any.

    The advantage of "vertical" is, I think, mainly to give a co-axial connection to the load from a connector at the top.

    I have an old article detailing construction of this kind of load; if you PM me with your e-mail address I can send it to you. It is for 100 watts but you can scale it up.

    I built one many years ago; it worked fine at HF and was "acceptable" at two-metres.

    Re the diode; watch the voltage rating. The site that COR posted talks about this.

    You can get data sheets from here;


    If you calculate the peak voltage for your max. power into the load you should find that the BAV21 is suitable.

    The 1N34 is not; it's a low-voltage signal diode. To use this device you would have to make a voltage divider to drive it and there would be no point in that.
  8. K9FV

    K9FV Ham Member QRZ Page

    VK2TIL, You are correct the 1N34 will NOT work - I had an RF probe in my mind when I wrote the suggestion.... thanks for catching that.

    73 de Ken H.
  9. W4INF

    W4INF Ham Member QRZ Page

    Wow! That was a great link, thanks much!!

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