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Thread: RF power resistor for SMPS dummy load test

  1. #1
    diysmps Senior Member res_smps's Avatar
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    RF power resistor for SMPS dummy load test

    We know that there are several methods for testing SMPS.
    In this post I want to ask, has anyone ever used an RF dummy load power resistor?
    I can get this used resistor at a very cheap price (under 1 Euro each).
    I am thinking of paralleling and placing it on the CPU heatsink and installing the fan
    rf dummy load.jpg
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    Last edited by res_smps; 09-04-2019 at 03:33 AM.

  2. #2
    diysmps Senior Member res_smps's Avatar
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    Electronic Dummy load concept

    I have a plan like this:
    electronic dummy load.jpg
    Using an electronic switch (Mosfet) to eliminate sparks
    M1,M2,M3 fully on when switched
    M4 as variable resistor depends on gate voltage

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    .... Silvio's Avatar
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    Hi Res although I am a radio amature I never operated at microwave frequencies. I also never come by these kind of resistors.
    I made an electronic DC load but due to the low voltage capability of the mosfets (45v) I never use it to load smps.

    As load resistors I had 4 resistors X 0.4ohms and 4 mosfets with a 70 amp capability. It was quite some time ago since I built this dummy load. To my experience there is a lot of heat dissipation and it has to be expelled somewhere. Knowing all this I suggest you divide your load resistors between two cpu heatsinks and not one

    Here is the link where you can find the 200w electronic dc load. You can download the pdf file and see what I made. There is a full explanation.

    https://www.instructables.com/id/A-2...ronic-DC-Load/

    Regards Silvio

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    .... Silvio's Avatar
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    Hi Res although I am a radio amature I never operated at microwave frequencies. I also never come by these kind of resistors.
    I made an electronic DC load but due to the low voltage capability of the mosfets (45v) I never use it to load smps.

    As load resistors I had 4 resistors X 0.4ohms and 4 mosfets with a 70 amp capability. It was quite some time ago since I built this dummy load. To my experience there is a lot of heat dissipation and it has to be expelled somewhere. Knowing all this I suggest you divide your load resistors between two cpu heatsinks and not one. I guess water cooling will be better than a fan

    In your case I see that the mosfets are fully switched on so there will be no heat dissipation as much as when driving them in the linear range. However the one mosfet that will be driven in linear range tend to get really hot as the dissipation will have to be expelled as heat. Regarding the resistors themselves well as I said I never tried them and you have to try them to see what they can handle. All in all with 4 resistors at 150w each the dummy load will not sustain more than 600w continuous or maybe less.
    Another approach to this will be PWM to the load and controlling the mosfets this way instead. I am afraid that the peak current will still be quite high but the mosfets will be happier driven hard than in linear mode. I guess you have to experiment a bit and see the results yourself.

    Here is the link where you can find the 200w electronic dc load. You can download the pdf file and see what I made. There is a full explanation.

    https://www.instructables.com/id/A-2...ronic-DC-Load/

    Regards Silvio
    Last edited by Silvio; 09-05-2019 at 07:41 AM.

  5. #5
    diysmps Senior Member res_smps's Avatar
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    Quote Originally Posted by Silvio View Post
    Knowing all this I suggest you divide your load resistors between two cpu heatsinks and not one. I guess water cooling will be better than a fan
    yes I still have enough CPU heatsinks

    Here is the link where you can find the 200w electronic dc load. You can download the pdf file and see what I made. There is a full explanation.

    https://www.instructables.com/id/A-2...ronic-DC-Load/
    very good article silvio, i will consider your concept. Have you ever tried Saltwater dummy load? https://www.youtube.com/results?sear...ter+dummy+load

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    .... Silvio's Avatar
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    @ Res


    Hi Res, No I never tried a salt water dummy load, if that is worked for a long period it will tend to emit chlorine gas which is very poisonous.

    I had a dummy load which I made with one watt resistors in a parallel combination which where mounted on 2 copper discs, these were fitted in coffee tin can. I had the SO239 socket soldered to the lid. Finally the coffee tin was filled with some clean engine oil. (that what I had available at that time) I left a little air space on top for expansion when the oil heats up. I soldered the lid. This made my 100 watt 50 ohm dummy load for my transceiver.

    Building a dummy load with a 200v capability and about 1Kw of power is not an easy task I guess. Remember energy cannot be destroyed.
    The heat has to go somewhere. I guess some heating element is the best choice.which one could adjust the resistance according to his need. It could always be dipped in a bucket of water if its to be used for a prolonged time. Microsim use these heating elements.

    Regards Silvio
    Last edited by Silvio; 09-06-2019 at 06:46 AM.

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    diysmps Senior Member res_smps's Avatar
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    Dear Silvio,

    I think I would not consider using saltwater dummy load again. from what I read yesterday this method is suitable for AC voltage, if using a dc voltage electrolysis will occur.
    from the article in the following link saltwater dummy load can be used to test smps but directly from the secondary transformer
    https://www.powerelectronics.com/pow...tat-dummy-load

    I agree that the heating element is the best choice

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    .... Silvio's Avatar
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    Hi Res here are some ideas where to source some heating elements, Old electric oven, old water boiler heater this can be dipped in water. electric kettle, electric iron, old microwave oven with grill, Old space heater fitted with ceramic bars, halogen lamps etc. practically anything that has a heating element can be utilised.

    Regards Silvio

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    diysmps Senior Member res_smps's Avatar
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    Dear Silvio.

    I used nickel wire for a long time, glowing red when it was hot, it burned my table several times
    nikelin wire.jpeg

    yesterday I bought some 1000W 220V (around 400W when I tried to 140V smps) water heaters for EUR 1.28 each
    water heater1.jpeg

    water heater3.jpeg

    I measure this heater has a 1.1mH inductance
    water heater2.jpeg

    and this is the rf power resistor I will try next
    rf res.jpeg

    regards,
    res

  10. #10
    .... Silvio's Avatar
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    Hi Res I see you found a solution at last without burning the table LOL. Those immersion heaters seem a good idea. Regarding the RF power resistors well I found a video on youtube using these but this chap had an engineering workshop and had access to a lathe and milling machine. He did a fine job and I think it is worth seeing this video It may inspire you with some ideas.

    Link https://www.youtube.com/watch?v=tp_hhWXyHmk

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