Huge tubular resistor for a DIY kW dummy load?

Discussion in 'Amateur Radio Amplifiers' started by SP4IT, Jan 16, 2022.

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  1. SP4IT

    SP4IT Ham Member QRZ Page

    So I've bought some cheap huge non inductive resistors shown below. For some reason their power rating is only 50W and 25W despite their size. I wonder if I could use the bigger one in a 1.5kW dummy load by immersing it in oil (and putting a steel band in the right place to achieve 50 ohms instead of 75).

    It has lots of surface area, but I found some guy online that wrote he tried doing that and the resulting dummy load had bad swr after oil was added(he didn't say why).

    Also, I wonder how the power rating is calculated and could I use much more power in air by providing a blower (and how to calculate the power rating with a blower.

    What do you think?
    20220116_220415.jpg 20220116_220437.jpg
  2. VK4TUX

    VK4TUX Ham Member QRZ Page

    If you have 6, connect 2 in series in three sets then connected in parallel to obtain 50 ohms R with,
    a considerable dry power rating and immersed in oil in a metal container will achieve what you want.

    KA0HCP and K7TRF like this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If oil changed the SWR, I suspect it wasn't the right kind of oil.

    Real dielectric oil is pretty inert.
  4. SP4IT

    SP4IT Ham Member QRZ Page

    Unfortunately I have just two, one larger (10in/25cm long) and one smaller. The larger one is almost 2in,(40mm) in diameter. So I'm quite surprised it is rated only at 50W (in air).

    So I'm thinking about making a container from a 3in (75mm) internal diameter stainless pipe closed on both ends and filling it with oil.

    However, before I spend lots of time on this I was hoping it would be possible to calculate the power rating or resulting dummy load.

    The total area of the carbon film on the larger resistor is around 320cm.sq (a bit under 50in).

    So I figured they rated it in air one watt per square inch. I would be using just 2/3 of the area (as the resistance value is 75 ohm end to end) so let's say 213cm.sq.(33in.sq).

    I found an article that measured heat transfer coefficient of various oils and most values were around ~100W/m2/K so plugging my usable area this comes to about 2W per degree C( at 100deg C temperature difference between the resistor surface and oil we get 213W) this is not enough.

    However, water has a lot better heat transfer coefficient. Plugging 2400 (distilled water) I got 51W per degree C. At 30 deg C different rence between the water and resistor temperature we get 1500W.

    With ethanol we get 95W per degree(for this resistor) , but it is very flammable. Perhaps mixing it with water 50/50 could be usable.

    How about distilled water as a cooling liquid in a dummy load?
  5. W1TRY

    W1TRY XML Subscriber QRZ Page

    Bird and Coaxial Dynamics have water cooled terminations (dummy loads) for broadcasting that used this type of resistor that was designed to be used with running water or 50/50 Dowtherm coolant at around 10 gpm. Power handling was typically 50kW or more depending on the model of load. It could NOT be used unless the water was flowing at the appropriate rate. (Ask me how I know...)
  6. G0OIL

    G0OIL Ham Member QRZ Page

    You just need the right mineral oil or this stuff 5-1070-2, PMX-200 Silicone Oil | Bird - The RF Experts (

    If you immerse it in oil and (even better) put some sort of impeller in there to move the oil about together with some sort of external heat sink you should be able to run it at a great deal more than its rated power.
    SP4IT likes this.
  7. N0NC

    N0NC Premium Subscriber QRZ Page

    From some info I found on the Heathkit Cantenna. The resistor needs to be coated.

    A non-coated version of this resistor (such as type A or AS) will absorb cooling fluids causing a significant increase in resistance (up to 100%) see page.

    73, Chuck
    SP4IT likes this.
  8. EW3KO

    EW3KO Ham Member QRZ Page

    Добрый день всем.
    Эти парковочные резисторы не подходят.
    Углеродный незащищенный УВЧ.
    Есть ряд резисторов на разные значения сопротивления и мощности.
    Они допускают десятикратную перегрузку по рассеиваемой мощности. Очевидно, температура рабочей поверхности резистора не более 155°С/311°F.
    Отвод можно сделать из посеребренной медной полосы....
    Резисторы с жидкостным охлаждением получили название УВ1-5.
    Они производятся по другим технологиям.

    Last edited: Jan 19, 2022
  9. EW3KO

    EW3KO Ham Member QRZ Page

    Good day to all.
    These parking resistors are un.
    Carbon unprotected UHF.
    There are a number of resistors for different values of resistance and power.
    They allow a tenfold overload in terms of power dissipation. Obviously, the temperature of the working surface of the resistor is not more than 155°C/311°F.
    The tap can be made from a silver-plated copper strip....
    Liquid-cooled resistors are called UV1-5.
    They are produced using other technologies.
    I apologize for the previous post.
    What's wrong with the translator...

    Attached Files:

    SP4IT likes this.
  10. SP4IT

    SP4IT Ham Member QRZ Page

    Great, thanks for the files. I was wondering what was the original use of those resistors and the explanation is right there on the first page:
    (Google translated)

    Resistors UNU (carbon unprotected ultra-high-frequency) are designed to operate as non-reactive absorbing resistors and UNU-Sh (washer) as active loads in high-frequency coaxial lines.

    BTW, for anyone wanting to be able to read documents saved as pictures in languages they don't speak there is a neat way to translate them with Google translate. It is not possible to copy the text from such files to use standard translation apps. But the app "Google translate for mobile" (on android smartphone) has the "camera" button. When pressed it goes into camera mode where it can translate any text camera sees. You can point the phone at a computer monitor with text to be translated, or you can click the "import" button and import a screenshot. I use this feature a lot for Chinese, Russian etc.

    I was thinking about using an impeller. A modified pc fan would make a good one perhaps. However, I still haven't given up on distilled water it has 10-fold better heat transfer capability than oil. If I'm coating the resistor anyway the dielectric properties of the cooling liquid may matter less.

    Do you know what kind of coating would be used for this? Just a coat of polyurethane varnish maybe?
    Last edited: Jan 22, 2022
    KD2ACO likes this.
  11. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    It's amazing, right? :p
  12. EW3KO

    EW3KO Ham Member QRZ Page

    You can still find suitable options on the radio components market.
    Both with forced air cooling and with liquid.
    In the photo UNU 200 watts, PVS-3, UV1-5.
    The latter is cooled with distilled water.
    Altronic also produces the same products.
    But the prices for these products are quite high ....

    Attached Files:

    Last edited: Jan 22, 2022

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