Sunday, August 16, 2015

Homebrew dummy load

I have created a simple 50 Ω dummy load to test transmitters. I also added a simple RF diode detector so I can measure the peak voltage, and calculate the power.

  

The dummy load consists of eight 100 Ω resistors rated at 2 W so the load should handle 16 W, at least for short periods. I constructed the dummy load using a combination of ugly construction and Manhattan style, by gluing pieces of PCB  (as isolation pads) on top of a ground plane PCB.  Then I soldered the components directly on the copper without drilling holes.


RF Probe


The RF probe part consist of a simple 1N4148 diode and a 0.01 uF ceramic capacitor. I only had a 50 V capacitor in my junk box, but it should be sufficient given that this is a 16 W dummy load and 16 W translates to 40 V peak.

I added female banana connectors, which connects to a multimeter. The power can be calculated by Ohms law by subtracting the forward voltage on the diode from the measured voltage, then multiply by 0.707 (to get RMS), then square the result and finally divide by 50 Ω. Some homebrewers add a voltage divider to their RF probes using a 4M7 resistor to get RMS voltage directly. I did not bother since I am sure the input impedances on my multimeters varies.

The calculations might seem a bit cumbersome, but I might print out a small lookup table and glue it to the box to have some ballpark figures. However, this is not a precision instrument. The forward voltage on the diode varies with load and can be somewhere between 0.4 and 0.7 V. I simply use 0.5 V in my measurements.

So far, the dummy load has been very convenient when testing my Softrock RXTX and its low pass filter,

Further reading


1. Jeelabs, Forward voltage drop on a diode
2. NXP, 1N4148 data sheet
3. N5ESE, Classic RF Probe

5 comments:

  1. Be sure to use non-inductive resistors like carbon composite. Often, the 1 & 2 watt resistors are wire-wound.

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  2. Those resistors and their declared wattage look like what I used once and there was a lot of parasitic inductance.

    Also the big plates of copper sum up to some substantial capacitances. Make sure you expect some parasitics as this won't be a clean 50Ω load to much more than couple of MHz. I have something similar (20x 1kΩ blue metal film resistor 0.5W each) and it is good up to 30 Mhz but not more.

    I used two plates with holes arranged to a circular form and the resistors are soldered between the two plates (resistor sandwich :). The center then has a thick wire connecting the connector's center conductor to the back plate. The front plate is connected to connector ground. The plate distance is much higher (and capacitance drops with the power of distance), but it is still not good enough even for VHF.

    Btw your setup will only handle 8W (4x 2W)! The series resistors do not count.. only those in parallel.

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    Replies
    1. Hi. I beg to disagree with your statement about series vs parallel resistors and so would ohms law. Putting 50 1 Watt 1 ohm resistors in series would give a 50 watt dummy load. So would 50 2k5 resistors in parallel.

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    2. I was too fast in saying that because the series sections truly lower the voltage applied to the resistors and thus the applied power. I was thinking in terms of current only. Sorry about that. The rest still applies though, check for parasitics.

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