# You Can Learn a Lot from a Dummy (Load)

If you work on RF circuits–even if you aren’t a ham radio operator–you ought to have a dummy load. A dummy load is a non-radiative “antenna” with known impedance that you can use to test your RF circuit without radiating. For radio work, you usually just need a 50-ohm resistor that is non-inductive (at least at the frequencies you are interested in) and that can dissipate the amount of power you’ll expect it to handle (at least for a short time). [VO1PWF] wanted a dummy load and built his own.
The Cantenna (not the Pringle’s kind; see left) was a famous dummy load design when Heathkit was in business. It was a single carbon rod immersed in a paint can full transformer oil (which we now know was full of dangerous PCBs; and we don’t mean printed circuit boards). [VO1PWF’s] design is a little more practical, using some resistors in parallel (20 1K resistors), a plastic pipe housing, and mineral oil to keep it all cool.

The reason for the parallel resistors is to maximize the power handling capability. The resistors are 3W units, so the dummy load–in theory–can handle 60 watts. Often, high power resistors are wire wound and thus have a good bit of parasitic inductance that makes the dummy load reactive (not a good thing since that makes the load impedance vary by frequency). They do make non-inductive wire wound resistors, but these aren’t truly non-inductive. The wire winds in two different directions, so the inductance tends to cancel out. We wouldn’t trust them to be a pure resistance in a high-power dummy load design.

# A constant resistance dummy load design

This constant resistance dummy load has not yet been tested in the real world. [YS] was inspired to come up with the circuit after reading Wednesday’s Re:load dummy load post. That was a constant current load, not a constant resistance load. [YS] started with the schematic for the Re:load and made his changes to arrive at this.

For him the exercise was just to alter the design to achieve constant resistance. He didn’t actually build and test the hardware because he doesn’t really have a need for it. This image was exported from Proteus, which includes a ProSPICE circuit emulator. His slides run through test voltages from 5V to 50V, maintaining a constant 10 Ohm resistance.

When studying this project we needed a little refresher on the different varieties of dummy loads. We found this post very informative about the differences and uses of Constant Current, Constant Power, and Constant Resistance (Impedance) loads.

When testing power supplies or LEDs, a constant current dummy load is needed. These devices will draw a constant amount of current, regardless of the voltage at the input terminals.

[Nick] was looking for a load to test out a power supply, and found commercial offerings to be too large, too powerful, and most importantly, too expensive. This lead to the design of the Re:load, his open source alternative.

Like other constant current sources, the Re:load uses an opamp to control a pass element. While most constant current loads will just use a transistor, [Nick] opted for a BTS117 smart low side switch IC. This device has a built in current limiter, over-voltage protection, over-temperature protection, and short circuit protection, which makes it much safer. The project write up goes into detail on how the device works.

If you need a constant current load, [Nick] is selling kits on Tindie. All the design files are available on Github so that you can build your own.

We’re not really interested in building a dummy load like this one for ourselves. But the concepts behind its design make for a nice little mental exercise as you read your way through the build description. [Pabr] wanted to build a dummy load which could be used to test a cheaply made gas generator. He wanted it to be as simple as possible, while providing a range of different loads. What he came up with is this monotonically adjustable load tester which uses gray codes for switching.

The video after the break does a good job of explaining the motivation for the design. Grey coding ensures that just one bit changes at a time. The example he uses to show the importance of this is when binary code transitions from 7 (0b0111) to 8 (0b1000). Three digits have been turned off and one has been turned on. Since he’s using light bulbs for his load this will turn off 700 Watts and then switch on 800W. That sudden jump in power draw can cause all kinds of problems with the generator’s engine. But the system he wired up will ensure that each flip of a switch moves in smaller steps.

# Dummy loads and heat sinks

In [Dave’s] latest episode of the EEVblog he takes a look at constant current dummy loads. These are used to test power supply designs and instead of just chaining resistive loads together every time he’s decided to look into building a tool for the job. What he ends up with is a reliable constant current load that can be dialed anywhere from 1.5 mA up to just over 1A. There’s even an onboard meter so you don’t have to probe the setting before use.

It may look like he sent his design off to the board house for production but that’s actually a re-purposed PCB. In walking though his junk-box assembled dummy load [Dave] shares some great tips, like using multiple 1% resistors instead of shelling our for one large and accurate power resistor. But our favorite part comes at about 12:00 when he takes us through some rough math in calculating heat sinks. We’ve always just guessed, but like any good teacher, [Dave] explains the theory and then measures the actual performance taking the guesswork out of the design. See for yourself after the break. Continue reading “Dummy loads and heat sinks”