What are dummy loads and How are they used?

Dummy loads are tools used to load down a device in order to do testing on amplifiers or RF systems. Some devices can be damaged if no load is used while testing. RF devices will always have to be tested with dummy loads attached.

In audio, a dummy load is used to test an amplifier’s handling capabilities without producing the intense sound and to also diagnose problems within the amplifiers.

In RF, dummy loads are used to simulate a load of an antenna on a system without producing radio waves. They also use these for tuning the RF device before putting it online. They simulate a perfectly tuned antenna that is connected to the amplifier.

Dummy loads for amplifiers

When testing audio amplifiers, a power resistor used if the user does not want the excessive noise from testing over speakers. During testing, speakers can introduce excess noise into the system possibly causing the user to falsely diagnose problem areas that may not even be the true problem. If testing an amplifier, a connected speaker can act as a microphone and can pick up on audible noise and show up on an oscilloscope causing false diagnosis.

I recently witnessed a person repairing an older tube guitar amplifier that produced excessive noise when in use. The repairman first tested the amplifier with the speaker hooked up to the output. As he tapped on the tubes, the speaker picked up the noise from light taps on the tubes and caused them to show up on the oscilloscope. He proved this was happening by coughing into the speaker which caused a large spike on the oscilloscope. He then unhooked the speaker and plugged in a dummy load to the output and performed the same test. Once the dummy load was hooked up, the tap test was completed again. This time the only part that showed noise was the faulty tube. If the dummy load was not used the user could have easily misdiagnosed the other tubes as being the problem when only one was truly the problem.

Dummy loads for amplifiers can range from a single power resistor up to an array of resistors or even a benchtop electronic load. The dummy loads can be as simple or as complex as the user wants to make them. I have seen versions that the designer has used a switch to let a user choose between 3 different load resistances. The designer set up the resistors so that the load is either 4, 8, or 16-ohm dummy load depending on which position the switch is placed in. This is accomplished by placing the 8-ohm resistors in series, parallel, or series-parallel configuration.

Benchtop Electronic Loads are specialized test equipment that can be tuned into a number of different load sizes to accommodate a number of different tests. If you need to test a range of different loads, this may be the best way to go unless you have a large stock of power resistors. Another nice feature of this type of device is that the load can be changed without having to rewire to the device. If you were to use multiple resistors, you would need to hook them up to the system each time it was changed. Digi-Key does stock a number of different electronic loads located here https://www.digikey.com/short/z9zvb4

One thing to keep in mind while designing dummy loads with power resistors is the derating of power resistors. For example using part number RHRD-8.0-ND (Vishay part number RH0508R000FE02)( https://www.digikey.com/short/zhvq4h ), the datasheet lists that with the proper heatsink this resistor is rated to 50W. Without that heatsink, however, this resistor is only rated to 20W. Digging a bit deeper into the datasheet it states that the recommended heatsink size has an area of 291 sq. in with a thickness of 0.059”. If this is a flat aluminum panel, it would be 12” x 12” x 0.059”. The designer would have to keep this in mind if they are trying to make a miniature dummy load as it may not be able to perform up to expectations and could fail if these specifications for the heatsink is not met.

Another thing to keep in mind is that when choosing load resistors, it is important that the resistors are non-inductive type. This is especially important when the device you are testing gets to the higher frequency ratings.

Dummy load applications in RF projects

A dummy load in an RF application is used to simulate an electrical load for testing. In the case of RF, the dummy load is used in place of an antenna. Because the dummy load is used, the system does not transmit any radio waves. If an antenna was used in this case, the system could transmit unwanted transmissions and could interfere with other devices or technologies. If no load was used it could be possible that the system could become damaged as a result.

Dummy loads for RF applications can be made from a few parts. The basic pieces of a dummy load for RF is a connector and a resistor. Most times the resistor will be 50 ohms. The other rating that will have to be known is the systems transmitting power because the resistor will need to be able to handle that amount of power. If the power rating of the resistor is close to the output rating of the transmitter, the heat that is produced by the resistor may become an issue. In that case, it would be best to design the device with additional heatsinking.

Most of the low cost builds that I have found mount these two devices into an enclosure and there are no other parts used. Even though there are very simple builds such as the one above, more thought has to be put into loads used in higher frequency projects. This is due to the amount of inductance that is introduced into the device at the higher frequencies. It is especially important to used non-inductive resistors for these builds. Good RF practices must be followed to ensure that signal is transferred from the connector to the dummy load with no mismatching.

The application that came up most in my searches on this topic was HAM radio users use dummy loads to tune their devices before putting them live. There are also a number of other applications such as radar systems during matienence.

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