Author Topic: Dummy load for testing PA equipment.  (Read 345 times)

Offline eskoilola

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Dummy load for testing PA equipment.
« on: October 02, 2017, 03:06:05 PM »
These are some images of a dummy load that I made some years ago. Basically it is just another adjustable resistor with true RMS power measurement (frequency range 0 - 10kHz).
What is special is that it can dissipate about 3 kilowatts of raw power.
There is an Atmel microcontroller hiding behind the scenes and the huge "analog" meter has actually a tiny stepper inside. The original mechanism was so corroded that I had to toss it and make a new one for this. Lot easier to swing the needle with a stepper than with a coil and magnets and whatnot.
The resistor is actually wound using quite thick resistor wire and then soldered to steel sticks. And to those that wonder the solder - it was branded Alusol and it seems to be able to solder almost anything.... but not the material You are trying to solder with it.
Actually this solder works well but seems it works better with steel than copper. This might e because steel does not conduct heat well.

The images below are stored in my personal server - so they do not waste MadModder space - they waste Your time since the connection is rather slow.

The resistor network.


Bevel of the meter was so corroded that I had to clean and repaint it.


The mechanism was junk but with some gears and a stepper motor it was like new :)


I really hate wires. It seems to be impossible to make this scene neat and tidy.


The two holes are for XLR connectors for audio monitoring.


It is not that heavy. Handles were there so why toss them.


Offline PekkaNF

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Re: Dummy load for testing PA equipment.
« Reply #1 on: October 03, 2017, 03:01:31 AM »
Thank you for showing.

You have that 3 kW toaster on same enclosure than control electrocity? Doesn't it build up some heat even when you use those fans to move some air?

500W+ SMPSU heats up tower PC enclosure nice and toasty in no time.

That meter concept is interesting, any more pictures on innards? You have mechanical home stop and/or home position detection to "set" it when system is powered up?

Pekka

Offline eskoilola

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Re: Dummy load for testing PA equipment.
« Reply #2 on: October 03, 2017, 04:42:07 AM »
This device is in active use by a music repair shop. He asked me to make one. Before this he had some resistors submerged in oil barrel. After days work it was hot enough to make french fries....

The four fans at the bottom blow air into the cabinet. The resistor network is attached on the back cover in a way that air escapes through it. So it forces the heat out of the cabinet the only way out being through the resistor network.

I really wish I had some pictures of the stepper setup. The stepper was cannibalized from a 3 inch disk drive. There is a hard stop and the meter is "calibrated" by making it bang it's head on that stop. The very same concept was used by IBM XT "personal" computers in the early 80's. Those made a very recognizable sound when booting up.