Free Adjustable Dummy Load

A project log for Bench Power Supply

Designing an open source, modular bench power supply to rule them all.

The Big OneThe Big One 08/23/2015 at 01:242 Comments

For those who have ordered the PCBs, you probably have a bunch of boards left over. Rather than just let them accumulate dust in your desk, you can use one of the channels as an adjustable dummy load. It's not the best dummy load, as it was not designed specifically for it, but it does the job decently.

Basically, you just need to populate the top of the board and add a few jumper wires. Place a jumper between OUT and IN on the voltage regulator chip, and between GND and VOUT at the very bottom. Just as with the power supply channel, you can control the current by applying a voltage on the I SET pin. The lower the voltage the less current will pass. If you use the recommended component values, it will allow for 5v I Set = 5A current. I put a potentiometer on this pin to let me easily control it, but you could also use a DAC or something.

To use it, you plug in the power supply under test to VA / GND, and supply a negative voltage (I use one equal to VA in magnitude) to VB. (Yeah, it's annoying that you need to provide a dual supply for a dummy load... but that's the way I designed the power supply.)

If you wanted to improve on things a bit, you could not solder on the top 0 ohm resistors, and instead connect the logic supply (+/-/GND) to those pads; you can then connect the power supply under load to the VA / GND pins. This should give better results since the logic supply is separate from the main supply. Take a look at the PCB layout to see exactly where to solder things...



Michael Vowles wrote 03/29/2016 at 12:10 point

This is a great idea, might give this a go as I'm not far away from testing things. Im not sure I follow when you say logic supply, +-GND. 

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The Big One wrote 03/29/2016 at 14:48 point

There are two portions of the board; the high current part, and the logic / controller section.  When using the board as a PS channel, you connect V+ / GND / V- to the high current section, right by the heat sink.  However, if you look at the PCB layout, you will see that the V- completely bypasses the high current area, and that V+ and V- are brought out to the controller section via some 0-ohm resistor bridges.

What I am suggesting is that if you don't connect the bridges, you can instead connect a low current power source to the controller section (run it at, say, + / - 12V).  You can then connect the high current V+ / GND lines separately, at potentially a lower voltage (say, 5V).  The only thing you need to be aware of is that GND is shared between these two sections of the board.

Hope that makes a bit more sense.


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