Saturday, July 23, 2011

Everything Including the Kitchen (current) Sink!

With almost every project/design I work on there is a switch-mode PSU that needs testing. In order to quantify the switch-mode PSU's performance you have to load it down to typical and worst case load conditions. This will allow you to make sure the Inductor is properly sized, the ripple current is acceptable, and just to make sure the supply is stable. In the past I would either grab a variable power resistor box or buy some 25Watt resistors and just make a load brick for the project.

The down side with using resistors as loads is you have to buy new values of resistors for every supply voltage you are testing. There has to be an easier way.... and there is!

I got the idea to make a variable Current (Sink) Load from watching one of Dave Jone's EEVBlogs.

This Variable  Current Load takes out all the "Ohm's Law" guess work when applying loads. If I need a 0.5Amp load, I just adjust the pot to set 0.5V across my 1 Ohm Current Sense Resistor and I have a 0.5Amp load irregardless of the Load Voltage. The Load voltage could be +3.3V, +5V, +12V, +24V and I still have a 0.5Amp Load...GREAT!
I grabbed some spare parts I had in my lab and an Altoids case and started hacking. I stacked 3 different size heatsinks and attached them to the pass FET; this will help pull the heat away better.

The terminal block provides the +10V, 0V, and -5V supply voltages. The Bar Graph panel meter displays the load current from 0-5Amps. Below I am using my Current Load to test a Linear Tech Flyback PSU eval board..

This was a fun project and the current sink works really well. The 10 turn pot allows some pretty precision current settings. I am pretty pumped to use my new Variable Current Load on Monday to test my new project at work.

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