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Variable Power Supply 

I got tired of cobbling together some sort of power source every time I needed something other than 12 volts for a project so I tackled the construction of my own version of a variable bench power supply.  I thought it would be handy to have both a volt and amp readout and a few other amenities.  So here is the final product.   

Sorry, no labels one of these days I will get a "round to it" and label my projects.  The left meter is voltage and the right meter is amps.  The voltage control is a 10 turn pot for more resolution on the voltage setting.  The switch above the main binding posts turns the output off so you can set the voltage before applying power.  The switch and binding posts on the right allows an external ammeter to be used.  The panel meter will only read down to 10 mA so this allows lower readings with an external meter.

My first try with the panel meters was to install the current sense resistor on the "high" side of the power supply.  This resulted in releasing the magic blue smoke from one of these cheap panel meters.  After trying a few remedies to this, I finally moved the current sense resistor to the "low" side so that the volt meter and ammeter grounds were the same as the power supply ground.  You can see this in the schematic below.

The main power source is a 90 watt laptop power supply.  The voltage control device is an LM350 giving a capacity of 3 amps.  There is lots of filtering to keep any hash from the laptop switcher out of the final DC output.  I used an old 9 volt power cube as a power source for the panel meters.  I had a nice aluminum enclosure in my supply of stuff that would hold everything.

It is probably overkill, but I used a big processor heat sink with a fan for the LM350.  You can see the AC fuse and I have a 3A DC breaker to keep from damaging anything on the DC side.  

Here is the schematic.



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