Monday, December 28, 2009

one dual rail 12V PSU on the bench and working

Spent a couple of hours researching and then building, testing/destroying, then researching and re-engineering a dual rail power supply for +12 volts & -12 volts.

Not sure what kicked it off, but I am kind of over my 10 meter QRSS receiver not working. I was looking at a ham radio blog, found a video of NE555 and 4017 based synth sequencer. Down the rabbit hole I went reading dozens of web pages, until finally finding the coolest sequencer I've ever seen!

On the Natural Rythm Music site I found a 16 step rotary analog sequencer. It is a visually amazing device, certainly feels more intuitive than you regular 4 blocks of 4 linear sequencer. The builder has a great presentation style and can put together some cool tracks.

This discovery then in turn inspired more and more reading until I found some basic building blocks. Everything seemed to need a dual rail power supply, so I though I should start there. Some web searching, followed by checking a couple of different editions of the ARRL handbook led to a simple dual rail regulated supply. I carefully noted down the schematic, made a parts list and marched off to the shed/shack to build.

Stepping back a ways, I purchased for two dollars, a two amp twelve volt power supply at the radio club meeting. I was a bit disappointed that it didn't work, but put it aside for a rainy days hacking I suppose its the risk you take when you buy gear from a deceased estate, no real history on the item. So yesterday was that day, except it was 32c, no rain and no wind. Makes the shack a little warm.

I amassed all the parts, dissected the old power supply, noted the two 10,000uf 24 volt electrolytic capacitors and carefully discharged then with a long screw driver... the spark was impressive for a power supply that had been unplugged and idle for six months. Note to self, be careful around these caps!

Spent a while working out the arrangement on tag strip, then manhattan style, but didn't find a nice way to mount everthing in the space available. Got a coffee then came back with the lost thoughts of some veroboard of yore...

Found the vero board and worked out a layout on the board from the schematic. Assembled it, checked it twice. Carefully hooked everything up and turned on the power. Everything looked Ok until the LM7812 regulator launched a small part of itself into orbit!

Ok, this wasn't part of the plan! Turned everything off, disconnected everything and started the autopsy. Everything was correct according the schematic. So I checked the schematic against the original. Sure enough the pin out of the LM7812 was wrong and had exchanged the ground and incoming unregulated power line, pins 1 and 2. No wonder it went FIZZ POP!

I downloaded the data sheets, just to be sure. Who would have thought that the LM7812 and the LM7912 would have different pin outs? I mean really?!?!?

Anyhow, I spent about an hour looking for a replacement regulator. Almost despondent that I'd never found one, I happened to find a little plastic draw that had been missing out of the component cabinet. Interestingly I must have put it aside when I first looked at the power supply and thought about reworking it as a regulated unit for 12 volts for my 2 meter radio.

Anyhow, it all went together more easily the second time and it passed the smoke test the second time, keeping the smoke on the inside! Along with providing 11.99 volts and -12.01 volts. The simple test load this time was a pair of 12 volt 20 milli amp four inch fans from a peecee power supply.

So, now to find all the parts for an audio VCO for tone generation, an LFO for effects, mixer for multiple channels, various high and low pass filters and an amplifier... The aim is that all of these will be building blocks for test equipment for RF down the track. Cunning plan huh?

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