Bench Power Supply 0-30V@1.5A
16.10.2006
23.09.2006
First off, the PCB images below are different to what I’m getting made as a first run of two boards, without the ground plane. It seems that BatchPCB.com has some issues if I include a ground plane, but for final production runs, if this catches on, I’ll obviously get them made from a company such as Advanced Circuits.
So what’s all this about then? Some time ago I got Bob to buy me some ultrabright blue 7-segment LEDs off eBay (8 of them infact) as I saw these on a timer in the TV series “24″ and instantly fell in love ^^. I’ve already designed a different PCB shown below,
12 control lines, multiplex the four digits, allowing them to be driven via MCUs/FPGAs - whatever really. That is why you see a 16F88 on the PSU PCB above. The beauty of the design above is that if you ‘don’t’ want to have this ‘old skool’ type voltmeter display, you can simply *not* solder on the MCU components, and you can connect a voltmeter of your choice across Vo, which varies from 0 - 30vdc via a 5K pot, which in turn controls the LM317 vreg. If one likes, they can include the MCU circuitry and directly interface with the TX pin on the 12-pin jumper block (meant to interface with my display) and get serial output via the PIC, and display this on an LCD for example - actually the 12-pin jumper block let’s you access 12 of the available 13 i/o lines on the 16f88, while RA0/AN0 is used as the A/D converter which is connected via a potential divider across the output.
So back to the PSU PCB - What you want is a 3A 24VAC transformer. The one I got does 3A with two 12VAC output terminals, and by connecting a (+) to (-), the output is increased to 24VAC. 24 * root(2) = ~33.9 so 34VDC after rectification via the FWBR (KBL04) which is rated at 400V/4A. The SW/FUSE connection is meant to be connected across a switch capable of 240V/3A and a slow-blow 3A fuse. The huge electrolytic cap should be 2200uF/64V (anything over 40V really) and then we have the LM317 with a 1N4001 power diode to absorb back emf (thereby protecting the regulator), and the two silicon 1N4148 diodes provide a total forward voltage drop of ~1.4v to correct the output scaling (this is due to the voltage regulators setting up a reference voltage of 1.2v).
Another slight note, some may think I’ve forgotten to include a decoupling cap for the MCU, but I think it’s far simpler to solder it by hand across the pins on the reverse side of the PCB, a 0.1uF ceramic jobbie would do fine.
- Bench Power Supply 0-30V@1.5A
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