Straight from the horse’s mouth over on another board, update your bookmarks! The site is now www.spookshow.net.
That was a timely update. I took a peek at your site, Trainboy, and found the regulated power supply circuit. I was just thinking about illuminating some structures.
http://www.spookshow.net/lowvcircuit.html
Even more amazing, the Shack had everything in stock, except the circuit board (which I subbed another part# for.) Note that the specified circuit board will make things easier, but you can also kludge it together another way if you pay attention to the circuit traces in the pics.
I had a 1 amp 7.5 volt DC output power supply. Put it together and boxed it in an old RS project box. I dialed it down to 1.25 volts, so works great. I may actually end up using it to drive some resistors, rather than 1.5 volt bulbs, so may have to turn it back up a little.
My only question is the rating on the resistor. I used the 220 ohm part number, but noted these were only rated at 1/4 watt, so paralelled them to get 1/2 watt equivalent. That is IIRC my ELEC 101 right – and if it is, that’s about all I remember…[:S][:P] Maybe overkill, but I’m going on a hunch that they could stand to be 1/2 watt just to give a little more headroom for heat.
You’re good on 1/4 watt there - the current flow there is very feeble, just to feed back to the regulator. The full load current does not pass through those resistors.
Paralleling a pair of 220 ohm 1/4 watt resistors does not get a 220 ohm 1.2 watt resistor - it gets a 110 ohm 1/2 watt resistor. Parallel resistance is 1/(1/r1 + 1/r2 + … + 1/rx).
Theoretically you can eliminate the potentiometer if you want about 1.2V out, that is the lowest the 317 can deliver. It will require about a 4ma minimum load to work like this though.
–Randy
Randy,
Thanks for the insight into the resistor issue. One of the reasons I decided to become a historian, I suppose…[:$]
Doubt if I have any need to go lower than 1.25 volts, even with bulbs, but good to know I’m near the bottom on voltage out of the 317.