I was testing out my solar charger circuit tonight. Unfortunately, we’ve had so much cloudy weather lately, I haven’t been able to put the panel on it yet. In the meantime, I have plenty of old wall-brick power supplies lying around, so I decided to see if any had a reasonable voltage/current to test with. The first one I tried was my old speaker set brick: 14V 1100mA. Since I was cutting down the voltage and current so far, this brick forced the LM317 to dump way too much waste heat. I took to looking for something a bit more reasonable. I found it in an old cell phone charger: 5V 100mA. This was much better to test with as the low current couldn’t hurt a fly. Unfortunately, the 5V wasn’t really anything special as I was looking for close to 5 volts for the output. Tonight, I tried a 12V 1000mA supply. This was much better, but much like the 14V brick, too much waste heat.
My panel is a 1W panel. In average practice, I’ve watched it dump a very solid 10V at give or take 100-120mA. Those are the low currents I’m expecting to be up against, but I don’t have much to replicate that. Still, I did make some changes tonight, like lowering the R1 on the voltage divider on the adjust pin of the 317 from 180 to 100 ohms. Not a big change, but enough to give me the better range of voltages I want from the 1K variable resistor.
Speaking of variable resistors. I knew I should’ve bought more than two. I was testing some reference specs on the 317 and my less than stellar wiring caused the probe leads to short sending a wonderful 1A at 12V straight to my variable resistor. There was smoke. I didn’t realize what happened at first, but I’ve burned those things out before. At least I had a spare. I’ll have to order more.
I wasn’t planning on it (with the low currents and voltages) but I may need to heatsink the 317. If only when I’m testing with these high current power supplies.