With electronics I've mostly put circuits together straight on copper strip board. Really I should have used bread boards more often. It's only due to the power supply circuits drawing a fare bit of current, I thought bread boarding would just melt jumper wires.
So this evening I tried something I thought would be straight forward, but it was far from that.
It was just a simple L7812 with a pnp pass transistor, I was trying to limit the current through the L7812. But no mater what value resistor from 10 ohms to 150 ohms the measured current done by the regulator was still 720mA there where probably losses as my load was a 20 watt halogen lamp. I stripped it back multiple times, checked and double checked all the connections. I tried different power rated resistors from 0.5 watt to 10 watts, but nothing limited the current between the base emitter chain. I was starting to think the breadboard was dodgy. Another strange thing, I opened the circuit to see the current the regulator was passing which showed the 720mA current it was passing. But when I removed the series meter connection the lamp was still alight but dimmly. Almost like there was some kind of low current path with the base and input to the regulator completely removed. To be honest I feel a bit silly asking, but I've no idea why I couldn't get this regulator to drop it's work load.
Any help appreciated. What I was trying to do was get the regulator regulating with out a heatsink, and the pass transistor doing most of the current driving the lamp. If I disconnected the regulator from the heatsink,, it heated up just short of thermal throttling itself. As I said I'm completely at a loss as to why it wouldn't current limit the LM7812. I've got some L7812L 100mA regulators, and some LM317L 100mA regulators I was hoping to use. But no chance of that if I can't keep a TO220 device cool.
Thanks for reading.
It was the circuit below with out R2.