09-11-2012, 02:09 PM
Hi Joe,
Looking good - sounds like you're getting there, and understanding what you're doing, which is the important thing...
I've just noticed how high your UNREG supply is, and that's pushing things a bit for a 741. I'd be tempted to install a resistor in series with the supply pin to drop this down a little - perhaps 1K or 2K2 - maybe a bit more. You'd need to put a capacitor between pins 7 and 4 of the op-amp - perhaps 10uF would be OK here.
Alternatively, a Zener diode might be better - a 10V or thereabouts should do. This has the advantage of being pretty constant in light (sorry!) of the changing current consumption caused by the LED being on or off.
If you don't get much joy playing with the hysteresis resistor, adding a resistor between +IN and ground might work. Perhaps try 100K... It's just to set up an offset that corrects for the fact you have that voltage across the sense resistor...
Of course, you could consider adding current metering to your PSU...
Looking good - sounds like you're getting there, and understanding what you're doing, which is the important thing...
I've just noticed how high your UNREG supply is, and that's pushing things a bit for a 741. I'd be tempted to install a resistor in series with the supply pin to drop this down a little - perhaps 1K or 2K2 - maybe a bit more. You'd need to put a capacitor between pins 7 and 4 of the op-amp - perhaps 10uF would be OK here.
Alternatively, a Zener diode might be better - a 10V or thereabouts should do. This has the advantage of being pretty constant in light (sorry!) of the changing current consumption caused by the LED being on or off.
If you don't get much joy playing with the hysteresis resistor, adding a resistor between +IN and ground might work. Perhaps try 100K... It's just to set up an offset that corrects for the fact you have that voltage across the sense resistor...
Of course, you could consider adding current metering to your PSU...
