23-04-2012, 08:00 AM
The transistors are getting hot because the quiescent current is too high - power = volts times amps. Yes, those transistors are rated at 800mA, but because they're only in a TO92 package, they're not intended to dissipate much power. In other words, they can pass 800mA, but only if the voltage drop across them is very small - i.e. they're really good at switching loads.
You'll find that's often the case - a given transistor will have a voltage rating and a current rating, but there's no guarantee it'll do both at the same time. The power rating is the first check - 0.625W in the case of the BC327/337 - but there are second-order effects as well, which are best "parked" for now
Why did the output pair fail, but not the "weedier" BC108? Well, that's because of where the transistors are in the circuit, and their ability to pass damaging currents. The output pair are directly across the power supply, so the only limit is the power supply - probably in excess of 1.5A if you used your LM317T PSU. The BC108 is surrounded by "padding", and even when saturated, the maximum current that can flow is only a few milliamps.
That PSU looks reasonable enough, and of course you can retro-fit it to your existing design. Without thinking too hard about it now, I'd say that Rsc should be ~0.8 ohms (1.25V / 1.5A). A couple of 1.5 ohm resistors in parallel would probably do the job - best to use 1W or higher wire-wound resistors for that. The FETs are just being used as current sources, and normal bipolar transistors can do that job with the addition of a couple of extra components.
Cheers,
Mark
You'll find that's often the case - a given transistor will have a voltage rating and a current rating, but there's no guarantee it'll do both at the same time. The power rating is the first check - 0.625W in the case of the BC327/337 - but there are second-order effects as well, which are best "parked" for now
Why did the output pair fail, but not the "weedier" BC108? Well, that's because of where the transistors are in the circuit, and their ability to pass damaging currents. The output pair are directly across the power supply, so the only limit is the power supply - probably in excess of 1.5A if you used your LM317T PSU. The BC108 is surrounded by "padding", and even when saturated, the maximum current that can flow is only a few milliamps.
That PSU looks reasonable enough, and of course you can retro-fit it to your existing design. Without thinking too hard about it now, I'd say that Rsc should be ~0.8 ohms (1.25V / 1.5A). A couple of 1.5 ohm resistors in parallel would probably do the job - best to use 1W or higher wire-wound resistors for that. The FETs are just being used as current sources, and normal bipolar transistors can do that job with the addition of a couple of extra components.
Cheers,
Mark
