Originally Posted by Syzygies
While I wouldn't bet on your rule of thumb staying linear in a short circuit, it makes it clear that for purposes of this discussion, an NiMH battery pack might as well be viewed as capable of infinite current. Something else will give first.
Sure. Also keep in mind, my rough-and-ready 35A estimate is based on the pack impedance, and that is further reduced by the rail cap bank's parallel impedance. So while I'm not sure what the actual number is, I can't see that I'm greatly overstating the potential.
I think you're right, though, that it's effectively infinite. To get 8A (the max peak the MJE243/253 allows) through 2.2 ohms, you need to put 17.6V across it. That just can't happen in a typical PPA. Ohm's Law poops out on you first. Now, that doesn't put us fully in the safe zone, because between that and the 4A continuous rating there's a danger zone. But I'm optimistic that the supply voltage and discharge current will be dropping at this time, so the ability to sustain more than 4A for a prolonged time should be difficult.
The less studly BD139/140s would be easier to kill. In fact, I think it wouldn't be all that hard to kill them with 2.2 ohms of output resistance.
It's worth pointing out that the resistor can easily die before the output transistor, even with 2W resistors. If you're somehow putting 4A through them, the wattage is 35.2W. Fortunately, film resistors tend to fail open; they're hard-to-replace fuses, in effect.
During the PPAv2 development, we went through several design changes to the buffer, and at several points I had 10 ohms on the output. This will raise your distortion a bit but it will ameliorate these problems greatly. And because it's inside the feedback loop, it doesn't hurt as much as putting resistors inline with the output wires. That's why the distortion doesn't go up much.
|As a rough rule of thumb, I'd say that additional R ohm output resistors should be rated for 4/R watts, e.g. 1/2 watt, 10 ohm output resistors can handle the same scenarios envisioned by the 2 watt, 2.2 ohm on board resistors.
Careful there. We didn't pick 2W through any special wisdom. It's just the largest value that fit in the space we had.