Why is is that when you are thinking, working productively on one things, four other ideas come to you? And why, when you are roadblocked on one thing, you're roadblocked on everything?
Sigh.
Amy, I'll do two reviews for you. I have to do a couple more for other people today, and that won't even catch me up.
Meanwhile, while I was working on a 'final' demo box for my flasher circuit--weeks ago, and I still have to cut the opening for the switch--I got the flywheel diode backwards. It actually managed to flash, but drew so much current it was probably responsible for blowing the output of my little battery-powered power supply. That has to be fixed, and I'd rather it be resistent to future damage of that sort.
It looks like the bootstrap transistor is blown to a short. If it's not the bootstrap, it's the main output. Either way, I'm redesigning that part, using multiple bootstrap transistors in parallel. That will allow it to carry more current with less current on the 'main' output. But ... paralleling bipolar junction transistors requires current sharing resistors. The low values I need are only available in surface-mount, about half the size of a grain of rice. Since all of that has to be mounted on the heat sink, I have an interesting physical design problem. I know what I want, but not yet how to get it.
The main output transistor will also get a heatsink. There's only one kind available for TO-92 package devices, and it's a flat teaspoon-sized blade, so I have to move things around on my little circuit board. (I'm starting with a new board.)
I want to put a low-pass filter on the voltage-regulating section so that brief current spikes don't let the output drop as the internal resistance of the batteries cuts in--but I want to be able to switch it out. It looks like it will take less board space with the parts soldered point-to-point on the switch.
The battery circuit switches were a beast last time. They'll probably be a beast this time.
And now ... as I was trying to work on the physical design of the heat sink-and-bootstrap unit ('power block') I got nagged by the thought that I could still overload my main output transistor. The solution is a current limited between the output and the current bootstrap. The current source for the output transistor goes through a resistor that develops a voltage as the output current rises. When it gets up around the working range for junction voltages, the current bypasses that sensing resistor, feeding the base terminals of the bootstrap transistors. Their output--up to 200 times the current they receive--is added to the output transistor's current.
So the current limiter has to go between the collector of the output transistor and the current bootstrap. It will drop probably about 1.4 volts, reducing the output voltage I can get with a given battery voltage. It will also have interesting effects as the output transistor goes into saturation with its collector drive cut off.
So I need to test and tweak the limiting circuit (3 to 9 hours) and figure out how to squeeze it on the circuit board. It needs just four nodes, but it involves two transistors tied to each other and the output transistor ... and that makes for a hard fit, especially since the output transistor has to sit in a way that will make the flat heatsink mounting work. (The heatsink for the power block is a finned chunk of extruded aluminum. I'd prefer a black finish, but this is what I got cheap.)
AND ... while I was working on that problem I suddenly got A Very Good Idea to increase the tangle Merran and Pausonallie will get themselves into. I'm struggling to get stuff revised and get to that part. And I can only remember one of the orders I need to place today ... sigh!
This after four days when I couldn't think long enough about one thing to make progress.
On to reviews!
