Hello again,
Thanks for the replies. Some very good ideas here. I tried to point out the various shortcomings of various ways
to handle this but keep in mind im searching for a *very* good way to get around this problem...one that will be
simple and cheap and solve the problem perfectly for a wide range of output voltages. I am hoping it does not
boil down to needed an op amp to get it right because i felt that that would just clutter up the circuit too much
when it is such a nice and simple circuit to begin with...i hate to do that.
Of all the ideas it's starting to sound like the secondary circuit idea is the best as long as that secondary
circuit can be simple too. We'd have to start looking at circuit ideas for this too then i guess, the simplest
crowbar or other secondary regulator circuit. Anyone mention a shunt regulator on the output, set to slightly
higher? But then again that circuit would have to be adjustable too and i guess the dual ganged pot comes back
into the picture. I was hoping to use a regular ol' pot you can get almost anywhere too so other people can
build this easily without ordering any special parts. Of course then we also have to have a fail safe mechanism
for the second section of the pot in case it goes bad and we dont know it, or that part of the circuit defaults
to 0v instead of some 22v or so.
More ideas or additions to the ideas already presented here are certainly most welcome.
[1]
Multiple Throw Switch With a Bunch of Resistors:
Not a bad idea, but then we have to worry about the switch contact instead of the pot 'arm' contact.
[2]
Bypass Cap Across Pot:
That's a good idea too, but unfortunately it doesnt help the completely open 'arm' problem.
[3]
Higher Quality Pot:
Not too bad either, but again it could open up and besides the higher quality pot will come with a higher price tag.
But on the other hand i do have to wonder now if anyone makes a high quality pot that is guaranteed not to open up
and cause this kind of problem. That would certainly solve it although we'd have to pay more for the pot.
[4]
Choosing R2 (the pot) For Max Safe Value:
Well, that would defeat the purpose of being able to adjust to a higher voltage (as others pointed out).
[5]
Roff Crowbar:
That is very interesting...in the form of a secondary adjustable circuit to guard against the first one failure.
It would have to be cheap too and simple, and fast to react.
[6]
Brevor/Make R1 Adjustable instead of R2:
Yes, that's a very good idea. The only problem is that with some required output voltages the current drawn through
the resistor to ground will have to be up around 500ma. That's a bit high. We want to get down to around 1.5v and
up to about 22v or something like that. Because of the various problems this would only work for limited output
voltages. For a dual plus and minus version, we want to get down to 0v and up to say 22v or around there, but dont
worry about the dual version just yet.
[7]
Hero, Dual Ganged Pot, One For Primary and One for Crowbar:
That's sounds interesting too, except for the fact that we would have to purchase dual pots. We'd also have to have
a way to keep track of the second section of the pot to make sure it didnt go bad and we wouldnt know it.
Maybe two separate pots to adjust the two circuits individually? That would be a bit of a hassle i guess.
Thanks again for all the ideas and comments, and any more ideas would certainly be appreciated.