bbs00068@uafcseg.uucp (Joel Kolstad) (07/14/90)
Hi... recently, I made a circuit consisting of an Intersil 7660S voltage
doubler/inverter (inverter in my case), hooked up to an op-amp. The circuit
had a +12V power supply, the 7660S changed that to -12V, and feed it to the
op-amp. The op-amp was then supposed to take any +/- level and kick it to
+/- 12V (for RS-232 conversion). The whole thing worked fine, EXCEPT...
...I tried to use it with one of those "12V lighter batteries" that Radio
Shack sells. It sure didn't work! After checking through the circuit, I
determined that the problem was that the battery seemed to be shorted. I
checked where it was sitting, and it wasn't. I ran long wires to the battery
that was then sitting on absolutely NOTHING metallic, and it still happened!
A multimeter showed a current draw of about 160ma -- about short circuit
current for the battery. Needless to say, the battery quickly would have
died if I hadn't disconnected it.
I wanted to make sure I hadn't accidentally done anything to the circuit
itself, so I hooked it back up to the bench power supply -- still worked.
I hooked it up to 8 AA's in series. Worked fine. I hooked up a 9V battery --
worked fine (except now I was only at +/- 9V, of course). I tried two
lantern batteries in series -- worked fine. ARGH! I was getting quite
frustrated at this point. I should mention that with all of the above
tests, the current was approximately 2ma, which was reasonable for what the
circuit was doing.
Just to make sure I hadn't been hallucinating, I hooked up the lighter battery
again -- it still did the same thing. What the hey!? At that point, totally
baffled, I gave up...
So, now, all you gurus and guru-wanna-be's, can any of you tell me what went
wrong? I pryed open that 12V lighter battery, and it turned out to just be
8 "watch" batteries in series -- nothing unusual. I am at a total loss
as to why that battery wouldn't work. Can anyone out there help me?
Thanks... ---Joel Kolstad
P.S. -- Don't hit reply unless you change the address! Please mail me at
(as) kolstad@cae.wisc.edu!johne@hpvcfs1.HP.COM (John Eaton) (07/17/90)
<<< < Just to make sure I hadn't been hallucinating, I hooked up the lighter battery < again -- it still did the same thing. What the hey!? At that point, totally < baffled, I gave up... ---------- I suspect your battery has a high internal resistance that is being killed by the starting surge of your circuit. The other sources have a low enough resistance that they can put out enough voltage to put the circuit into its normal operating region. Your battery cannot and so it simply remains in the startup state. Try putting a large cap across the battery before connecting it to the circuit. Your circuit is probably acting like a negative resistance. John Eaton !hpvcfs1!johne