I have a security system in my home that I built around a car alarm
design which I modified. Since the original had none, I designed the
120.0 VAC power supply to be used with this unit. It is a basic full
wave bridge, feeding a 2000 uf electrolytic, and a simple two
transistor series regulator which uses a 15 volt zener diode as a
reference from base to ground of the driver transistor. The voltage
across the filter is about 16.0 VDC. The voltage as measured at the
emitter of the output pass transistor is about 13.80 volts, just about
what I expected it to be. In my first trial design I connected this
output through a 5.0 ohm resistor which was in series with a 12.0 volt
7.0 AH sealed rechargeable lead acid battery. I placed a known good
battery which had been sitting on the shelf for two months into the
cicuit and powered up the system. The initial charge rate was about
180 ma. After charging for a day the charge rate dropped to 70.0 ma
with a battery terminal voltge of 13.6 volts. Noting that this trickle
charge, as well as my battery voltage was much too high, I ran the
battery down to 9.50 volts and this time instead of the 5.0 ohm
resistor, I used a small # 354 incandescent light bulb which is in
series with the battery. I then powered up the system again. My theory
is that the bulb will now act as the current limiter, and now for
example with the battery in this serious discharged state, the initial
charge current will be high, and drop to a trickle of 10.0 ma as the
terminal voltage rises. So now, with the battery at 9.50 volts, my
charge rate started out at around 20.0 ma, and after two days the
battery has come up to 11.20 volts and the charge rate has dropped to
about 15.0 ma. I should back up here at this point and mention that
across each side of the bulb are connected the anodes of two diodes.
The cathodes are connected together and this is where my 12.60 volt
output feeds the alarm circuit. I have also placed a 250.0 ma load at
this point to simulate the alarm equipment when it will be connected.
So my question is: with the charge rate running so low now, will this
battery eventually fully charge up, and if so does this sound like a
good "float" type situation. I would like the eventual float voltage
to be 12.60 volts so as not to overheat the battery thus shortening
its life. I know that this charge circuit can be better regulated
electronically but I really didn't want to get that involved. I've
seen this simple bulb scheme used before with success and thought that
I'd try it for myself. If anyone has any further thoughts or ideas on
this I would sincerely appreciate any and all comments. Thanks, Lenny
Stein, Barlen Electronics.