More on more battery...

George - I just finsihed the quest for a simple battery charger that you are enterring! Power zeners didn't work for me either and for the same reasons. I tried the LM317 a couple of different ways. It worked but not well. The results appeared to show too much effective series resistance in the LM317's output. From a later view, I suspect use of caps to provide a dc source was the basic cause. Basically, the final version assumes the transformer provides the current and choice of the 12v ac transformer specifies the current and the current limit. Now, the transistors do nothing but limit the output voltage.

13.0 volts sets the final current limit from the battery's point of view. for the circuit components (...): a bridge rectifer from said transformer connects to the +&- rails. a pnp to220 (Tip42) device's emiter connects to the + rail a 1k resistor connects from the + rail to the pnp base the pnp collector connects to a 1n4002 diode. the other end of the 1n4002 goes to the output. the collector of a 2n3904 npn to92 device connects to the base of the pnp device. the npn emitter connects to a small schottky diode and the other end of the diode connects to the output. a 220 ohm resistor is connected to the + rail and the other end connects to a 15v zener. the other end of the zener connects to the - rail which is also the negative output. from the zener/220 ohm junction a 100 ohm resistor connects to the base of the npn device. From the base of the npn device a 1.8k resister connects to the - rail. That should do it. The 1.8k resister does some fine tuneing to 13.0 volts. The circuit basically tracks the up&down of the rectified 60 cycle so be carefull to not put a capacitor in the circuit. The battery (this is lead acid) does the deciding as far as current goes and 13.0 volts seems to saturate at ~0.1 amp with the voltage returning to ~12.6 after the charger is removed.

Hul

-------------------------------------------------------------- From: George Herold snipped-for-privacy@gmail.com Newsgroups: sci.electronics.design Date: Sat, 6 Mar 2021 09:25:01 -0800 (PST) Subject: More battery charger, voltage shunt reference.

Hi all, this is mostly a continuation of CD's solar battery charger thread. I was thinking of using a power zener as a shunt reference... but this has some problems. I think #1 is that as the zener sucks up the power it will warm and go up in voltage... which is exactly the opposite of what I want. So how about a zener turning on a power npn. I think someone mentioned this idea in the previous thread, but I couldn't find it in a quick rescan. Maybe the TL431? Anyway I drew this,

Power npn across the rails, with base feed by zener and some series diodes and maybe led and ?10 ohm base R. Then two or three diodes from base to emitter to protect b-e when it turns on. And maybe a collector resistor. ~50 ohms 3-5 watts I'd like the shunt to turn on at 13.5 Volts at room temp. with a negative tempco.. less voltage at high temp. I'd like some knob (pot) to tweak the voltage. I was thinking if I used diode connected transistor (as part of diode chain) I could put a little bit of R in the c-b connection... that seems like a beta dependent tweak. Which might be OK if beta goes down as the temperature goes up. But maybe there is a better way. Ideas?

George H.