Simple circuit explanation, please.

Thread Starter

zophas

Joined Jul 16, 2021
115
I have a very old garage type battery charger which has the following little circuit in it besides the main charging stuff. Now I don't know what the circuit is for because I don't understand how it works. I assume it is either a "battery full" circuit or a reverse polarity circuit. The 12v relay activates on switch on and the charger works. There are no lights or led's to indicate anything. It's old, built before the led era. Gives me a few ideas for modifications. Can someone explain what this circuit does?Circuit1.jpg
 

Audioguru again

Joined Oct 21, 2019
3,516
When the battery voltage is low then the two resistors turn on the transistor. The zener diode does not have enough voltage so it does nothing.
When the battery voltage is high enough then the zener diode conducts and turns off the transistor. Then the relay can disconnect the charging.

The circuit is extremely simple and the voltage that the transistor turns off is affected by the temperatures of the transistor and the zener diode.
 

Thread Starter

zophas

Joined Jul 16, 2021
115
When the battery voltage is low then the two resistors turn on the transistor. The zener diode does not have enough voltage so it does nothing.
When the battery voltage is high enough then the zener diode conducts and turns off the transistor. Then the relay can disconnect the charging.

The circuit is extremely simple and the voltage that the transistor turns off is affected by the temperatures of the transistor and the zener diode.
Ok, thanks. So it's a "battery charged" circuit. Would I be able to put a LED and resistor in the emitter leg without upsetting any existing thresholds?
 

AnalogKid

Joined Aug 1, 2013
9,353
Are you sure about the battery connections in your schematic? It makes more sense if the two "out to battery" connections are in series with the battery rather than across it.

ak
 

Thread Starter

zophas

Joined Jul 16, 2021
115
Yes I seem to have labeled that a bit wrong. Sorry. The two circles with the + and - signs are what go to the battery under charge. The other side of those leads go into the guts of the charger. The "out to battery" should have been under the circles.
 

dcbingaman

Joined Jun 30, 2021
334
No.
Something is not correct with the circuit as drawn.
The transistor is always on and the relay is energized.
Correct. It looks that way. Modeled it up in LTSpice with similar components. The relay is always on regardless of the input voltage. When redrawn it is easier to see, R1 and R2 supply nice bias to transistor keeps it on no matter what. D1 limits base current to around 2mA as input voltage goes above certain threshhold.
StrangeCkt.PNG
 

MisterBill2

Joined Jan 23, 2018
9,267
At one time quite a few battery chargers had a circuit that would not allow the charging power to the battery terminals unless a battery was connected correctly. That would prevent damage to the battery and also protect the charger against short circuits from the clips touching each other The circuit needed just a bit of voltage from the battery to operate. There was a button to force a start if the battery was far dead.The first version looks like that circuit. It was rather importand with some of the higher power chargers that could deliver 50 amps but had diodes thatdid not have much safey margin for overloads.
 

Dodgydave

Joined Jun 22, 2012
10,050
I would say it's a battery reverse protection, without the battery first connected the relay won't pull in and power up the charger.
 

Thread Starter

zophas

Joined Jul 16, 2021
115
Thanks. I think it's looking more like a reverse polarity protection than a charge control. I have not noticed if it will charge without a battery connected but I will check on that. This charger is marked as a TI Crypton 41 and is pretty old. Made in England. I am pretty sure it could handle a lot of current. I can not find a document about the charger but I did find some interesting info on the Crypton company.
 
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