battery charger - can a transistor work on an AC supply?

[deepak007]

battery charger circuit by scr

hello

could some one explain me this circuit. here 12v AC voltage is directly given to the transistor collector. can a transistor work on an AC supply? please help.

 

[VVV]

scr battery charger

The SCR will be the rectifier.
The top two transistors are actually powered at DC by the battery itself.
If the battery voltage is below the threshold set by the pot (it's discharged), then the top right transistor will be off.
The left transistor will then be on and it can turn on the SCR through the LED and diode. That however can only happen when the AC input polarity is such that the top is positive. At that point the SCR fires and power is supplied to the battery for one half of the aC cycle. When the polarity reverses the SCR turns off. It will be on the next AC cycle.

If the battery current is too high, the voltage across the two 1R8 resistors will turn on the BC547 transistor and charge up the lower 47uF cap. The transistor cannot turn off the SCR, but it will prevent it from firing the next time the AC has the right polarity. The cap can keep the transistor off for several AC cycles.

When the battery has charged up the top right transistor will be on, keeping the top left transistor off. That means the SCR can no longer fire and the battery will no longer charge. But if its voltage drops, then the charging cycle will begin again.

Not that since the battery powers the top of the circuit, it will not work with a completely dead battery. That is why you have the switch which will temporarily turn on the top left transistor with help from the AC when the polarity is top positive.

I am not 100% sure about the role of the top 47uF cap, but I suspect it's meant to prevent the top right transistor from turning on when the SCR fires, because the voltage increases when that happens.

 

[banjo]

battery charger help

The trick to understanding this circuit, is I think it only works if the battery has some residual voltage left. Then the battery is supplying a voltage to the top BJT for bias purposes. The designer took advantage of the fact that batteries are storage devices and is using the battery for where we would normally place an electrolytic capacitor.

Based on the battery voltage, the SCR conduction angle is varied which determines the amount of charge per 1/2 cycle is put into the battery. The designer saved both the filter capacitor from a normal DC supply and the rectifier.

Note: when I was a kid, I took apart my grandfather's charger for car batteries to see what the circuit was. I was disappointed to discover almost nothing in the box. It was just a transformer, a large diode and an ammeter. That was the entire circuit. The only magic is in the turns ratio of the transformer. It was tweaked so that the peak ac output voltage from the secondary was close to the final voltage they wanted on the battery.