Another Battery Charger Circuit

Circuit diagram:

A Low Cost Universal Charger Circuit Diagram

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Parts:

R1 = 120R-0...5W
R2 = See Diagram
C1 = 220uF-35V
D1 = 1N4007
D2 = 3mm. LED
Q1 = BD135

I will make the following changes to the circuit.  I will substitute a 50 ohm potentiometer (rated at 1/2 watt as all the various R2 values are in that range.  I have also substituted BD137G transistor as it has a little higher voltage ratings, it is basically the same transistor.   I have added a full wave rectifier rated at one ampere and a filament transformer (step down from 115 to 24~25 volts at 800 milliamperes.

I will move the  diode D1 up next to output jacks.  It will act as a blocking diode vice rectifier as shown.  The full wave bridge will be the rectifier coupled with the capacitor C1. 

I will add the jumper so that I can insert a ammeter to set the current values.  Current measurement requires one insert the current meter in the circuit.  So I will remove the jump, insert my VOM, set the value I need and then reinstall the jumper.   I could add a meter but that runs the cost up to high.

The transistor is in a TIP case, so one bolt will hold it to the chassis.  The case will be the ground side for everything.

I am not sure just how the LED works.  It is a device that allows positive current to follow one way, it is a current device.  As it is shown in the circuit, it appears to be fed by the negative output of the transistor.  Of course R1 has a voltage drop so the potential at the face to the LED is less than the output of the transistor.  I suppose the difference in potential is sufficient to show a positive to negative flow, that is the anode, the positive side of the LED will be greater than the negative side.  So in the world of real numbers it is positive (that is a less negative value. ergo is positive in relation to the remainder of the circuit).  The LED is there to keep the base of the transistor positive and allows the transistor to conduct current in relation to its input via the potentiometer.  It will also indicate the circuit is active (e.g. pilot light).

I have been getting components form Radio Shack, sort of expensive.  I found I could get the parts form Mouser, much cheaper if one does a "Will Call."   Will call means one will pick up the merchandise at the warehouse.  The warehouse is located in Mansfield not to far away.  The transistors, I got two of them, cost me $0.49 a piece and the 120 ohm resistors cost $0.14 cents each.  There was a little tax but the whole cost of the four parts was around a buck and half, about 4 to 5 times cheaper than Radio Shack and it was higher grade stuff.