Battery Charger , , ,

A few years a go I got a schematic for a trickle battery charger for NiCd/NiMh batteries.  The charger could charge 1 to 20 cells.  It was a current adjustable DC power supply.  The elderly gentleman that gave me the circuit has, I believe, passed on.  He was a instructor pilot in WW-II so I am sure when I last talked to him he was well into his upper 80's.  And as with all of our Veterans of the Great War, he has passed on.  At least I can not locate him anymore so that is my sad conclusion.

The circuit is pretty simple.  I use a filament transformer to step down the input voltage from 110VAC to 25VAC.  For those of you that do not know what a filament transformer is, it is a hold over from the vacuum tube days where they had a filament circuit hooked up to the cathode of the tubes.  Filaments were power by 12.5 volts which heated up the cathode.  So the secondary of this transformer has a winding that supplies 25 volts across the entire winding.  It is center tapped to provided two separate 12.5 circuits.  You just ignore the center tap and use the two outputs that crosses the entire winding and you have 25 volts.

The next part of the circuit is a full wave rectifier.  In the vacuum tube days you used a dual diode tube that basically had one cathode/filament and two positive plates.  Now one uses a single solid state device that has four diodes embedded and provides the full wave rectification of the AC current.  Then you use a 100 or better microfarad electrolytic capacitor as filter to smooth out the rectified DC.  The capacitor needs to be rated higher than 25 volts, they come in ranges from 25, 50 and up, anything above 25 volts will do.  Simple enough, even an old geezer like me understands it immediately.  Not a big deal and quite impressive since now the whole shebang is just two parts, a rectifier and a capacitor.

Now comes the interesting part to me.  The part I clearly do not understand how it works but can easily wire it up.  It uses a 10,000 ohm resistor hooked up to the positive side of the filtered 25 vdc.  The other end of the resistor hooks up to the base of the 2N3055 power transistor.  The transistor is in a T0-3 case and is mounted directly on to the aluminum box that will house the charger.  The case of the transistor is the collector and grounded to the case of the charger.  That takes care of the heat and establishes the ground for the transistor.  The negative side of the full wave rectifier filter is also grounded.  My thinking is that the current path is through the ground or negative side of the circuit.  And the transistor conducts based on the biasing of the emitter.  The base input is fixed.  So the current flow is regulated in the collector-emitter flow.  Changing the bias on the emitter allows more or less current to flow through the transistor and circuit.

Here comes the hard part, the emitter of the transistor is hooked up to a 25 ohm potentiometer or pot as we call them.  That is a variable resistor.  The center tap, or wiper of the pot is hooked up to ground along with the one end of the pot.  Basically that allows the pot to vary the resistance in the emitter circuit to ground.

A light emitting diode (LED) is wired in parallel with the transistor base and goes to ground.  The LED conducts and lights up indicating the circuit is active.  So it shares the 10K resistor with the base of the transistor.  LEDs are current deivices and usually need drop in the avialable current to keep it in the bounds of operation or it will burn out (open).  So the resistor does double duty.

The positive output has a blocking diode in it to prevent back voltage from the battery pushing back into the circuit.  It simply allows positive voltage to flow one way to out put terminal.  A simple protective device and probably costs you a volt or two drop across the diode.   Use one rated about say 40 volts, you can higher but it costs more for the part.

The negative output is wired directly to ground.  That is the case as well as to the negative out put of the rectifier.

I put a jumper in the positive output circuit.  I use my VOM to measure and set the current flow.  One varies the pot and reads the output.  To measure current one must insert the meter in the circuit.  So I set the current value I want and then hook up the battery to be charged. It can charge up to 20 cells.

The circuit allows the 25 vdc to "float."  As long as the voltage output exceeds the voltage of the battery at full charge the battery does not care.  Of course, excessive voltage is not a good thing but 25 volts is acceptable.  The batteries all have rated capacities in milliampere hours or ampere hours. 
One divides that number, say 800 milliampere hours, by 10 which give the ideal trickle charge rate or basically a tenth of the batteries current output capacity.  This is known in the battery chemistry world as C-10.  All NiCd and NiMh batteries can easily tolerate that rate.  Fast chargers use higher rates but have a voltage cutoff circuit so that when the battery is at full capacity as measured by voltage, it will shut down.  But fast charging can be damaging to the battery chemistry.

If you want you expensive batteries to last a long time, you trickle charge them.

My problem has been to determine the polarity of the LED.  I am pretty sure I have that down now,  The anode must be connected to the positive source of power in the circuit.  The gentleman had it connected to the base of the transistor.  Thus the 10,000 resistor does double duty, one for the base and one for the anode of the LED.

So I think I have reconstructed the circuit.  Not to fix it.  I know it worked as the old worked a long time.  I think something got kicked or trampled and did damage to the old charger.  Anyway  it currently does not work.  So I am reverse engineering circuit so to speak.