Cheap charger for Tadiran cells
This is the recipe for making a charger for N Tadiran cells in
parallel. It may charge the AA cells as well as the newer 2/3 AA ones.
It's principle is to have N independent current sources, and to
monitor the voltages of the N cells by comparing against a reference
voltage (3.4 V) provided by a voltage source. When this voltage is
reached the cell is disconnected using a simple relay.
Ingredients
For N cells, you will need the following ingredients :
- N+1 LM 317
- (N for the current sources, plus 1 for the reference voltage)
- 1 LM324 for 4 cells
- (the LM324 contains 4 op amps, which will make the 4 comparators).
- N relays, 9V or 12V
- (take the cheapest your local dealer has)
- N adjustable res, 100 ohm
- N res, 220 ohm
- (optional)
- N LEDs, 220 ohm
- (optional)
- 1 adjustable res, 1k ohm
- 2 res, 1k ohm
- N push switches
- (well, I made mine out of paperclips, because it was late and the shops were closed)
- 1 piece of cardboard
- (you make holes in it and plant the components and it looks very professional, too)
A nice schematic (if you can't see it try clicking here or here)
![Nice schematic of the charger](charger.png)
The top part is to replicate N times to get a charger for N cells.
How it works
The bottom part is a constant voltage sources, which places at point A the switch-off voltage.
The top part is mainly a constant current source (left). Initially the
battery is discharged, so voltage at point B is lower than at point
A. You press the pushbutton, which brings this voltage to point
C. This brings the voltage at the output of the comparator to a value
close to the alimentation, which switches on the relay. You may
release the pushbutton. The voltage at point B/C will raise as the
battery charges, and when it reaches the voltage at point A, the
output of the comparator will go to 0V, which will switch off the
relay and stop the charge.
Adjusting and tuning
- Before powering up, place the 100ohm adjustable somewhere on the middle position.
- Fist power up without the Tadiran.
- Adjust the resistance to get 3.4V at point A.
- Place an amperemeter (?currentmeter?) where the battery should go (between point B and ground). Initially no current should me measured.
- Push the button. The relay should close.
- Adjust the 100 ohm adjustable to get the desired current (from 30-40mA for slow charging a 2/3 AA Tadiran, to 120mA for fast charging a AA Tadiran).
- That's it. Now you may replace the amperemeter with the Tadiran, push the button again, and wait. The voltage at point B/C will raise as the battery charges, and when it reaches the voltage at point A, the comparator will switch off the relay.
Most relays can open/close more than one circuit. In this case it is very helpful to have a LED (in series with a 220 ohm resistance) on the other circuit, so that the LED shows when the charge is finished. Maybe it even works to wire this led in parallel with the relay, at the output of the comparator. Who knows.
About the LM317
The LM317 basically maintains a constant 1.25 V between pins 1 and 2.
Hence,
1/ The current of the current source is given by
1.25 Volts
I (Ampere) = --------------
Radj
2/ The voltage of the voltage source is given by
1k+Radj
U (V) = 1.25 ( 1 + --------- )
1k
The pinout of the LM 317T is as follows:
--------- _________
> O < 3 [ ] 2
--------- O------[ LM317 ]------O
| | PIN 1 - ADJUST [_________]
| LM317 | |
| | PIN 2 - OUTPUT | 1
--------- |
# # # PIN 3 - INPUT O
# # #
1 2 3
About the LM324
My favorite quad op amp. It works with a single alimentation.
Here is its pinout (view from top).
---0---0---0---0---0---0---0--
[ 4o 4- 4+ gnd 3+ 3- 3o ]
[ ]
> ]
[ ]
[ 1o 1- 1+ VCC 2+ 2- 2o ]
---0---0---0---0---0---0---0--
VCC may range from 3 to 30V.
Florent de Dinechin
Last modified: Mon Sep 27 09:40:46 MET DST 1999