Category Archives: Power Supplies

Universal Nicad/NiMH Battery Charger

Sometimes its more convenient to recharge batteries ‘in place’ rather than removing them and putting them in an external charger.

This device is built around a PIC12F675 (a dandy little part from Microchip). The number of cells ( 1 to 8 ) is programmed in using the one button. The cell count is saved in EEPROM the next time you power up the device.

Using just one button for all operations is a bit tricky, but easy once you try it a few times. Here is the operation as best I can explain it…

There are 2 types of button operations: Hit (less than 1 second), and Hold (over 1 second). Here is what to do.

Apply power (12 to 24 volts DC) to the jack. Then, connect the batteries.

The green light comes on (indicating power applied)
The red light flashes once for each cell.
Hit the button to display the cell count again.
Hold the button until you see one flash of the red light.
Hit the button to enter the program mode – one flash of the red light.
Hit the button once for each cell – light flashes for each hit.
Hold the button to store count and return to start[/list:u:691555831a]-or-
Hold the button until the red light comes on steady – unit will start charging.
Wait for charge to end.
Hold down to return to start.[/list:u:691555831a][/list:u:691555831a][/list:u:691555831a]
During charging the red light ‘winks’ out about once per second for each volt short of a full charge. The result is that it flashes faster as full charge is approached. A timeout is provided if full charge is not acheived after several hours.

Once full charge is complete. The red light repeatedly flashes once and a small charge current (short pulse) is applied to the batteries. Two flashes indicates that the unit timed out.

This design could use some improvements. However, it does work for me quite well at present, so I releasing the design at this time.

Print the [url=]layout[/url] at 52%


[url=]Source File[/url] (CVASM16)

[url=]Object File[/url] If your programmer gives an error, try removing the 2nd to last line used to specify the PIC type.

Copyright 2006 [url=]Luhan Monat[/url]

Constant Current Battery Charger

A simple method of charging a battery from a higher voltage battery is shown in the circuit below to the left. Only one resistor is needed to set the desired charging current and is calculated by dividing the difference in battery voltages by the charge current. So, for example if 4 high capacity (4000 mA hour) ni-cads are to be charged at 300 mA from a 12 volt battery, the resistor needed would be 12-(4*1.25)/0.3 = 23.3 ohms, or 22 ohms which is the nearest standard value. The power rating for the resistor is figured from the square of the current times the resistance or (0.3)^2 * 22 = 2 watts which is a standard value but close to the limit, so a 5 watt or greater value is recommended.

The circuit below (right) illustrates a constant current source used to charge a group of 1 to 10 ni-cad batteries. A 5K pot and 3.3K resistor are used to set the voltage at the emitter of the TIP 32 which establishes the current through the output and 10 ohm resistor. The emitter voltage will be about 1.5 volts above the voltage at the wiper of the pot, or about 1/2 the supply voltage when the wiper is in the downward most position. In the fully upward position the transistors will be turned off and the current will be close to zero. This yields a current range of 0 to (0.5*input)/10 or 0 to 850 milliamps using a 17 volt input. This produces about 7 watts of heat dissipation at maximum current for the 10 ohm resistor, so a 10 watt or greater rating is needed. The TIP 32 transistor will also dissipate about 7 watts if the output is shorted and needs to be mounted on a heat sink. If more than 4 cells are connected, the maximum current available will decrease and limits the current setting to about 100 milliamps for 10 cells. The usual charge rate for high capacity (4AH) ‘D’ cells is 300 to 400 milliamps for 14 hours and 100 milliamps for (1.2AH) ‘C’ or ‘D’ cells. For small 9 volt batteries the charge rate is 7 milliamps for 14 hours which would be difficult to set and probably unstable, so you could reduce the range to 0-20 mA by using a 750 ohm resistor in place of the 10. The charge current can be set by connecting a milliamp meter across the output (with the batteries disconnected) and then adjusting the control to the desired current, or by monitoring the voltage across the 10 ohm resistor (1 volt = 100 mA) or (1 volt = 1.33 mA using a 750 ohm resistor). The current control should be set to minimum (wiper in uppermost position) before power is applied, and then adjusted to the desired current.

The circuit (lower right) illustrates using a LM317 variable voltage regulator as a constant current source. The voltage between the adjustment terminal and the output terminal is always 1.25 volts, so by connecting the adjustment terminal to the load and placing a resistor (R) between the load and the output terminal, a constant current of 1.25/R is established. Thus we need a 12 ohm resistor (R) to get 100mA of charge current and a 1.2 ohm, 2 watt resistor for 1 amp of current. A diode is used in series with the input to prevent the batteries from applying a reverse voltage to the regulator if the power is turned off while the batteries are still connected. It’s probably a good idea to remove the batteries before turning off the power.

Copyright 2006 Bill Bowden

Generating Negative 5 Volts from 9 Volt Battery

A 555 timer can be used to generate a squarewave to produce a negative voltage relative to the negative battery terminal. When the timer output at pin 3 goes positive, the series 22 uF capacitor charges through the diode (D1) to about 8 volts. When the output switches to ground, the 22 uF cap discharges through the second diode (D2) and charges the 100 uF capacitor to a negative voltage. The negative voltage can rise over several cycles to about -7 volts but is limited by the 5.1 volt zener diode which serves as a regulator. Circuit draws about 6 milliamps from the battery without the zener diode connected and about 18 milliamps connected. Output current available for the load is about 12 milliamps. An additional 5.1 volt zener and 330 ohm resistor could be used to regulate the +9 down to +5 at 12 mA if a symmetrical +/- 5 volt supply is needed. The battery drain would then be around 30 mA.

Copyright 2006 Bill Bowden