While waiting for parts, I decided to try the Dust Buster 18 volt AC charging adapter with this charging circuit.
Since it is AC, I added a 1N4007 diode and a 10 uF electrolytic capacitor after it by polarity + to cathode of
the diode. This reads over 30 DC volts unloaded, but only 24 volts connected to the regulator cut off circuit
board. The green ground wire goes to - of capacitor and to common black wire of regulator cut off circuit.
Cathode of diode and + of capacitor are jumpered to the yellow input wire of the circuit. Connecting a
variable power supply to the circuit's green jumper I set it to trip at 12.6 volts, dropping LM338 regulator
output voltage to 1 volt on red wire. Turning the voltage down to 11.7 volts turns off LED and charges again.
The 100 uF capacitor on op amp pin 2 keeps it low on power up to briefly light the LED indicating power.
If another LED indicates power, the 100 uf capacitor need not be added. I added it because otherwise a
user may have no other indication that the charger is powered up. A larger value will keep it lit longer
if you have the space for it. A cheaper BC547 transistor can be substituted for the 2N2222
Testing a 12 volt DC, 1 Amp
wall supply, it output 15 volts DC. The cutoff voltage set worked at 12.6 V,
but the restart voltage went down to 11 volts. I will probably need this supply as the original supply is
only 100 ma
and would take 40 hours
to charge 4,000 ma lithium batteries. This would only take 4 hours!
Here is the final hard wired circuit with the 100 uf startup capacitor added to indicate power on.
Used a pre-drilled and IC formatted board from Radio Shack with some lead to lead soldering.
Note that the LM338 regulator is on the side of the board in case it needs a heat sink bolted on...Yellow
goes to charging input AFTER the diode in the tool if the charger is AC. Add diode when necessary.Black
goes to common which should also be black in most tools. Only one common connection should be required.Blue
goes to the indicator LED anode(long leg). A current resistor should also be in series to common.Red
is connected to the + battery. The regulator voltage output can also be read before connecting to batteryGreen wire
is normally connected to the battery with the red wire. It can also be wired to a variable power
supply or know fully charged battery to check shutoff and restart voltages.
To set the LM338 Output voltage
: With volt meter on red and black and input to yellow, adjust the P1
trimmer to 1 + 1/6 battery voltage. Once set, the output voltage on the red wire should not vary.
The rectified charging voltage coming in to the regulator will normally be much higher than battery
voltage. The charger current should remain the same as the current the charger delivered previously.Highest DC output battery charging voltage range is 1.2 volts less than the DC input voltage................To set the cutoff voltage........................
Turn on board with yellow wire to charging input and black to common. Blue to LED to 1K ohm to black.Then...
With a variable power supply
+ to the green and - to black wires, set the cutoff voltage desired.OR
With the battery fully charged
and red and green wires to the positive, common to negative battery. Then...
If LED is on already adjust trimmer down to turn it off. Then turn opposite way until LED turns on again.
Turn trimmer one quarter turn in opposite direction. This may take several adjustments to work properly!
Cutoff voltage will turn on LED and cut voltage to the battery down to 1.25 volts on red W/O battery.
The voltmeter on right shows the actual charge output voltage(red wire) set previously above and cutoff below.
Charge voltage will return when battery voltage is about one volt less than cutoff with 1M ohm hysteresis .
A 2 mega ohm resistor from op amp pin 6 to 3 will cut voltage difference between shutoff and restart in half. Note: LED may stay on after starting a new charge cycle. If so just stop and start it again quickly.
can deliver 1.2 volts to 32 volts at 5 amps with the device reducing input 1.2 volts itself.
op amp can operate at supply voltages from 4V to 36V (either single or dual supply).Thermal Design with Linear Voltage Regulators (heat sinks)