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BrianG · 12:23 PM

Made a V2 charger today. Similar concept except a little better:

The same basic constant current scheme, but the thermal shutoff is accomplished via an LM339 comparator. The LM339 actually contains 4 independent comparators, but I am only using two of them. I used a diode in the positive feedback loop to accomplish the latching. Resetting the latch is done via a NO momentary switch. Also, this comparator has "rail to rail" output capability, which simply means the output can go "almost" as high as the supply and "almost" as low as ground. This is due to the open-collector output, but does need that pull-up 1k resistor. Most other comparators/op-amps have push-pull outputs and cannot swing the voltage nearly as much.

Operation is pretty simple. The 470 resistor and LED provides the bias for the TIP120 transistor creating a constant current of around 310mA under normal operation. The two 47k voltage divider resistors set up a reference voltage of 1/2 supply voltage to the inverting input of the LM339. The thermistor, 56k resistor, and 10k pot set up another voltage divider to the non-inverting input of the LM339. When the thermistor is cool, the LM339 output is low, which means the 2N3904 transistor is off, so the constant current circuit functions. Once the thermistor heats up, the voltage at the non-inverting pin of the comparator exceeds the reference voltage and the LM339 ouput goes high (through the 1k pull-up resistor), which turns on the 2N3904 transistor pulling the collector to ~0.1v thereby shunting the LED close to ground, and shuts off TIP120. The 1N914 diode provides an isolated feedback loop to keep the LM339 latched. This avoids having the charge come back on one the thermistor cools. Using the reset switch brings the + pin back low resetting the LM339.

Improvements in this circuit design:

- Fewer components per charge bank. Last design had 17 for one charge bank, this one has 29 for two banks.

- Uses a normally-open momentary switch which are much easier to find than normally-closed types.

- The use of a comparator IC makes the temperature trip point independent of the supply voltage, so there is no need for a tightly regulated supply.

- Used a Schottky diode for battery isolation, which provides a lower voltage drop than a regular diode. This allows me to use a lower supply voltage for better efficiency.

- More effectively shuts off charging since it brings the base of the TIP120 to ~0.1v (last design brought the base to ~0.8v). The last design still had a ~1mA charge current even when the TIP120 was off. This design still has a charge current, but is miniscule being around 45uA (that's 0.000045A). I don't think the TIP120 could ever be completely shut off (short of removing power) since there are a couple integrated bias resistors as it is a Darlington transistor.

I gutted an old RadioShack charger to make this version:

	(#14)
BrianG RC-Monster Admin Offline Posts: 14,609 Join Date: Nov 2005 Location: Des Moines, IA	04.20.2010, 10:15 PM Made a V2 charger today. Similar concept except a little better: The same basic constant current scheme, but the thermal shutoff is accomplished via an LM339 comparator. The LM339 actually contains 4 independent comparators, but I am only using two of them. I used a diode in the positive feedback loop to accomplish the latching. Resetting the latch is done via a NO momentary switch. Also, this comparator has "rail to rail" output capability, which simply means the output can go "almost" as high as the supply and "almost" as low as ground. This is due to the open-collector output, but does need that pull-up 1k resistor. Most other comparators/op-amps have push-pull outputs and cannot swing the voltage nearly as much. Operation is pretty simple. The 470 resistor and LED provides the bias for the TIP120 transistor creating a constant current of around 310mA under normal operation. The two 47k voltage divider resistors set up a reference voltage of 1/2 supply voltage to the inverting input of the LM339. The thermistor, 56k resistor, and 10k pot set up another voltage divider to the non-inverting input of the LM339. When the thermistor is cool, the LM339 output is low, which means the 2N3904 transistor is off, so the constant current circuit functions. Once the thermistor heats up, the voltage at the non-inverting pin of the comparator exceeds the reference voltage and the LM339 ouput goes high (through the 1k pull-up resistor), which turns on the 2N3904 transistor pulling the collector to ~0.1v thereby shunting the LED close to ground, and shuts off TIP120. The 1N914 diode provides an isolated feedback loop to keep the LM339 latched. This avoids having the charge come back on one the thermistor cools. Using the reset switch brings the + pin back low resetting the LM339. Improvements in this circuit design: - Fewer components per charge bank. Last design had 17 for one charge bank, this one has 29 for two banks. - Uses a normally-open momentary switch which are much easier to find than normally-closed types. - The use of a comparator IC makes the temperature trip point independent of the supply voltage, so there is no need for a tightly regulated supply. - Used a Schottky diode for battery isolation, which provides a lower voltage drop than a regular diode. This allows me to use a lower supply voltage for better efficiency. - More effectively shuts off charging since it brings the base of the TIP120 to ~0.1v (last design brought the base to ~0.8v). The last design still had a ~1mA charge current even when the TIP120 was off. This design still has a charge current, but is miniscule being around 45uA (that's 0.000045A). I don't think the TIP120 could ever be completely shut off (short of removing power) since there are a couple integrated bias resistors as it is a Darlington transistor. I gutted an old RadioShack charger to make this version: Last edited by BrianG; 04.21.2010 at 12:23 PM.