Hello all,

I have a 15 watt solar panel and am trying to build a battery charging circuit to charge a 6v 3.5am battery. I found a circuit online and have built it on a breadboard. The circuit that I built is located below

 

The max charging current when hooked to a power supply is 15mA and I am having problems with both led's being on at the same time. I cannot get the led's to swap illumination. One led will get brighter then the other but the other led remains on dimly.

I have been playing with this circuit on a breadboard for over a week now with no satisfaction. Any help would be greatly appreciated, thanks

 

For a start the 741 is not a good choice for this circuit. It needs higher supply voltage and its output will not go fully to either the positive or negative rail hence the LEDs do not go out.
Try with a TS912. It is a dual amplifier so it is not pin compatible with the 741, so you will have to do some rewiring.

 

You have been beating your head against a brick wall.
The problem is that the circuit can't work as shown.
The errors include wrong op amp, wrong feedback connection, and wrong LED designation.
Apparently it is just a paper design that was never actually built or even simulated.
I suggest you not build any more circuits from that website.

Below are modifications to the circuit that should work.

I changed the op amp to a single supply type that will operate in that circuit whereas the 741 won't.
Notice you now don't need the zener diode at the op amp output.

I also changed the feedback to positive from negative (by adding R6 and moving the feedback form the minus input to the plus input) to get the proper snap action comparator response with hysteresis you want from the op amp.
The value of R6 determines the difference in battery voltage between the charging stop and the charging restart (the hysteresis).
5k gives about 0.5V difference.
Increasing the value of R6 will increase this difference.

The two LED functions were also reversed.

Let us know if that helped.

 

There may be a problem with either of those op-amps as the output won't go near enough to the positive supply rail to fully turn off LED D3.
That's why I chose the TS912 as it definitely will.

 

Thanks for the replies guys

I am newbie and have been beating my head of the wall with the led's and the low output. I have checked and rechecked every component and connection numerous times

Will swapping out the op-amp to another variety cure the low output to the battery also? 15mA is is not enough to keep the battery charged. I would like to get to 300-350mA range

 

There may be a problem with either of those op-amps as the output won't go near enough to the positive supply rail to fully turn off LED D3.
That's why I chose the TS912 as it definitely will.

The LM324 output should go high enough to be above the forward drop of the LED, thus D3 should effectively turn off.

Certainly the TS912 should work well also but it's more expensive and not as commonly available.

 

...............
Will swapping out the op-amp to another variety cure the low output to the battery also? 15mA is is not enough to keep the battery charged. I would like to get to 300-350mA range

The trickle-charge current after the charge circuit shuts off is determined by the value of the 470 ohm resistor.
Just reduce its value to increase the trickle-charge current.
You many have to experimentally change the value to get the desired current.
The value will likely be less than 100Ω.

The normal charging current (when the transistor is ON) is affected by the pot R2 which controls the LM317 output charging voltage.

 

The normal charging current is only 15mA when the output voltage of the lm317 is set to 7.2 volts. How can I increase the current during normal charging?

Would I be better off scraping this circuit and finding another more favor design?

 

Would I be better off scraping this circuit and finding another more favor design?

The problem is that the circuit can't work as shown.

I thought that question was answered properly by crutschow.

 

The normal charging current is only 15mA when the output voltage of the lm317 is set to 7.2 volts. How can I increase the current during normal charging?

Would I be better off scraping this circuit and finding another more favor design?

The normal charge is low because the circuit is not properly designed as I stated.

You don't need a different design.
Just make all the changes I suggested in post #4 and then get back to us if you have any problems.

 

The value of R6 determines the difference in battery voltage between the charging stop and the charging restart (the hysteresis).
5k gives about 0.5V difference.

Because of the way the 470R trickle charge resistor is connected, when the transistor across this resistor switches off, the current from the charger will fall and its voltage will rise. The voltage sensing circuit is actually connected to the charger not the battery, so this will provide extra hysteresis.

 

Hello all,

I have a 15 watt solar panel and am trying to build a battery charging circuit to charge a 6v 3.5am battery. I found a circuit online and have built it on a breadboard. The circuit that I built is located below

I see a problem trying to run the charging circuit off of the battery you are charging. Run the op amp and the reference zener off of the +12 V line. LM741 will probably be okay but I agree an LM358 would be better in some ways.

 

I see a problem trying to run the charging circuit off of the battery you are charging. Run the op amp and the reference zener off of the +12 V line. LM741 will probably be okay but I agree an LM358 would be better in some ways.

Good point.
As is, if the battery voltage is too low it may come up in the trickle charge mode instead of the full charge mode.
Everything but the 10k pot and transistor should be connected to the +12V.

But I don't think the LM741 will work properly even then because its output can't go to 0V, unless some additional level shifting zeners are added.

 

Just wanted to thank everyone that has given me advice. Think that I know what to do to fix the circuit, just need to find or buy a LM358 to complete the changes and see if the circuit works.

I'll let you know how I make out, thanks again guys

 

Just wanted to thank everyone that has given me advice. Think that I know what to do to fix the circuit, just need to find or buy a LM358 to complete the changes and see if the circuit works.

I'll let you know how I make out, thanks again guys

I don't think the implication was that the LM358 was the only or best op amp to use. Just that the LM741 was the worst choice. Most any good op amp will work. Understand what your needs are and pay attention to the details in the data sheets. Or try the LM741 and see if it works.

 

I don't think the implication was that the LM358 was the only or best op amp to use. Just that the LM741 was the worst choice. Most any good op amp will work................

Nope. Only single supply or RR type op amps will work properly in the circuit shown.
Standard op amps require additional level shifting zeners to shut off the TIP122 transistor since their outputs don't go to the negative rail.

 

Nope. Only single supply or RR type op amps will work properly in the circuit shown.
Standard op amps require additional level shifting zeners to shut off the TIP122 transistor since their outputs don't go to the negative rail.

Yep, you are right. LM741 does not work well there. I had to build it to convince myself.

 

Yep, you are right. LM741 does not work well there. I had to build it to convince myself.

It also doesn't work properly in simulation.

 

It also doesn't work properly in simulation.

Okay, starting from scratch using an LM311 voltage comparator ...