Will LM308 work with single polarity supply?

I just found this circuit of emergency light employing automatic battery charger. It uses LM308 Op-amp for over-charge protection. As far as I know and what datasheet shows, LM308 is a dual supply op-amp. So my question is, will this circuit work since LM308 is used with single polarity supply?

Link:
**broken link removed**

lm308 pin

Dear CMOS,

Voltage is measured in a differential form.
The OP AMP cannot "tell" whether it is connected to a dual power supply (For example +6V and -6V) or one power supply (for the sample example +12V).

The magic word here is bias.

Usually a dual power supply OP AMP is used when the input and output signals are not biased (DC component is 0V).
You can use a single power supply instead of dual if you bias the signals.
For example:
Using a +12V single power supply, and biasing the DC components on both entries to +6V equals to using a dual power supply of +6V and -6V, and not biasing the DC components at all.

Always remember that even the single power supply OP AMP entries must be biased in order to work within the linear zone.

Regarding the specific circuit, there is an explanation within the article:
"Battery overcharge preventer circuit is built around IC1 (LM308). Its non inverting pin is held at a reference voltage of approximately 6.9 volts which is obtained using diode D5 (1N4148) and 6.2-volt zener D6. The inverting pin of IC1 is connected to the positive terminal of battery. Thus when mains supply is present, IC1 comparator output is high, unless battery voltage exceeds 6.9 volts."

Regards,

Pazam

integrated circuit lm 308

Yes, I got your point. Thanks.

lm 308 schematics

Hi,

The LM308 is a single 8pin OP-Amp from Nat. Semi., and not a dual one.

Cheers.

ic1 lm308

I was talking about "Single Supply" and "Dual Supply" not "Single", "Dual or "Quad" op-amps.

tlc272 vs tlc2272

Any dual supply OP-AMP can be used as single supply by biasing the non-inv input at vcc/2

auto cut off using zener diode

EcraZ said:

Any dual supply OP-AMP can be used as single supply by biasing the non-inv input at vcc/2

Click to expand...

I know this trick, but I was doubtful about the mentioned circuit since input is not biased at Vcc/2.

lm308 simgle supply noninverting

The Input with the zener is acting as Vref and so the op-amp is used as a comparator.

transistor feedback to avoid relay chattering

Now that I have built this circuit, I am facing a problem. Even when the battery is fully charged, the transistor does not go into cut-off because output oof LM308 is 1.9V which is sufficient to keep it on. What do I do to turn-off this trannsistor when battery is charged to 13.8V?
I did following modification in given circuit to make it for 12V battery.
1) Replaced 0-6V transformer by 0-12V
2) Replaced 6.2V zener by 13V
Rest everything is same. I am using only the charger part not the inverter section.

lm308 power supply

Dear CMOS,

You can add a couple of 1N4007 diodes between the output of the OP AMP and the base resistor.
It should solve your problem.

Good luck,

Pazam

lm311 battery charger

Hi Pazam,
I already tried that before posting my question, but it causes relay to chatter when battery voltage reaches 12.5V . I also tried to put 3.3V zener in reverse at the op-amps output but it also had the same effect.

ca3140 drive transistor

No solutions?

Re: LM308 Question

Dear CMOS,

It seems you have a current issue.

After adding the diodes, have you lowered the series resistance ?
It should solve what you're describing.

Regards,

Pazam

Re: LM308 Question

No still there is problem

I tried removing the base resistor, put two 1N4007 diodes and then three diodes in series. The relay still chatters.

I think since LM308 is a dual supply, non-rail-to-rail op-amp, the output never go to zero volts in single supply operation. Using either single supply or rail to rail op-amp may solve the problem. What do you suggest?

Re: LM308 Question

Mmmmm....

Have you done any measurements using DVM or oscilloscope ?
More data would be useful.

When the output of the OP amp is high (10V I guess) the base current should be something like 90mA.
When the output of the OP amp is low (2V I guess) the base current should be something like 10mA.

It's a big difference.

What you should try is to forget the diodes at the base.
Try to increase the base resistor more and more.
(Use a potentiometer - It should be easier).
There should be a point were the relay will only respond to the high output of the OP amp (All depends on the relay resistance, and the amplification factor of the transistor).

About your question - rail to rail OP amp could simplify the circuit, hence solving the problem.

Regards,

Pazam

LM308 Question

Use three in4001 diodes in series at the emitter of the transistor instead of the base this should reverse bias the transistor when the out of ic is 1.9v

Re: LM308 Question

Man..........even diodes in emitter are not making it work.
I never thought battery charging is so painful. Does anyone have a tested battery backup power supply schematic? I am tired debugging the above circuit. Last try remaining is to use rail-to-rail op-amp. I'll try it tommorow and give my feedback.

Again, if anyone has a good tested SLA backup power supply schematic with charging auto-cut-off, kindly share it with me. My project is pending just because of power supply.

Thanks.

Re: LM308 Question

Tried with precision TLC272 op-amp (couldn't find TLC2272 rail-to-rail). This circuit didn't work.

So then I swithched to LM311 comparator, changed zener to 6.2V, and put a voltage divider at the inverting pin of it to divide battery feedback voltage by half. Also used a PNP transistor for driving the relay. Due to this I had to swap the NC-NO relay contacts. This worked better to a extent that it is capable of turning the relay on and off with change in batery voltage. But for very small range of voltage (20-30mV) near the threshold (6.9V), the relay begins to chatter and as voltage across battery changes very slowly, chattering goes on for a long time until comparators output permenantly changes state. Using 10uF capacitor at the base of relay driving transistor slowed the chattering but couldn't eliminate it.

Is there any techniqe to avoid this?

Re: LM308 Question

Dear CMOS,

If you're going to 6V you can use a schmitt trigger based CMOS gate (Such as the 74HC14). Put it between the OP Amp and the transistor (Don't forget to raise the value of the resistor - You don't want to over load the gate).

Regards,

Pazam

Re: LM308 Question

I think you misunderstood my problem. Due to very small difference at input of op-amp (20-30mV), the output of op-amp is oscillating between 1.9V and 14V (Vcc is still 15V for op-amp). So using hysterisis inverter at its output will make no difference.

For example, non-inverting pin is held at 6.90V using zener diode. Battery voltage is divided by two and applied to inverting pin. Now when feedback voltage (inverting) reaches 6.87V, op-amp's output starts oscillating and keeps on doing this until feedback voltage crosses 6.9V boundry. So the region between 6.87 to 6.90 volts is causing trouble of chattering. How can I eliminate this.

Thanks.