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[SOLVED] Emergency light circuit explaination

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Full Member level 4
Sep 3, 2011
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Bangalore ,India
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Hello Everyone!

I came across this circuit in Electronics for you Magazine.
LED emergency light.png
Can someone explain me how that 6.8V zener is stopping the battery from overcharging.

Also why do we need the 15ohm/5W resistor along with the diode.

When battery voltage rises above (6.8V + 0.6V/0.7V) = 7.4V/7.5V, the ADJ pin is pulled to ground, and the LM317 output is around 1.25V. 1.25V won't charge the battery. So, charging is stopped.

I think the 5W resistor is for current limitng. It looks like this circuit was taken from EFY. You should read the entire article. You might find your required information there.

Hope this helps.

Thanks Tahmid!

Indeed the circuit is taken form EFY.

Yes in the document it is given that resistor is used for current limit.

Why do we need current limit resistor when we are capable to vary the charging voltage.Also we have overcharge protection[by sensing the voltage across the battery terminals].

How do we calculate the value of the current limit resistor if at all necessary in this case.

The value of the resistor will depend on the LM317 output voltage. The resistor is there mainly to limit charging current when battery is (nearly) fully discharged, during which time charging current will be the highest.
6.75V to 7.2V is a common charging range. The higher the charging voltage, the higher the charging current. However, don't try to charge the battery too fast.
Also how do you do measure the charge state of sealed lead acid batteries.

During charging they will show terminal voltage which is different from their actual terminal voltage.

One trick I used was that, during charging after some time (between 10 seconds to 1 minute), I turned charging off, waited some time and then measured the voltage. This was of course done by a microcontroller that then decided if charging is to be resumed.

You can also estimate the state by measuring the charging current.

You could stop charging, connect a load (like a lamp) for 2-5 seconds and then measure the voltage.

It seems that it's a general technical problem to measure the state of charge while charging.Does this problem happens for all kind of batteries?

In that case this circuit is not very good but otherwise we do have to consider the cost factor involved in developing a microcontroller based emergency light.

Do you have any working circuits?

It's not too big a problem at slow charging rates. If you're charging at C/10, it might not be a problem.

You could do one thing.

Adjust the output of the LM317 so that the 12V battery is charged at 13.5V (or 6V battery at 6.75V). Then, the battery will essentially be "float" charging. It can be left charging without being damaged. So, you won't have to turn charging off and so won't need to check battery voltage.

Hope this helps.
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