[SOLVED] Simple zener diode question

Hi all, i have currently work on a solar charger circuit which the circuit is shown below.

But kinda not understand is,the zener diode is act like cut off voltage which cut off the charger voltage when the battery is 6.8v which is already full. The concept is when the terminal voltage is full which is 6.8v then the voltage can pass through the zener voltage and cause the transistor open and cause the input voltage to ground.

But..have a question here..the input charging voltage is always higher than 6.8v in order to charge the battery what? wont the charging voltage pass through the zener voltage and make a wrong sense about the battery is already full too?

Your circuit works like a stabilizer with an output voltage (appr.) 6.8 + 0.7 v.
Use a bistabil control element to make a permanent cut off.

i update the original link here : Solar Charger Circuit

May i know how is my question answer will be? cause i nt very know about bistabil control element =)

Reik, you are correct but not fully understanding how the battery works.

If you disconnect the battery, the circuit works as a simple voltage stabilizer. As the output reaches 6.8V + 0.7V (the base-emitter voltage of T1) the transistor starts to conduct and it pulls the ADJ pin of the LM317 nearer to ground. This in turn drops the output voltage and the Zener stops conducting. In reality what happens is the circuit finds a balance where ZD is just conducting enough to keep enough voltage to keep itself conducting - in other words it stabilizes the output.

When the battery is attached something different happens. Unless fully charged, the battery voltage will LESS than ZD so the transistor does not conduct and essentially does nothing. Now the voltage is set only by the adjustment of VR. The LM317 voltage rises to whatever level VR lets it reach and current is allowed to flow through the battery to charge it. As the battery reaches full charge, in other words near to ZD+Vbe the Zener and transistor start to conduct again and prevent the voltage rising any further and overcharging.

It might help to think of it this way: The battery drags the voltage down as it draws charge, as it fills up, ZD and T1 put the brakes on to prevent it overfilling.

Brian.

OH ! so u mean the output voltage is not that high due to the voltage has been drag down by the battery? Am i correct betwixt? =)

Correct !

The resistor R3 is to allow the battery to be at a lower voltage than the regulator without is drawing too much charging current, this protects the LM317 and the battery. As the battery charge increases, the current it draws gets smaller and the voltage dropped across R3 gets smaller as well. The reason R3 is rated at 1W is so it can dissipate enough heat when a completely discharged battery is attached and the current is high.

Brian.

WOW ! Im totally understand! Thanks Brian. Last more question, if my solar input the current is very low, is just around 150mA. So do i still need the R3 ? =)

You might be able to remove it but I would recommend you leave it in place. Without it you could still see a problem if the battery was completely discharged because the LM317 would see a short circuit across it's output and might not operate properly. Besides that, it gives you the option to increase the size of your PV panel in the future without needing to fit the resistor again. I've got 30 A coming out of mine!

Brian.

ok ! thank u !!!