why 7812 get damaging

[ismu]

please check bellow circuit .
this is a part of solar charger. when i connect 12v battery and 20v solar panal this circuit is working fine [removed 7812 and shitted "A" and "B"]
The problem is when i connect 24v battery and 40 V panel [series] and put 7812 same as per this circuit. then suddenly 7812 getting damage and too hot.
why this happen ?

 

[tpetar]

You have huge power dissipation in 7812 when you connect it to 40V, regulator need to dissipate voltage difference between 40V and 12V. And check 7812 maximum input voltage, if you use standard 7812 max input voltage is around 30-32V and you give 40V.

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You need higher voltage then 12V to charge 12V lead acid battery, around 13,5V-13,8V.

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What I see now, in best way you will kill 7812 regulator, maybe battery later.

 

[kripacharya]

thats because you are dramatically increasing the voltage drop across the 7812 with the higher batt/ panel. Vdrop ~ 12v upto 28v. The max current 7812 can supply without a heatsink with a 28v drop will be ~75mA. If your current draw is greater than this, then without heatsink the power dissipated will be > 2watts in the 7812, and it will get hot

 

[ismu]

No for charging voltage is not taken from out put of 7812 .just check battery is directly connected with solar pannel through mosfet. So what ever the voltage in pannel should by pass to battery[no need to worry about 13.5v for charging].
when i connect battery 24v only connected and solar is removed same time regulator goining to damage , its not deppend solar so leave it about 40v panal also.

 

[kabeer02]

check the max input of 7812 from its datasheet ,if the input voltage goes beyond it max input it will definitely get burnt..

 

[ismu]

check the max input of 7812 from its datasheet ,if the input voltage goes beyond it max input it will definitely get burnt..

Maximum voltage is 35V, But i am givig only 28V [battery ] solar is just removed at that time , without solar 7812 is getting damage

 

[kripacharya]

there is such a thing like max power rating.
Please learn how to calculate this.

Power = Voltage x current

just in case you don't know the formula

 

[ismu]

ya i got the point ,
maximum power dissipoation without heat sink is 4W .
so pd=(28-12)*I =4W so I =250mA maximum .
i think bcs this may be regulator bcme damage .


Any body know which one is the highst pd regulator IC for 12v ? then only i can solve may problem

 

[kripacharya]

maximum power dissipoation without heat sink is 4W ...
so pd=(28-12)*I =4W so I =250mA maximum .
i think bcs this may be regulator bcme damage .


Any body know which one is the highst pd regulator IC for 12v ? then only i can solve may problem

I'm curious to know where you got your 4W-max number from ? This could be anything from 2W to 20W depending on heatsink & ambient as per the specs.
Also the 78xx series is supposed to have internal thermal protection, so maybe something else is going on here - what is your 'damage' like ? Maybe you are connecting it wrongly ?

Lastly, it is not necessary to connect the input directly to the power source (28v). You could always insert a voltage drop before the regulator - say by use of a properly selected resistor.

 

[Tahmid]

Keeping in mind the large voltage gap between input and output, the power dissipation in the 7812 as well as the fact that the only purpose of the 7812 is to reduce the voltage gap for the 7805, you could use a switching regulator here to provide the 5V instead of the 7805, and you won't need the 7812 at all. With efficiency > 70% (common of switching regulators), heat won't be too big an issue any more.

You can take a look at LM2575, L4971, L4962, etc.

Hope this helps.
Tahmid.