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LM7812 and LM7805 Linear voltage regulator

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Kranthikkenoch

New Member
Hi all,

I have a 36V 10amp battery pack and a small switching circuit. I need 5V to power my microcontroller and 12V to power my mosfet.
Can anyone suggest me if I can connect my 36V battery to LM7812 and get 12V required or my input voltage is too high?
 
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kubeek

Well-Known Member
Most Helpful Member
What kind of microcontroller needs 5W of power? :eek:
What current do you need for the 12V? Please show your circuit, it may be possible that you only need the 5V.
 

Kranthikkenoch

New Member
sorry to mention 5V 1amp, I'm using atmega32 MCU. My question is I need 12V for mosfet and I have 36V battery pack.
Can I use LM7812 for which input is 36V will the IC survive if the current is 500mA?
 

JLNY

Active Member
The LM78XX series is rated to a maximum input voltage of 35V, so yes, your 36V battery pack is too high. On top of that, all of that voltage drop from 36V to 12V or 5V will be burned off as heat, so the power dissipation could also be a concern depending on how much current the circuits will draw.

Would there be any way to configure the battery pack for a lower voltage, such as 18V? 15V might also work depending on how much the voltage of the pack drops over its discharge range. If the voltage drops any lower than 14V, you may run into issues with the dropout voltage of the 7812.

Edit:
okay, so for 500mA @ 12V, that would be a voltage drop of 24V, which would be 12W of heat dissipated at the regulator. This would probably overheat unless you had reasonably good heatsinking on the 7812. For an 18V pack, the dissipation @500mA drops to 3W.
 
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kubeek

Well-Known Member
Most Helpful Member
You can turn on some mosfets with 5V or even less, are you sure you need 12V?
 

tomizett

Active Member
Some LM78 regulators have a max. input of 35V, and for some it's 30V... always check the manufacturer's datasheet for the exact part you're using!
 

JLNY

Active Member
Actually, thinking about it some more, depending on the voltage range/regulation requirements of your MOSFET circuit, it might be possible to use a 12V pack directly and eliminate the 7812 regulator altogether. Depends on the application, though.
 

cowboybob

Well-Known Member
Most Helpful Member
Just but two of these of your local ebay.

Mike.
Those are outstanding buck PSs. I've used them for all manner of projects, especially battery driven units. And their efficiency is pretty much as stated (≈92%).

They do take up a fair amount of space (vs. a 7012 or '05), but that never bothered me, given its advantages...
 
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