lm7805

Using Arduino General Electronics
1

Hi guys so i made a obd socket for my car that will capture some data, I ended up using an lm7805 to regulate the charging voltage of my car which will be around 14.1volts to 5volts to run atmega328AU chip. I have also soldered in protection diode with the additional capacitors aswell although i am worried it will create alot of heat due to having to drop near enough 9volts. I unfortunately do not have the space to add a heat sink so would i be ok with adding a resistor in series with the power line going into the lm7805 to roughly drop the input voltage down to 8volts? I was thinking of adding a 13ohm resistor which should if calculations are right dop the voltage down to 8volts while the vehicle is not running/charging.?

fyi (the only space i have for any heatsink would be to place it on top of the lm7805 with paste but i do not fancy doing just incase)

2

IF you don't have room for a heat sink, what are you mounting the 7805 to? Is it just hanging on wires

Do you have all the capacitors as the data sheet requires?

If you don't have room for a heat sink, how do you have room for resistor that will generate the same amount of heat?

Paul

3

hi paul, Its a pcb that was made a while back and the lm7805 is soldered onto the pcb facing down, Their is a diode on the input that i would not mind removing and placing a resistor their instead so the input is at lower value and hopefully not create as much heat, And yes the correct capacitors are all included in the design.

Would i get away with using thermal glue and just sticking it on to the top of the lm7805 or do you think its not worth risking ?

4

How much current will the 7805 (unknown package) be handling?

5

This is 2020. We have now switching regulators that are the same size of an ancient 7805.
They don't get hot (if used within their limits).
Leo..

6

Yes man i know i do not know what i was thinking at the time of making the design, For now i will just add a heatsink and TVS diode to prevent any damage, Thanks guys

7

Hi,

RyanAES:
hi paul, Its a pcb that was made a while back and the lm7805 is soldered onto the pcb facing down, Their is a diode on the input that i would not mind removing and placing a resistor their instead so the input is at lower value and hopefully not create as much heat, And yes the correct capacitors are all included in the design.

Would i get away with using thermal glue and just sticking it on to the top of the lm7805 or do you think its not worth risking ?

Wasn't the project breadboarded and these things checked?
Can I ask why small is important?
Put it in a bigger metal case and bolt the 7805 to it.
Tom... :slight_smile: