12V Output Powers Supply design question

Thread Starter

James 26

Joined Mar 26, 2020
Hello all,

I am building a power supply that has a voltage regulator that will have up to 1amp flowing through it.

Is it necessary to include a heat sink?

Your input much appreciated,




Joined Mar 19, 2019
My crystal ball is a bit cloudy... How about a schematic and part numbers? Most 78xxx regulators will need a heatsink to reach max current output. If not their internal protection will shut them down before getting there.


Joined Oct 2, 2009
Calculate the power dissipated by the voltage regulator and then calculate or use an on-line calculator to determine the temperature rise.


Joined Feb 24, 2006
Just in case you don't know how to do that:
  1. Subtract the the output voltage Vout from the input voltage Vin
  2. Multiply that voltage difference by the current to get watts of power to be dissipated
  3. Multiply watts by thermal resistance to get temperature rise.
  4. If the temperature rise means the junction temperature will be at 125 °C or greater, I would be sure to use a heatsink

Thread Starter

James 26

Joined Mar 26, 2020
So have tracked down the data sheet, see attached screenshot for relevant values:


my circuit has 16.8v dc in 12v dc out.
1 in/out
4.8 watts disapated = 288 degrees if I use the value of 60.
So I need a heat sink.

I assume this is right? Thoughts?

Thread Starter

James 26

Joined Mar 26, 2020
So I have build the circuit, Decided to stay with the LM2940C regulator.

I have 12V out but for some reason there is no current flow to charge the battery, have tried other loads including light bulb etc and that works fine.

Thought the battery might have been too heavy a load for regulator (1 amp limit) so tried creating some parallel branches to decrease load but that didn't work either.

I have had issues with this battery only being able to charge on certain chargers, some electrical characteristic of the charger maybe? impedance matching issue with the battery?

Your help much appreciated,



Joined Jan 29, 2010
hi James.
You cannot really charge a 12V battery with a 12V supply.
If it is a lead acid 12Vbattery, you need approx 14V charger.