TO-220 Regulator Question

[chrisaxeman]

Hi Everyone,

I'm screwing with power supply designs.

Anyone have any idea how much current draw a T0-220 regulater(78xx)series can stand before I have to start using  a heatsink?

Every other design I built has works fine with the 'sink,but I've gone nowhere near the 1amp rating of the regulators-I could save some space and simplify the build without 'sinks.

Are they a must for currents of 200-250ma max?

Cheers

Chris.......

[chrisaxeman]

Hi Everyone,

I'm screwing with power supply designs.

Anyone have any idea how much current draw a T0-220 regulater(78xx)series can stand before I have to start using  a heatsink?

Every other design I built has works fine with the 'sink,but I've gone nowhere near the 1amp rating of the regulators-I could save some space and simplify the build without 'sinks.

Are they a must for currents of 200-250ma max?

Cheers

Chris.......

Anyone?

[Paul Perry (Frostwave)]

There's a lot of variables here..... in particular, the power that the regulator dissapates, is the current multiplied by the voltage drop across it. So if you have 200mA, and 12V input, and 9V output, that is .200 X 3 =600mW.
But if the input is 15V, that's 1,200mW!!
But, there is a minimum drop you need across the regulator for it to work....

And, as far as a bare TO220 is concerned, the only way the heat can get out is by radiation and by conduction. It's not going to conduct much via the legs (but give it big pads, it all helps). Plus convection (air moving past it) which is better if therei s room for the air to move.
As for the radiation, that depends on how hot the surroundings are in the box.

What helps & is free?
1. drill holes in the box. And the bottom of the box. And put it on feet.
2. bolt a chunk of scrap metal to the TO220 case (must be smooth to make a useful thermal contact).
3. minimum workable raw input voltage.
4. read the application notes for the regulator.

[Peter Snowberg]

Try LDO (low drop-out) regulators like the LM2940. They waste less energy in the process of regulation as comparred to good old 78XX series parts.