power supply heat sink question

[mikepukmel]

Hi All!
So I got a used atmel programmer/dev board a few years ago, no power supply. I got some parts from ebay and made a 5v ps. The cheap ps kit came with LM340 5v 1a and LM7812 12v 1.5a regulators, and the smallest heat sinks I ever saw. Idle it was OK but as soon as I tried to do any activity with the programmer board, the heat sink would get piping hot too hot to touch. I dug around, found bigger heat sinks that fit the ps board (can't remember think it was mouser). They are over 3x the height, and probably 10x the mass. The sinks don't get too hot to touch but I think still way too hot, although I don't have any way to measure the temp.
I keep an eye out for throw away electronics, and lucked into an old Samsung stereo system. A cheapie, not worth fixing. But the audio amp box has two HUGE heat sinks. Big fins, The back of the heat sinks are probably 3" tall x 8" wide, with big heavy fins on the other side.
I have not the slightest idea what size to cut these down to for this 2 x LM340 (double) power supply. I have a couple of old computer power supply cases that Im going to mount the boards in, it has a fan iec power connector and switch. (benfits of rooting through the trash).
If I cut the heat sink the width of a standard ATX power supply, would that do it for both the 5v and 12v regulators?
Thanks
Mike

[Helmholtz]

If your regulators get too hot you might have too high regulator input voltage. Regulator dissipation is proportional to the DC voltage difference between input and output.

[mikepukmel]

Hi Helmholtz, thanks. I don't think I did something stupid, but I don't know what I don't know.
The 7812 is running on a 16v ac rms secondary transformer I measured 18.98v ac. at the transformer. I think our house voltage is a little on the high side. After the diodes I measure 24v. DC This is all no-load.

The data sheet reference for the 7812 is Vi = 19v. Output is 500ma. The absolute max input on the data sheet is 35v.
I did poke around on the internet, and suggestions were to not let the input voltage go "to low" or the regulator could not work.

[mikepukmel]

They list dropout voltage as 2v. This isn't absolute input voltage, its difference between O and I, right? So, if output is 12v for the 7812, it will fail to regulate to spec when the input is 10v or lower?

EDIT: OK I just reread articles on stack exchange re drop out. So, for this linear 12v reg, it needs at least 14v to regulate. So I don't want the input to drop to near 14v under load. But how much it drops, eek, don't know. I haven't measured with load. Will do that tonight.\

EDIT again: Im not quite up to the calculations, but I think the input should be closer to 16 or 18v but not 24. So, 12 or 13v ac secondary?

[J M Fahey]

Your post is confusing because you do NOT tell us about the actual LOAD needs.

What does the "atmel programmer/dev board" need, as far as voltage and current?

Describing ONLY the supply bu not the load is half the data we need.

Thanks.

[mikepukmel]

Thanks JM looking . . .

[mikepukmel]

Ah, says 500ma min. So, Im assuming that would power the board and be able to power one chip, but not external stuff. So I thought 1.5a would be good enough for the task, and if I wanted to, say, run a camera, Id put that on its own ps.