I've built this regulator circuit to give me 7.5v from 12v. It works, but it gets very hot, not just hot, like i'd be worried about it melting through things. I've drawn a diagram showing the values of components I used, can anyone suggest any improvements that might make it run cooler/more reliably, it's for the car so I want to be able to just put it in and leave it alone. Maybe the R values I used were too small? The 135R is just 5 27R in series. Thanks.
i guess it's the regulator itself that gets hot... how many Amp's are you pulling out, and what cind of cooling do you have?
The 27R resistor is far too low, it's taking 46mA alone. Use around 200R-500R. A 240R and 1k2 will give you 7.5V output. Apart from that, temperature rise depends on the current you're using, watts = current x 4.5. Over 1W and you need a heatsink.
Thanks, I thought the resistor values would have something to do with it. I have put a heatsink on it because when I used it without it would cut out, then come back on when it had cooled down (I assume the 317T has a thermal cut out). Just a standard clip on heatsink with some AS on there. Anyway, will try some bigger values tomorrow and report back, thanks!
Yes, the 317 has thermal and current overload protection, which makes it a tough little bugger to kill without doing something like having it get hit by lightning or smashing it with a hammer, although, too be honest, the 317 doesn't have that much defense against hammers.
well.. the TO220 casings can take quite a beating... but a hammer might be a little bit too hard for it..
Don't rely on the thermal protection though, I decided that I wouldn't mount the heatsink while I was testing a circuit (for some reason ) and after a minute or so it proceeded to desolder itself and drop out the PCB (though I guess that's some kind of protection ). Same has happened with a 7805 with 30V supply
the 7805 can handle up to 35VDC IIRC... but i guess you can't pull much current before it gets hot... if you pull 1,5 amps from the regulator it puts out about 20 Watts!
The 27R resistor is only contributing 0.2W to the heat so a decent sink is the way to bring the temperature down. But it's 0.2W you can reduce to 0.023W with a 240R resistor and every little helps...
Well I replaced it with a 330R and 5 of those in series, seems to have made a difference because I can actually hold the PCB now, which I couldn't do before! Still far too hot to mount it tidily in a box though, maybe i'll see if I can get a better heatsink, or stick it under the bonnet instead of keeping it inside the car, then it'll get some airflow.
