On Dec 1, 7:01*am, Bruce Esquibel wrote:
klem kedidelhopper wrote:
I need to replace this TO220 regulator in a power supply I'm working
on. The original is a "CT" suffix however looking through the books I
see that there are several other versions of this part. The original
seems to be a 1.0 amp device while several others appear to be able to
handle 1.50 amps. I would like to buy a few extras to have on hand.
Aside from any appreciable price difference, would it make sense to
buy a bunch of "CT's" that will only handle 1.0 amp when there are1.50
amp parts available that will do the equivalent job? * Thanks, Lenny

We used to use that LM78xx family in the 80's, a lot of them. The 05, 08,
12, 18 and 24's.

I don't remember a 1.5A version in the TO220 case, but technology marches on
as they say.

One hint, they aren't rocket science. It's a cheap regulator that does the
job simply, but I'm unsure if better ratings would give better results.

The main problem we found with those is the source voltage, I think they are
all rated with Vcc max at 36v, even the 5v ones. There is a min of something
like 3v over the regulation voltage (7805 needs min 8v in, the 7812, 15v and
so forth). The closer you can get to that, the better the life span.

The heat sink is also important, it needs one. I think the data sheets from
National said something about 100ma-250ma "free air", and the full 1A draw
with some minimum postage-stamp sink, but don't beleive it.

They are/were supposed to be over-current and short circuit proof, but they
do pop like firecrackers when they let loose.

One handy trick with them, if you lift the ground pin and insert a pot or
some diodes, you can make it regulate at different voltages, keeping all the
ripple rejection and other specs within specs. So a 7805 with a 30v source,
a few caps and like a 500 ohm pot makes a quick and dirty 5v-24v variable
regulated power supply, 1A max as usual.

I don't know what you found with the 1.5A rating but it would just be cost
considerations, if there is a 10 cent difference, fine, if it's $10, forget
it. This is a 30 year old comment that probably doesn't apply but we did
find differences depending who made them. Both the National and Motorola
versions were fine, the ones by SGS stunk. There was another by Fairchild
but I think they were SGS's with a re-ink job on them.

-bruce


That's very interesting about the 500 ohm pot. I've never heard of
doing that before. I would have thought that the regulated voltage
would have been dictated by some internal zener diode or something.
But in any case I have a bag of 7805's and I'm going to experiment
with them. As far as the capacitors go are you talking about using any
additional from the usual input and output caps? Lenny.