Power Supply Issues

Well, I finally returned from working abroad and I've gotten to start actually making stuff, instead of just reading the forum while procrastinating at work. Now this means I'm having problems, however.

I decided to build a simple power supply for experiments cloned off of the GSSL powersupply (mainly because I had parts and the schematic). The 15V rails seem to work reasonably fine. I'm getting +14.8 and -14.7. It's the 12V rails that are giving me issues.

I'm using an amveco toroidal transformer to step down the 115V. I read 19VAC off of the transformer.

After the bridge rectifier, I'm getting about 26VDC and a few volts AC on my meter. This seems a little high and I believe (if I read the spec sheets right) is just at the top range of the 79L12 and 78L12's tolerances.

I checked the circuit on the scope, and the seem to be doing a good job of smoothing out the DC. I also noticed that the 79L12 and 78L12's were getting very hot. The +12V rail was reading 800mV and the -12V rail gave me 26V, or just a little less than the input.

I did try switching the regulators out and I got exactly the same result on both rails. Hmmm.

Any help would be greatly appreciated!

Thanks - Andrew

btw - has anyone noticed that the seem to have tried to make the pin assignments for these things as confusing as possible. Not only do they change the pin numbers for each leg between these, but they also change the functional designation of each pin number. Arghh.

No takers yet huh? Come on, it's been at least 10 hours. Haha. Just joking. :grin:

After further review, I'll ask another question. Does anybody know how sensitive to heat and static these chips are? How careful do I need to be while handling and soldering these components? Are there any typical precautions I should take besides touching something grounded before handling the chips and keeping the soldering as short as possible?

I ask because the more I think about it the more I'm convinced that the 12 volt regulators are fried, and if I fried two of them, I'm obviously doing something wrong.

Cheers - Andrew

If it were me, I'd be running the 7xL12's from AFTER the 7x15 regulators.

Keith

what he said. 78LXX are wimpy compared to the non L versions, they often also more noisy. if it's just an experimental PSU use 78/79XX only

i've nver destroyed a Vreg by heating them too much. it's always too much V/A that kills them. they're not sensitive to statics, maybe the MOS types are

yes, they're tough. 78/79(X)XX are the Behringers of Vregs

Thanks for the suggestions. I'll look into ordering some LM3x7's to see how those do.

Running the 12's after the 15's makes a lot of sense. Good idea.

So if nobody seems to like the L-series regulators, why are they used? What's the benefit?

It still annoys me that the circuit doesn't work, esp. because I was just trying to copy the GSSL power supply, but at least it sounds like I'm probably not killing the regulators. How much does it matter that there isn't a load on the chips? I guess I'll keep messing with it and try and figure it out. It is most likely something painfully simple and obvious.

Thanks - Andrew

the only benefits i can think of is cost and size, TO92 vs TO220. the like to be loaded with at least 5mA. some Vregs don't work without load, but 78/79s can, dunno 'bout the Ls. Also, they can easily go into self oscillation if you got bad grounding, long wires on/to the Vregs and maybe no bypass caps. depends on the mfg though