78L05 regulator is burning up with a Murata +/-15V DC converter...

[aion]

I'm using a Murata NKA0515SC to get +/-15V from a 9V source. It works and I get a stable +/-15V from it. But, within about 10 seconds the 78L05 starts to warm up and after 30 seconds it's too hot to touch. Haven't given it a chance to go any longer than that. The DC converter doesn't even get warm, just the regulator.

Here's the relevant portion of the circuit:



Basically I'm just regulating 9V down to 5V before it goes into the DC converter. The 78L05 is giving me 5V just fine—it's just getting really hot!

I'm keeping the power ground plane separate from signal ground just in case, so that's what PGND is in the schematic - it connects up with everything else at the input jack. Also note that I'm actually using 10uF in C12, not 100n as listed, though I also tried it with 100n and it was the same.

It's only driving two TL072's, so we're way under the maximum current rating of the DC converter. I've tried a couple of different 78L05s and it does the same thing with each of them, so it's not the component itself. What could be causing this?

[aion]

One more bit of possibly helpful information. I have the 78L05 and the DC converter socketed, and when I pull the DC converter, the 78L05 runs cool. So it's something about the DC converter. But looking at the datasheet, it doesn't seem like there's any good reason it'd be sucking that much current. And I've seen a couple other circuit examples using a regulator with a DC converter just like I am...

[PRR]

So how much is a few '072s? I'll pencil 10mA.

Back-jacking from 30V to 5V is 6:1 ratio. So there's gotta be 60mA at the 5V point.

The converter is 80% efficient at best, near full power, less at part-power. I'll pencil 67%. So current must be 1.5X, 90mA.

The 7805 drops 9V-5V= 4 Volts.

4 Volts times 90mA or 0.090A is 0.36 Watts.

0.36 Watts in a TO-92 package is HOT. (I call them 0.3W blobs, though newer sheets show a little more power handing.... by letting them get HOTter.)

I say working as expected.

If my guess of '072 drain is wrong, the results be wrong. Stir your own numbers.

[PRR]

> I'm just regulating 9V down to 5V

And throwing-away 44% of your input power as heat.

Or: whatever heat is in your '072s, there's about 0.8 as much heat in your 7805.

Note that a TO-92 is much smaller than two 8-DIPs. So same (or 0.8) as much power in the smaller package is a much higher temperature rise. Imperceptable heat in two 8-DIPs can be flesh-stinging heat in a TO-92.

The TO220 version of that regulator is the best fast-hack.

If energy consumption is an issue (as in battery work), a switching regulator may be wanted.

I can't think of much that needs +/-15V which can't be re-rigged to run on +/-12V, even +/-9V (which means only jacking-down one rail, not both).

[greaser_au]

have a look at:
http://www.diystompboxes.com/smfforum/index.php?topic=100385
and..
http://www.diystompboxes.com/smfforum/index.php?topic=97310.msg858029#msg858029



best wishes,
david

[peterg]

Have you tried an LM7805? Its a lot bigger than the 7805 but I used one in place of the 7805 that was overheating and it solved the problem.

[jk-fm]

TL072 will take up to 36V, could you use a standard doubler/inverter on the 9V?

[aion]

Really appreciate the explanations and advice, all of you... I had no idea it would multiply quite like that, but Paul's math makes perfect sense. I think I'll switch to the 7805 and call it good.

[R O Tiree]

Beware - the pinout is different between the TO-92 package and the TO-220 one.

[bluebunny]

Beware - the pinout is different between the TO-92 package and the TO-220 one.

Yep, spin it around - if the T092 package was "facing" you originally (flat side towards you), then you want the TO220 facing away (writing on the other side, metal tab nearest you).