Stumped by odd LM7815 output -- request help

use a scope instead of a meter, is the regulator oscilating? does it need better bypassing?

double check the pinout

are you sure you have an LM7815?

apply a small load to the output

one of these issues is probably going to be the answer.

Mark

Have you checked the current ratings for your rectifiers or regulator?

First: how much current are you pulling? Might you have just burnt the thing up with too much current? An 18V drop means that to dissipate 5W you only need to pull 360mA. There's a huge variation in how much power you can dissipate from a given package, but for 5W with a TO-220 with natural convective cooling I'm pretty sure you'd need a heat sink a couple of inches square.

Do you have an oscilloscope? If you're loading the output of your rectifier at all then you've probably got pulsating DC; if the average is

If your transformer is truly 30V RMS then with zero-drop diodes you'd expect to see a peak of 42V at the rectifier output. That translates to around 41V with your 1N4007 diodes. While I would expect most chips to be able to sorta-kinda tolerate this much overvoltage, the 7815 may be on the delicate side.

You've included some big caps after the bridge, and whatever output caps the 7815 wants?

George H.

Max voltage of the lm7815 is 35 V

There's a HV version of the LM317 that goes to 60 V.

George H. (I'm thinking he might have let the magic smoke out and just didn't notice.)

The peak of a 30V sine wave is 30*sqrt(2) ~= 42.4 V, and an unloaded transformer is probably higher than this--depending on quality it could easily have been 50V.

You very probably blew it up from input overvoltage.

Cheers

Phil Hobbs

I don't know if the 78xxHV is actually a different die, or just tested to higher input voltage.

The other thing it might be is that failure mode of George's, where it hit the thermal limit, tried to turn off, and the leakage inductance of the transformer caused a spike that blew it up.

How big is your filter capacitor?

And it's probably getting very hot, because it's dragging that +42V down to +33V.

The only ways I know of to blow up a 78xx regulator are:

1. Input overvoltage
2. Pushing current back into the output (e.g. by shorting the input to ground when there's a big output cap
3. Ground pin overcurrent (positive or negative)

Unless the regulator was already defective, you can't hurt it by overheating. It'll happily sit in thermal limit for a _long_ time.

Cheers

Phil Hobbs

Me too. I guess I didn't say that clearly enough.

Post a schematic somewhere. How are you getting +/- input voltages from the transformer using a bridge? You need to establish +/- raw voltages _relative_to_ground_ first.

A center-tapped (output) transformer is easiest. ...Jim Thompson

If you measured 33V I'm guessing that was without any smoothing capacitors and your DC voltmeter meter was reading an average. the peak would be much higher, upwards of 40V.

Cheers

If you measured 33V I'm guessing that was without any smoothing capacitors and your DC voltmeter meter was reading an average. the peak would be much higher, upwards of 40V.

Cheers

I think a 78XX needs a minimum load, around 5 mA. Are you loading it?

Wow, cool, I didn't know that was my failure mech. :^) I was thinking the filter cap would cut down the overvoltage, (But on second thought that depends on the load.)

George H.

OK if that's right I voting we all send JL a beer! George H.

could be shorted, that is too close. 33.0 Volts most likely still have a ripple on it so yes, 30 volts at the peaks would be ~ 42 volts less minus a couple of diode drops.

Yes, I would say you shorted it or at least leaking through. Did you attach the center leg to your common?

Jamie

I think you're thinking of the LM317 (or is that 337? -- one's + and one's -, and if I cared I could look it up). The adjustable ones need put their "ground" return current into the output as much as possible, so they will disturb the adjustment-pin current as little as possible.

Thanks to each of you for your helpful feedback. The basic setup is as follows : Transformer input is 230 Volts. The bridge rectifier output is connected to a bus, a simple 3 cm x 5 cm single layer copper clad piece with 3 0.5 cm parallel strips etched on it, the first for the positive rectifier output, the second for the negative rectifier output and the third for common ground (connected to the center tap of the transformer) There are two 1000 uF 50 V filter capacitors, one for positive-ground, and the other for ground - negative. So, ideally ripples at this stage are controlled. I use 3 LM7815 (TO-220 package) chips, for which pinout is in, ground and out (left to right, looking at the device non-tab side) I have three of these devices in parallel, because the absolute maximum current rating of the device is 1 A, and the maximum current output of the transformer is 2 A. I do not have an oscilloscope at home, so I have to do the testing with the oscilloscope only over the weekends, when most of the office oscilloscopes are not used.

As others have pointed out, the peak voltage of a 30V sine wave is about

I believe the input voltage must be at least 2V higher than the output rating WRT its common pin. Perhaps the voltages WRT the CT tap ("ground"?) are about 17V, the positive one feeding the LM7815 and thus "staving" it. Double check wiring and voltages.