linear regulator crapping out

Could it be a short on the common terminal? Would that cause on of these things to quit?

Could it be a short on the common terminal? Would that cause one of these things to quit?

What, you are using a inductor on the input pin of the regulator? That isnt exactly what its supposed to be and likely oscillates. Standard connection is as follows: transformer-rectifier-large electrolytic-100nF-input output-100nF-10-100µF electrolytic the 100nF can be ceramic or polyester film. Inductors usually are no good idea. Grounds should be connected with short wires, use a ground plane or single point gnd. Also, a diode is recommended from out to GND with cathode on out (on dual supply systems this is necesary, on single supply it doesnt hurt) and one from in to out with cathode on in. The large electrolytic should be calculated based on load current and other requirements, last time i did a 5v supply i came up with 2200µ 16v (input from 9v Transformer, load current 0,6A).

isn't a 100nf a bit small to keep the ripples low? is there a reason the capicatance is so much higher on the output side? those cap I mentioned on the input is to convert the rectified circuit from rippling. Right now I'm looking at between .01 and .03 amps on the output side at 5 Volts. What size cap do you recommend? Thanks!

On the output, theres no ripple (well, almost no) since the regulator reduces the input ripple (around 10-20%) by its ripple rejection ratio of around 70db. The output capacitors only serve to keep the regulator stable and improve transistent response.10µ usually is enough, if it has low ESR (tantalum) or 10µ in paralell with 100n ceramic/film if the 10µ is electrolytic.100µ only improves transistent response slightly, probably not needed.

On the input side, you need a capacitor sized to get the ripple voltage you need (ensuring the voltage never drops below Uout+3v under all circumstances), a generally accepted value is around 20% maximum. Since your current is so low 220µ (rough guess) do nicely. The 100n capacitor needs to be close to the regulator, again to keep it stable.

Inductors if any sort should not be used unless one understands exactly what they do and why it is needed.

Both 100n capacitors should be right at the regulator pins. The electrolytic caps can be at some distance.

So thats it all again,to be viewed with fixed font:

Diode +-----AK--------+ | ______ | in-------+-----+---|7805|------+-------+----+----out | | ------ | | | === === | === === K | | | | | A | | | | | | gnd------+-----+-----+---------+-------+----+----gnd 220µ 100n 100n 10µ Diode

Sorry, the reason I had the inductor in there was because the load will be plugged in and unplugged. Also, the load may be doubled at any moment as well. THANKS FOR THE ADVICE!!!! it is very helpful.

How about dumping the linear part and go for a TI switching module like the PT5105A? That's a 6.5V output module, but you can find a 5V part. Best thing is you can get a free sample from TI. Read the datasheet carefully and see if it fits your needs, the big plus here is very much reduced power dissipation.

No problem, the inductor will probably cause problems and you dont need it.The 78xx regulates quickly and the output capacitor keeps the voltage up during that time.With your low load currents the drop probably is in the microvolts. One last Q: is the 7805 connected the wrong way round?Input is on the left side if the heatsink tab points away from you. If it still doesnt do, try a new regulator.

It's properly connected. This is the 4th one that has crapped out.

History. Been in this movie long time ago. Try a small resistor on input line ~10Ohm value to reduce inrush current.(Check values according to manufacturers data taking into consideration a dead short presented by the electrolytic capacitor(s) on power-up.) It helped in my/our case.

Have fun

Stanislaw Slack user from Ulladulla.

Did you remove the inductor yet? Is the input cap reasonably close to the regulator? Put a scope on it. It could be oscillating.

Is it possible the output is even briefly seeing > 7V with the input off? If that's even a remote possibility, put a diode such as 1N4001 from input to output (reverse biased, obviously). It usually is unnecessary for 7805 parts, but you've got funny things going on.

Best regards, Spehro Pefhany

Get rid of the inductor- it is worthless at this frequency and does nothing for ripple reduction. Insert a series 47R at 1/2W rating between the common cathode junction of the rectifiers and the input to the first

Jimbo, Now, you have a centertapped transformer, and you are full wave rectifying it. What is the actual voltage at teh input to your regulator- 14 or 28 volts? A ways back, I tried this with 28 volts, but forgot that this is RMS voltage, not peak. The actual measured voltage was 38 VDO at the input of the 7805, which was above the operating point. Regulators usually lasted about a day, at best, before giving up magic smoke...

Charlie

I never needed the diode on the output pins to ground. and in your diagram the one across the regulator is backwards. other then that it is fine by me. Your real problem is most likely the input voltage is too high. 7805 ic's are hardy little bugs. Jtt

The problem can come if the input has a heavy load on it or can be shorted, in which case current can flow back from output capacitors through (IIRC) some B-E junction breakdown. But that won't happen on a

Well inside.

Best regards, Spehro Pefhany

Yes, sorry, the one diode is reversed. Im using these diodes from out to gnd for some time now but they are only necessary in dual-supply applications, where overloadung one rail could lead to reversed voltage across the regulator and damage it.This blows both regulators most of the time and also damages the parts connected to it because the regulators shorted input/output, resulting in increased output voltage with loss of current limiting.

On single supply applications that cant happen but during testing stupid events happen, like connecting wires to a power supply instead of a meter...

For the input voltage, if its too high this can cause problems, although i dont think this happens here, since the transformer is mentioned to be

Another problem (which is why i put in the second diode from out to gnd) only happens in dual supply systems and will cause extensive damage without this diode. This happens if a load connected between both rails draws excessive currents and the regulators limit current.Usually one regulator has a little lower current limit and then its output gets negative, blowing the regulator and making it short from in to out, putting unregulated input to the output, which causes high current to flow and does the same to the opposing regulator. Then one has both regulators shorted and until the fuse blows, damage will occur and expensive chips fail if present and no cheap part shorted out already. The diodes avoid this condition by keeping high reverse currents from flowing into the outputs. This always served me well in the past, and i did a few power supplies by now. As always, one can often get away without these diodes but then dont cry around if it blows up.A 4001 costs around 1 cent and can save rather expensive loads connected to the regulators from blowing.

Sorry; I just saw this... nobody has posted the correct answer yet, so I'll chime in. Your problem is the 680 uF cap on the *output* of the regulator, and your failures are occuring when you *unplug* your power supply, not when you plug it in.

Basically, you don't want current flowing from the regulator's output back to its input. That lets the smoke out. Lose the 680-uF output cap, or install a diode across the regulator with the cathode towards its input pin, and you should be OK.

-- john, KE5FX