Wierd overheating of LM780x's

Do you have the recommended capacitors on the input ant outputs of the regulators?

Any capacitance on the input (across the supply rail)? You may have invented a new kind of 780x push-pull oscillator!

Any oscillation observed when they're getting hot?

Not really surprising !

What made you think that was a sensible thing to do ?

Graham

Wait, the original post is a bit disjointed, but it seems he's not talking about "in parallel" as we might think, but simply feeding two regulators from the same source, which of course is legit. After all, they were originally referred to as "on-card regulators", the intent being that you'd have one master supply feeding a bunch of small regulators on each board.

And way into the post, he does mention seeing oscillation on an oscilliscope, which went away when he bypassed the regulators.

The things oscillate, because they are high gain devices, and everyone has known from the start that they oscillate.

Michael

As others have suggested but not quite stated outright: these regulators need capacitors to stabilize them. Otherwise they oscillate, which is what you're seeing.

You need to read the data sheet for the chip, and do what it says. It will tell you what values of capacitors to use, where to place them, and so forth. Go to the manufacturer's web site to get the data sheet.

Reading the data sheet is a pretty good idea for *any* chip you use, not just regulators.

Indeed. I guess he was just lucky to begin with.

Graham

all voltage regulators deviate from the voltage they are supposed to provide. So when u connect two in parallel you get a one at say ...12V the other at 12.1 V. These regulators have a very small internal resistance as seen by the load ... so a 0.1 V diff cld be working across a few milliohms ! hence the heat ... (mercifully these regs have thermal runaway protection so they dont just give up the ghost right away)

u shld nev parallel regulators without an active circuit that ensures equal current sharing between the regs (automatically ensuring equal voltages).

Yes, but the OP said that only the inputs were in parallel. The problem seems to be no capacitor on the inputs.

mc with just the inputs paralleled, how does he hope to draw twice the current at the outputs ?

Mark What is your input voltage ? Linear regulators essentially burn off power @ (Vin - Vout) Iout, which, at 200mA and a diff of say ...10V (5W) can be enough to roast your fingers on, not to mention the device. Do you have adequate heatsinking ?

sorry ...didnt read the entire thing thru ...

seems like u have a decoupling problem here ....try 0.1u ceramics or tantalums at the input to decouple the regulators (10n arent quite good enuff). Also, increase ur smoothing caps to 1000u min. If they still heat up .....puke ...

You may have another problem here, if you're using a traditional multi-voltage PC switcher supply. Many of these PSUs *need* to supply a significant amount of current to their 5-volt output - if the load falls below this point, the 12-volt supply may not regulate properly, and the supply may either feed you bad voltages or may shut down entirely.

It's also a good idea to put a small bypass cap right at the input terminals, with a minimum amount of lead length (no more than an inch at most).

I tried it again last night, a 5v and a 9v reg connected to teh same supply rail, 220uf cap on the supply rail, a 10nF cap from the output of each regulator to ground and a load on each regulator - oddly the 5v was cool to the touch, but the 9v was getting hot with about 200mA running through it - although not super hot as before. I will dig out the spec sheet when I get a spare five minutes and have a browse through it, see if I cant pick up a few clues there.

Originally I was hoping to use these regulators to build myself a regulated power supply with various output voltages that could all be used at once. I wasnt really expecting these kinds of thermal issues though... I just expected it to get hot when I drew a lot of current through them.

Thanks for all the advice people! got a few nice pointers there. Mark

I was perhaps being a little vague, my appologies. I had hoped to draw twice the current with the outputs in parallel as well using similar regulators, although I understand now that it is perhaps not as simple as that. The overheating problem I was experiencing was observed with just the inputs wired in parallel though.

Input voltage is around 12v, from an old switch mode AT computer PSU. I wouldnt say i'm heatsinking the regulators adequately at the moment, but like I stated before individually they seem to run fairly large loads (I think about 500mA) and only get "slightly warm" without a heatsink

I will try 1000uF, I just grabbed a 220uF at random and noticed the oscillations dropped off when I connected it so I used that.

I think perhaps in light of the posts i've recieved on this subject there's a lot I dont know about these little devices which perhaps I should. So i'm gonna take some good advice and bury my head in the datasheets before I continue experimenting.

thanks.

Outputting how much current?

I've built hundreds of similar supplies, & always put 100nF monoblock caps directly between each input pin & ground, & each output pin & ground. If you're running long leads between the unregulated supply & the 78xx reg's, it may be helpful to place 10uF (approx.) tantalums between the 78xx input & ground. If you do all that, your PSU should work fine. Don't forget to heatsink the reg's & remember that if you're using any negative reg's (79xx series) you'll need to isolate the heatsink tabs.

If you've bypassed the input and output as described and you're still having the overheating, then you're right, there's something more going on.

It does seem possible that part of the problem is the PC supply you're using. Take a look at the output of the PC supply with your scope, to see whether there's oscillation. You might briefly try powering your circuit from a 12V lantern battery, or from two 9V batteries strung in series, to see if the problem goes away; if so, then it's an interaction between the PC supply and the downstream regulators. But in general, as long as there's enough load on the PC supply you should be okay. You may need to put a load on the 5V part of the PC supply to get the 12V to regulate properly - try for something that draws a couple of watts, e.g. a 4 ohm 10W resistor, or even a 10W automotive bulb (turn signal, overhead lamp, etc.).