DC Power Supply

Use same 5K pot (if not burned-out from previous connection) and pot always sees the output voltage on its ends and not the output voltage at 240 Ohm that gives too much pot current at pot position zero SM

I checked the National Semiconductor G.P. linears and I saw that your schematic corresponds to a LM117 suffix T or K (TO-220 and TO-3 respectively), but a schematic for a similar regulator using a LM317 is somewhat more complex, if you have the National "General Purpose Linear Devices" goto page 1-45 or download a datasheet from the web.

Suffix is important, it tells you the power rating:

K TO-3 20W

H TO-39 2W

T TO-220 20W

MP TO-202 2W

Yahlasit

P.D if your pot smoked, replace also the regulator.

I had the lm317 outpot and adjust legs backwards. I was wiring it toward the schematic drawings. I can guarantee this is something that will never be forgotten.

Thanks for your help.

I don't remember off the top of my head what the adjustment voltage level will be but since you have the data sheet you can look this up. I suspect that your problem though is that you're not taking into account the changing power dissipation that your pot must handle. If your potentiometer is rated for 1/2W, thats the power the potentiometer can dissipate across the fixed resistance leads, not the wiper to fixed resistance end. Now don't forget that most transformer manufacturers rate the output in RMS voltage out. This will mean that you will get a √2*25.2VRMS≈35VDC after your diode bridge when the transformer is providing rated current. While you're circuit is drawing less current this will be a larger voltage. So the linear regulator will have to dissipate nearly the same amount of power as all of the rest of your circuitry.

The problem is pretty clear. You are smoking pot (sorry, I couldn't resist the pun)

Before providing a robust solution, more information is needed.

>What output voltage range do you really need?
>Does the output have to be continuously adjustable or are a few discrete output voltages acceptable?
>How much current do you need ?

These are critical pieces of information which significantly affect a real circuit design. Simplified cookbook circuits can work fine, but reliable circuit design requires well defined numbers and careful consideration of power dissipation, temperature rise, and heat flow.

Your last Paragraph was perfect, seen it many times. Also the reaon why may companies go to the wall!!!

It would actually work provided the output voltage required is almost the same as the input, or within 5 to 7 volts.....but not if you try and screw the voltage down any lower....

The other circuits shown from other posts are much better, have you tried any yet? Are you now happy?

Hey Andy, no i have not used any other schematics, i had to use this one because it was a class project. I was wiring the lm317 exactly how it is shown on all schematics that i have seen. Imagine my surprise when i actually looked on the back of the lm317 package that it came in. Will go from 28v down to 1.25v (the output of the lm317 i imagine).

Thanks for the info.

If Pot is 18 years old and above, smoking is leagel.

Selling and growing is illegal, but smoking OK?

I could never figure that logic.

Most things have been mentioned- lm317/lm350 between mans could be inferior-v/r- pot you are using could be lo rate- could be dry solder joint or incorrect hookup- before applying smoke test-check= check-check again!

I notice that you show no filter capacitor for your rectifier. The 0.1 uf capacitor will act as a noise filter but leaves energy storage undone. The transformer selection is also a likely problem since 25.2 VAC into the rectifier will give peak voltage output of nearly 35V. This 35 V exceeds the input rating on the LM317, unless you only adjust output down to 10V or so.

You should (1.) replace the transformer with a lower voltage device, then (2.) support the 0.1 uf capacitor with a parallelled 1000 uf device with, say, 35VDC rating.

When, with the present circuit, you measure 28 VDC, your meter is lying to you for reasons you possibly haven't yet covered in class. The DC voltage you expect from the rectifier will, in fact, be unfiltered pulses of (25.2 V * 1.4) - 1.2 V at 120 Hz. This will be read as an average voltage of around 25 VDC. If you switch the meter to read the AC component, instead of a small ripple value of, say, less than 5 VAC (also a lie but you need to figure this one out) the reading will probably exceed 12 VAC with any significant load on the supply. But this is supposed to be a DC supply - output AC equals zero.

The 317 regulator is internally defended against over-current and excessive dissipation (a problem I sometimes run into - joke). It passes excessive voltage along to the load. With the present transformer you will always be on the ragged edge of circuit failure, particularly when you adjust the output voltage low. Reducing the input to the regulator, to 25 VDC (filtered) from its present 24 VDC (pulsing average), will make the circuit perform as you think it should.

Though for class, this is maybe stuff you haven't seen yet which will come to take its pound of flesh from your backside.

Good luck with the circuit and with your newly chosen interest. Any additional questions - feel free.

Thank You very much, i believe you have explained why my lm317 only worked for a few minutes. i'm getting the full 37v now just as you mentioned and i checked the lm317 output and it was over 3v, i think it should be around 1.25v.

gotta get to class now. will inform of good fortune latter.........Hopefully .

Mjb, i'm sorry i didn't answer those questions. I guess i overlooked them and was kinda overwhelmed with all the responses that were given. I did not have any requirements other than connecting a DVM to the output of this device and showing the instructor that it worked. Most of the others have a hi around 27V and a low of 1.25V. Depending on the transformer they used..i suppose. My problem of course was not realizing the pinout could be different than the schematics (which again, you made me feel a lot better about doing). Running behind as i have been doing i just wanted this thing to work for a few seconds ( is fine with me). So i did not use a heat sink and i think that is why it no longer works. So i will deal with that this evening.

One guy mentioned this morning about the voltage being too high and it all makes real good sense except for the fact that i know of about 20 of these things that work fine at least long enough to get signed off on .

I think this thing is way too simplistic, i'm guessing that our instructor just wanted us to do some heavy thinking and learning. I know it has work on me.