Wiring a Linear Pot for a Power Supply

Use pin 1 and 2 OR pin 2 and 3. The other pin not in use for either configuration is not connected. Fiddle with the potentiometer to get desired resistance once connected.

Two seconds on Google got me this LINK

I want to thank you for your excellent answer which allowed me to make my first power supply project a complete success. Your answer was easy for me to grasp and the link was also useful. Thanks again.

Cheers, glad it worked...I too was in your shoes once :)

Well, I guess I spoke too soon. It seems I have a problem somewhere so I'll try to explain it and maybe you can help. The supply works as it should at first but after a while of testing, the solder trace on the voltage out leg, right under the capacitor begins to get hot...hot enough to melt the solder. In fact, I let the first one burn so I could maybe find the fault. At first I thought it was the cap so I built another one only to have the same thing start to happen.

What could be causing this? Here is what I am using: For the transformer I am using a 1 amp wall wart. I built the board using using a 100 ohm resistor across adjust/out legs, a 4.7uf polarized cap from the out leg to ground with the + side toward the LM317, and a .01uf cap from the input leg to ground. I have a 1k pot that attaches from the output leg at the + side of the cap to ground and then the feed and output voltage wires. The max voltage out is 12.6 volts. Again, it works right for a while but if I turn the voltage down with the pot is when things start to go south.

I should have had this posted with the other but here is a picture of the transformer. R1 was not there when I opened up the transformer but there is solder on the trace side of the board. Also, are those diodes the rectifier or should I build one for it? Thanks.

The dismembered wall-wart has two diodes, suggesting that the single phase input from your mains supply is converted to DC on output after passing through the diodes. I say this, as after enhancing the image you provide, I see the transformer is center-tapped, with a transformer not center-tapped, you will need to have four diodes. Which is not the case here. The resistor absent (R1) is at a guess, a current limit...not needed, therefore a solder trace has replaced it. Do place the hacked wall-wart into a container or pack it somewhere safe

Above photo, I have shown the center tap I refer to...I believe this is used (well assume really) to allow a rough DC out using only 2 diodes.

...MODIFY YOUR CIRCUIT...to the one above. That way you keep it simple...you can add extras later. Also, You can use an old computer power supply (ATX OR WHATEVER) for a clean and "safer" DC power supply.

Also have a look at LINK

Yep, tie the wiper to one end. A GA for you. 99% of the time, that hanging pin causes no problem, but then....

Connect the PIN 1 to ground. PIN 3 to Vcc. THen use PIN 2 (+Ve) and 3 (Gnd) for the variable voltage source.

So that Always the load (Linear POT) is connected across the power supply. According to the POT adjustment you will get 0 to Vcc supply.

This will not be harmful for the circuit.

Except you'll have one smoked Potentiometer quick time. Bearing in mind you have very limited current carrying ability of the average carbon track potentiometer. Not to mention that if the Potentiometer is of a low value(1kohm) and the feed voltage of the transformer/rectifier/filter capacitor is high (in dc terms anything over 9v is high) then Ohms law dictates that the potentiometer will draw significant current and turn that current into heat. Perhaps even flames (seen that happen). That's without asking it to provide power through the wiper contact (pin2).

Using the LM317 you will be able to deliver up to 1 amp of current at the desired voltage output. Note that the output voltage will be dependant on the input voltage from the transformer/rectifier/filter capacitor.

Ideally if you want a power supply to deliver 12Vdc then you should provide around 14Vdc at the input of the LM317.

The purpose of the potentiometer in this particular circuit is to adjust the reference voltage setpoint. The potentiometer does not carry any of the delivered current.

If you have any problems download the National Semiconductor datasheet for the LM317, preferably from the National Semiconductor website.

Is the drawing showing Knob-side view of pot?

If so then connect 1 7 2 together & 3 to Ground .

When you will see it from the front the the arrow will be reversed.

See the direction of rotation from front.

Regards

Hi, connect pins 1 & 2 to Gnd and connect pin 3 to the LM317, if the pot and voltage work the wrong way round, IE on CW rotation of the pot, output voltage reduces, then connect Pins 3 & 2 to Gnd and connect Pin 1 to the LM317.

Try it

Cheers