I'm At A Loss: Variable Resistor

[Honduras]

Honduras, You might want to read this also.
http://chemelec.com/Projects/Potentiometer/Potentiometer.htm

That link may be the final solution.

Let's see if I can post an image.

**broken link removed**

This is actually close to what I was trying to build. If I can get down to 1.25 vdc I can work with that.

Heat transfer will be a big problem. Maybe two LM317Ts with heat sinks?

 

[audioguru]

Repeat:" The more expensive LM117 (shown in every circuit in the datasheet) uses 240 ohms. The Less expensive LM317 uses 120 ohms."
Then the resistance of the pot also needs to be halved.

The circuit you posted uses a LM117 resistor but uses an LM317, but since the input voltage is fairly low then maybe it does not matter. With a supply that is only 12V then the maximum loaded voltage will be about 10.25V (because it has dropped-out) with very poor voltage regulation because it will try to go to 11.7V.
When the pot is zero ohms then the ADJ pin is grounded and the output will be 1.25V.

 

[KeepItSimpleStupid]

I know you wanted to build something, but this **broken link removed** is actually pretty cool. I have one. I think it's based on one of the brothers of the LM117. As you select the voltage the secondary gets selected too. I'm permanently using it on a portable radio plugged into a UPS that didn't like a standard wall wart. Too much noise and the batteries didn't last long. Elderly mom has radio, a high intensity lamp, personal emergency response system, answering machine and a handset base on a UPS in her room and a flashlight nearby. The other UPS runs my network except a wireless repeater.

You could always use it as a source of DC (like you said two LM117's).

 

[Honduras]

Repeat:" The more expensive LM117 (shown in every circuit in the datasheet) uses 240 ohms. The Less expensive LM317 uses 120 ohms."
Then the resistance of the pot also needs to be halved.

The circuit you posted uses a LM117 resistor but uses an LM317, but since the input voltage is fairly low then maybe it does not matter. With a supply that is only 12V then the maximum loaded voltage will be about 10.25V (because it has dropped-out) with very poor voltage regulation because it will try to go to 11.7V.
When the pot is zero ohms then the ADJ pin is grounded and the output will be 1.25V.

The circuit I posted was posted in a tutorial.

 

[audioguru]

The circuit I posted was posted in a tutorial.

I know. The authors for most tutorials about the LM317 do not notice that the datasheet shows the more expensive LM117 and did not read the dropout voltage spec'd.

 

[Honduras]

I know you wanted to build something, but this **broken link removed** is actually pretty cool. I have one. I think it's based on one of the brothers of the LM117. As you select the voltage the secondary gets selected too. I'm permanently using it on a portable radio plugged into a UPS that didn't like a standard wall wart. Too much noise and the batteries didn't last long. Elderly mom has radio, a high intensity lamp, personal emergency response system, answering machine and a handset base on a UPS in her room and a flashlight nearby. The other UPS runs my network except a wireless repeater.

You could always use it as a source of DC (like you said two LM117's).

I have more than 4 laboratory quality power supplies. I need to provide more power supplies than I have.

1. 9 vdc for my Ransey array.
2. Calibration voltage and current supplies for a number of items.
3. Various supplies for a precision vacuum tube tester.
4. Power supplies for experiments.

 

[Honduras]

Double post.

 

[Honduras]

I know. The authors for most tutorials about the LM317 do not notice that the datasheet shows the more expensive LM117 and did not read the dropout voltage spec'd.

I didn't either. That is why I prefer to do things for myself.

 

[KeepItSimpleStupid]

Be careful with tube testers. I have one that would be affected. I have been cautioned that a modern day 10 M input Z meter can seriously affect the calibration on some models if say the instructions say use a 50K ohms/Volt meter. You would need to load each measurement appropriately.

 

[Honduras]

Be careful with tube testers. I have one that would be affected. I have been cautioned that a modern day 10 M input Z meter can seriously affect the calibration on some models if say the instructions say use a 50K ohms/Volt meter. You would need to load each measurement appropriately.

Not working on this alone. The other guy is Russian and has a lot of lab grade stuff, but it is hard for him to make reference standards. Prices are high even for basic regulator parts.

I was trained in the USMC as a ground to air (NOT the same as air to ground) radio repairman in 1968. Then I was sent to a place where all of our radios had been discarded by the Air Force in 1957. All tubes. Fifteen years ago I had a business repairing tube testers, mainly, but also any other piece of vintage test equipment I could find.

I may not be good, but I'm not new.

 

[Honduras]

Double post.

 

[Honduras]

The middle and the back should change too but that all depends on how you have it plugged into the breadboard. I think you want the longest dimension of the POT parallel to the longest dimension of the breadboard and the pot on the divided side if you have the standard breadboard.

Here is a breadboard pic. http://wiring.org.co/learning/tutorials/breadboard/ Note the break in the parallel busses. A lot of people think that there are 4 bussed parallel to the longest dimension. Note that there are eight busses.

Let's ask what are your resistor values? Something about 220-270 and what else?

The regulator sometimes needs a minimum load to work, otherwise it tends to stick close to Vin.

The load is 1.5 kOhm, 10 Watt. I hate to see things go up in smoke.

 

[Honduras]

If the input is 18V and the output is 2V at 0.5A then the poor little LM317 will be dissipating (18V - 2V) x 0.5A= 8W which is a lot of heat so it needs a pretty big heatsink.
When the trimpot is set to 1k then the 240 ohm resistor has 1.25V across it so its current is 1.25V/240 ohms= 5.2mA and the 1k trimpot will have a voltage across it of 5.2mA x 1k ohms= 5.2V. Then the maximum output voltage from the LM317 is 1.25V + 5.2V= 6.45V.

Why is the input voltage so much higher than 3V? All it does is make lots of heat. For a maximum output of 3V the input should be about 5V then the heating when the LM317 output is at its 1.25V minimum is (5V - 1.25V) x 0.5A= 1.875W and only a small heatsink will be needed.

Actually, the input is 3 vdc from a commercial regulated source. I'm hoping that's enough. Idon't remember seeing any heat sinks for the TO-22 package. Now that I think of it, they wouldn't do much good. This is, I believe a TO-92 case. All plastic.

Actually, I just Googled it and found some. My bad.