voltage controller circuit

Update

I built following circuit on a breadboard and voltage regulator (5K LM317 regulator) was burned -- sparking were coming out it...

Sparking? Sounds as if your input voltage is far beyond 30 volts.

how much current are you using in your load check the spec sheet for maximum input to output voltage work out the maximum power dissipation for a 317, did it have a heatsink?

martin

I have no idea how to check that. The only formula I think of is i=v/ r.

I am very new to this stuff, beside parallel and series circuit my knowlege is nil :-)

Correction:

LM317 does have a head sink and it is okay. Sparks, and smoke were coming out of 5k potentiometer. Why did I do wrong?

PS: My regulated adaptor has two output 15v, and 30v. According to my DIMM one is 15.11V and second is 31.70V. Adaptor came with my HP printer.

Sorry, sparks were coming out of potentiometer. Voltage regulator is okay, although it was very hot.

The diagram is correct, as far as it goes, except that your input voltage is way higher than it needs to be at

40 volts, which would cause a whole bunch of extra heat.

For a maximum of 12.71 volts output, you want to keep the input voltage to no more than ~16 volts. The 220 ohm resistor is not a problem. Your table of resistances and output voltage, if those are measurements, suggest a high current draw and no heatsink. For example, with

220 R1 and 2200 R2 and a measured output of 12.71, you are over a volt low. Makes me believe you are cooking the LM317.

What to do? 1) Make _absolutely_ sure you set the 10-40 volt input supply to *no more* than 16 volts when you want

Tomorrow, I will buy two new potentiometer. In some of my lab expirement I need 20-25v output. I still don't understand how my potentiometer end up sparking, and smoking.

I am sure I put it in a wrong way, I just don't know which way would be right though...

Where do you have the potentiometer connected. I don't see it in your schematic. The schematic is correct. Try putting the potentiometer in place of the two resistors: put the pot's wiper to the ADJ terminal on the LM317, one end of pot to OUT and the other end of pot to grond. That should vary the output voltage.

No, Don't do that! It won't work. The 317 wants the 240/220ohm from out to adj. Connect the pot as a rheostat betwee adj and Gnd.

Mike When truth is absent politics will fill the gap.

Your diagram does not show a potientiometer. It sounds like you have connected the potentiometer to the wrong place. Post a schematic that shows the potentiometer.

You can get 20-25v output. But first, you need to get the basic circuit to work properly. Then you can add pass transistors.

You can measure how much current is drawn to get a fairly close ball park figure this way: put a 1 ohm 5 watt resistor in series between the Vout from the 317 and the load. Connect your voltmeter across the 1 ohm resistor. The reading you get is the current in amperes that the load is drawing.

Ed

I haved replaced the R2 resistor with 25k potientiometer (i will be using 5k one later). Right now I am using 25k potientiometer. I have also change the input voltage to 4v from a unregulated adaptor (for testing). Both the potientiometer and regulator get really hot.

I think i am putting potioentiometer in a wrong way...

Mine looks like this

It has three pin a,b,c this is the way i put them.

a went to ground b went to output c --- where does this go?

I have also bought 50k and two 5k potioentiometers.

The pot should go in place of R2, with the center terminal connected to the LM317 "ADJ" pin, and one end terminal connected to ground.

You probably don't want a pot of more than 5K, or the useful range of the post will be squeezed down at one end.

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[... LM317 ...]

You may also want to include a 0.1U from the ADJ pin of the LM317 to circuit ground. The LM317 has useful gain up into the many MHz range. This is good because it makes the output voltage very stable with changing loads but also can cause oscillations if the leads are too long.

If the LM317 is sharing a source with other loads, you will also want to think about what happens when the power switch is turned off. You may want to add a diode (1N400X) from the output bact to the supply (Cathode to supply). This way the output of the regulator is pulled down if the input to it goes down.