Slow Smoke

[MrWild]

I made up a LM317 circuit so I can power some solidstate selinoids. My target voltage is 12-15v and math says R1 is 100 ohms, and R2 is 1000 ohms and I should have 13.75v. I had some 1/2w 100 ohms resisters, but only 1/4w 1000 ohm.

I also added a LED to the circuit and used another 1000ohm resister for it.

When I power it up, the two 1/4watt resisters slowly start to smoke. The R2 smokes first, but after awhile the LED resister starts smoking also. I haven't left it on until it chars and quits, but I'm open for any ideas anyone might have. The base voltage is 48v. Sorry I haven't checked the output of theLM317 yet, but it can't be too far off if the LED is lighting without blowing... (I think).

[Eurisko]

The 48 volt input is the problem. Double check the specs on the LM317, the maximum input is probably 36 volts. Also, with an output of 13 volts, it is best to keep the input voltage just a few volts above the output.

Excess input voltage is magically converted to heat. And smoke.

[MrWild]

My data sheet says a range of 1.2-37v and no more than a 40v differential (Vi-Vo). I'd use a 7812, but that has a max in of 24v. In. Nowhere in the data sheet does it say a max In for the LM317, but if the differential is 40v, and it will out put up to 37v, wouldn't the max be up to 77v input (depending on output)? With a 48v input, it should be able to handle a 8v output. This is why I went for near 14v. I think I'll try a higher output as the solid state selinoids have a range of 3-33v. The LM317 isn't even getting warm, it's the resisters that are getting hot.

[Al_The_Man]

Are you using a Toroidal transformer? If so it is far easier and more efficient to add an overwind for 12v, or why not use the 12v in the PC?
For SSR's, you do not need regulation.
Al.

[MrWild]

No, I'm using a 48v 7.6a power supply from Keling. I could use a wall wart for the SSSs, but thought I'd try to learn something and melt some solder. There is no real reason to make this thing other than trying to learn something new and a, "Looky there, I made that" aspect.

Here is the data sheet for an LM317

http://www.onsemi.com/pub_link/Collateral/LM317-D.PDF

Here is a calculator for a two resister set (no variable resister) for fixed voltage

http://www.cpemma.co.uk/317calc.html

This is a LED calculator for single LEDs and their resistor needs for different voltages and ma.

http://led.linear1.org/1led.wiz

[cgm003]

MrWild I think your resistor values are OK. The problem is the 1/4 watt resistors are trying to disapate too much power. The numbers I got from your discription calculate they are trying to drop .2 watts. The resistors are almost at their limit. Try some 1/2 watt resistors.
CGM003

[Eurisko]

My mistake. The newer LM317's can operate with a differential voltage of 40 volts.

Double check everything on the circuit, including the input voltage ( don't assume 48 volts ), the resistor values & connections, and most important, the output voltage.

13 volts should not be burning up these resistors.

Let us know how this turns out.

[MrWild]

Thanks for all your help everyone. I went to 1/2 watt resisters and upped the resistor to the LED to 1.2k as this was more in line with the math. I found the power supply while rated at 48v puts out 64v when it has little to no load. This time when I tried it it didn't come on and I found a bad solder joint for the neutral post. I get 18v with the higher input (no load) and as it puts out 48v when wired to the G540, I figure things will be fine once everythingis all put together.

Things I learned were well worth the time and fooling around.

Thanks,

Bill