Re: 0-12 Volts LM317 PSU

[ashishamours]

hey friends,
i m  in need of a variable power supply (0-12v) , using lm317, n current booster ,hv output capacity of 1.5a.
 plz tell me if u can....

[gogo2520]

hello ashishamours
   The lm317 to 220 package will handle a 1.5amp. If you need more try the lm350 to-3 package that will handle 3 amps.
                                                 gogo

[audioguru]

The lm317 TO-220 package will handle a 1.5amp.

Not when it has 15V across it because then it will be too hot and a huge heatsink and a fan might not help.

[gogo2520]

Hello
 

The lm317 TO-220 package will handle a 1.5amp.

Not when it has 15V across it because then it will be too hot and a huge heatsink and a fan might not help.

     Wounder why the data sheet doesn't say that
                                     gogo

[Hero999]

It does, for a start 1.5A is only guaranteed at (V_IN-V_OUT) ≤ 15V, see page 5.

And there's a graph showing the safe area protection, see page 6.

http://www.national.com/ds/LM/LM117.pdf

[audioguru]

If the output is 1V at 1.5A and the input is 18V then the power in the LM317 is (18V - 1V) x 1.5A= 25.5W.
The thermal resistance of the TO-220 case is 4 degrees per Watt and if the ambient temperature is 30 degrees C then the chip will be at (4 degrees x 25.5W) + 30 degrees= 132 degrees C. But its max allowed temperature is 125 degrees C.
Adding an electrical insulator will make the temperature of the chip higher.
A heatsink is not perfect (it also heats up) so the temperature of the chip will also be higher.

[gogo2520]

Sorry
  I don't know were the 18 volts came from the original post shows a 12 volt input. I figured he would use a 12 volt transformer  to start with and then use the regulator to lower it. And that would of been (12 - 1) x 1.5 = 16.5W
 ( 4 degrees x 16.5) + 30 degrees = 96, well under the max allowed temperature. Ya but still hot.
                     gogo                  

[audioguru]

An LM317 with a 12V output needs an input of at least 15V.
I was mistaken when i thought the output should be 15V then the minimum input must be 18V.

A 12V transformwer has a peak voltage of 17V which is reduced to about 15VDC by the rectifiers and filter capacitor.
If the output is set to 1V then the LM317 will have 14V across it and will try to dissipate 21W.

You must include the thermal resistance of the heatsink and electrical insulator into your temperature calculation.

[Hero999]

This can be solved by using a centre tapped transformer and taking the power from the tap when the output voltage is set below a certain level. I'll post a schematic if you're interested.

[gogo2520]

Thanks Guys
    I'm not real good at building power supplies, mine are pretty simple. When I need a good large one I would use a prebuilt by someone who has the right stuff to do it with.
    I think this fall I am actually going to break down and invest in a oscilloscope and study power supplies along with a lot of stuff.   The reason is because I am getting ready to retire and am going to need something to keep me busy, so why not a hobby I enjoy.
   When I see something I don't understand I tend to ask questions and I might be a little crude about it. Sorry about that.

    Yes Hero999 if you don't mind please your schematic for the center tap power supply.
                                     thanks again
                                        gogo

[Hero999]

This will work, when the output voltage is set to under 12V the power is drawn from the centre tap cutting the power dissipated by U1.

It's designed for a 15-0-15V transformer, for a 9-0-9V change the D2 to 6.2V.