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1999\11\08@215756 by

Hey guys, i built a real (cheep) quick adjustable power supply using an LM317...
i basically followed the schematic at:

http://expert.cc.purdue.edu/~bymaster/power_supply.gif

anyway, when i check the output voltage on the 317, no matter what values of res
istance i put the adjust pin and the output pin always read 16.8v.  There is a 1
8v input.  The 5v 7805 regulator works fine...  Now i know i probly blew out the
chip because while testing i turned the 5k pot to 0, there should be 20ohm in s
eries wit the 390ohm resistor and when u do the math

1.25(1+ 20/390) = 1.31v

In my circuit..the two 10ohm resistors started to smoke and turned black.  Why??
They are 1/4 wat resistors (probably why but all i had on hand)..???

mikE

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Mike M wrote:
>
> Hey guys, i built a real (cheep) quick adjustable power supply using an LM317.
..i basically followed the schematic at:

If you use a 120R resistor (between VOut and VAdj) then use this simple
formula to find the other resistor value. (between VAdj and Gnd)

R = (96 X Vout) - 120.

Eg VOut = 5V to 15V, use 360R in series with a 1K trim pot.

--
Best regards

Tony

http://www.picnpoke.com
Email salespicnpoke.com

part 0 8726 bytes
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">The schematic is incorrect.&nbsp; As wired the switch will not affect the voltage in any way.</FONT>
</P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">The correct verison would be as follows</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; _______</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; |_1_2_3_|</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | | |_____From transformer/rectifier</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | |_______Output</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | |&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | /&nbsp;&nbsp; ---</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | \&nbsp;&nbsp; ---</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | /390 |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | \&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |_|&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; /</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \ 10R</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; /</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; /</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \ 10R</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; /</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp; To switch and variable resistor</FONT>
</P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">Note that a 2200uF capacitor on the output is really way too much.&nbsp; This amount of capacitance does not help the regulator work better, it slugs the transient response.&nbsp; A single 100nF will suffice in most cases with maybe a 10uF cap if you are expecting big load spikes.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">Also note that when the switch is inbetween positions, i.e. when you switch it, the output voltage will briefly rise to almost the unregulated voltage, probably destroying any sensitive equipement on the output.&nbsp; You can use a make before break switch to stop this, or you could change the whole scheme and switch the 390R resistor to other values.&nbsp; This way the voltage will fall to 1.25 volts during switching.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">Regards</FONT>
</P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">Mike Rigby-Jones</FONT>
</P>
<BR>
<UL>
<P><FONT SIZE=1 FACE="Arial">{Original Message removed}

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