regulator voltage resistor

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raysolinski

Well-known member
Joined
Jun 16, 2004
Messages
329
Location
Cincinnati, OH
Hey Guys,
I am building a power supply that doesn't need to be adjustable..I am using 317"s for the positive rail and 337's for the negative..rather than use a multi turn pot I would like to use a resistor since I have a pile of these guys around.. ...what resistor value(s) would I need to get +/- 24 on the other side? I see a 5k trimmer is used in most schematics which I can get but would rather use something already on the bench if I can :)...

Ray
 
Hi Ray,

there is a formula in the datasheet, but this table might save you the math (scroll down near the end): http://www.derose.net/steve/guides/universalpower/buildit.html

HTH,
;Matthias
 
Hi Ray,

formula for this is U out = 1.25 V ( 1 + R2 / R1 )

keep in mind that these regulators require a min.load of typ 3.5 mA to max. 10 mA, giving max.values of R1 (1.25V / 3.5mA) = 357 Ohm to (1.25V / 10mA) = 125 Ohm for stable operation.

To get out 24V you may use these pairs for R1 / R2;
120 / 2184
150 / 2730
160 / 2912
180 / 3276
200 / 3640
220 / 4004
240 / 4368
Power rating for R2 min 0.5W

If load-capacitance is larger than 20uF, don't miss protection-diode between Vin and Vout.

Harpo
 
Thanks Guys!
I guess I missed the math on the data sheets (I looked at them briefly)..but in my defense I did have a 3 yr. old tugging at my sleeve wanting me to play trains with him :)....

Cheers,
Ray
 

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