LM317 adj pwr supply fix

[circuitmangler]

Hi,

I have a used Elenco Digital/Analog trainer, and on board there is an adjustable 0-20 VDC power supply. However, ever since I've had it it's only been able to adjust from 0 to 10 V. I've just gotten around to looking at it, and figured out the following:

- It seems like a standard LM317 setup - there's a 120R resistor between the adj. and output pins and a 1.8K pot between adj and gnd. (The pot is probably meant to be a 2K pot, but it only measured 1.8K full scale.)
- The 120R measured 120 ohms (measured in-circuit)
- The input voltage measured about 24 V.
- There is a protection diode from gnd to out as well as a filter cap on the output.

I replaced the 120R with a 51R, and now it's adjustable to 20 V, but I'm wondering if the real problem is with the 317 itself or some of the other components in the circuit. After all, 120R is the correct value to use with a 2K pot to achieve 0 - 20 V.

Any ideas?

 

[jimkirk]

Have you looked at the output with a scope? For example, if the input voltage was actually a half wave rectified voltage (maybe a single rectifier on the input side and a faulty or absent filter cap), then the output could be regulated on one half cycle, but zero on the alternate half cycle, making a voltage that appears to be about half what it should be when measured with a DC volt meter. Even a full wave bridge with no filter cap could give suspect readings like that.

 

[itsmoked]

You are probably overheating it and it's current limiting which will reduce the output voltage. Can you put your finger on the tab when the voltage is not reaching the desired value?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[circuitmangler]

jimkirk - that's a good idea and I'll be sure to check that out.

itsmoked - I thought I gave it the ol' touch test to see if it was overheating, but I can easily check that again.