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Topic: LM1117-ADJ woes! (Read 2015 times) previous topic - next topic

thegeekway

This is a bit of a strnge one, I have a handful(50) LM1117s from RS and when choosing resistors for it I used the datasheet VREF to work it out as so:

1.25(1+330/200) = 3.3v

But when I did I was only getting an output of 0.22v.  So I measured the VREF myself and found it was 0.08v so then I worked it out from there:

0.08(1+8200/200) = 3.36v

It seems stable, no heat etc.  I have test 5 from the strip of reel and they are all the same +/- 0.1v.  Have National updated their product and kept it quiet or have I got a bunch of faulty parts?

Just wanted to check before sending an email to RS.

Cheers, Phil

RuggedCircuits

Not all LM1117's are adjustable. Any chance you got a fixed-output version?

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thegeekway

I though that too, so I double checked without the voltage divider and the 4.9V input was 4.45V on the output and as far as Im aware they only sell 5V then 3V.

RuggedCircuits

Really confused now. If you apply a 4.9V input without a voltage divider to an adjustable regulator, I'd expect the output to be 4.45V as the regulator tries to (without success) increase the output voltage to maintain VREF at 1.25V. But the same would occur for a 5V fixed regulator -- you'd just be seeing the dropout voltage.

What do the part markings on the body of the LM1117 say?

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