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Old 12th January 2012, 11:17 PM   #1
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Default Voltage regulator issue

I'm trying to build one of Rod Elliott's P05 preamp power supplies and have hit a bit of a snag. Using 7815 (+15v) and 7915 (-15v) gives me +14.5v and -17.4v. The first is within tolerance but the second isn't. I've swapped out 7915s and get the same thing. Set up a third 7915 with the test circuit shown in its spec sheet and, again, get the same.

1. Why does my 7915 give ~17.4V instead of 15V?

2. Does it matter for an opamp based pre-amp? Elliott's P88.
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Old 12th January 2012, 11:39 PM   #2
jaycee is offline jaycee  United Kingdom
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Are you measuring the outputs with any load? Try a 1K resistor from the -v output to ground and see if it's in spec then
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Old 12th January 2012, 11:40 PM   #3
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Do have any load on -15 volt regulator?

Connect an led (with correct polarity) in series with 2k7 resistor and connect between +15v and -15V.

Then measure the -15v supply voltage.

Art
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Old 12th January 2012, 11:42 PM   #4
jaycee is offline jaycee  United Kingdom
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Also watch the pinout. 78xx's are, looking at the front of the chip and reading left to right - Input, Ground, Output. The 79xx series are - Ground, Input, Output.
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Old 13th January 2012, 12:06 AM   #5
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Quote:
Originally Posted by jaycee View Post
Are you measuring the outputs with any load? Try a 1K resistor from the -v output to ground and see if it's in spec then
Awesome. Hooked up 1k resistor and measured 14.99 across the leads. Thanks guys.
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