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Old 19th April 2012, 10:47 PM   #1
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Default power on?

Hi i have buit standed power supply set up for my ESP active crossover using 7815/7915 regulators.when i turn on nothing happens but if i unplug then plug back in i get power.
any ideas why this might be happening?
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Old 20th April 2012, 07:51 AM   #2
Mooly is online now Mooly  United Kingdom
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Do you mean the voltage regulators fails to produce output ?

There can be a problem if one regulator "starts" before the other. Have you tried reverse biased diodes across each regulator output to ground.
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Old 20th April 2012, 11:07 AM   #3
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hi

i hAVE NO DIODES ON THIS ITS JUST SIMPLE bridge ,largecap , regs followed by 0.1uf caps then 10uf caps .
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Old 20th April 2012, 11:41 AM   #4
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Mooly is suggesting you put reverse biased diodes IN .....something like 1N4001 (reverse biased ) from each rail to earth after the regs on the regulated output sides . This will probably cure your problem......especially if your following circuitry has rail capacitors connected directly from rail to rail.
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Old 20th April 2012, 11:49 AM   #5
DF96 is offline DF96  England
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Is this caused by the regulators latching into 'protection' mode?

A circuit diagram might help. We are good at guessing, but why should we have to?
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Old 20th April 2012, 02:08 PM   #6
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Hi put in diodes as per national data sheet.diodes position with banded end towards positive rail.
It seems to be better will turn on first time.
but after switching off it will only come back on within 3 seconds or after 10 seconds.
the times it doesnt come on and is switched off it powers up briefly as the power drains away
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Old 20th April 2012, 05:57 PM   #7
Mooly is online now Mooly  United Kingdom
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You can also try two more diodes... it's a recognised method... by fitting one across each reg between input pin and output pin.

For the 78 reg the banded end goes to the input pin of the reg and the other end to the output pin.

For the 79 reg the NON banded end goes to the input pin and the banded end to the output pin.

Leave the other diodes in place.

This odd problem, and I must admit it's never happened to me, can also be fixed by sometimes using a different brand of reg. But that's not really the answer.
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Old 20th April 2012, 08:59 PM   #8
DF96 is offline DF96  England
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Would a soft-start circuit help, by slowing things down?
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Old 20th April 2012, 11:35 PM   #9
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hey thanks for advice guys.
working well enough in normal use but still a strange one.
i think a have another reg to try.
they get pretty hot by the way!the trafo is 15v to match so should be too much!
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Old 21st April 2012, 12:23 AM   #10
imix500 is offline imix500  United States
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You have them on heatsinks right?
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