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Old 3rd May 2011, 07:52 AM   #1
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Default Pre amp PSU

I have a preamp with a +/- 31V regulated supply. The power on-off LED indicator illuminates fine when powered on but does not extinguish when powered off. Well, it does, it just takes a few mins (dims VERY slowly). Not a huge deal but I'd like to fix it if possible. I know it stays illuminated due to the residual charge in the filter caps, so I'm wondering how I could have the LED turn off immediately when the PS is powered off (like my other components) and not several minutes later???

I'd like to avoid using bleeder resistors across the filter caps if at all possible!

thanks.
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Old 3rd May 2011, 08:44 AM   #2
hesener is offline hesener  Germany
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you may want to connect it to the AC of the input transformer (secondary side, please), with a resistor (e.g. 15K) in series, and a 1N4148 anti-parallel to the LED, as per attached proposal. adjust the resistor for brightness as needed.

hope that works, have fun
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Old 3rd May 2011, 12:11 PM   #3
alexcp is offline alexcp  United States
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Quote:
Originally Posted by hesener View Post
you may want to connect it to the AC of the input transformer (secondary side, please), with a resistor (e.g. 15K) in series, and a 1N4148 anti-parallel to the LED, as per attached proposal. adjust the resistor for brightness as needed.

hope that works, have fun
This will work, although there will be visible flicker. You can add a small (10uF) electrolytic cap across the LED to avoid it.

As an alternative, you can add a zener diode in series with the resistor limiting LED current. When the voltage goes down, zener shuts off, and so does your LED. The resistor will need to be recalculated, of course, and be careful with the dissipation in the zener - use 1W or more.
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Old 5th May 2011, 12:09 AM   #4
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Quote:
Originally Posted by hesener View Post
you may want to connect it to the AC of the input transformer (secondary side, please), with a resistor (e.g. 15K) in series, and a 1N4148 anti-parallel to the LED, as per attached proposal. adjust the resistor for brightness as needed.

hope that works, have fun
Thanks for the advice.... Yeah, I have a small Power-Amp with a set up like this and as pointed out, there is (was) a noticeable flicker (60hz) ALTHOUGH that set up came straight from the AC (primary -120V). It had a 5K6 / 1W resistor in series and a 1N4004 diode. That LED was extremely susceptible to burnout (DED) and so I replaced the 5K6 with a 15K 2W flameproof resistor.
SEE attachment @ Bottom!




Quote:
Originally Posted by alexcp View Post
This will work, although there will be visible flicker. You can add a small (10uF) electrolytic cap across the LED to avoid it.
Yep, you guessed it! I'll have to try that (cap across the LED) for the Amp (mentioned above) to see if that "stops" the flicker!

Quote:
Originally Posted by alexcp View Post
As an alternative, you can add a zener diode in series with the resistor limiting LED current. When the voltage goes down, zener shuts off, and so does your LED. The resistor will need to be recalculated, of course, and be careful with the dissipation in the zener - use 1W or more.
Someone else mentioned this too but I wasn't sure if it would work. What "size" zener would you recommend?


Thanks...

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