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 Forum index » DIY Hardware and Software
+/-15V PSU power-on LED's
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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Tue Oct 10, 2006 11:31 am    Post subject: +/-15V PSU power-on LED's
Subject description: Is this right?
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I'm building a bipolar psu for my modular-to-be, and want a power-on LED for both pos. and neg. voltages, so I don't have to get my dmm out to see if it's working, should I have reason to doubt it.

Does this look right? I would think limiting the current from each rail to ground would be the way to go. I don't want the LED's to suck too much current (better to save it for the modules), but how much is enough for them to be visibly on?

2mA as suggested in the Super Klee thread?

C


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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Oct 10, 2006 1:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

if we talk about bipolar PSU, we talk about relative voltages: GND is positive in relation to -15V and GND is negative in relation to +15V so you want rather this:

+15V - LED>| - R1 - GND - LED>| - R2 - -15V

hope the encryption is understandable...
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Blue Hell
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PostPosted: Tue Oct 10, 2006 2:11 pm    Post subject: Re: +/-15V PSU power-on LED's
Subject description: Is this right?
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Coriolis wrote:
Does this look right?
Ok, except that (as fonik already said) the LED for the negative supply should be turned around as current flows from positive to negative and GND is more positive than -15V
Quote:
2mA as suggested in the Super Klee thread?C

Depends on the LED's efficiency, 2 mA seems OK for a modern LED, an older one might need more current (like 5 mA) to be comfortably visible.

To calculate the resistors you can assume that 2V will drop over the LED and so 13 V has to be dropped by each resistor, which at 2 mA means R=V/I -> 13V/2mA -> 6.5 kOhm, experiment a bit around that value to see how bright you want the LEDs to be.

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Coriolis



Joined: Apr 11, 2005
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PostPosted: Tue Oct 10, 2006 9:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok guys, get it! Very Happy
Guess negative voltage confuses me - I would think any voltage (pos or neg) would be considered HIGH relative to ground, which is why I oriented the LED's the way I did.

Live and learn. Laughing

Thanks, both of you!

C
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yusynth



Joined: Nov 24, 2005
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PostPosted: Wed Oct 11, 2006 2:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi

The ground is the reference voltage assumed to be zero volt. Any other voltages are relative to that 0V ref. Therefore, negative voltages are lower than the ground reference and positive above. That's as simple as that Wink

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Uncle Krunkus
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PostPosted: Wed Oct 11, 2006 3:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Almost all LEDs can handle up to 30mA. So I tend to start at 10mA and adjust if necessary.
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Coriolis



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PostPosted: Wed Oct 11, 2006 12:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, I don't like LED's to be too dominating (I'm going to avoid blue LED's completely for the same reason), so I'm going to try and run them as low as possible and still be able to see them ok.

C
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