bleeder resistor for front panel LED? - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th July 2005, 01:16 AM   #1
cowanrg is offline cowanrg  United States
diyAudio Member
 
cowanrg's Avatar
 
Join Date: Apr 2002
Location: Lafayette, CO
Send a message via AIM to cowanrg
Default bleeder resistor for front panel LED?

just the other night i was watching a movie and noticed that my aleph3 LED's kinda stay on when the amp is off.

they arent noticable during the day, but at night, its very dim and you can see them when they are off. can i just toss a small-value resistor across the leads to drain them down all the way? ive measured it, and there is just a very small voltage on them, enough to make them glow a tiny bit. it drains very slowly over time (like a half-day or something). would a resistor drain it quicker?
  Reply With Quote
Old 5th July 2005, 01:21 AM   #2
Duo is offline Duo  Canada
diyAudio Member
 
Duo's Avatar
 
Join Date: Jun 2002
Location: Victoria, BC, Canada
Send a message via ICQ to Duo
I've found a few ways I like to deal with that problem. One is to run the LED right from the AC from the transformer with its own diode, cap, and resistor so that it dies immediately after shutdown.

I suppose another way would be to use a transistor or a diode gate to turn the LED off when the power is switched.

These of course require slight modification to the original device.

Adding a resistor in to drain the LED is unlikely to be enough to drain it quickly unless it draws lots of current.
__________________
-- Duo, W1ngs, VA7MON, and lesser known handles. --
-- http://www.w1ngselectronics.com -- My Work and Projects --
  Reply With Quote
Old 5th July 2005, 08:21 PM   #3
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
Tack a 1K resistor across the LED. If that makes the ON-state dim, try 2K.

I don't know the amp. But I'll guess that it is all MOSFETs. Say it runs a 50V supply voltage. When you turn it off, the active devices suck the power caps down. The caps are large, but not infinite: it will go down fairly quick, no more than minutes.

Except when it gets to about 5V, the MOSFETs turn OFF. Now only leakage flows. Good MOSFETs and good caps have low-low leakage. You could stall at 5V-4V for days.

The LED needs at least 1.6V to turn on, explodes at 1.7V-2V. You set brightness by setting current. 5mA to 20mA makes a nice glow. Since you have much more than 1.7V available, the simple way to set the current is with a series resistor. With 50V supply, 50V-1.7V= 48.3V across the resistor. 10mA is a nice glow: 48.3V/10mA= 4K7 resistor. Resistor heat is 48.3V*10mA, 1/2 Watt, you need a 1 Watt resistor. (This part was all done by the Designer.)

Now turn-off. Rail voltage falls to ~5V and stalls. The resistor sees 5V-1.5V= 3.5V, current is 3.5V/4K7= 0.7mA. A very dim glow, and some old LEDs would leak more than this, but yours apparently glows fine with 0.7mA.

Now add the 1K across the LED. It sucks 1.5V/1K= 1.5mA. If the rail voltage is stalled at 5V, it won't put enough current into the 4K7 to keep the LED voltage above 1.5V. It will turn completely off. In fact it can't turn-on if the rail is less than about 1.5V*(4K7/1K) or about 7V (actually 8.5V).

If the LED current was designed for 5mA (still bright in good LEDs, and less heat in the series resistor), then this 1.7mA bleeder will reduce brightness enough to notice. From 10mA to 8.5mA is probably not visible except side-by-side in matched LEDs, but 5mA to 3.3mA might be "dimmer". If objectionable, try 2K (2K2 is a standard value). Now the rail has to sag to 4.7V to fully extinguish the LED.
  Reply With Quote
Old 5th July 2005, 11:18 PM   #4
cowanrg is offline cowanrg  United States
diyAudio Member
 
cowanrg's Avatar
 
Join Date: Apr 2002
Location: Lafayette, CO
Send a message via AIM to cowanrg
wow, thanks.

that helps a lot. its not a big deal, but its just kinda annoying that the light is barely on.

and yes, it is a mosfet amp, so i assumed it stopped playing when it got very low in voltage, and it has a 208,000uf supply per channel, so the little LED has a lot of power to feed from.

ill have to re-read your post to let it all sink in, but thats good info, and that will help me in the future for other things as well.
  Reply With Quote
Old 6th July 2005, 02:51 AM   #5
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
> it has a 208,000uf supply per channel

Yowsa.

Say the amp proper becomes an open-circuit below 10V.

Using my estimate of LED current, we have 0.2Farad(!) times 5,000Ω time constant: 16 minutes! At 16 minutes the rail voltage will be 3.7V, after 16 more minutes 1.37V.

Hmmmm.... that says that the LED should be dead-out after a half hour. I may have mis-guessed the LED current, so it might be an hour or even two. But probably not a "half day".

Big electrolytics have a lot of "soak". Short them to zero volts, let them sit open, the voltage creeps up again. Charge buried deep in the dielectric comes out slowly. But I don't see why that should allow any great extension of LED-linger.

I computed time-constant as if it was draining from 10V to zero. In fact it drains toward 1.5V, the LED threshold. So it may have a long linger around 1.5V. If we assume you can see (in dark) a current of 1% of "normal" current, we get 0.1mA. The slew-rate of 0.2F and 0.1mA is 0.000,5 volts per second. Assuming the LED passes roughly 0.1mA at voltages of 1.5V to 1.6V, a 0.1V spread, it would take 0.1/0.000,5= 200 seconds or 3 minutes. Is it possible you can see an LED with 0.01mA current? Seems unlikely; anyway that still gives ~30 minutes.

I'm baffled.

Three tricks:

AC-operated relay to short-out the darn LED when the power fails.

Give-away that 0.2F cap; 2,000uFd is all anybody really needs.

Duct Tape.

This is one of Nelson's boxes? See if he has a brilliant idea. ("PASS-Tape": looks like ordinary duct tape, but torn into 1"x1" squares and signed by Nelson Pass.)

> its not a big deal

Of course not. I'm just curious. I never saw such a long-linger. And if you have gone to some trouble to darken your movie-theater, a little red eye under the screen is going to stand out like a sore thumb.
  Reply With Quote
Old 6th July 2005, 03:25 AM   #6
Account Disabled
 
Join Date: Aug 2002
Location: Near to the Pacific Ocean
Is it real concern . . . ?

Make this. The light will pop off.

Regards
jH
Attached Images
File Type: jpg led.jpg (8.3 KB, 497 views)
  Reply With Quote
Old 6th July 2005, 08:48 AM   #7
diyAudio Member
 
Greg Erskine's Avatar
 
Join Date: Jan 2002
Location: Sydney/Australia
Am I misunderstanding? Don't you just need bleeder resistors across your 208,000uf filter caps. Seems to easy, so I must have missed something??
__________________
Greg Erskine
  Reply With Quote
Old 6th July 2005, 04:40 PM   #8
jane is offline jane  Norway
diyAudio Member
 
jane's Avatar
 
Join Date: Jul 2004
Location: Norway
What about a zener diode in series with the LED?
If you choose a value of B+/2 for the zener, you just have to wait for the MOSFETs to suck the caps half ways empty before the LED stops lighting...
__________________
Life is hard - Then you die.
  Reply With Quote
Old 6th July 2005, 09:31 PM   #9
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
--Put the LED on the other leg of a DPDT power switch.
--Use a switch to disable some or all lights on the equipment when watching video. Flip the lights on for audio use.
--Don't use an LED at all. Use a light bulb. It will work all the way down to 0V.
...didn't we just go through all this for a preamp that played after the power switch was thrown? So much angst over such a...oh well...never mind...
You folks have fun.

Grey
  Reply With Quote
Old 7th July 2005, 03:10 AM   #10
cowanrg is offline cowanrg  United States
diyAudio Member
 
cowanrg's Avatar
 
Join Date: Apr 2002
Location: Lafayette, CO
Send a message via AIM to cowanrg
there are a million ways to do what im asking.

i guess im just trying to learn electronics a little bit better. now that i kinda understand how bleeder resistors work, im trying to learn more about their applications, and other methods to do similar things. ultimately, im asking these questions so sometime down the road, i wont have to ask them, or if somene else asks them, i can fully answer.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Bleeder resistor! lanchile Chip Amps 15 24th February 2009 10:06 PM
Bleeder Resistor Placement dsavitsk Tubes / Valves 2 31st December 2005 12:38 AM
Bleeder resistor action sam9 Solid State 8 15th May 2003 08:48 PM
Bleeder Resistor Advice KevinLee Solid State 17 23rd March 2003 10:15 PM


New To Site? Need Help?

All times are GMT. The time now is 01:05 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2