LEDs in vintage receivers - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 7th February 2013, 11:28 PM   #1
srinath is offline srinath  United States
diyAudio Member
 
Join Date: Feb 2010
Default LEDs in vintage receivers

Hi,
I am trying to understand how LED's work.
I have looked into the LED resistor calculator and I know what resistor I need in the circuit.

However my question is a little fundamental ...

The LED measures something like 1200 ohm.

So why does it measure 1200 ohm in both directions ? Shouldn't it measure open in one direction ? its a diode after all.

Then the thing measures 1200 ohm, put across say a 3.4v source - its correct forward voltage, it should work just fine right ? However this is what I get from the calculator -
The wizard recommends a 1/8W or greater 1 ohm resistor.

What does a piddly 1 ohm do to a 1200ohm circuit ?

Thanks.
Srinath.
  Reply With Quote
Old 7th February 2013, 11:34 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
They're diodes, which means that ohmmeter resistances aren't particularly significant.

LEDs emit light when forward biased. The voltage drop depends on the color- red LEDs, for example, drop 1.6-1.7V when they're on. 5mA is a typical current and safe 99% of the time. So you can calculate the series resistor from Ohm's Law by subtracting the diode's forward drop from the source voltage (that gives you the voltage you need to drop across the resistor), set I = 5mA, then calculate the resistor from R = V/I.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 8th February 2013, 01:01 AM   #3
srinath is offline srinath  United States
diyAudio Member
 
Join Date: Feb 2010
OK so now I got more questions - What does drop 1.6-1.7v mean ? As in a 3.4 v dc source will lose 1.7v ?

My led's are 20ma - does that mean something ? These are 3mm dia, If= 20ma Vf - 3.4 - 3.6 V.
They are supposedly blue. So I should follow that calculator pretty close huh ?

Thanks.
Srinath.
  Reply With Quote
Old 8th February 2013, 01:23 AM   #4
diyAudio Member
 
Join Date: Nov 2005
The forward drop means you will have that much voltage across the diode before it goes into forward conduction. 3.4 to 3.6 volts forward drop is typical for blue diodes, but it also means that if you only have a 3.4 volt supply you will only be able to force microamps to 1 mA through the diode, with no dropping resistor added. The lower and higher voltages might be measured at a minimum versus maximum current respectively.

If you have a DC source it's not an issue, but blue diodes have about a 5 volt reverse breakdown voltage, so be careful not to connect the diode backwards with the power on, if you have, say, a 12 volt or more supply, especially not without a current limiting resistor in series.

Normally you subtract the expected forward drop from the power supply voltage when calculating the resistance needed for a specific current. So for instance 5V - 3.5 = 1.5 / 100 ohms = 15mA. For high life you probably shouldn't stuff more than 10mA through a diode rated 20mA max. As was already implied, 5mA is usually enough for indicator lights.
  Reply With Quote
Old 8th February 2013, 01:43 AM   #5
srinath is offline srinath  United States
diyAudio Member
 
Join Date: Feb 2010
Oooo that is what forward voltage means.
So let me rephrase it. Below voltage that it will not light.
Above that - it will light, but unless you soak up the excess with a resistor that keeps the current down below the forward current (like way below the forward current), it will fry the diode.
If its equal to that, since the diode resistance is 1k ohm or so, you wont get enough current through the diode to light it up worth a darn.

Cool.
Srinath.
  Reply With Quote
Old 8th February 2013, 03:38 AM   #6
diyAudio Member
 
Join Date: Nov 2005
I attempted another reply but decided to check and sure enough this does a better job:

Light-emitting diode - Wikipedia, the free encyclopedia
  Reply With Quote
Old 8th February 2013, 10:48 AM   #7
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Wow! All those fun facts, pics and references and not a jot of maths...
__________________
Ian
  Reply With Quote
Old 8th February 2013, 11:07 AM   #8
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
....of course one have to ask why the Japanese choose to run their bulbs at AC and not DC

after that we may be able to discuss how and if its possible to replace them with LED

kind regards
sakis
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 8th February 2013, 01:33 PM   #9
srinath is offline srinath  United States
diyAudio Member
 
Join Date: Feb 2010
My LED says AC or DC.

Also I measure DC in the yamaha cr840 I am working on. But they have a 27v dc line running 2 14.4 v bulbs in series.

Cool.
Srinath.
  Reply With Quote
Old 8th February 2013, 01:53 PM   #10
diyAudio Member
 
KMossman's Avatar
 
Join Date: Jan 2013
Location: now in Suzhou, China
Send a message via Yahoo to KMossman Send a message via Skype™ to KMossman
Quote:
Originally Posted by srinath View Post
My LED says AC or DC.

Also I measure DC in the yamaha cr840 I am working on. But they have a 27v dc line running 2 14.4 v bulbs in series.

Cool.
Srinath.
LEDs are diodes. and like tunnel, zener, etc they have unique properties.

So, sure they can be used in AC circuits. On the website Instructables there was a guy who build a 220VAC LED lamp - runs off VAC directly - no rectification at all.

As long as you are careful and aware of their limitations/constraints.....
__________________
Getting back into business, though non-audio related. Have excellent contacts for NEW [OEM] Russian and Chinese tubes. Custom chassis, and parts [MOQ applies]
  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
help 2 receivers connect 22306 PA Systems 2 25th November 2012 03:27 PM
Anyone Collect Vintage Receivers? VintageReceiver Everything Else 0 7th November 2010 01:57 AM
FS: Transformer for vintage Pioneer receivers maurycy Swap Meet 2 26th August 2010 02:43 AM
Trade my blue LEDs for White LEDs CaliforniaBob Swap Meet 7 26th March 2004 08:28 AM
WTT: Blue LEDs for Red & Red/Green LEDs CaliforniaBob Swap Meet 2 9th June 2003 03:58 PM


New To Site? Need Help?

All times are GMT. The time now is 07:40 AM.


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