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Author Topic: LED replacements for dial lamps  (Read 1779 times)
K5NT
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Posts: 25




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« on: June 14, 2009, 03:42:55 PM »

My station runs on battery power, and I would like to replace the 6-8 incandescent dial lights in my equipment with LEDs because of their low current requirements.  Are LED replacements available?  It would be great if they had bayonet sockets to facilitate installation.
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N5RMS
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Posts: 37




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« Reply #1 on: June 14, 2009, 04:06:33 PM »

Yes, they are available but rather expensive.  You can make them yourself using the old lamp base, a 1/8 resistor and a small LED.  I did this in my Dentron MLA2500 amp.
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KZ1X
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Posts: 3227




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« Reply #2 on: June 14, 2009, 04:30:06 PM »

Chicago Miniature Lamp

http://www.cml-it.com/cgi-bin/start.cgi/home.html
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K5NT
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Posts: 25




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« Reply #3 on: June 16, 2009, 02:59:19 PM »

FYI:

A friend of mine emailed the following:  "Most LED's operate at 1.2 volts at about 20 ma.  If your battery is supplying 12v, subtract 1.2 from 12 which leaves 10.8v.  Divide 10.8 by 20 ma to get the value of the resistor you need(10.8/.02=540. A 560 Ohm resistor will work fine here.  The power dissipated by the resistor will be about 1/4 Watt.  You might want to use a 1/2 Watt resistor since the light will be on for long periods of time."  This is the info I was looking for.  Thanks to those who posted earlier replies.
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KZ1X
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« Reply #4 on: June 16, 2009, 04:13:13 PM »

"Most LED's operate at 1.2 volts at about 20 ma"

Really?  I've never seen an LED that would draw any current at all at less than 1.5 volts and even then it was so dim I could barely see it.
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WA7NCL
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Posts: 625




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« Reply #5 on: June 17, 2009, 08:00:11 AM »

1.2 or 1.5 is not really very important if you run from 12V.  The drop across the LED is small compared to the current setting resistor.  Mathematically you can see that the voltage drop across the resistor sets the LED current.  10 or 20 mA for a garden variety LED seems about right.  

If you want to be accurate you should consult the data sheet of the LED.  It will give the values of light output vs current.  Once you select the current, consult the voltage vs current info in the data sheet and get the voltage drop on the LED and do the calc. per the other post.

If the purpose is to increase efficiency, you might consider putting more than one LED in series.  Then the drop across the current setting resistor will be less and less power will go into the current setting resistor.

As you add more in series, you will not get as good current regulation with changing 12V because the diode drops are now a larger percentage of the total drop.  You could design a low dropout current source with a PNP transistor to allow many LEDs in series and just enough voltage across the PNP to regulate the current.  This would result in maximum efficiency.

There, I think we've beat this topic to death.
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N4TTS
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Posts: 151




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« Reply #6 on: June 17, 2009, 02:25:02 PM »

Not quite beaten to death, here are the sexiest LED lamp replacement solutions for meter illumination I have found.

These are max cool. Everyone who has seem my shack loves 'em.

http://www.environmentallights.com/categories/1303_2334/led-ribbon-flex

Don N4TTS
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KB1LKR
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Posts: 1899




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« Reply #7 on: June 17, 2009, 02:59:26 PM »

"Most LED's operate at 1.2 volts at about 20 ma"
"Really? I've never seen an LED that would draw any current at all at less than 1.5 volts and even then it was so dim I could barely see it."

Correctly stated:
Most LED's operate at about 20mA and the voltage drop across them at that current is about 1.2 volts"

(or perhaps a little more, say 1.5-2 volts -- it may vary a little w/ LED color too), but they are fixed current devices.

The critical parameter is 20mA, though it is also important to have the available source voltage be greater the nominal diode junction drop.
For stable supply voltage, a simple fixed current limiting resistor is sufficient.
If much more that a couple volt supply is available (say 12VDC) than LEDs should be wired in series to add up to most of the available voltage, e.g. 6 LED's w/ a 1.5V drop per LED will drop about 9 volts, so would be fine on a 12VDC supply with an appropriate series resistor to drop 3V at 20mA (e.g. 150 ohms for 20mA dissipating 0.06W) will work well. This way less power needs to be dissipated in the current limiting resistor. All the LED's will see the same current.
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ZS5WC
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Posts: 410


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« Reply #8 on: June 18, 2009, 01:03:15 AM »

Do a search on EBAY under the CB section for LED, hundreds to choose from , pre-wired and most already have the resistors in series..
Check here:
http://electronics.shop.ebay.com/items/?_nkw=led&_sacat=40054&_trksid=p3286.m270.l1313&_odkw=&_osacat=40054

73 de William
ZS4L / ZS5WC
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