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24V LED bulbs

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rd2(at)evenlink.com
Guest

Posted: Wed Aug 16, 2006 4:26 am Post subject: 24V LED bulbs

Anyone know of a good source for 24V LED bulb replacements?

I found some 24V bulbs (at) superbrightleds.com but not in the needed config.

What I am looking for is 15 mm bajonet base, 2 prongs at the same level, 2

contacts on bottom. Also smaller bajonets like 1820 bulb.



Intended to replace e.g. 306, 306, 1820 bulbs.



Any pointers?



Rumen

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Aug 17, 2006 6:16 am Post subject: Re: 24V LED bulbs

LEDs have a series resistor to limit the current. That is the only difference in LEDs run at different DC voltages. These resistors are often built-in so check the specs. So if you have a 24V LED you need a resistor to limit the current to what it would be at 12V. This does not have to be exact. The only caution is that the wattage must be calculated. W=I X I X R



For example: If the 24V LED has a current of .030 Amps, and a Vf of 2V then the needed resistor is--



R=(12-Vf)/.030    R=330 Ohms approximately. For 24V this would be:

R= (24-Vf)/.030   R=750 Ohms approximately. So you would need to add R=750-330=430 approximately (Forgive the "resistor-math") if the resistor is built into the 12V LED.



For the 430 Ohm resistor you will need a 0.030 X 0.030 X 430 Watt resistor, or 4 Watts.



Remember, you can put two 12V LEDs in series pairs to avoid having to mess with a resistor.



Search Google for "automotive LED". There are more every day.

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

rd2(at)evenlink.com
Guest

Posted: Thu Aug 17, 2006 10:42 am Post subject: 24V LED bulbs

Thanks, Eric.

I was really looking for a lazy way out - direct replacement, but absent

that will have to pull out the meter and do some work 

Rumen

do not archive



_____________________Original message __________________________

                  (received from Eric M. Jones; Date: 07:16 AM 8/17/2006

-0700)

________________________________________________________________




LEDs have a series resistor to limit the current. That is the only

difference in LEDs run at different DC voltages. These resistors are often

built-in so check the specs. So if you have a 24V LED you need a resistor

to limit the current to what it would be at 12V. This does not have to be

exact. The only caution is that the wattage must be calculated. W=I X I X R



For example: If the 24V LED has a current of .030 Amps, and a Vf of 2V then

the needed resistor is--



R=(12-Vf)/.030    R=330 Ohms approximately. For 24V this would be:

R= (24-Vf)/.030   R=750 Ohms approximately. So you would need to add

R=750-330=430 approximately (Forgive the "resistor-math") if the resistor

is built into the 12V LED.



For the 430 Ohm resistor you will need a 0.030 X 0.030 X 430 Watt resistor,

or 4 Watts.



Remember, you can put two 12V LEDs in series pairs to avoid having to mess

with a resistor.



Search Google for "automotive LED". There are more every day.



--------

Eric M. Jones

www.PerihelionDesign.com

113 Brentwood Drive

Southbridge, MA 01550

(508) 764-2072

emjones(at)charter.net





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=55311#55311







--

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bob(at)bob-white.com
Guest

Posted: Thu Aug 17, 2006 3:39 pm Post subject: 24V LED bulbs

On Thu, 17 Aug 2006 07:16:13 -0700

"Eric M. Jones" <emjones(at)charter.net> wrote:



 	  Quote: 	
		   

 

 LEDs have a series resistor to limit the current. That is the only difference in LEDs run at different DC voltages. These resistors are often built-in so check the specs. So if you have a 24V LED you need a resistor to limit the current to what it would be at 12V. This does not have to be exact. The only caution is that the wattage must be calculated. W=I X I X R

 

 For example: If the 24V LED has a current of .030 Amps, and a Vf of 2V then the needed resistor is--

 

 R=(12-Vf)/.030    R=330 Ohms approximately. For 24V this would be:

 R= (24-Vf)/.030   R=750 Ohms approximately. So you would need to add R=750-330=430 approximately (Forgive the "resistor-math") if the resistor is built into the 12V LED.

 

 For the 430 Ohm resistor you will need a 0.030 X 0.030 X 430 Watt resistor, or 4 Watts.

 

 Remember, you can put two 12V LEDs in series pairs to avoid having to mess with a resistor.

 

 

 

 Search Google for "automotive LED". There are more every day.

 

 --------

 Eric M. Jones

 www.PerihelionDesign.com

 113 Brentwood Drive

 Southbridge, MA 01550

 (508) 764-2072

 emjones(at)charter.net

 

 

 

Math check.  0.03 X 0.03 = 0.0009
	


0.0009 X 430 = 0.387, so a 1/2 W resistor should be OK.



Bob W.

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Aug 17, 2006 3:48 pm Post subject: Re: 24V LED bulbs

Bob,



Thanks, 



Yes the decimal point vanished someplace.

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Sat Aug 19, 2006 7:59 pm Post subject: 24V LED bulbs

Remember, the forward voltage drop on most red LEDs is about 1.5V.  

The forward voltage drop for white LEDs is about 3.0V. You can put  

these LEDs in series such that the combined forward voltage drop is  

less than the minimum bus voltage and then add a series resistor to  

control current.



For instance, if you are using a group of red LEDs that need a 20 mA  

current and you want to power it from your 14V bus, you can do the  

following. Since you have a minimum bus voltage of 11V, you have to  

have a string of LEDs that needs less than 11V. That means you can  

put 6 red LEDs in series (6 x 1.5 = 9V) and then put a series  

resistor that will limit the current to 20 mA for the max voltage.  

The max voltage on your bus is probably 15V. That means that you need  

a resistor that will pass 20mA for a drop of 6V (15V-9V). That means  

a 4V/.020A = 300 ohm resistor.



If you want to do this with white LEDs use 3V per LED instead of 1.5V  

per LED.



If you want to do this for a 28V electrical system then you have a  

minimum voltage of 22V and a max voltage of probably 30V. You want a  

string of LEDs to be less than 22V forward drop so I would pick  

probably something like 14 red LEDs with a forward drop of 14 x 1.5  

or 21V. Now I would need a series resistor to limit the current to  

20mA for a drop of 30V-21V or 9V. That would be 9V/.020A = 450 ohms.



If you are doing this for a single LED just change the voltage drop  

value.

Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

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