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Reduce LED Brightness

zoom38

Mar 30, 2021
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Good evening,

I have (2) red/green LED panel indicator lights that put out 250mcd (each) of light. For my application, this is too bright. They are 24vac that utilize 15ma (each) for a total of 30 ma. My 24vac power supply is rated at 50ma. How can I reduce the brightness? I've been looking around the web and it looks like inserting a power limiting resistor is the way to go but I am not sure.

Thank you for your help in advance.
jw
 

Bluejets

Oct 5, 2014
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Resistor is one way.......but LEDs are a DC device.
Why you would need 24v in any case is beyond me as it means large power disipation in the resistors.

Do you have a drawing of your circuit..??
Include as much info as you can.
 

Martaine2005

May 12, 2015
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If your panel indicatators are indeed 24V, they are probably AC or DC compatible.
But as @Bluejets already stated, a resistor is one way.
For a 2.8V LED @ 15mA, a 1.5KΩ half watt resistor is required. As we don’t know what resistor is in place now or how bright your liking is, you can add another and reduce or increase the value until you’re satisfied.

Martin
 

zoom38

Mar 30, 2021
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Mar 30, 2021
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Ok, sorry for the confusion. Bear with me I'm new to electronics so i'm learning as I go along.
I have an Stancor STA-3524 24v power supply. Supplies 24vdc directly to (2) Dialight 645-621-1124-304F Red/Green panel indicator lights. I chose 24volt because in my inexperience, I was concerned of voltage drop across my house. However the voltage drop turned out not to be a concern because i ended up moving the power supply to within 30' of the panel indicators. I chose the STA-3524 because it had the closest ma rating to the load. There currently are no resistors installed.
 

Martaine2005

May 12, 2015
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I chose the STA-3524 because it had the closest ma rating to the load

And the load being just the LEDs?.
According to the data sheet, the resistor is internal.
And it’s 24VDC.
Kind of OTT for just two LEDs.
So add another series resistor.

Martin
 

dave9

Mar 5, 2017
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I would use a series resistor. The question is how much (%) brightness do you want? If you want 50% of the current brightness, then as Martaine2005 mentioned, it is probably about 1.5K resistor internal so add a 1.5K 1/2W in series.

Another way to reduce brightness is just put a thin coat of low-medium pigment paint over them. ;)
 

Externet

Aug 24, 2009
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Feeding LEDs with AC may become lethal to them. I would replace your 24VAC supply with a 5V usb (phone) type of wall transformer available at many forgotten drawers and evaluate brightness.
 

dave9

Mar 5, 2017
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^ It is a 24VDC supply, which I would not have chosen, along with not choosing 24V LED modules, but zoom38 already has them, they have integrated resistors (don't know if they can be opened and swapped with a different value resistor), and are fairly expensive for what they are.

If it were something higher powered, I'd be concerned about the wasted power, dropping from 24V to around 2V, but in this case is only 15mA (30mA if both lit simultaneously, except the goal is even less current for less brightness), a fraction of a watt so (depending on regional power cost) it could take decades for equivalent replacement parts more than a resistor added, to pay for themselves in power savings, plus the effort to do so, I mean this must already be installed so putting a resistor in series is as easy as it's going to get.
 
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