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IR LED, do I need a resistor?  

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MadMisha
(@madmisha)
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Joined: 7 months ago
Posts: 44
2020-07-12 12:31 am  

I am building a remote for my AC that uses IR. I am planning on using a 2N3904 transistor and I need to calculate the resistor for the IR LED. I have chosen one that has a 1.3V forward voltage (1.6 max) and a forward current of 800mA. I calculate it to need a 4.265 ohm resistor and it will consume 2.96W. I don't want to have to get a resistor that can handle more power. I could run 2 2ohm 1/2W resistors but that is still 1/3 of the power.

 

My question is, since it is such a low value resistor needed and an IR LED would not be continuously lit, could I get away without the resistor? Or could 2 in parallel at a 1/2 watt rating work considering the short duration of use? The AC unit does have a long code to send compared to a normal remote.

 

Any advice would be greatly appreciated.


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triform
(@triform)
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Joined: 1 year ago
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2020-07-12 3:17 am  

Hi @madmisha,

You would want one. Damage is cumulative and at some point, the LED will probably stop working before it's time.

thanks.


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MadMisha
(@madmisha)
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Joined: 7 months ago
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2020-07-12 9:24 pm  

Thank you @triform,

 

I was hoping that I could sacrifice the longevity on a component that only costs a few cents and could be replaced easily. I don't think anyone can answer how long it will really last so I guess the best decision would be to use a resistor.

 

But that leaves me the question of the resistor. Can 2 half watt resistors handle the short bursts? Otherwise I would have to use 3 1ohm 1 watt resistors. Unless there are 3 watt resistors. I was hoping to use common components.


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triform
(@triform)
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2020-07-13 2:05 am  

With LEDs, I typically add a resistor that I like for the brightness. For IR, I would use the datasheet recommendation though. For small bursts that happen occasionally, you could probably get away with a .25watt resistor in the 10R or so.  You never said what your circuit voltage is? 


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MadMisha
(@madmisha)
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Joined: 7 months ago
Posts: 44
2020-07-15 3:49 pm  

@triform, Yeah sorry for leaving out that bit of relevant information. I am using a 5 volt supply.

 

I have some .5 Watt carbon film resistors I will probably use. I might use 2 5R in parrallel. I will begin testing of that soon. I do have few lower ohm resistors to bring it closer to the calculated voltage but I will play it safe at first.

Thanks


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