# LED Optimum Voltage

Discussion in 'The Projects Forum' started by roscirc, Apr 30, 2012.

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1. ### roscirc Thread Starter New Member

Apr 30, 2012
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Hi All,

I need to calculate the resistors for an LED with the following parameter as per datasheet:

-Optimum Voltage 24V dc
-DC current 21 mA (max)
-Reverse Voltage 5V

I will have to use a power supply of 110V , this and LED type as above come from the design of an existing project.
In the LED datasheet there are no indication about forward voltage but as far as I understand it should be always in the range 1,5/3.8V.

So my question is , what does the Optimum Voltage stand for? would it be possible it is the forward voltage I will have to use to calculate the resistors?

Thanks,

Ros

2. ### #12 Expert

Nov 30, 2010
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Without seeing the document that said, "optimum voltage" I don't have the context to work with. Can you post it?

3. ### roscirc Thread Starter New Member

Apr 30, 2012
9
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Here is the original datasheet

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4. ### #12 Expert

Nov 30, 2010
16,665
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That one already has a resistor in it to compensate for 24 volts.
Just take your 110 volts DC and subtract 24, then calculate the added resistance for .021 amps.

110-24/.021 = 4095 ohms. Next closest value is 4200 ohms, 5%, 4 watts

I sincerely hope you are talking about 110 DC volts! This isn't built for AC.

5. ### roscirc Thread Starter New Member

Apr 30, 2012
9
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Thanks #12.

The strange thing though is that in the original design they connected in series a 150kOhm resistor with which the LED works fine.
The reason why I was trying to calculate it, is that this LED keeps failing over time and I wanted to check whether they sized the resistor correctly but with 150kOhm there is actually a current of 0.57 mA far less than the 21mA.

Another reason for the failing I suspect is a transient reverse voltage of about 200V for few milliseconds which was recorded with a data logger in some case. Would this short time be enough to cause a breakdown?

thanks,

Ros

6. ### #12 Expert

Nov 30, 2010
16,665
7,313
Oh yes. You should put a diode in parallel with the LED to bypass the reverse voltage spikes around it.
or a series diode worth about 400 volts. 1N 4005 or better.

You know how?

7. ### roscirc Thread Starter New Member

Apr 30, 2012
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That's interesting but not sure how I should connect it..

8. ### roscirc Thread Starter New Member

Apr 30, 2012
9
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I guess the Diode in parallel should be connected as "inverse parallel" but what type should I be looking at?

Would you have some internet resources about protecting LED this way?

Thanks,

Ros

9. ### #12 Expert

Nov 30, 2010
16,665
7,313
No. I never even looked for a web page about paralleling a common rectifier with an LED to protect from reversed voltages.

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Apr 30, 2012
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Thank you!!

11. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Please read our ToS, if I interpret this correctly powering LEDs from mains is directly mentioned. Am I mistaken about where the power is coming from?

Power anything like this from mains is a major safety hazard. If I am you may contact the mods via PM or email.