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Resistor Value for LED series? Answered

I am new to electronics, just started it as a hobby, and tried my hands to make a LED Series. As I am not aware of the exact calculations to figure out the resistor value, I am facing problems - resistor is getting burnt. I am just seeing a led series bought from the market and trying to make another one based on its circuit & design. The overall scheme & design of the circuit was: 

- I made a series of total 44 white led lights.
- Added a bridge rectifier first (4 diodes joined in a systematic way),
- then came three led's with 1K Ohms resistors each
- then followed 38 plain led's,
- finally three leds again with 1K resistors each.

The last three resistors got burnt after 10 odd minutes. Obviously the resistor value is not right. What should be the general thumb rule to figure it out.

I wish to make few smaller series of 12, 24 & 36 LED's, how many resistors do I add in each?

Will be thankful for some guidance. 

Discussions

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-max-

3 years ago

I actually covered this exact topic in my first Tutorial Tuesdays video. You can see even the thumbnail has 3 LEDs in series with the resistor I calculated!

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-max--max-

Answer 3 years ago

Skip to about 7:37 which is where I start talking very basic LED calculations and theory.

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-max--max-

Answer 3 years ago

Skip to about 10:18 (very next LED example) where I did correctly figure out the resistance needed but neglected the power rating. Watch the video to see what happens!

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bwrussellrickharris

Answer 3 years ago

+All the numbers.

Can we have HQ set up a macro that just auto-responds with this link to questions that contain: LED and series or parallel in them?

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rickharrisrickharris

Answer 3 years ago

As your a beginner I will expand a little - You need to use Ohms law (best look it up.

Voltage = Current x resistance Often printed as V=IR - If you know 2 values you can calculate the other one.

Your LED currents in series will add up so lets assume 10mA each - 12 x 10 Ma = 120 Ma total current.

If your supply is 10 volts. Each LED needs say 2 volts so you resistor will need to drop 10-2=8 volts and pass 120 Ma.

So from Ohms law : R=V/I

R=8/0.120 because a milli amp is 1/1000th of an amp.

= 66.6 Ohms In reality I would go for the next higher standard value - This will give slightly lower current but that is safer.

Is this helpful?

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Yonatan24

3 years ago

Burnt resistors: The resistor values are probably correct, but aren't capable of handling the watts, try a higher wattage resistor

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seandogue

3 years ago

A word of general advise regarding calculations for leds from an experienced electrical engineer. NEVER, EVER use the max forward current value to calculate load resistors, and NEVER listen to advise that suggests you 'should'.

Maximum forward currents are just that. They are the value at which exceeding will almost guarantee failure. I recommend using about 3/4 or max or less to ensure safe operating currents.

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seandogueseandogue

Answer 3 years ago

3/4 OF max not 3/4 or max..

once again, I sure do wish Instructables would offer us the opportunity to edit our posts at least for some period of time following submission. this requirement to either delte and repost or add followups is silly.