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# LEDs to indicate rotation of motor

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#### 3dOptics

##### Member
I would like a LED to turn on when a motor used for positioning is turning clockwise or counter clockwise. Ideally I would like to use only one LED, but if it would be easier to use two LEDs that would be okay. Since with a LED power only flows from one lead to the another I plan to hook one up backwards. This way when the motor turns one direction one LED will turn on while the other will turn off. I hooked up the LEDs with a 1k 1/4w resistor and LED does not get hot, neither does the resistor.

Here are the specs on the LED:

LTL-4234

Later on I used the resistor calculator here: and it calculates I should use a 1.8k 2w resistor. So should I buy two 1.8k 2w resistors or do you think the 1k 1/4w resistor would be fine?

Thanks.

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it calculates I should use a 1.8k 2w resistor. So should I buy two 1.8k 2w resistors or do you think the 1k 1/4w resistor would be fine?

I presume big resistors are undesired. How long is the motor typically on for ? If it's less than a few seconds, then the Resistor Calculator (which de-skills the process of design) won't take advantage of pulse rating. Traditional doorbell transformer is given high overloads but it survives fine because is occasional use.

You can often user larger resistor values without any noticeable drop in LED intensity. Larger resistors result in less current and less heat.

You said the calculator result was 2.1K but went with 1K resistors which will result in 2X the current and heat you had in the calculator.

Go the other way and try a 2.8K resistor to start with. The higher the resistor value the lower the wattage will have to be. But at some point it will not be visible.

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Connect two LEDs wired back to back across the motor. There's no need for two resistors, they can both share the same resistor.

You can buy bipolar LEDs which have two LEDs (the same colour) in the same package or bicolour LEDs which have two different colour LEDs in the same package.

I presume big resistors are undesired. How long is the motor typically on for ? If it's less than a few seconds, then the Resistor Calculator (which de-skills the process of design) won't take advantage of pulse rating. Traditional doorbell transformer is given high overloads but it survives fine because is occasional use.

I do not mind using a big resistor. The problem is all I have are 1/4w resistors so I would have to order one from the internet since I do not have any local electronic suppliers. The motor will be turned on for 1 minute max(the time it takes to move from one extreme position to the other).

You said the calculator result was 2.1K but went with 1K resistors which will result in 2X the current and heat you had in the calculator.

Go the other way and try a 2.8K resistor to start with. The higher the resistor value the lower the wattage will have to be. But at some point it will not be visible.

I first used the 1k 1/4w resistor since it was attached to the LED and I thought the worst case I will burn out the LED. So far the led and resistor do not get hot. Then I thought it might the a good idea just to check online and see if 1k would be fine. I found the online LED resistor calculator and it suggested I use a 2.8k 2w. I would use a 2.8k 2w if I had one but I do not. I am just wondering how critical it is that I use a 2.8k 2w instead of the 1k 1/4w.

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Assume your motor is running at 36V and you want ~10mA to light the led.
Ignoring the small voltage drop (1.7V) across the LED:
36V/0.01A=3600Ω ...... 3300Ω is a standard value.
(36V*36V)/3300Ω=0.39W ...... 0.5W is a standard value.

So a 3.3KΩ-1/2W resistor in series with the anti-paralleled LEDs should work.

ken

With the LEDs connected as per your crude sketch the LEDs will be destroyed because each LED will be subjected to a reverse voltage of 36V when it's only rated for 5V.

You need to connect the LEDs back-to-back or use a bpolar LLED as I suggested in my previous post.

Simple arithmatic shows a 1k resistor dissipating 1.15W when the supply is 36V and it is inn series with a 2.1V green LED. A 1/4W resistor will be smoking.

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