Which resistors to use for LED?

Hello All

I am building a simple motor controller and wish to add LEDs to indicate when the motor is energised one way or the other.

I am aware of online utilities such as http://led.linear1.org/1led.wiz which assist in calculating the resistor required for an LED at a given voltage, but my problem is the given voltage (motor supply voltage) can range from 3v to 36v.

What is the recommended approach in selecting resistors when the voltage is not known in advance?

The only option I can think of is to use a resistor for the maximum rated voltage and make do with very dim LEDs (if they light at all) at the lower voltages.

Alternatively I gues I could use a pot but that is a pot per LED which will mean 4+1 pots for a dual motor + motor power supply LED. Seems a bit over the top to add 5 pots and then top expect the user to set all 5 pots depending on the motor supply voltage...

Any advice would be appreciated.

What is the recommended approach in selecting resistors when the voltage is not known in advance?

Use a constant current source.

rather than drive the LEDs directly from the motor drive them from the motor control signal

Use a constant current

Something like this:- http://uk.farnell.com/on-semiconductor/nsi45030at1g/ic-ccr-led-driver-linear-sod-123/dp/1787605

@AWOL & Mike Thanks regarding info about constant current source. I have never heard of such so I do appreciate your link to clarify Grumpy. :D

@mmcp42 I could do that, and it is a possible solution, but if I wish to use the LEDs as a debugging tool to ensure the motors are actually getting current I need the LEDs on the motor wires rather than on the control signal. I will consider it since it is a cheaper solution at this time - I do not have any LED drivers, but next time I place an online order for parts I will include the LED drivers.

It might be that I end up simply putting on a resistor as applicable to the largest voltage and replacing with a constant current source when such time comes... I will need to see if the LED lights up with the lowest voltage.

Always learning more :)