# LED resistor

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#### DSGarcia

##### New Member
Not quite sure, but I have an illuminated switch with an LED. Brightness specs are given "@20mA" and the forward voltage is given as 2.3V. I have a 24VDC supply.

What is a good value for the resistor? 24V@20mA gives me 120 ohms which seems low to me.
Thanks,
Dale

#### Diver300

##### Well-Known Member
You have a decimal point in the wrong place. It should be 1200 hm:.

In fact, if you allow for the forward voltage, only 21.7 volts is left across the resistor. So that should be 1085 hm: to get 20mA

However, you won't loose much brightness at lower currents so you could use 1.5 khm: to 2.2 khm:. That would also be less likely to damage things if the voltage is a bit above 24V.

#### shaneshane1

##### New Member
If your not to sure how that works out its done like this.

24V(power supply) - 2.3V(the LED's voltage drop) / 0.020(20mA) = 1085ohms

so

24 - 2.3 / 0.020 = 1085

anywhere close to that will be fine

hope this helped you as well

#### DSGarcia

##### New Member
Thanks for the help.
Dale

#### Hero999

##### Banned
Also note the power disipation of the resistor will be 21.7^2/1200 = 392.4mW so a 1/4W resistor won't do, a 1/2W resistor will work fine.

#### slosjo

##### New Member
It may or may not be important for your application, but it dosen't hurt to also add a small ceramic cap (0.01uF or so) across the anode and cathode of the diode. It will help absorb voltage spikes that could be present if the LED was part of a larger system that could be subject to EMI.

#### Hank Fletcher

##### New Member
Hero999 said:
1/4W resistor won't do
But two in parallel will. In that case, double the resistance.

#### slosjo

##### New Member
Hank Fletcher said:
But two in parallel will. In that case, double the resistance.

yes, or half the resistance and put them in series...same effect.

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