## what size resistor

## what size resistor

(OP)

I have an illuminated pushbutton switch. It uses a LED which requires 3.5 VDC. I have 2 power source available.

24 VDC and 12 VDC. What size resistor should I use to drop the voltage to this level? The resistance thru the LED is 2.18 Kohm.

thanks,

Bob

24 VDC and 12 VDC. What size resistor should I use to drop the voltage to this level? The resistance thru the LED is 2.18 Kohm.

thanks,

Bob

## RE: what size resistor

Take the power supply (12 VDC is reasonable) and subtract the LED drop (at a reasonable current). This leaves the voltage you want across the resistor.

Then use Ohms Law to work out the resistor value based on the desired current and calculated voltage.

Finally, double check the power on the resistor.

Using a lower voltage supply means that your resistor can be dissipating less power as heat. But if you get too close to the LED drop, then the current can get unstable if any voltage changes.

## RE: what size resistor

## RE: what size resistor

For 24Vdc, (Vsupply - Vledvf-3.5V)/Iled = resistor

For example, (24 - 2.0-3.5)/.005 = 3.7 kOhms. I guessed

5mA should keep the LED lit and resistor to 1/4W.

3.7 kOhm - 2.18 kohm = is the resistor you need.

Check resistor for heat dissipation.

P = Iled^2 * resistor

e.g. P = .005^2*3700 = 92.5 mWatts

You might still be able to push more current to get brighter

LED. Same steps for the 12Vdc.

## RE: what size resistor

## RE: what size resistor

it came with no details at all. I dont have internet access at work so I take the first one, hook up to my 24 vdc test box and you can guess the rest Im sure. After finding the company and getting the pdf it says "an appropriate resistor must be series connected by user." that would have been useful info to send with the sensor.

thanks again,

Bob

## RE: what size resistor

So resistance = (Vsupply - 3.5) / .02 as a minimum value.

For 12 volts that's 245 ohms at .17 watts.

For 24 volts it's 1.025 kOhms at .41 watts.

So using real world values, try 270 ohms 1/4 watt resistor if you've got 12 volts. For 24 volts, 1 kOhm or 1.2 kOhms.

1/2 watt should work, but a conservative application might want more margin.

These are minimum values. If it's too bright, increase the resistance.

## RE: what size resistor

Keith Cress

Flamin Systems, Inc.- http://www.flaminsystems.com

## RE: what size resistor

they are about $16 each.

thanks for all the help guys. Not only an answer but also how you came up with the answer so I can do this myself in the future.

Bob

## RE: what size resistor

He meant 425 ohms at 12V.

## RE: what size resistor

reallybright. (for a while)Keith Cress

Flamin Systems, Inc.- <http://www.flaminsystems.com>

## RE: what size resistor