Led resistor question for electrical expert

[Kevin K]

I have a Casita trailer with a water pump switch.

I want to add a LED light to show when the water pump switch is on. The switch runs on 12 volt.
Most 12 volt systems charge from 12 volt to 15 volts.
My LED bulb is rated at
Forward voltage is 3
and
Reverse current is 30
Using this web site
LED calculator for single LEDs

What resistor should I get?
1/2 watt 330 ohm for 12 volts

1 watt 390 ohm for 13 -14volts

1 watt 470 ohm for 15 volts

[Darwin Maring]

Go to CrapShack and purchase a LED pilot lite that has the resister built in and will be labeled as a 12VDC operating voltage.

[Thomas G.]

These $2 modules are really handy for electronics. You adjust the voltage that you want out with a little screw driver. So for instance, you could adjust this to drop the 12 to 15 volts to 3 volts. If you wanted to charge a phone or iPod, you could adjust one to 5 volts. More expensive than a resistor, but more useful, too.

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[GeorgeR]

I would go with resistor for 13 or 14V or simply replace the switch with a switch with LED built-in such as this one: LED Dot Rocker Switch
George.

[Kevin K]

GeorgeR
Thanks
I will use the 13 to 14 volt

This is the LED I have
with a blue LED light.

[Darwin Maring]

R U Sure that thing does not have the resistor built in?

[Kevin K]

Bought 10 for .99 cents.
Tried one on a 12 volt battery charger and it got TOASTED.

[Thomas G.]

Originally Posted by Kevin K

Bought 10 for .99 cents.
Tried one on a 12 volt battery charger and it got TOASTED.

What exactly did you do to make it toast?

[Kevin K]

Hooked it up to 12 volts. That's why I need a resistor to bring voltage down. I found out the bulb can only handle 3 to 3.4 volts.
Description ~

- RoHS : Yes
Life Rating : 100,000 Hours
Emitted Color : Blue
Size (mm) : 5mm
Lens Color : Water clear
Peak Wave Length (nm) : 460 ~ 470
Forward Voltage (V) : 3.0 ~ 3.4
Reverse Current (uA) : <= 30
Luminous Intensity Type Iv (mcd) : 6000(Typical) ~ 8000(Max)
Viewing Angle : 20~25 Degree
Max Power Dissipation(PM): 80mW
Max Peak Forward Current(IFP): 75mA
Lead Soldering Temperature : 240 Degree (<5Sec)
Operating Temperature Range : -25 ~ +85 Degree
Preservative Temperature Range : -30 ~ +100 Degree

[Raz]

The 30 microamps is a reverse bias leakage current. Since you are using DC, the diode is never reverse biased. Not an issue.

Try (15-3)/.02=600 ohms I would go to the lower standard value which is 560 ohms. The .02 is 20 milliamps, a typical LED forward current. The 470 will make it a little brighter. Power is 1/2 watt in either case. Have fun. Raz

[ericw]

You are correct that you definitely want the resistor. Since you are using the LED for indication and not illumination you can back off of the maximum current rating significantly. In fact at 14V and 390 ohms you are technically over the max power dissipation of the part (but I'm guessing it will not blow up being only a little over). Since running cool generally relates to longer life, I usually try to stay away from the limit and have a safety factor. Personally I would use something around 1,000 ohms (1k resistor), but it depends on how bright you want it to be.

If you do go with the adjustable resistor, I would put two resistors in series. I would put ~390 in series with the adjustable resistor in series with the LED. That way if the adjustable goes to 0 ohms it won't blow up the LED.

A couple of other notes. First, you mentioned the reverse current of 30 uA. Your forward current max is 75mA (micro vs milli). Obviously you want to use this in the forward current mode, not reverse. However, the power dissipation and not the current is what I would pay the most attention to (but preferably pay attention to both). Second, the 1 Watt resistor is more than enough. 1/2W is sufficient and if you are happy with the 1000 ohms you can use 1/4W (but bigger wattage always gives you more safety margin).

Good luck and let me know if you have problems or questions.

[Byron Kinnaman]

Originally Posted by P. Raz

The 30 microamps is a reverse bias leakage current. Since you are using DC, the diode is never reverse biased. Not an issue.

Try (15-3)/.02=600 ohms I would go to the lower standard value which is 560 ohms. The .02 is 20 milliamps, a typical LED forward current. The 470 will make it a little brighter. Power is 1/2 watt in either case. Have fun. Raz

The circuit diagrams indicate milliamps not microamps. Typically in all the years of designing electronics I always set for 20 milliamps at the highest expected voltage, which in the case of your trailer is about 14.1 volts. Which would require about a 470 ohm resister. Typical forward voltage drop across an LED is 2 volts. Therefore it's 12 volt drop across the resister, which would be 1/4 Watt.
A larger resister might make the LED too dim to see, even at nominal 12.6 your not going to be very bright and dimmer as the battery voltage drops.

[Raz]

Originally Posted by Byron Kinnaman

The circuit diagrams indicate milliamps not microamps. Typically in all the years of designing electronics I always set for 20 milliamps at the highest expected voltage, which in the case of your trailer is about 14.1 volts. Which would require about a 470 ohm resister. Typical forward voltage drop across an LED is 2 volts. Therefore it's 12 volt drop across the resister, which would be 1/4 Watt.
A larger resister might make the LED too dim to see, even at nominal 12.6 your not going to be very bright and dimmer as the battery voltage drops.

1) Kevin used the spec. sheet reverse current of 30 microamps to do his calculations. I corrected him.
2) He is using a blue LED. They typically have a barrier potential of 3 volts.
3) He specified the 15 volts.

Spec's are in post number 9.

[Hambone]

Kevin, I just went and bought a automotive led indicator light think it was at fleet farm, drilled hole in sink cabinet front and wired it in, if i did it again I would have moved the switch (on sink cabinet side) and would have mounted a combo light and switch on the front