Hope I can ask this here. I just put electric fans on my radiator of my truck. I want to know when they turn on and off. Long story with temp control sending unit switches that I won't go into. The fans draw 13 amps according to the paperwork. I want to put a 12 volt LED in my dash to tell me when they're on. Got the 12 volt LED but it's rated at 25mA max forward current. So if I install this LED with its "HOT" lead to the 12 volt side of my fans hot wire and the other lead, neg. to my trucks ground will it blow my LED?

One person said yes but I say no because the LED will see minimal current wired in the way I want to wire it. Am I correct?

Thanks for taking the time away from what I'd rather be working on too to look at this question.
Johnny

:Hope I can ask this here. I just put electric fans on my radiator of my truck. I want to know when they turn on and off. Long story with temp control sending unit switches that I won't go into. The fans draw 13 amps according to the paperwork. I want to put a 12 volt LED in my dash to tell me when they're on. Got the 12 volt LED but it's rated at 25mA max forward current. So if I install this LED with its "HOT" lead to the 12 volt side of my fans hot wire and the other lead, neg. to my trucks ground will it blow my LED?
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:One person said yes but I say no because the LED will see minimal current wired in the way I want to wire it. Am I correct?
:
:Thanks for taking the time away from what I'd rather be working on too to look at this question.
:Johnny
:
Johnny,
Alot of us on the Forum know electronics and tubes.
We have shared LED monitoring circuit designs with much success. We will need some more details, schematic, equipment ratings, manufacturer. Your design? The LED will fry if you hook it up directly to the circuit. You will need a current limiting power supply to drive the Led's. Would you like them to flash, multi color, this could be a fun project.
Look forward to more info.
Mitch

If your LED is designed to run at about 12VDC, and comes fitted with an appropriate current limiting resistor, connect the positive wire of the LED to the fan side of the thermostatic switch, and connect the negative wire of the LED to the negative side of the wiring system, which is usually part of the metal chassis, and the chassis will be a fine point to connect to for this purpose.

If you are concerned that the fan draws a lot of current and the LED is only supposed to take a small amount of current, current is the amount of electricity flowing through an object. With the LED wired in parallel with the fan, each will draw its necessary current separately, and the two will, for the most part, not affect eachother in any way.

If your LED is not designed to run at from 12 to 14VDC, then you will have to supply it with a current limiting resistor. Often LEDs are designed to run at from 1.5 to 2.5 volts DC. Find out what your LED is rated at for voltage, and calculate what resistance will be necessary to drop the remaining voltage at the maximum allowable current. Use the formula 'E=IR,' where 'E' is electromotive force, or voltage, 'I' is current, and 'R' is resistance. The electrical system in your car probably runs at 13.6VDC, 14 to be safe. Let's say your LED is rated for 2VDC at 25mA. That leaves 12V to be dropped by the dropping resistor at 25mA. 12/.025=480, so you need a 480 ohm dropping resistor. EI=W, so 12*.025=.3 Watts. You will probably be fine with a 1/4 watt resistor.

This is all approximate. Hopefully the information you have gives a normal current rating in addition to the maximum current rating so that you may more safely calculate the required resistance.

:Thanks guys! My truck is very customized. '93 F-150 with a '66 289cid high performance engine just for starters. I have an identical LED that is wired into my fog lamps in the front bumper. Tells me when the fog lamps are on so I don't forget to turn them off. This LED has worked fine for 5 years now. Don't know what kind of current the fog lamps draw.

The LED I have for the fans is rated at 12 to 16 VDC 25mA max current. I plan to wire it just as Thomas stated. It is a panel mount LED that is fitted with an internal current limiting resistor. I can see the resistor and it appears to be blue, gray, red, gold. 6.8K 5% tolerance or 680 ohm? It doesn't measure when I attach my meter leads to the red and black lead coming from the LED. I'm not familiar with how this resistor is designed into the LED. I believe that if I wire it in as Thomas stated it should work just fine.
Johnny
:
:
:If your LED is designed to run at about 12VDC, and comes fitted with an appropriate current limiting resistor, connect the positive wire of the LED to the fan side of the thermostatic switch, and connect the negative wire of the LED to the negative side of the wiring system, which is usually part of the metal chassis, and the chassis will be a fine point to connect to for this purpose.
:
:If you are concerned that the fan draws a lot of current and the LED is only supposed to take a small amount of current, current is the amount of electricity flowing through an object. With the LED wired in parallel with the fan, each will draw its necessary current separately, and the two will, for the most part, not affect eachother in any way.
:
:If your LED is not designed to run at from 12 to 14VDC, then you will have to supply it with a current limiting resistor. Often LEDs are designed to run at from 1.5 to 2.5 volts DC. Find out what your LED is rated at for voltage, and calculate what resistance will be necessary to drop the remaining voltage at the maximum allowable current. Use the formula 'E=IR,' where 'E' is electromotive force, or voltage, 'I' is current, and 'R' is resistance. The electrical system in your car probably runs at 13.6VDC, 14 to be safe. Let's say your LED is rated for 2VDC at 25mA. That leaves 12V to be dropped by the dropping resistor at 25mA. 12/.025=480, so you need a 480 ohm dropping resistor. EI=W, so 12*.025=.3 Watts. You will probably be fine with a 1/4 watt resistor.
:
:This is all approximate. Hopefully the information you have gives a normal current rating in addition to the maximum current rating so that you may more safely calculate the required resistance.
:

Johnny,
As Thomas said, if your LED is current limited (resistor) for 12V operation, install it as you suggested. Just like your fog lights install.

marv

::Thanks guys! My truck is very customized. '93 F-150 with a '66 289cid high performance engine just for starters. I have an identical LED that is wired into my fog lamps in the front bumper. Tells me when the fog lamps are on so I don't forget to turn them off. This LED has worked fine for 5 years now. Don't know what kind of current the fog lamps draw.
:
:The LED I have for the fans is rated at 12 to 16 VDC 25mA max current. I plan to wire it just as Thomas stated. It is a panel mount LED that is fitted with an internal current limiting resistor. I can see the resistor and it appears to be blue, gray, red, gold. 6.8K 5% tolerance or 680 ohm? It doesn't measure when I attach my meter leads to the red and black lead coming from the LED. I'm not familiar with how this resistor is designed into the LED. I believe that if I wire it in as Thomas stated it should work just fine.
:Johnny
::
::
::If your LED is designed to run at about 12VDC, and comes fitted with an appropriate current limiting resistor, connect the positive wire of the LED to the fan side of the thermostatic switch, and connect the negative wire of the LED to the negative side of the wiring system, which is usually part of the metal chassis, and the chassis will be a fine point to connect to for this purpose.
::
::If you are concerned that the fan draws a lot of current and the LED is only supposed to take a small amount of current, current is the amount of electricity flowing through an object. With the LED wired in parallel with the fan, each will draw its necessary current separately, and the two will, for the most part, not affect eachother in any way.
::
::If your LED is not designed to run at from 12 to 14VDC, then you will have to supply it with a current limiting resistor. Often LEDs are designed to run at from 1.5 to 2.5 volts DC. Find out what your LED is rated at for voltage, and calculate what resistance will be necessary to drop the remaining voltage at the maximum allowable current. Use the formula 'E=IR,' where 'E' is electromotive force, or voltage, 'I' is current, and 'R' is resistance. The electrical system in your car probably runs at 13.6VDC, 14 to be safe. Let's say your LED is rated for 2VDC at 25mA. That leaves 12V to be dropped by the dropping resistor at 25mA. 12/.025=480, so you need a 480 ohm dropping resistor. EI=W, so 12*.025=.3 Watts. You will probably be fine with a 1/4 watt resistor.
::
::This is all approximate. Hopefully the information you have gives a normal current rating in addition to the maximum current rating so that you may more safely calculate the required resistance.
::
:

When I wrote my response I was just going by what information you gave, including 25mA maximum. However, that is a maximum, and anything after that will blow the LED. Your LED (the bulb itself) is probably rated to normally run at a current much lower than 25mA. The 6.8K ohm resistor will probably maintain this normal current rating throughout the 12-16V range of your car's electrical system. Most generating systems also don't go much above 13.6V, though occasionally they do, so it's good that your LED was designed with a current dropping resistor to allow for this voltage range.

...And if current and voltage may still confuse you (thinking that the LED may blow because other items in the car draw more current), remember that current is -amount-, and voltage is -strength-. If your car's generating system started producing too much voltage, the LED would burn out. However, no matter how much current your car's generating system produces or how much current is used by the other devices in your car, your LED will not burn out if the generating system still maintains the same voltage or pressure. Think of it like water. Increase the amount of water in the system but maintain its pressure, and all of the 'holes' or 'resistances' in the system will maintain the same water flow. Increase the pressure at any amount of water, and the holes or resistances will increase water flow. ...And all of the current drawing devices in your car are paths that the electricity can flow. Each one will not increase flowage in another, but if they all draw too much from the generating system, flowage will be -reduced- in all of them. If your fan were to draw too much current, your LED would actually get -dimmer-.

T.

T.

Also, you will probably not be able to measure the resistance of the resistor within your lamp assembly because the LED has resistance and voltage drop of its own. You may be able to measure the resistance of the entire circuit if you orient your meter leads correctly, as the LED has infinite resistance in one direction, and conducts in the other. However, if your meter's battery voltage does not exceed the voltage drop across the LED, you might not get a resistance reading in any direction. In any case, if you needed an accurate reading of the resistor, you'd need to measure directly across the resistor. Most likely the resistor is a 6.8K ohm, as the colors seem to indicate. You really do not need to know the resistor's value for any purpose, since the lamp assembly was most likely properly designed at the factory. However, for curiosity's sake, if you wanted to measure the resistance, this is what you'd have to do.

T.

:Also, you will probably not be able to measure the resistance of the resistor within your lamp assembly because the LED has resistance and voltage drop of its own. You may be able to measure the resistance of the entire circuit if you orient your meter leads correctly, as the LED has infinite resistance in one direction, and conducts in the other. However, if your meter's battery voltage does not exceed the voltage drop across the LED, you might not get a resistance reading in any direction. In any case, if you needed an accurate reading of the resistor, you'd need to measure directly across the resistor. Most likely the resistor is a 6.8K ohm, as the colors seem to indicate. You really do not need to know the resistor's value for any purpose, since the lamp assembly was most likely properly designed at the factory. However, for curiosity's sake, if you wanted to measure the resistance, this is what you'd have to do.
:
:T.
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