12vdc circuit for LED indication in car

Since the LED and fan voltage are equal, parallel should work. One thing to check is if the additional current to the LED overloads the fan fuse. I think this is unlikely, but....

You can very well connect the LED circuit in parallal with fan circuit. Go a head

Anand.Mandarapu

cheers thanx for the help

Just to be sure:

A parallel device as you indicate will normally only show that power is available to the fan. This may be all the indication you want, but it is not a fail safe design. This is because any form of open circuit failure in the fan will cause it to stop without there being any change in the voltage supply that is measured by the parallel device.

One possibility for a fail safe design is something that also depends on current flow for an "ON" indication to be given. This of course assumes that in the event of a motor short type failure, that a supply fuse will blow and the supply current will reduce from a high value to zero.

The other way is: inserting an air flow sensor in series with the LED.

am i missing somthing here. why not just use a lighted switch. Those switches light up when you turn them on,and have no light when they are off. or did you just want to know when your fan actually comes on?If so, just run a wire from the hot lead close to your fans motor through a fuse,for safety, then to your light,then ground. That way, every time your fan kicks on, your little light will glow.

You are probably missing a couple of things.

First is that it may not be possible to swap out the existing switch.

Second is that your solution is not fail safe - an open circuit motor will not work but will still have power to it and your design will still show "OK"

Couldn't you just connect the LED in series on the ground side of the fan motor? that way as long as the fan makes a complete circuit the LED should light up.

Yep, it'd probably light up alright. This would be followed by a puff of smoke!

At an educated guess there are two big reasons why "a LED is series" is a bad idea. First is the "technical" issue that it would take away some of the voltage supply to the motor, but teh big big reason I would guess is that the motor current would be way to high for the LED to accommodate

Not quite but yes, you would need a series resistor and diode with the led. The resistor to limit current +- 1K ~ 10mA and the diode 1N4007 forward biased to stop any reverse voltage across the LED (they don't like much reverse voltage!!!)