led woes

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iblis

Member - Posts: 19

Joined: April 06, 2006

Location: Ireland

Posted: February 24, 2008 at 3:33 PM / IP Logged

Hello all. Hoping someone can tell me what I'm doing wrong here.

I've been experimenting with trying to put an LED (built in resistor for 12V) in place to indicate whether the fog lights are active.

The two scenarios are as below:

In the series scenario, the LED lights, but the fogs do not come on.

In the parallel scenario, the fogs come on, but the LED does not.

Neither of these are quite what I had hoped for. led woes -- posted image.

I'm sure I've done something silly here, but my brain has turned to mush after ages of looking at this. Any help very much appreciated! Would be nice to be able to reassemble the interior of the car led woes -- posted image.

Thanks

i am an idiot

Platinum - Posts: 13,667

This member consistently provides reliable information

Joined: September 21, 2006

Location: Louisiana, United States

Posted: February 24, 2008 at 5:49 PM / IP Logged

Connect the positive lead of the LED to the same place you connect the fog light positive wire. Ground the LED to ground.

iblis

Member - Posts: 19

Joined: April 06, 2006

Location: Ireland

Posted: February 24, 2008 at 6:02 PM / IP Logged

Thanks for the reply. Do you mean to connect it in parallel with the fog lamp?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: February 24, 2008 at 7:06 PM / IP Logged

If you want to stay on the trigger side wire the LED in parallel with the relay coil (the + of the LED to 12vdc and the - of the LED to the (-) switch).

Or, you could do the above and you would be on the switch side of the relay, which, at the end of the day, is the important part to monitor.

Kevin Pierson

i am an idiot

Platinum - Posts: 13,667

Joined: September 21, 2006

Location: Louisiana, United States

Posted: February 24, 2008 at 7:09 PM / IP Logged

As Kevin mentioned either way will give you what you need.

iblis

Member - Posts: 19

Joined: April 06, 2006

Location: Ireland

Posted: February 25, 2008 at 1:57 AM / IP Logged

That's great, thanks guys! Your help is much appreciated.

Any ideas on what went wrong the first time? Be great to learn something from this experience.

iblis

Member - Posts: 19

Joined: April 06, 2006

Location: Ireland

Posted: February 25, 2008 at 4:14 AM / IP Logged

Would I be right in assuming that in the series situation from my original post the resistance was too high to energise the coil, but there was enough to light the LED? And in the parallel case, the current was finding earth so never going through the LED?

Thanks again!

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: February 25, 2008 at 5:30 AM / IP Logged

Since you have a 12vdc LED it has a current limiting resistor built in. When you put that in series with the coil of the relay it prevents enough current from flowing through the relay coil to energize the relay. Had you gone with a 'normal' LED it may have worked.

The parallel setup has no way for current to flow, as you are connecting both leads to the same signal. You need a difference of potential to turn the LED on.

So your assumptions are correct!

Kevin Pierson

iblis

Member - Posts: 19

Joined: April 06, 2006

Location: Ireland

Posted: February 25, 2008 at 5:42 AM / IP Logged

Thanks Kevin!

Really appreciate you guys taking the time to reply. This could have taken quite some time for me to resolve on my own!

Ah well, at least I'm learning led woes -- posted image.

Cheers

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led woes

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