Resistor help needed

[sladey]

I recently changed the 911R style lights on the back of my car - I fitted lights (following the recommendations of our very own Mr Keith) with a unique feature - they can be seen at night time.

More specifically they are LED lights from a Land Rover - they fit the 911R frame perfectly.

However there is a slight problem. The indicator units flash rapidly as if the earth connection wasn't good ( it is good - it's perfect).

I believe the problem is that the new LED indicators aren't providing enough resistance thereby causing the flash unit to flash too quickly.

I think I'll need to add an in-line resistor to the bulbs but does anyone have an indication of what size resistor I need to go for?

Thanks in advance

Sladey

[Jonny Hart]

You'll need to add a resistor in parallel (i.e. across the LED unit) to simulate the load of a filament bulb. 5 to 10 ohms should be about right but it will need to be a power resistor (something like 50W) and it may get quite hot.

Alternatively, you might find an LED compatible flasher unit.

[sladey]

Thanks Jonny

BY "across the LED unit" what do you mean? There are two wires going into the unit - live and earth - I was thinking of routing the live wire through the resistor - is that not right?

[Bruce M]

Nope.
The led doesn't draw enough current, I think, so you need to provide a low resistance bypass that will let more "juice" flow, simulating the old bulb.

Might be better to get a more suitable relay.

[Bruce M]

Like this:

http://www.autobulbsdirect.co.uk/LED-Flasher-Unit.html

[sladey]

Thanks for that as well Bruce - you don't happen to know which of the three would replace the 911 one do you?

Also will it cause problems that the front indicators are non-LED?

[sladey]

OK I've ordered one of the 3 pin ones now - I'd already ordered the resistors as well!

Prefer the relay option but lets see if that works

Thanks both

[DustyM]

Can you not fit Led bulbs to the front to keep things consistent?

[Jonny Hart]

The flasher unit will probably work fine with a led/bulb combination. Be aware that these units often make no sound. The 'tick tock' indicator sound originally is made by the bimetallic strip inside the flasher unit heating / cooling which causes the contact to make / break. A solid state flasher uses an IC to do the switching, so is silent but some do have a buzzer/sound option.

[Google parallel vs series resistor to see what I mean't about connecting 'across' the led]

[sladey]

Thanks everyone

With the help of uncle Google I've sussed the parallel setup if I need to go down that route

I'm hoping the new relay will just do it

With regard to just putting LED bulbs in the front I'm guessing that the ones you buy as replacement have resistors built into them so they work with an old relay? Just guessing here.

Anyway I've already got 2 solutions in the post to me at little cost (£8 for resistors, £10 for the relay)

Thanks again everyone

Mark

[Bruce M]

The modern relays use a simple timer IC like the 555 chip instead of the mechanical approach so work with normal bulbs or LED (check the amp rating though).

[sladey]

Cool thanks

[smallspeed]

take a look at the "motogadget" flasher unit - its for motorbikes but works on anything - it is completely smart and maintains flasher speed regardless of load.. I was going to use one on my car, but it can't handle the load of 4x 21W plus 2x 5w side repeaters for hazards, so have gone the traditional route. should be fine if you have 2 or more LED's in the circuits though

[sladey]

Thanks

[sladey]

The relay arrived today - plugged it in lunchtime - worked perfectly front and back

Fantastic - thanks for the help guys