# Resistor for LED indicators



## Pudsey_Bear

I've finally got around to fitting LED rear lights including indicators, However the MOT regulations state they they must flash between 60 and 120 times per minute, mine are doing somewhat more than that so I need a resistor.

Any one know what value they would need to be, the flasher can is in the worst possible position I can't even see it or feel it so I would need to fit to each light.

Kev.


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## pippin

The Law as determined by Herr Ohm is what you need to apply.

You should have paid more attention in your physics classes at school!

R=V/I

W=VxI

Unfortunately, I have to dash so I don't have the time to work it out for you now.

I will be around later to do it if no one else beats me to it.


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## rayc

Ascertain the wattage of the bulb you are replacing e.g. 15w.
Divide it by 12 [volts] which is 1.2 [amps] approx.

Divide the volts by the amps to give the resistance which in the above example is 10 [ohms].

Are the resistor kits listed at the bottom of the link below of any help to you?

http://www.solware.co.uk/customise-car-part-accessories/stop-and-tail-led-light-bulbs.shtml


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## Pudsey_Bear

rayc said:


> Ascertain the wattage of the bulb you are replacing e.g. 15w.
> Divide it by 12 [volts] which is 1.2 [amps] approx.
> 
> Divide the volts by the amps to give the resistance which in the above example is 10 [ohms].
> 
> Are the resistor kits listed at the bottom of the link below of any help to you?
> 
> http://www.solware.co.uk/customise-car-part-accessories/stop-and-tail-led-light-bulbs.shtml


The bulbs being replaced are 12 v 21watt

I didn't want to pay through the nose for a kit, I hoped I could go to maplins and get them there.

The ones on the web site link are 33 Ω +/- 5% 10 w and should be wired across the turn signal bulbs to simulate a filament bulb load, but not sure what that means exactly, but sounds like I should connect the earth and positive through the resistor, I has assumed I would just put it in line with the positive lead

21w/12v = 1.75 amps

12v/1.75amps = 6.857Ω or 7Ω

Now I'm stuck as I don't do electronics, do I just ask for 2 x 7ohm resistors, or is it more complicated than that.

Maplins resistors seem to start at 
10Ω to 30Ω Metal Film 0.6W Resistors, and go up to 1MΩ to 10MΩ Metal Film 0.6W Resistors, all way over my head.

interesting if you understand it

Kev.


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## rayc

You are trying to make the circuit think that there is a bulb there with a resistance of approx 6 - 7 ohms. To do this the resistor must be connected across the bulb and not in line with the + leg.

I think that the resistors that are shown in the original link are for side light bulbs which are 5w, which at 12v give a resistance of approx 30 ohms. They are intended to make bulb failure circuits think that a bulb is present. See this link for an explanation.

http://www.phoenixautobulbs.co.uk/led-resistor-kit-p-306.html

Be aware that the resistor must be rated as aminimum for the same wattage as the bulb i.e. 20 watts. See the link below which says ideally they should be 50watt:

http://www.ukclassifieds.co.uk/pair...d_indicator_stop_tail_light_bulbs-o51002.html

Ray


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## Pudsey_Bear

Thanks Ray,

Do you know what the correct resistor description (size/value) should be, as I don't want to pay £6 for something which cost less than £1 for 2.

Kev.


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## Pudsey_Bear

I might have the answer to part of my question below why I need a proper load resistor, not a ceramic one...

Can anyone beat this price on ebay

Kev.


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## CliveMott

YES you need an aluminium housed resistor which needs to be bolted to some stout chunk of metalwork to dissipate the heat. But before you start drilling any holes just lash it up electrically and check that it works. The resistance of an illuminated 12 volt 21 watt indicator bulb will be about 7 ohms. However this is not a "prefered value" so go for a 6 ohm resistor. This will pass 2 amps and dissipate 24 watts when the volts are 12.
BUT some flasher units rely on the INRUSH CURRENT of the bulb to cause it to flash at the correct rate. The resistance of a 12 volt bulb is approximately between one tenth and one twentieth of its hot resistance. So if not limited by wires and the like the instantaneous current when the volts are applied to the bulb can be as much as 40 amps until the filiament warms up.

So, get a contact cooled 6 ohm resistors and try before you go any further.

Some of the "dummy loads" for this duty include a couple of resistors and a capacitor to simulate the inrush scenario.

Or Change the flasher unit to one which does not monitor the bulbs. After all you don,t have a bulb to fail.

Hella part number 4AZ-001-879-011 could be interesting if you can graft it in.


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## Pudsey_Bear

CliveMott said:


> YES you need an aluminium housed resistor which needs to be bolted to some stout chunk of metalwork to dissipate the heat. But before you start drilling any holes just lash it up electrically and check that it works. The resistance of an illuminated 12 volt 21 watt indicator bulb will be about 7 ohms. However this is not a "prefered value" so go for a 6 ohm resistor. This will pass 2 amps and dissipate 24 watts when the volts are 12.
> BUT some flasher units rely on the INRUSH CURRENT of the bulb to cause it to flash at the correct rate. The resistance of a 12 volt bulb is approximately between one tenth and one twentieth of its hot resistance. So if not limited by wires and the like the instantaneous current when the volts are applied to the bulb can be as much as 40 amps until the filiament warms up.
> 
> So, get a contact cooled 6 ohm resistors and try before you go any further.
> 
> Some of the "dummy loads" for this duty include a couple of resistors and a capacitor to simulate the inrush scenario.
> 
> Or Change the flasher unit to one which does not monitor the bulbs. After all you don,t have a bulb to fail.
> 
> Hella part number 4AZ-001-879-011 could be interesting if you can graft it in.


Thanks Clive, that's a different slant, I've just had a quick search for a local stockist, but found none, I'll have a ring round on Monday.

Would that be a straight replacement for the std flasher relay or an add on.

If it's an add on I might need a bit more info on connecting it into the system.

Kev.


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## CliveMott

Hi Kev,
Its a replacement job but its almost certain that some re-wiring will be required to make it work. Its not straight forward but that is a flasher unit to use if one is designing a non load sensing system.

Hella UK are in Banbury, Wildmere Ind est.

C.


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## Pudsey_Bear

Hmm, that might be a bit awkward to do, as it's a very tight area under the dash, as it's got electric everything, and just full of wires.

The resister route might be the safer option, at least I won't need to work upside down.

Shame though a fit and forget relay would have been preferable.

Kev


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## sallytrafic

Can I just ask Kev why did you fit the LED indicators?


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## dwwwuk

If the circuits need to be biased for the bulb failure detection electronics to work (i.e. by drawing the same current as with filament bulbs)

What's the benefit of fitting LED's to these? Is it a style thing or functional benefit?

I can see the benefit for simple installs without bulb failure detection - including less current = less fuel (in theory anyway)


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## cater_racer

sallytrafic said:


> Can I just ask Kev why did you fit the LED indicators?


Absolutley ! If youre now going to simulate a bulb by putting aload resistor in coverting the energy to heat instead of light, the whole exercise has been an expensive waste of time, you've just made life more complecated and likely to fail.......


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## Pudsey_Bear

I did cover the reasoning behind this in another thread, oddly I can't remember anyone saying it was a waste of time/money.

The original tail lights were of a bad design internally as they would keep blowing the fuse, the contacts were arranged in such a way that +/- were so close that a simple pot hole would be enough to make contact, I had tried everything to stop this but nothing worked.

I also needed to replace the rear fog as it had broken off.

I now have combined stop/tail/flasher in one unit, totally water proof, absolutely no problem with connections, it also gave me two redundant indicator pods and two redundant red lenses, which became to rear fogs instead of the single £2.99 after market crap the importers fitted.

Once the flash rate is sorted I should never have a problem with any of the rear lights.

As a bonus, while working around the rear end underneath I noticed 
that the way the rear valance was held on was less then perfect and would likely fail, it is now rock solid, and held together with stainless nuts etc instead of pop rivets.

I also was able to remove and repair the damaged corner post, as I could see how they were fixed on, which is now ready for paint which will save me in excess of £1,000 and/or increased premiums for the next few years.

Kev.


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## Pudsey_Bear

I went with RayC and his Link

All fitted and working, Thanks to RayC, and all others who offered advice.

I now have nice LED flashers, and stop/tail lights.

Hopefully, they'll last more than the mile the others did before blowing fuses.

Kev.


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