Are resistors needed?

[Cornish4]

Hi folks, I need some help please. Having fitted front fog lamps a while ago, and recently added roof mounted spots to my R4, I thought I would convert them to LED bulbs for in theory a brighter light, less load to the system and longevity.
Now fitted they are no way brighter than the filament bulbs, in fact they are dimmer.
My question is, do I need to fit a resistor in parallel with the feed to the lights to make them function correctly?
I know a resistor is required on the indicator circuit to make them flash correctly, as I found on the Twingo I once owned, but when I fitted front and rear sidelight LEDs on it they were very bright without any mods to the circuitry, as are the the brake/tail LEDs I've experimented with on the 4.
So, any info would be much appreciated, many thanks, regards Brian.

 

[JdeW]

Hmmm, don't think you need a resistor that's only done in modern can-bus system cars for led-bulb recognition. Modern day cars are merely computers on wheels, if you change just 1 thing another thing fails
On the R4 if you install a resistor parallel over a led-bulb then the resistor heats up and thus more load on the 12V system.
For our cars there are 12V Led-bulbs with bayonet available, should be relativily plug&play
Another option is a more powerfull alternator for installing a battery of halogene spots.

 

[KeithS]

My 1975 Porsche is very much pre-CAN-BUS but the LED bayonet bulbs caused 'hyper-flash' – the cure is to use an LED-friendly flasher unit, which you can buy on line very easily. It worked a treat on the Porsche and adds no load to the system.

 

[Cornish4]

Thank you for your replies chaps, yes the 50ohm resistors I fitted to the Twingo did get very hot, and at the time I didn't know about the flasher units designed to be used with LEDs. So I fitted them well away from any cables and all worked well.
Getting back to the auxiliary lamps, the LEDs have normal H3 type fittings, nothing unusual, and I would have thought it was simply just plug n play. To be fair I've only tested them in my garage in the dark, so maybe a road test is called for, but I just thought I would run this by our resident electrical boffins first.

 

[JdeW]

yes the 50ohm resistors I fitted to the Twingo did get very hot...

Ohm's law is easy incalculus, those 50 ohm resistors heat up with 2,88 watts so a bare minimum is at least a 5W resistor, 10W to be safe.
I just stick to filament bulbs as my alternator generates 500 watts and that's more then enough for an extra moderate subwoofer audio system

 

[AdamWilkes]

Use no resistors, they are just a burden on the system. Resistors used with after-market LED bulbs are only for fooling the in-car computers that the (filament) bulb is still unbroken. LED bulbs usually consume far less power (Watts) than the filament bubs they replace - the computers measure the power usage to tell if a bulb has blown - so an LED bulb 'looks' like a blown bulb to the computer. Renault 4's have no computers, so nothing to worry about

 

[KeithS]

Use no resistors, they are just a burden on the system. Resistors used with after-market LED bulbs are only for fooling the in-car computers that the (filament) bulb is still unbroken. LED bulbs usually consume far less power (Watts) than the filament bubs they replace - the computers measure the power usage to tell if a bulb has blown - so an LED bulb 'looks' like a blown bulb to the computer. Renault 4's have no computers, so nothing to worry about

Well, in most cases, if a bulb blows, the indicator on that side will flash faster. Fitting LEDs can have the same effect, fooling the flasher relay into acting as if a bulb is blown. This is what happened with my Porsche. Fitting the LED-friendly flasher relay solved the problem.

 

[JdeW]

Well, in most cases, if a bulb blows, the indicator on that side will flash faster. Fitting LEDs can have the same effect, fooling the flasher relay into acting as if a bulb is blown. This is what happened with my Porsche. Fitting the LED-friendly flasher relay solved the problem.

Hmmm, so the old simpler filament technique is still the cheapest solution

 

[André4Renaults]

Use no resistors, they are just a burden on the system. Resistors used with after-market LED bulbs are only for fooling the in-car computers that the (filament) bulb is still unbroken. LED bulbs usually consume far less power (Watts) than the filament bubs they replace - the computers measure the power usage to tell if a bulb has blown - so an LED bulb 'looks' like a blown bulb to the computer. Renault 4's have no computers, so nothing to worry about

Your fitting extra bulbs, be they ordinary 55 watt H3 bulbs or L.E.D. equivalents is simply adding an extra item to the circuit from which they are running, be it the main beam or an extra one. No resistor is necessary in any part of the circuit as the L.E.D. will draw only that current it needs, as would any other bulb. As long as the wiring is able to handle the load, the circuit is correctly fused and any relays are correctly connected you should have no problems. Any perceived dimming of the bulbs might be due to poor earths or insufficiently thick wiring.

 

[Cornish4]

Thanks again gentleman for your input, and I must admit when I posed the question of the need to fit resistors it was a stab in the dark, no pun intended.
The relays are all correctly wired up, and earths are good, proven by the fact that the lamps are up to spec, brightness wise when the filament bulbs are fitted, and I'm pretty sure that the cable size is also correct. So, I will do a road test, and if the LEDs don't perform well I guess it's back to old school bulbs. Hey ho

 

[angel]

Any headlight or auxiliary unit designed for use with filament bulbs works on the same principle: The reflector (the concave silver plated back of the light) reflects light from the bulb and directs it forward where it's being diffused to the desired beam pattern by the lens. This means that the lamp fitted has to give light at almost 360 degrees, which is the case with filament bulbs.
However, LEDs give straight line light, so they need special light units, with lenses rather than reflectors, in order to produce an accebtable beam. Substitutes for filament bulbs have a number of LEDs around the lamp to try emulating a filament bulb, but obviously these will never become the "real thing". You get fooled by the cooler beam colour, and that's all, no real improvement over bulbs.
In my opinion, there is nothing better than good reflectors paired with 80W bulbs and careful installation.

 

[Cornish4]

Thank you angel, that makes a lot of sense, so it is indeed back to good old filament bulbs. Anybody want to buy some LEDs? hardly used
PS The label on the wrapper they were in said, made in China, say no more.
Do they produce anything that is any good? I don't think so, and as for their latest export........

 

[angel]

That said, a friend of mine fitted cheap 21/5W bulb LED substitutes in the rear lights of his R4 and this made them really bright without looking modern-ish.

 

[AdamWilkes]

Can of worms right there Unregulated brightness of LED bulbs that dazzle or confuse traffic? Traffic is bl88dy easy to confuse, let's face it.

 

[Leonardo.IT]

Maybe the issue is also the type of led bulb used, not all the bulbs and not all the LEDs are the same

 

[Cornish4]

It's interesting you say that angel, because I've fitted some 21/5W tail LEDs too, and they do give a good light, although the brake light side of the LED needs a reflective surface to show up more, which I'm experimenting with at the moment, and I agree Leonardo, it's a bit of pot luck to get the right LED for the job I think.

 

[André4Renaults]

The MOT rules in this area are clear: (paraphrasing) lights must be "matched pairs", or either white or yellow light and must not dazzle on dipped beam. In other words, your dipped headlights, together with ANY other lights which illuminate with them must dip correctly and be wired in the same way. Main beam lights must be "matched pairs" so you can't have 1 on one side and two on the other!

 

[Cornish4]

The roofrack mounted spotlights, which I'm upgrading to 100w, are probably illegal on the Queens highway, but I'm sure they will come in handy at times They are on a separate switch. The bumper mounted foglights are on a switch too, but only work on sidelights and dip beam. So all is legal, I think