Thread: Discovery 3 LED trailer lights

Originally Posted by Graeme

A resistor will not prevent the reverse EMF generated by a coil when the current is interrupted from affecting electronic components at the supply source - it requires a diode correctly connected across the coil. A diode passes current in 1 direction only whereas a resistor passes current in both directions, albeit with some resistance!

Yes, that is correct, the diode is the most effective design to totally eliminate the reverse EMF. As in all things automotive, less than ideal solutions are often implemented in the name of cost or long term effectiveness.

Not too much goes wrong over time with a resistor and it appears that the effectiveness of the resistor depends upon the resistor chosen. In the case of the original 70 amp air compressor relay R7, part # YWB500040, per the pdf below, the chosen resistor is 560 ohms in parallel with the 90 ohm coil to yield a total Resistance of about 78 ohms, + or - 8 ohms.

I was not able to find any data on the new replacement 70 amp relay, part number YWB500220, but there is some indication the 560 ohm parallel resistor is now fairly common in these type of relay designs.

This then brings to mind an interesting question. Does the nominal 6 ohm 50 watt load resistor in the LED adaptor effectively provide EMF suppression and hence a plain relay is OK? The answer may be yes.

A second question also arises. If one only used the resistor style relays, would the internal resistor be big enough to soak up the milliamp signals sent down the signal light wires by the computers to effectively arrest the "test blinking" of the indicators; also is the resistor big enough to fool the electronics into cancelling the backup sensors and the like? In this case, probably the answer is no.

I wasn't aware that resistors are now being used but it appears that 560 ohms must be a value that is low enough to allow sufficient reverse current flow to prevent damage to the power source components yet not so low as to impose an unnecessary current draw when the relay is operated. A distinct advantage of a resistor over a diode is that no polarity for the coil has to be observed because current flows in either direction, which I expect is the reason for the change from a diode rather than cost or longevity.

A resistor of lower value than 560 ohms will have an increased ability to allow reverse current to flow so the 6 ohm resistor in the LED flash canceller circuit will indeed provide the necessary protection.

Edit: The flashing is prevented because the relays don't operate in the very short time the flash power is supplied, but the omission of the 6 ohm resistors will cause the electronics to think no trailer is attached because the relay doesn't draw enough current to be seen as an incandescant globe.

I hope that the resister in a box was cast alloy.the resisters i fitted have a warning that they can operate at 140Cel.

I burnt a finger on one.I tryed to use one on a flasher timer control and not thinking mounted to the floor inside the dash.

Resister protected relay coils--not realy intended to protect electronics from spikes-used with systems like air conditioning clutch coil and fan motors that use mechacical contact switches that may give irratic switching that may create spike surges.

some relays have diode[zenor??] acros the contact [or coil]? -but that wont earth the spike..these are a pain when you guess the wrong pin layout when trying to work out the circuit set up due to the feed back nature of diode.comon on radiator fan control systems.

Hi Guys,

I fitted the linear electronic design module and it worked perfectly, used the in car mount version which has the switch to turn the unit on when towing (a trailer with led's) which disables the reverse sensors and allows the rest of the system to work correctly Then turn off when not towing to allow reverse sensors to operate again. You only need to connect a wire to each blinker (yellow & green) cable in the rear left compartment where the tongue is stored and the earth which has a connector already fitted ready to use a existing earth in the rear compartment, fitted the switch in the underside of the left rear indent side cover, then turn off when not towing. I could fit one in under 15 minutes very neat and solved all the problems. Did cost $265 but worked a treat and was delivered the next day. Thanks LG Fossicker for the info previously in this post.

Originally Posted by Davehoos

some relays have diode[zenor??] acros the contact [or coil]? -but that wont earth the spike..these are a pain when you guess the wrong pin layout when trying to work out the circuit set up due to the feed back nature of diode.comon on radiator fan control systems.

Not a zener diode, just a switching diode. The diode is used to provide a path for the reverse EMF that has less resistance than the path back through the electronic switching component. Zener diodes don't conduct until their trigger voltage is reached which then provides a limit for over-voltage situations. The diodes and resistors have nothing to do with any voltage spikes across the contacts that can occur when a coil or heavy current load is switched by the contacts, where a capacitor is often used to absorb the charge to prevent arcing.

Originally Posted by Davehoos

I hope that the resister in a box was cast alloy.the resisters i fitted have a warning that they can operate at 140Cel.

I burnt a finger on one.I tried to use one on a flasher timer control and not thinking mounted to the floor inside the dash.

Yes the load resistors can get hot, hence I used what is called here an FS box. It is not very delicate or cheap to purchase either; made of sand cast aluminum and with or without the cover can be driven over with no damage to the box but maybe to the tyre. This is another reason I tend to think that the pre-made units are not that bad an alternative.

I am starting to think that for our purposes, a smaller load resistor might be OK - say the 21 watt variety as we are only trying to duplicate a single 21 watt filament in a P21 bulb. Using a pair of the smaller resistors should reduce the current draw and hence the heat buildup to be dissipated.
LED Load Resistor - Products - Narva

I am inclined to think the 50 watt ones are to duplicate front and rear signal bulbs operating at the same time on the same side.

The resistor wattage is how much power they will withstand, not draw. The resistance value determines how much current will be drawn (V=IR & P=VI). At a nominal 12V a globe that consumes 21W of power draws 1.75A which means that it has a resistance of approx 6 ohms. The 50W resistor can dissipate 50W of power so really only a 25W 6 ohm resistor is needed but it will get a lot hotter because it cannot dissipate the heat so readily and will be working near its rated maximum power.

Edit: One cannot be sure whether the Narva specs are referring to a heat dissipation capacity of 21W or to replace a 21W globe as its 8 ohms resistance equates to 18W at 12V but when added to a LED light cluster might total 21W as per an incandescent globe. 8 ohms will only draw 1.5A at 12V so less heat will be produced.

Another edit: The Narva resistors must be referring to simulating a 21W globe rather than a statement of their power dissipation capacity as they are also marked as 12V and voltage is not a characteristic of a resistor.

I appreciate that clarification, particularly the comments re the Narva resistors. I have been comparing various load resistor advertising and was getting even more confused.

In short, a resistor is not an incandescent light bulb that the more the watts, the more current draw and hence more heat. (I did actually know that but was starting to question it.) A resistor is more like a water tap - you can have a lake behind it but only so much water will flow thru it.

Hence regardless of wattage rating, a load resistor as long as they are around 6 ohms across 12 volts will approximately simulate a single 21 watt filament. The resistors ability to dissipate the heat generated will be better the larger the rating. That makes sense. Thanks.