One for the electronics wizz kids

[Steve_D]

The car I'm building now has LED lights all round but in its former life had a 5 pin flasher relay to provide flashers to the towing socket.
I will now have LED on the car and filament bulbs in a trailer.

Old flasher units required the current of filament bulbs in order to work so will not work with LED. Later came electronic flashers units.
The fact that companies now sell flasher units for LEDs leads me to think I cannot just fit an electronic flasher to deal with both.

The messy solution, as I see it, would be to retain the existing flasher and fit resistors to my LED lights.

Is there a better way forwards?

Thanks in advance.
Steve

[stevieturbo]

What make is the car, and what does a dealer offer for towbar fitment ?

[Steve_D]

It was a Range Rover Classic and is now becoming a Dakar.

The OE fitment has all the towbar wiring already in the loom and includes all the wiring and warning lights in the dash for the trailer.

I'm now fitting LED flashers but these will not be on the trailer as I may tow a variety of different trailers.

Steve

[dilbert]

Can you not just use an electronically timed flasher unit for both?

[Steve_D]

dilbert said:

Can you not just use an electronically timed flasher unit for both?

Well basically that is the question but if it were possible then why are manufacturers now making units specifically for LEDs?

I asked Vehicle Wiring Products today and they said not. I don't want to shell out £15 just to find that it doesn't work.

Steve

[dilbert]

Steve_D said:

dilbert said:

Can you not just use an electronically timed flasher unit for both?

Well basically that is the question but if it were possible then why are manufacturers now making units specifically for LEDs?

I asked Vehicle Wiring Products today and they said not. I don't want to shell out £15 just to find that it doesn't work.

Steve

I guess that's fair enough. From here though I cant tell what the details of the electronically timed units actually are. Obviously the critical parameter is going to be that of load. I don't know what the details of your setup are, although I suspect that the LED's are a smaller load than the older bulbs. If the timed unit can only deal with the LED loading, the you could use a relay or a solid state amplifier to allow the flasher unit to drive an extended load.

Presumably you are proposing to put resistors in parallel with the LED indicator units and use the older flasher? I guess it would work O.K., but those resistors could be geeting mighty hot depending on the difference in load between the LED's and the ordinary bulbs. If you just subtract the wattage of an LED unit from that of an old bulb, that's how much power the resistor will have to dissipate as heat.

If it turns out to be much over 5 Watts, the resistor is going hot get very hot indeed. Bear in mind that the older flasher units often used a bimetallic strip. Under load it would get hot, bend and break the circuit. Once the circuit was broken, the strip would cool and bend back again. This gives some idea of the energy that you're dealing with in this sort of scheme. The heat in the bimetal strip, is parasitic to the power of the incandesent bulbs.

I figure that the best plan is to use a transistor to drive a relay from the electronic timer unit. To the timer, the transistor will look similar to an LED. To the ordinary bulbs on the trailer, the relay will look like the old flasher unit did.

If you go to those lengths it might also make sense to build the whole of your own flasher unit, with decent outputs on it.

How about... http://www.ledshoponline.com/electronic_flasher_ca...

Edited by dilbert on Saturday 12th January 06:59

[Steve_D]

Dilbert. Just spotted the edit.
I have bought an LED specific flasher unit (3 pin) which is marked Max 10W so I think I could just connect that straight to a relay for the trailer as I doubt they draw that much current. I also have (in a box somewhere) one of those trailer warning devices with a buzzer that I can also wire in to get over not having an indicator on the dash.

Will have a look at the circuits tonight and see if I can modify the existing wiring which is for a 5 pin flasher unit.

Steve

[dilbert]

Steve_D said:

Dilbert. Just spotted the edit.
I have bought an LED specific flasher unit (3 pin) which is marked Max 10W so I think I could just connect that straight to a relay for the trailer as I doubt they draw that much current. I also have (in a box somewhere) one of those trailer warning devices with a buzzer that I can also wire in to get over not having an indicator on the dash.

Will have a look at the circuits tonight and see if I can modify the existing wiring which is for a 5 pin flasher unit.

Steve

Hmmm.

10Watts is certainly not enough for four or even eight indicator bulbs. So the man on the phone looks to be correct. Obviously it all depends on the total rating of the LED's, and the relay that you use, but as long as the total is not greater than 10W you'll be OK. I think it's pretty reasonable to assume that you can run the LED's and a relay on 10W, but in the worst case you could always run 2 relays from the flasher unit, one for the LED's and one for the trailer.

The only thing to watch out for, if you connect a relay directly to the timer unit, is that you put a diode in parallel with the relay coil. The polarity of the diode should be such that when things are operting normally, the diode does not conduct.

I suspect that the output of your flasher unit is solid state (transistor). That would explain the 10W limit. If it is, and you don't use the diode, the relay could easily damage the output transistor in the flasher unit.

When the current in the relay coil is stopped, the magnetic field it was making to keep the relay contacts closed, collapses. As that field collapses, it generates an instantaneous pulse of current in the same direction as that which created it. If the current cannot flow, the voltage simply increases until it can flow. If you drive a relay with a mechanical switch you will see the spark that it generates when this happens. Clearly small transistors and sparks don't mix very well.

Edited by dilbert on Saturday 12th January 10:55