I've been buying led lights for the rear of my bus, and although the stop/tail, fog & reverse lights are all 24v, the direction indicators I chose were only available in 12v. If necessary I can organise a 12v supply to the rear just for these, but it's a hassle to do that - so my question is whether it's good practice to run two 12v led lights in series from a 24v source (fortunately each side of the bus has two of each light).
If they were incandescent lights I wouldn't have any concerns about wiring them in series, but I'm aware that led lights don't necessarily follow convention on such things, and might take offense at being wired like this.
Can anyone advise me on this? I've deliberately paid more to get 'brand name' led lights in the hope that they would be better designed and more durable, so I don't want to compromise that now by installing them wrongly.
Jeremy
Put a resistor in series with each LED assembly. I can't specify the size of the resistor. You, however can .E=IR so R=E/I. Find the current rating of the assembly, should be listed as ma (this is milliamps). An eaxample: 350 ma is 0.35 amps. Divide the current in amps into the voltage. Your voltage on the alternator is really around 27V DC. For our example 27 divided by .35 equals 77. This is the ohms of resistance necessary to protect the led at the specified current and voltage. Using the example I would use a 100 ohm resistor in series with the positive lead.
Bill
You can wire them in series, and we have several places where we did exactly this. Just make certain that the assembly does not have the negative wire connected to ground. We had to cut the grounding tang off some marker lights to wire them this way.
I do not recommend a resistor for anything other than turn signals. The resistors are enormous, waste a lot of power, and require open air flow to remove the heat.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
Thanks for the advice. On the subject of resistors, the manufacturers also sell an optional resistor to go with the direction indicators in case replacing the OEM incandescents with low-current-draw LEDs changes the rate at which the indicators flash. Whether this will be the case with mine and will cause an additional complication I don't yet know.
Incidentally, despite selling their own resistor for the purpose, the manufacturer then contrarily states in the installation instructions that no resistors should be fitted in series with the lights, as on some vehicles the resistor will cause the vehicle's 'blown bulb' warning system to think everything is operating normally even if one of the LED lights has failed.
Jeremy
Jeremy,
I think you may be looking at two different resistor functions. One would put a resistor in parallel with the LED to increase the current so the flasher will work properly (and disabling the lamp failure indication). The other puts a resistor in series with the LED to drop the voltage.
You're quite right of course. I knew that really, honest :-[
Jeremy
Quote from: Sean on September 15, 2011, 04:07:07 AM
You can wire them in series, and we have several places where we did exactly this. Just make certain that the assembly does not have the negative wire connected to ground. We had to cut the grounding tang off some marker lights to wire them this way.
I do not recommend a resistor for anything other than turn signals. The resistors are enormous, waste a lot of power, and require open air flow to remove the heat.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
If you have (2) 12v lights wired up in series to a 24v system, what happens if one of the lights go out?? Will the whole 24v's go to the remaining working light and blow that one?
Jimmy
Usually they fail open, not shorted. If one goes out, the other one gets no current and so does not light. I guess it's possible for one to fail shorted, just have never experienced that.
Brian
KISS: Take a 24Volt-coil relay and connect that to the turn signal circuit. Connect the 12V fixture to one of the switched terminals on the relay and the common to a fused center tap on your battery. Since you're talking LED fixtures on an intermittent turn signal circuit, that shouldn't put too much load on the lower "12V" battery.
Be careful with commecial LED fixtures, more of them are getting active circuitry inside, that does not lend well to simple series wiring. Many others are still internally wired with series parallel internal, which is also not a great idea to have wired in series - if one parallel string of LEDs in a string fails that will alter the current capacity of the fixture.
-T
Are for power you are talking about 5 watts for heating effect. I doubt the resistor will ever get warm. I thought about recommending a relay but that is another failure item, especially the 'made in china etc".
For a '24V' bus you have to use relays for the toad lighting so tap your LED's off that circuit. All the relays are Bosch or equivalent.
I converted all my lights to 12V LED's. The front clearance lights and the side markers are each installed with a 150 ohm resistor in series to each fixture.
Bill