Seeking Relay for Automatic Polarity Reversal

The TL;DR version is that I need some kind of simple relay that will take the negative and positive input and ensure that the output polarity is always forward, as the circuit occasionally reverses.

Long version: I'm trying to install LED highbeams (9005) on a Buick Park Avenue (1997-2005 generation, although it might apply to others). In this model, the highbeams double as Daytime Running Lights. In highbeam mode, they are run normally (forward) and in parallel like any other headlight setup, but to achieve DRL, General Motors implements a bit of trickery. The lights are run in series to halve the voltage, thereby reducing light output. I finally found some headlights that will actually operate in this way, but this model, when running those lights in series, reverses the polarity of one of them. Rather than having to start from scratch and find LED headlights that support both series wiring and automatic input polarity compensation, I'd like to see what can be done about building a simple relay device (preferably for a few dollars with basic & compact parts) that can flip the polarity anytime the input voltage is reversed.

Picture version of this model's unusual DRL functionality:


See my post in this thread ( https://buickforums.com/forums/threads/question-on-using-led-for-high-beams-and-drl.49407/#post-287212 ) for the lengthy explanation.

i am an idiot wrote:

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https://www.the12volt.com/relays/relaydiagram27.html

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I don't wish to switch from series to parallel because this already happens automatically. I just want to take a negative + positive input and ensure that the output negative + positive is always the same polarity, regardless of the input.

The simple solution would be to do away with the DRL's since LED bulbs will likely not dim when at a lower voltage like filament bulbs. They may run at full brightness (even at half voltage), blinding other drivers.

If you can't disable them by simply pulling a fuse/relay, a relayed harness will likely accomplish this. Use the input voltage of one high beam bulb to trigger a relay. Use that relay to power both bulbs. Since automotive relays typically trigger somewhere between 7.5 - 8 volts, the DRL's should not trigger the relay.

If you decide you want to keep the DRL's, it's probably best to switch to low beams.


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My ex once told me I have a perfect face for radio.

geepherder wrote:

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The simple solution would be to do away with the DRL

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This would be an option too. Most LED-based highbeams don't support dimming, so this would happen automatically as a result of the installation.

geepherder wrote:

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LED bulbs will likely not dim when at a lower voltage like filament bulbs. They may run at full brightness (even at half voltage), blinding other drivers.

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Up to this point, that has been the case. Despite that, I've found a resource that tracks this and located one set in particular that does dim properly when run in series (half-voltage). I tested it on the car and verified that it was not unnecessarily bright in this state of reduced output. I did, however, return that set when I found that it cannot accept reverse polarity. I have another set that does accept reverse polarity, but does not dim (they just won't light at all when given insufficient voltage).

To achieve the desired effect, I would need to either find a product that can accommodate for both irregularities or build a relay device that can accommodate for one.

geepherder wrote:

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If you can't disable them by simply pulling a fuse/relay, a relayed harness will likely accomplish this...

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It's not possible on this model, but also not necessary, as most LED headlights simply won't light at all when wired in series (giving them six volts each, effectively).

Here's something I just sketched real quick that will always provide the correct polarity using two SPDT relays. I'll create a better diagram later with a full description and add it to our relay diagrams section.


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the12volt • Support the12volt.com

Here is a much cleaner diagram.



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the12volt • Support the12volt.com

Here is another one with diodes. If needed and used, the current values of the diodes need to be greater than the load. You could also omit the diodes and use two additional relays, one for each output, if you want to prevent any possibility of momentarily sending the incorrect polarity to your LEDs.



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the12volt • Support the12volt.com

the12volt wrote:

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Here is another one with diodes. If needed and used, the current values of the diodes need to be greater than the load. You could also omit the diodes and use two additional relays, one for each output, if you want to prevent any possibility of momentarily sending the incorrect polarity to your LEDs.

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Excellent suggestion; thank you. I think we're really on the right track. My next question is if this can work in the circuit I (crudely) illustrated in the OP where the lights are wired forward and in parallel during normal (highbeam) operation, but wired in series with the polarity on one of the lights reversed during daytime (dimmed) mode.

I just read your original post again. I've never attempted to connect the coils of any relays in series with anything else (and never had any reason to do so), but both relays and the power source for the left relay would need to be 6V, not 12V, if you have 6V when you wire a relay coil and LED headlight in series. This would allow 12V to pass when the coils are not energized and supply 6V when the coils are energized while maintaining the correct polarity, however it is very unlikely you will have 6V when wiring a relay coil in series with one of your LED headlights.

Honestly, I would approach this differently to maintain the DRL feature by simply supplying lower voltage to the LEDs with an adjustable voltage divider and bypass the factory wiring to the headlights.

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the12volt • Support the12volt.com

the12volt wrote:

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Honestly, I would approach this differently to maintain the DRL feature by simply supplying lower voltage to the LEDs with an adjustable voltage divider and bypass the factory wiring to the headlights.

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Just for retaining DRL, I don't think it'd be worth it to make it into that much of a hackjob. If I did do any kind of non plug-n-play solution, I might consider going with something simpler, like sticking one of those inexpensive LED DRL strips under the headlight assemblies and just tap in to the driver-side highbeam ground (which becomes +12v in DRL mode) for power.

I'd use the same extension that I was planning to use for the relay setup to make it as non-permanent as possible. My only real concern about going that way is that the effect could be spoiled by how dim these products likely are.