Combining two +12v car outputs

[john.r2]

My car has a H4 3 pin connector which basically has a ground, and two +12v wires. The first +12v is the low beam and the second is the high beam. When the high beam is activated the low beam +12v has no power. I need to adapt it for a new headlight which has a flap type LED. It's also 3 pin, ground and +12v for low beam and +12v for high beam. The difference is that the new headlight requires the low beam to stay on because the high beam +12v only activates some sort of mirror flap. So if the low beam turns off then it's pretty pointless. But the car basically turns the low beam power off to the low beam +12v.

Can anyone advise, what will be the best way around this to allow the low beam to stay on when the +12v for the high beam is activated so that the new light can function?

Thanks

 

[crutschow]

The simplest way is to use a diode between the two wires (diode anode to high-beam wire). That will power the low-beam when the high-beam is on.
The diode needs to be rated for the low beam current, and Schottky type diodes have the lowest voltage drop when conducting.

 

[ahsrabrifat]

My car has a H4 3 pin connector which basically has a ground, and two +12v wires. The first +12v is the low beam and the second is the high beam. When the high beam is activated the low beam +12v has no power. I need to adapt it for a new headlight which has a flap type LED. It's also 3 pin, ground and +12v for low beam and +12v for high beam. The difference is that the new headlight requires the low beam to stay on because the high beam +12v only activates some sort of mirror flap. So if the low beam turns off then it's pretty pointless. But the car basically turns the low beam power off to the low beam +12v.

Can anyone advise, what will be the best way around this to allow the low beam to stay on when the +12v for the high beam is activated so that the new light can function?

Thanks

You can use a Schottky diode or a standard rectifier diode to allow power from the high beam +12V to flow into the low beam circuit when the high beam is on.

 

[danadak]

Keep in mind the 12V system in a vehicle is anything but reliably stable.



Your Schottky diode must be able to handle the breakdown.

https://www.ti.com/lit/pdf/snoaaa1

Regards, Dana.

 

[Bluejets]

Also be aware there are a variety of earth return arrangements in auto headlights.

 

[john.r2]

The simplest way is to use a diode between the two wires (diode anode to high-beam wire). That will power the low-beam when the high-beam is on.
The diode needs to be rated for the low beam current, and Schottky type diodes have the lowest voltage drop when conducting.

You can use a Schottky diode or a standard rectifier diode to allow power from the high beam +12V to flow into the low beam circuit when the high beam is on.

Thank you for the replies. Will a 1N5822 diode work for this application? If I'm understanding correctly, do you mean to do it something like this in my drawn diagram below? So the diode is essentially preventing the low beam power from ending up turning the high beam on all the time? so that's why it's essential to make sure the led flap for high beam is powered from the before the diode and the merging with the low beam happens after the diode with the cathode merging to the low beam wire?

Thank you

 

[danadak]

What is the current needed for low beam ? I would advise you use a diode with 2X current
rating, and it needs to have a heatsink, eg. you need to do the thermal design as well. Your
diagram seems OK, maybe check the 12V wiring gauge for high beam to see if its wire gauge
big enough to handle both low and high beam current.

https://www.analog.com/media/en/training-seminars/tutorials/MT-093.pdf

https://www.ti.com/lit/an/slva462/slva462.pdf

https://toshiba.semicon-storage.com/info/docget.jsp?did=141361

AWG - Wire Gauge Sizes: Current Ratings, Charts, Measurements, and Conversion Guide

Amp ratings vs. US AWG wire gauge.

www.engineeringtoolbox.com

Heatsink calculators :

Heat Sink Size Calculator | Celsia

Use heat sink size calculator to estimate required heat sink volume for heat sinks using input power, thermal budget, & airflow.

celsiainc.com

How to calculate heatsink - OnElectronTech

Thermal management of electronic devices is important for best performance. How to calculate heatsink is demonstrated by 7805 examples.

www.onelectrontech.com

Lastly you might want to fuse the composite supply line, eg. the diode path, just to add safety
to design.


Regards, Dana.

 

[Externet]

A relay can energize the +12V load you want when the other +12V line is active, with less diode voltage drop.

 

[crutschow]

What is the high and low beam current of the lamps?

 

[Externet]

From what I have seen in incandescent; limits are 55Watts, ~<5Amperes. If LED consumes a fifth... Brings another question, what limitations are headlights for LED emitters ... on lumens or watts ?

 

[Externet]

This rule of maximum brightness will get complex if efficiency improves
---> https://www.naoevo.com/blog/auto-le...lumens-are-in-your-car-headlight-quick-guide/