How can I use a diode to solve an issue on a vehicle?

Thread Starter

GDBD59

Joined May 3, 2022
16
Hi, I have very basic (& old) electronics knowledge, & hope I can use a diode to solve an issue on a vehicle.

A headlight type has been discontinued & I need to use a differnt one with a different bulb layout. I want to take a tap off a live wire to feed another bulb, the continuous current being approx 5 Amps & 60 Watts. Ideally a diode not requiring mounting, but if that was necessary to deal with heat dissipation that could be done.

Any item suggestions?
 

Thread Starter

GDBD59

Joined May 3, 2022
16
The newer headlight type has seperate bulbs. Both bulbs operate giving improved lighting in the later model of car. When hi-beam is turned on the newer headlight has two feeds to the different bulbs. The older version does not, - so I want to tap off the hi-beam live when turned on to feed the lo-beam.
 

dl324

Joined Mar 30, 2015
15,112
You didn't say it, but it seems that you want the low beam bulb to be on when the high beam is on, but not turn on high beam when only low beam is on.

Get a diode rated at 6A or more. It seems that even Schottky diodes have a large forward voltage drop (1.7-2.1V) at those currents.
 

Attachments

Thread Starter

GDBD59

Joined May 3, 2022
16
Thanks Dennis..... voltage drop..... meaning the voltage to the lo-beam will be reduced? And it'll require mounting to metal for heatsinking?
 

Thread Starter

GDBD59

Joined May 3, 2022
16
" You could use a MOSFET to do the switching, but that takes more than a single component."

Can you point me towards the type please?
 

Ian0

Joined Aug 7, 2020
6,293
Buy an automotive changeover relay. Connect the coil between the high-beam and ground.
Connect the new low-beam lamp to the common contact. Connect the n/o contact to the high beam supply.
Connect the n/c contact to the low beam supply.

High beam off: new low-beam lamp is on when the old low-beam lamp is on
High beam on: new low-beam lamp is on.
 

crutschow

Joined Mar 14, 2008
30,788
It seems that even Schottky diodes have a large forward voltage drop (1.7-2.1V) at those currents.
That has a high voltage drop because it's a high voltage silicon-carbide type for high reverse voltages.
Use a standard silicon Schottky diode such as this, which as less than a volt forward drop, but it will still need a heatsink.

If you want to avoid a heatsink, you can use a MOSFET circuit as dl325 suggested, LTspice simulation of a such a circuit below:
It uses a power P-MOSFET to carry the current and a small N-MOSFET along with one resistor to control it.
The P-MOSFET (M1) can be any with a voltage rating of 30V or higher, and an on-resistance of no more than 15mΩ to avoid requiring a heatsink.

1651602969937.png
 

ThePanMan

Joined Mar 13, 2020
494
Why not just use a relay? Turn the high beams on and that powers the relay coil. The contacts then connect the high beam feed to the low beam at the same time.

Please note: Such modifications to vehicle lighting may be illegal where you live. Same holds true for all those HID lighting kits that people install. Some people use HID in a standard lamp housing that does not block the higher beam pattern from blinding on coming traffic. I've seen AND heard of people getting pulled over and cited for the violation.
 

Thread Starter

GDBD59

Joined May 3, 2022
16
Buy an automotive changeover relay. Connect the coil between the high-beam and ground.
Connect the new low-beam lamp to the common contact. Connect the n/o contact to the high beam supply.
Connect the n/c contact to the low beam supply.

High beam off: new low-beam lamp is on when the old low-beam lamp is on
High beam on: new low-beam lamp is on.
Can you explain what n/o & n/c contacts are?

Supposedly someone has used one or more relays to do what's required. Another aspect that needs to be fulfilled is the hi-beam warning light has to function. There are 'rumours' that the relay conversion didn't do it.

I have wiring looms that were made to use the new lights, - the warning light works, but it does not operate both beams, hence my diode idea.
 

Ian0

Joined Aug 7, 2020
6,293
Can you explain what n/o & n/c contacts are?

Supposedly someone has used one or more relays to do what's required. Another aspect that needs to be fulfilled is the hi-beam warning light has to function. There are 'rumours' that the relay conversion didn't do it.

I have wiring looms that were made to use the new lights, - the warning light works, but it does not operate both beams, hence my diode idea.
Normally open and normally closed.
The warning light will still work.
 

Thread Starter

GDBD59

Joined May 3, 2022
16
That has a high voltage drop because it's a high voltage silicon-carbide type for high reverse voltages.
Use a standard silicon Schottky diode such as this, which as less than a volt forward drop, but it will still need a heatsink.


I got a couple of MBR20100 schottky but haven't looked at the job 'til now. Original arrangement is a 3 pin H4 bulb with low & high beams that work individually. The newer headlight has seperate bulbs, with the low beam staying on when high is selected, so my wish is to feed from the high beam positive to the low beam. But I'm unsure which to connect where. Attached is a pic showing the supplementary wiring loom, which plugs into the car loom. But it doesn't operate both bulbs simultaneously.

Can you advise?
 

Attachments

Thread Starter

GDBD59

Joined May 3, 2022
16
We really need to work from a schematic. Something's getting lost in translation.
I'll try expanding then. Original headlight is a twin filament H4 bulb. When hi-beam is used, low-beam goes off.
In the later version of the car Low beam stays on as well as high-beam, when 'high' is turned on because it uses seperate bulbs. The back of the light is shown above.
The aftermarket loom shown does not allow low beam to stay on when high is selected.

I want a way of feeding the low beam bulb from the high, when it is selected. I think the Schottky will do it, but don't know which connection goes where.
 

Tonyr1084

Joined Sep 24, 2015
7,093
Original headlight is a twin filament H4 bulb. When hi-beam is used, low-beam goes off.
In the later version of the car Low beam stays on as well as high-beam, when 'high' is turned on because it uses seperate bulbs.
A diode from the HB positive to the LB positive. When the HB is on the LB will be on as well. You don't need to worry about 6/10ths of a volt drop across the diode, the lights will be far more than bright enough. Choose a diode that can handle 12 amps. You likely only need just one diode on either of the headlight assemblies as whichever you put it on will back-feed the other assembly. OR use two 6 amp diodes, one on each assembly. The diode anode (left side in the drawing) connects to the HB positive and the cathode (right side in the drawing) connects to the low beam. When the LB's are on the HB's will remain isolated. But when you switch the HB's on the LB's will stay lit. Any time you flash your HB's at oncoming traffic you'll blast them with a fireball of light because all four filaments will light up.

Caution: This extra load might be more than the switch - or whatever control - can handle. You may burn something up. To avoid that you may want to use a relay instead. The relay can draw very little current from the HB feed and the LB can be switched directly from a fuse panel. Or you can just do it the down and dirty way it's drawn and risk burning something up.

1662510787801.png
 

MrSalts

Joined Apr 2, 2020
2,354
A diode from the HB positive to the LB positive. When the HB is on the LB will be on as well. You don't need to worry about 6/10ths of a volt drop across the diode, the lights will be far more than bright enough. Choose a diode that can handle 12 amps. You likely only need just one diode on either of the headlight assemblies as whichever you put it on will back-feed the other assembly. OR use two 6 amp diodes, one on each assembly. The diode anode (left side in the drawing) connects to the HB positive and the cathode (right side in the drawing) connects to the low beam. When the LB's are on the HB's will remain isolated. But when you switch the HB's on the LB's will stay lit. Any time you flash your HB's at oncoming traffic you'll blast them with a fireball of light because all four filaments will light up.

Caution: This extra load might be more than the switch - or whatever control - can handle. You may burn something up. To avoid that you may want to use a relay instead. The relay can draw very little current from the HB feed and the LB can be switched directly from a fuse panel. Or you can just do it the down and dirty way it's drawn and risk burning something up.

View attachment 275756
That design looks great except that the HB+ wire in the loom will be supplying nearly twice as much current as the original design.
Wire looms tend to melt when you try to push 2x current through a wire. Especially when all the wires are packed together in a plastic conduit that insulates the heat in the wire.
 
Top