Diode solution?

Thread Starter

GDBD59

Joined May 3, 2022
7
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
7
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
14,457
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.
 

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Thread Starter

GDBD59

Joined May 3, 2022
7
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
7
" 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
5,064
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
29,755
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
364
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
7
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
5,064
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.
 
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