Car wiring circuit replacing relays with transistors help - Page 1
Forum section: electronics / beginners

author: iconman26
author_rank: Newbie
timestamp: November 09, 2021, 04:14:16 am
content: ]I'm adding an auxiliary light (draws about 12 amp) to the car. When the key is in the ignition the aux light will turn on. When the low beam is on then the aux light will turn off. If the high beam is activated then the aux light will turn back on.I can accomplish this by using two SPDT relays but I would like to simplify this circuit (relays are physically big). I think I can use Mosfets instead but not sure how. Help needed.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 09, 2021, 04:41:20 am
content: While it can be done with Mosfets, it might not be the best newbie project. Would you consider using one relay to actually turn the aux light on and off and using transistors or even logic ICs to handle the logical sequences? The relay need not take up space under the dash. There is almost always plenty of room in the engine compartment for relays. And in a great many newer cars the lighting control relays are in a box that has empty relay positions in all but the most option laden models.

author: iconman26
author_rank: Newbie
timestamp: November 09, 2021, 05:18:05 am
content: Yea that will work. I think using two relays for this application is a little clutter

author: fordem
author_rank: Regular Contributor
timestamp: November 09, 2021, 11:10:47 am
content: Why do you need two relays?Supply power to the lamp from the main ignition fuse, that power will go away when the ignition is switched off. Supply power to the coil of a normally closed from the low beam circuit, when the low beams are on, the relay will be energized and open the aux light circuit so the lamp is off, switch to high beam, the low beam circuit and the relay are de-energized, the contacts close and the aux light comes on.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 09, 2021, 11:51:51 am
content: Some CMOS 4000 logic will fit perfectly here, as they work up to 16V.Then use a single beefy 30V P-ch mosfet for driving the light.

author: Ian.M
author_rank: Super Contributor
timestamp: November 09, 2021, 12:01:30 pm
content: Another approach if you don't want to add a 12A load to the switched supply from the ignition switch, and assuming conventional incandescent headlamp bulbs, not LEDs or HID units, is to wire the coil of a SPST-NO relay between the ignition switched supply and the low beam circuit. The relay coil will be energized when the ignition is on and the low beams off, grounding through their filaments. The coil current is very small compared to the low beam operating current so wont cause the low beam filaments to glow significantly or reduce their life. However it does have the slight disadvantage that if *both* headlights low beam filaments have blown, you'll also loose the auxiliary lights.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 09, 2021, 12:15:54 pm
content: You must consider that the when the engine is running, the ignition is also on.So the aux lamp would be turned on all the time.This circuit is pretty simple. The lamp is turned on when high beam and/or ignition are on. Low beam will force the lamp off.

author: Benta
author_rank: Super Contributor
timestamp: November 09, 2021, 05:05:34 pm
content: Even though I'm an electronics guy at heart, I vote for a relay or two here.The automotive environment is incredibly "dirty" from an electrical point of view. It's not about a bit of logic and a MOSFET or whatever. 90% of the design is about protection against spikes, undervoltage, overvoltage, overcurrent, load dumps etc., all those kind of things that semiconductorsreallydon't like.Relays are immune to all that. The only thing that will bother a relay is heavy vibration.