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How to prevent current feedback in a circuit with dual supply

pewe

Feb 4, 2016
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I am not an electrical or electronics engineer but am quite experienced in general DIY etc and can figure most things - but I do like to check before undertaking a job - especially if it is outside of my area of experience.

What I am looking to do is to convert a car wiring so that the headlamps come on every time the car is started without having to switch them on.

A simple solution I came up with is to connect the dimmed headlight supply to a normally closed relay fed from the accessory switch on the car.

Then, when the headlights are turned on, the relay is switched to 'open' and the headlights are powered as normal.

I have attached a diagram.

output.png

What I am wondering is if I need to put something else in the line to prevent power feedback to the normal headlight switch when power is fed to the lights from the relay.

Any thoughts would be welcomed.
 

Arouse1973

Adam
Dec 18, 2013
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Hello and welcome to EP. When you say dimmed headlights do you mean dipped beam. Or are you talking about side lights? If it's dipped beam what's the point of turning the lights on if they are already on? Lets just clear that up first.
Thanks
Adam
 

pewe

Feb 4, 2016
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Hi Adam.

Thanks for the response.

My car has daytime running lights (DRL) which come on with the ignition. These go off when I turn on my lights at dusk (a legal requirement that both cannot be on at the same time).

My brothers car does not have DRL so he leaves his lights switched on at all times so when he starts the car they are on - so he is using his headlights as DRL.

My wifes car has no DRL and if the headlights are left switched on there is a warning buzzer which constantly sounds when the keys are removed from the ignition - so she has to remember to turn her lights on every time she gets in the car.

So to simplify matters I thought why not have the dipped beam (no other bulbs) connected to the auxiliary switch so that those bulbs came on with the ignition, BUT the power from the auxiliary supply has to be switched off when she switches on her side lights/dipped beam in the normal way (ie using the light switch in the car).

Rather than have a permanent connection from the auxiliary to the lights when they are powered from the light switch I thought maybe use a relay to isolate the auxiliary supply when the normal supply is activated.
But then I thought that maybe it is not a good idea to allow the auxiliary power to back feed to the normal light switch - hence my question.

I hope this isn't too laboured and makes sense.;)
 
Last edited:

Arouse1973

Adam
Dec 18, 2013
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Your problem is going to be that with the relay powered off, the positive power from the aux power is going to switch off the relay and remove the aux power. Then off, then on.... Putting a diode in series is an option but it's going to dissipate power and also add a voltage drop to the light circuit. My preference would be to switch a transistor from the headlight switch and activate the relay using this. Have a look at driving relays with transistors.
Thanks
Adam
 

Bluejets

Oct 5, 2014
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Most auto headlight systems are not wired the way you show, they are constant power fed with return switching.
Can get complicated unless one starts with the correct wiring.
 

pewe

Feb 4, 2016
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Thanks guys.

I have had a second thought - mainly because I have been offered a good price for supply of some OEM lights with front panel mountings which I can install as DRL's, but need to wire them.

So would this work - fed from auxilliary supply, left permanently ON (but with isolating switch for use when needed) and powered down by a relay when the headlights are powered up.
output3.png

My preference would be to switch a transistor from the headlight switch and activate the relay using this. Have a look at driving relays with transistors.
Thanks
Adam

Would using a transistor still be preferable in the new configuration .
 
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