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Reverse current protection in car

K_Watson

May 14, 2018
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May 14, 2018
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Hello, I am adding a trunk release button to my car and I need some tips what to use to prevent current being sent back to unknown components in the car's key-fob radio receiver. The car currently has a wireless trunk release that operates a small motor (~1.5-3A @ 12V) to release the trunk latch when the button is pressed on the fob. The receiver may trigger a built-in relay and if so, I'd have no problem adding a switch... or it may be all solid state and require protection, I just don't know without buying another one and tearing it apart.

Assuming it needs protection... I will be tapping in to the wire on the harness that comes from the receiver which provides +12.6 to 14V for about 0.5 seconds and I am adding a 3A fused momentary switch that will provide the same voltage to the wire for as long as the button is pressed.

Will a simple diode placed between the tap point and the fob receiver will be sufficient to protect the unknown circuitry or should I run new power wires for both the switch and the motor and isolate the receiver by making it so it only trips a relay?... or is there a better solution?
 

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kellys_eye

Jun 25, 2010
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If you are driving an inductive load it is common to include a protection diode to prevent induced (back) e.m.f from causing problems - these can be high enough to destroy any switching transistor.

It really depends on how your fob receiver controls the load - does it have a set of volts-free relay contacts at the output? or is it a transistor (open-collector) or MOSFET etc.
 

K_Watson

May 14, 2018
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May 14, 2018
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I would have to go back to the junkyard and buy a fob receiver module to tear apart to find that out... I may go back Wednesday.

Do you mean I should add a diode to both the fob wire and the switch wire since they are linked?

Should I use a standard silicon or Schottky diode?

If you are driving an inductive load it is common to include a protection diode to prevent induced (back) e.m.f from causing problems - these can be high enough to destroy any switching transistor.

It really depends on how your fob receiver controls the load - does it have a set of volts-free relay contacts at the output? or is it a transistor (open-collector) or MOSFET etc.
 

K_Watson

May 14, 2018
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May 14, 2018
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I am remembering more of my electronics classes...

Since the receiver is driving what is basically a 12V brushed permanent magnet hobby motor with a spring loaded return... back feed would be expected from the motor acting as a dynamo when the spring winds the motor back and returns the latch to ready... so would the circuit likely already have protection or just simply be on a relay?
 
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K_Watson

May 14, 2018
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May 14, 2018
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Update... there is a relay that triggers when the button on the fob is pushed, its on the opposite side of the car's cabin from the receiver.

I will go back to the junked car and try and find out more about the wiring.
 
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