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I am a total beginner with electronics, so I apologize if this question has been answered already, I find similar questions but they seem to always be some variant and not quite what I am seeking, so I am asking here..

My automobile has power ports (cigarette lighter) that is always on. If I forget to unplug my dashcam I have a dead battery in a couple of days. To avoid this I would like to add a relay and tap into the accessory position circuit. Seems pretty straight forward, but then I see a bunch of videos on the importance of diodes due to kickback voltage as the magnetic field on the coil collapses which could cause a problem with the trigger circuit. When looking at the image below we see a manual switch activates the relay...

MGI Speedware Image

... but my automobile has something called "Retained Accessory Power" (A.K.A., RAP) which is what I will attempt to tap into. My concern is that a voltage spike on this RAP circuit could be problematic. So I am seriously considering using a diode. Then when I investigate all the circuits I find describing diodes includes the use of a transistor too. Question: Should I use a diode? Should I use a transistor? If so, what kind/size? Using this image as an example, how would the diode and/or transistor be wired to protect the trigger circuit?

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  • \$\begingroup\$ So you are planning on powering the relay coil from the RAP? \$\endgroup\$
    – TimWescott
    Aug 10, 2020 at 20:33
  • \$\begingroup\$ Hi @TimWescott - The power port has 12v on always. I will cut this 12v wire and place the relay in the middle. I will trigger the relay with RAP power, so yes, power the relay coil with RAP... \$\endgroup\$ Aug 10, 2020 at 20:35
  • \$\begingroup\$ @DKNguyen - thats the thing, I want to alter the circuit in the image - i want to use RAP power as the trigger instead of a manual switch. All my research shows novices trying to figure out how to create a power port that turns off automatically - i have not seen anybody using a diode for this particular application. \$\endgroup\$ Aug 10, 2020 at 20:42
  • \$\begingroup\$ I don't really know what a RAP is, whether its a battery replacement or a switch, but it still doesn't change things since the diode supresses the voltage spike at the source. Is a RAP a switch unto itself? Or does it only provide a control signal? Or does it just supply power? \$\endgroup\$
    – DKNguyen
    Aug 10, 2020 at 20:42
  • \$\begingroup\$ I would look at the automotive rated Hi side power switches \$\endgroup\$ Aug 11, 2020 at 3:41

2 Answers 2

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You should put one in though it is not as critical in this case since the a manual switch is a lot hardier to voltage spikes than a transistor which is often the first thing to blow from the spike. Even so, having one will prevent the switch from arcing and wearing it out.

You should put one in anyways just so the spike doesn't affect other things connected to your battery. Place the diode parallel to the relay coil in the direction that does NOT normally conduct current and short the battery out.

enter image description here

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  • \$\begingroup\$ For a possible (fused at 10A) 12v trigger on pin 85 what diode do you recommend? I see something called Schottky diode - is this a good choice? \$\endgroup\$ Aug 10, 2020 at 20:47
  • \$\begingroup\$ Also, are you saying don't use a transistor? \$\endgroup\$ Aug 10, 2020 at 20:48
  • \$\begingroup\$ @barrypicker A schottky is fine. I can't say whether or not to use a transistor yet because I don't know what the RAP actually is. What does it do? \$\endgroup\$
    – DKNguyen
    Aug 10, 2020 at 20:53
  • \$\begingroup\$ If you have a switch, why do you need a transistor? \$\endgroup\$
    – TimWescott
    Aug 10, 2020 at 20:54
  • \$\begingroup\$ I think it is my lack of understanding why I bring up transistor. It seems all the examples I have seen on the internet describing how to protect the trigger circuit using a relay use a diode and a transistor. Perhaps it is not needed. The trigger source is a 12v source controlled by the automobile computer. It is called "Retained Accessory Power". It stays on even if the key is removed from the ignition for 10 minutes, or until a door is opened. Given the complexity of the feature, and such that it is controlled by a computer I figured it probably needs some diode protection... \$\endgroup\$ Aug 10, 2020 at 21:00
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Given that the RAP circuit is powering other things as well, you can probably just use it to power the relay coil directly.

What will happen is that if the RAP circuit in your car shuts it off suddenly, the relay coil will continue to pull current for a bit, because it's an inductor. This could cause a negative spike on the RAP circuit -- but chances are very high that any such spike will be small, and will be absorbed by other devices on the network.

If you're worried, then put a diode in parallel with the relay coil, such that the diode is reverse-biased when the coil is energized, as in the diagram below. With the diode shown, the RAP will only be able to go about 1V negative. It is probably vast and paranoid overkill, given that most automotive circuits are already protected against things like inductive load dump.

Alternately, look around for an automotive-rated solid-state relay and use that instead of the Bosch unit; these gizmos act like relays, but don't have the inductive kickback of a relay.

schematic

simulate this circuit – Schematic created using CircuitLab

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