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i want to create a switch that will turn on/off the power to my OBD Port on my car when i turn engine on/off. reason being i have an OBD wifi dondle plugged in all the time. problem is the OBD Port has a constant 12v power so will always stay on.so the wifi dongle will drain he battery plus its not a good idea to leave it on as someone else could connect to it.

i am going to de solder the 12v pin inside the wifi dongle and put a switch in between . i was thinking on using a transistor. so when i turn engine on 12v from the cigarette lighter goes to the base of the transistor. that in turn will complete the circuit of the 12v into the wifi dongle.

i have bread boarded the following. 12v from car cigarette lighter goes to a 5v regulator. then 5v to 2.2k resistor to base pin of a MJD122G transistor collector pin is connected to OBD PORTS 12V then Emitter pin connects to the 12v inside the wifi dongle. This all works fine and turns the wifi on/off as expected. problem is if i run more than 500MA the transistor gets very hot. maybe all it needs is a little heat sink ?

i then thought why not use a small solid state relay like the 12v G3MB-202P DC-AC i could simply just connect 12v from cigarette lighter to the input of the relay. would this solid state relay get hot too ? im not sure what route to take with this.

iv started designing PCB for the transistor enter image description here

enter image description here

so which one would be best to use ssr or transistor ? my transistor design is smaller which is a plus and i do light building my own things

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    \$\begingroup\$ I tried to read this. To hard with lack of proper capitalisation, punctuation. Sorry. Why would you go to so much trouble and do it badly? \$\endgroup\$
    – Transistor
    Oct 2, 2017 at 22:30
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    \$\begingroup\$ Consider using a MOSFET instead of a Darlington: the lower voltage drop will translate into less heating. \$\endgroup\$
    – Abe Karplus
    Oct 2, 2017 at 22:46
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    \$\begingroup\$ A schematic is a much easier way to convey a circuit, it sounds like you are trying to highside switch with an npn \$\endgroup\$
    – sstobbe
    Oct 2, 2017 at 23:02

3 Answers 3

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Rather than what you’re trying to do, why not just power the dongle from the cigarette lighter? Just cut the connection to the ODB 12 volt pin and connect that instead to the cigarette lighter +12-volt. There is no need to connect the lighter minus wire, you can continue to use the minus from the connector (they both connect to the car body).

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  • \$\begingroup\$ i dont know why i didnt think of this before. makes so much sense thank you :) \$\endgroup\$
    – Mix Bag
    Oct 4, 2017 at 12:49
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Sorry, but the answer is neither.

The G3MB is an AC only relay. The relay can switch off only when the voltage being switched is zero. It is unable to switch off a DC voltage.

The MJD122G is an NPN darlington. Low-side switching the WiFi dongle will raise the ground voltage of the dongle and probably make the dongle unable to detect the data signals.

As others have said, use a PNP BJT or p-channel FET as a high side switch. Or if the cigarette socket is connected to the WiFi dongle, then power the dongle from the 12 volts from the cigarette socket.

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Alternatively, If you don't want to cut into the cigarette lighter harness, why not just use a 12 VDC relay? You won't need a pcb then. And you don't need a big relay, since the module/dongle won't draw much current.

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