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I have a circuit that should be turned on whenever the car stereo is turned on. It will draw ~300mA. My plan is to connect my circuit to the wire harness behind the stereo as it has +12v, ground and ignition swith-on signal available. Is there a reason why I shouldn't connect it directly to the car stereo wire harness as shown in the picture below?

Car stereo connection

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    \$\begingroup\$ Is your circuit prepared to handle transients of up to 60V as well as sudden removal of all power? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 30 '16 at 19:32
  • \$\begingroup\$ The car stereo is likely to have power all the time. Can you switch it on when the ignition is off? \$\endgroup\$ – Andrew Morton Oct 30 '16 at 19:41
  • \$\begingroup\$ @andrew almost no car stereos can be turned on without the car key in. \$\endgroup\$ – Passerby Oct 30 '16 at 19:43
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    \$\begingroup\$ @Passerby That is not my experience with BMW, Peugeot, Nissan, Vauxhall and VW cars in the UK. \$\endgroup\$ – Andrew Morton Oct 30 '16 at 19:55
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    \$\begingroup\$ @Ignacio The circuit has an extensive filtering section \$\endgroup\$ – JohnDonut Oct 30 '16 at 20:23
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That's exactly how most circuits in a car are wired. Parallel circuits. The stereo power line is normally wired to a 10A or higher fuse and 300 mA won't be a problem.

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Without qualifications, this typically works. As a note, the common circuitry implementations for cars seems to be: the unswitched power circuits provide the bigger, continuous power supply; the smaller switched, ignition-on, are controls circuits. Yes, they are both electrical power, but each has different intentions. Smaller control wire, connectors, labor, space and options can be done this way. E.g. several things may control the headlights, but they are powered by the same power circuit. You may also want to note the circuit protection size you are tapping into as it may not protect where the device needs it. You also may need to ground away from high current grounds (ground loop). (If you're possibly adding a steering wheel module.)

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