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What are the other alternatives to relay would be for switching 12V battery power rail with accesory (ignition) power? I'm trying to find an alternative, because the relay is too big for my application, I'm looking for something what's is more compact than the relay. Power load is about 150mA. I can't connect my application directly to accesory, because max rated power load for accesory is 150mA.

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    \$\begingroup\$ Things might be more compact, but are they also as robust? The ignition system likes its voltage spikes. \$\endgroup\$ – PlasmaHH Jul 20 '18 at 10:19
  • \$\begingroup\$ You didn't say how small your application requires the device to be. There are miniature automotive grade relays available such as the TE Connectivity Mini K series. \$\endgroup\$ – MIL-SPEC Jul 20 '18 at 10:40
  • \$\begingroup\$ "the relay"? What relay? How big? How small? \$\endgroup\$ – Andy aka Jul 20 '18 at 11:17
  • \$\begingroup\$ I want to fit everything onto 8x5cm single layer pcb, so regular size relay would take too much space. @PlasmaHH you can check how my power supply schematic look like here: electronics.stackexchange.com/questions/375541/… Btw what if I connect accesory directly to DC-DC switching regulator Enable pin? \$\endgroup\$ – user1258202 Jul 20 '18 at 11:19
  • \$\begingroup\$ @Andyaka for the prototype I used 12V 10A SPDT relay which is size 19 x 15.5 x 15.3mm. So I need an alternative which would be suitable for use in car and wouldn't take so much space. The power load is about 150mA \$\endgroup\$ – user1258202 Jul 20 '18 at 11:27
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Electronic switching typically relies on one of three technologies:

  1. Electromechanical relays - open/close the switch electromagnetically. Typical for appliance-scale applications like thermostats. Safest because they tend to fail open. This is not an electrically demanding application, so the smallest and lowest-power should do. You can find 1x1x1.5 cm or smaller (probably 5mm in height over the PCB). This related answer has some interesting options. For anything involving car batteries, I strongly recommend traditional relays unless you are sure that the rest of your system would withstand a permanent short at the switch.

  2. Transistors - popular in electronics. Search for 12 V transistors and pretty much any that you find will take 150 mA, often much more. This is as small and cheap as switching gets. However, transistors introduce a voltage drop. If a transistor fails, it is likely to short-circuit. Also, there is much weaker isolation between controlling input and controlled output. Still, plenty of automative applications use them, and the safety concerns can be somewhat mitigated with solid-state relays (generally at least as large as electromechanical).

  3. Vacuum tubes. They have their place in high-quality amplification and radiology, not here.

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  • \$\begingroup\$ Omron G6K-2F-Y-DC12 would be a good choice? \$\endgroup\$ – user1258202 Jul 23 '18 at 10:34
  • \$\begingroup\$ @user1258202 It looks compact, but you may want to check the coil voltage: a 12V coil voltage means that you should supply (close to) 12V at the input to close the output connection. Unless your control system already has a 12V output, one of the lower-voltage siblings (called something like DC3.3 or whatever is close to your logic level) may be better. If you want to be sure that a relay can withstand 12 at the output, check the contact rating. Note that a lower CV may need higher current to switch. If that exceeds what your output can supply directly, you need a transistor in-between. \$\endgroup\$ – MBer Jul 24 '18 at 15:01
  • \$\begingroup\$ I'm thinking about placing this relay at the very beginning (in front of step down switching regulator which will lower the voltage down from 12V to 5V). The accesory voltage I get from the car's head unit for switching is 12V (max. 150mA), so as I understand I should definetely use the relay with a 12V coil voltage. The contact rating of this particular relay is 1A at 30V, so it's more than enough for my application unless there're any other exceptions I don't know about. Maybe the relay should be protected somehow if I'm about to use it in the car? \$\endgroup\$ – user1258202 Jul 25 '18 at 6:42
  • \$\begingroup\$ Btw maybe I can just connect 12V accesory directly to BD9778F's ENABLE pin, so this won't require relay for doing the same thing? \$\endgroup\$ – user1258202 Jul 25 '18 at 6:49
  • \$\begingroup\$ Of course if all your parts are rated for 12V, then the relay's safety benefit is reduced, and the transistor path could be a fair option. Going into the details of the components is beyond the original scope. \$\endgroup\$ – MBer Jul 25 '18 at 14:32

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