I am trying to find a relay which will control the 12V power rail of a PCIe adapter so I can cycle power to a device during testing. The relay should be normally closed. I plan to control the relay using a GPIO from a Raspberry Pi so the signal should be 3.3V. I have tried looking digikey and narrowing down the search criteria, but I can't seem to find one. If one doesn't exist, what would it take to create a switch using components (i.e., MOSFETs). Thanks.
-
\$\begingroup\$ look for SSR ( Solid state relay), should be plenty that can do this, and quite small. They are just integrated MOSFET switches with a bunch of protection included. \$\endgroup\$– KyranFAug 17, 2017 at 9:49
-
\$\begingroup\$ A Raspberry Pi GPIO is not powerful enough to directly drive a relay. You will need to add a transistor to do that. Google for circuit solutions because there are plenty examples of this to be found. \$\endgroup\$– BimpelrekkieAug 17, 2017 at 9:50
-
\$\begingroup\$ Close. Google 3.3V coil relay. \$\endgroup\$– R DrastAug 17, 2017 at 9:56
-
\$\begingroup\$ I don't see how this question is off topic. I asked a specific question to the problem I am trying to solve. \$\endgroup\$– lordhogAug 17, 2017 at 19:39
2 Answers
A solid state relay rated for mains voltage will use a triac internally, so it will drop 2V, won't work for your 12V application. Also it is likely not to turn on at all.
Just use a PMOS as a high side switch. Pick a PMOS with proper RdsON rating according to your current for low voltage drop, and GND needs to be connected on both sides of course. Schematic example:
Or you can use a high side switch like this one.
I would use an N channel MOSFET connected as shown and a relay with a coil rating of 12 volt. +Vdd will be the 12 volt connection. Vcc will be your 3.3 volt logic supply.
Choose a MOSFET that has low enough gate threshold voltage to suit your 3.3 volt logic signals. A \$V_{GS(Threshold)}\$ of under 2 volts should be sufficient.
If the logic level is 1 then the relay activates and open circuits the normally closed contact.