diodesrelay
While working on a switching setup for a automotive cooling fan, I came up with a question. Can diodes be used instead of a relay when dealing with two switching sources? The main source would be the cars accessory line that is on when the key is turned on. The other source is a dash mounted switch that would let the fan be used when the key os off. Here are the two wiring diagrams.
No.
I checked google images for "Automotive Relay" and found picture of what is inside. There is no diode there. The reason can be that everything made for auto is massively produced, designed for low cost. It evolved over multiple generations into perfectly cheap, reliable and very low part count things.
Your circuit concept is starting in the right direction but it requires some important changes for it to work.
You will require series base resistors for all of the PNP transistors. As drawn you will cook them with no base current limitation.
Secondly the NPN transistor is drawn backwards. Its emitter needs to connect to the GND.
Third you need to do some serious work reorganizing the way the bases of the various transistor are biased. You need to separate the 12V and 5V PNP bases so that the 12V one can shut off when the base drive goes away to the 5V one. After this fix it is necessary to provide also a separate pullup of the NPN transistor.
Don't cringe. Resistors are incredibly cheap.
Lastly this schematic, to be bluntly honest with you, is horribly drawn being that it does a bad job of following conventional standards for how a good schematic is drawn.
Best Answer
Logically, yes, but in your circuit with no relays, note the diodes are in series with the motor, so that 1) the motor will see 12V minus the diode's voltage drop (say half a volt), and 2) that voltage drop multiplied by the motor's current (2A is it?) will generate heat (say 1 watt) within the diode and 3) 1N-series diodes are only rated for about one amp (at cooler temps).