diodesswitches
I want to connect two motors on an On-Off-On switch. On position 1, I want motor 1 to me active, on position 2, I want motor 1 and 2 active.
Here is the diagram I was thinking of implementing
My second question, if this work, what kind of Diode should I be buying? on the circuit lab I've just used the basic diodes.
My motors are 12V 25W water pumps
If you do not want to compromise on any of your design requirements this is what you are going to need.
The resistors may not be absolutely necessary, but when hard-switching between rails it's always better to have them, to limit current rush into the gate capacitance, which may degrade the channel. But you can put the one single resistor in the gate-path of the mosfet, no need for two in each leg of the switch.
If you want you can add a small NMost in stead of the switch and two touch pads:
simulate this circuit – Schematic created using CircuitLab
But that is entirely just for added spiff-factor. No function at all.
Your original schematic should, at currents up to 5A with low (but not ultra low) resistive losses in the MOSTS, not cost too much more than $5 to $7 though.
With regards to the fuse being all the way at the start, those are quite good instincts. I don't know the type of battery, but a TVS may also fail at some point, I have seen one or two go hard to ground at failure (they shouldn't, but hey), which with a good battery at least one MOST will start smoking as well.
This does not use a relay but a FET, seems to tolerate your voltage needs (from 4.5V to 20V). You have to check if 10A max is ok for your application.
http://www.pololu.com/product/750
If you bypass the push button with your momentary switch you should have what you need.
Best Answer
Jippie's answer (put as a comment) is largely correct.
Current = Power/Volts = 25/12 ~= 2A.
Substantially more current flows at startup and the diode must handle this.
Diode D2 is not needed to achieve your functionality (just replace it with a wire, as he says). Using D2 may be useful to provide both motors with the same voltage (as both then have 12V - 1 x diode_drop but you will probably not notice the voltage drop across the motor when the pump is pumping.
Many diodes will meet your need. Using Digikey's parametric selection guide I specified a diode of at least 5A rating and 40V reverse rating (safety margins never hurt) and asked for the cheapest silicon diode in a leaded package. Surprisingly it was [this 600V 5A BY500] (http://www.vishay.com/docs/88544/by500.pdf)
This is far more capable than needed but cheapest is cheapest :-). About 53 cents/1 at Digikey. (I tried specifying a >= 2.5A and >=20V part and the above was STILL cheapest :-) ). You can actually get a Schottky diode for less, and these are in fact superior technically BUT are more easily killed when playing so specifying a standard Silicon diode is safer here.
Be sure to be sure that the switch will handle the 12V inductive load of the motor.