power - How can I switch a high current with a low-rated switch?

I'm designing and building a breadboard power supply for myself, which draws power from an ATX-like SMPS (Mains in, single Molex plug out) via a 4-pin Molex connector. The design includes a switch to select either 12V or 5V output, but the only appropriate switch I can source is rated for a paltry 30mA!

Obviously I can't switch up to 2A with a 30mA-rated switch, but what can I do (that's cheaper than the ~$5-10 local vendors want for 2A-rated switches)? I thought of using an NPN transistor (see diagram, below), but in simulations that resulted in an unwanted voltage drop, and having the load connected to both emitters seemed to do weird things to the voltage.

^{simulate this circuit – Schematic created using CircuitLab}

(Note: Q1 & Q2 are generic NPN transistors, I've not considered part specifics yet.)

I thought of perhaps using a MOSFET, but decided that since I really don't know what I'm doing, asking for help was perhaps an even better idea.

Answer

To return to your transistor solution (as others have opted for a different mechanical switch), it might look like this. I included Wouter's hint for using a diode instead of the second transistor. Also I changed the bipolar transistor to a MOSFET. Reason for a MOSFET is because the base current of a bipolar transistor gets rather high with regular power transistors. Reason for P-channel high side switching is that this way the load can be attached to ground. If this is not a requirement, the circuit can be slightly changed to support an N-channel MOSFET.

• D1 should be rated > 2A


• T1 should be rated Id > 2A (P-MOSFET)

^{simulate this circuit – Schematic created using CircuitLab}