power supplyswitches
So I am designing a switchable voltage 9V/18V power supply and was wondering, what's the best way to switch between outputs, that's the current schematic I came up with:
Original image size
So my question is, will this work and is this the best way or is there a better one?
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.
simulate this circuit – Schematic created using CircuitLab
This will work but with a small modification. The output from the AC amplifier is decoupled via the output capacitor and therefore the voltage reaching the 4066 switch might have a strange dc value on it. The possible solution is to not ac couple but rely on the dc connection to the ac amplifier - there shouldn't be any problem designing the ac amp to have a "centred" dc rail coming from the device so that the 4066 switch is seeing an ac voltage mid-rail of its supplies.
On the most left-hand switch you ought to have a pull-down resistor after the switch and before it feeds to the AC amplifier, probably in the order of 1M ohm.
Any time you use analogue switches in this type of application you have to watch out for the switches inherent leakage currents charging up capacitors and this is always alleviated by dc coupling.
Best Answer
You should switch the outputs of the regulators. That way you bypass the problem relieved by AaronD and you don't take the risk to have the regulators doing weird things the first ms when they are powered since they are always powered (but I'm not 100% sure about that last part, maybe the regulators are stable even when they have just been powered, but prevention is better than cure...).
You can also switch both input and ouput (using two bridge rectifiers to avoid the transformer tap switching), you've already planned a DPDT relay. You don't get startup instabilities immunity but you get rid of the problem of backpowering the ouputs of the regulators and you don't power the unused regulator.