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: Current design Original image size

So my question is, will this work and is this the best way or is there a better one?


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.

  • \$\begingroup\$ It's possible that he's switching the transformer to get better efficiency. If the startup characteristics are okay, then that might be a good design choice. \$\endgroup\$ – AaronD Oct 8 '14 at 21:15
  • \$\begingroup\$ Better efficiency? how? \$\endgroup\$ – Biduleohm Oct 8 '14 at 21:16
  • \$\begingroup\$ Thank you for answer, that would probably save me from an output switch but set me back another bridge, I guess it's not that much of a big deal. Initially I was thinking of saving power by keeping the regulators unpowered when not in use. \$\endgroup\$ – Limiter Oct 8 '14 at 21:17
  • \$\begingroup\$ Assuming that they're linear regulators, which the 7800 series are, current out = current in. And they're dropping whatever difference in voltage there is between in and out. Less voltage drop = less power dissipated in the regulators = better efficiency. \$\endgroup\$ – AaronD Oct 8 '14 at 21:18
  • \$\begingroup\$ A powered regultor but not used should not consume much, maybe hundred of µA at max I guess. Edit: then you can switch both input and ouput, you've already planned a DPDT relay, switch only the positive and voila :) \$\endgroup\$ – Biduleohm Oct 8 '14 at 21:19

Hmm, I haven't seen someone switch it there before, but it looks like it should almost work. The only problem that I see is that the 18V setting will try to put that 18V back into the output of the 9V regulator. I don't think it'll appreciate that.

The 9V setting has potentially the same problem, but I think it's okay because it's less than 18V and the diode from out to in will effectively power the 18V regulator to the point that it doesn't blow up.

Fix the 18V setting, and I think you're good.

  • \$\begingroup\$ Great answer, how would I protect it, add a third switch pole to output? Diodes on both positive outputs(that'd create some unwanted voltage drop though)? \$\endgroup\$ – Limiter Oct 8 '14 at 21:12
  • \$\begingroup\$ DPDT switches and smaller are easy to get. A bit harder for 3P and above. I would go with the diodes and boost the regulators' outputs a bit to compensate. \$\endgroup\$ – AaronD Oct 8 '14 at 21:17
  • \$\begingroup\$ good point, but since efficiency is not that big of a concern, I might go with just switching the outputs \$\endgroup\$ – Limiter Oct 8 '14 at 21:19

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