Isolation transformer question

[aarons]

I'm trying to turn my adhoc isolation setup on my test bench into something a bit more convenient. I currently have a an isolation transformer, which has a variac plugged into it. Sometimes I put a dim bulb in series with it, and sometimes I don't want it isolated so I have to move the variac plug to the non-isolated outlet.

So I've mounted a switch box with 2 switches, one which bypasses the dim bulb, and one to switch between isolated and non-isolated. (I also have 2 banana jacks on there which need to be shunted for power to reach the DUT, so I can insert an e-stop for when sparks fly.

The plan for switching between isolated and non-isolated is currently to use a DPDT relay to switch the hot and neutral wires feeding the outlet the variac will be plugged into. However, in the middle of putting this together I started wondering if there is a more seamless way to handle this -- something that won't cause power to the DUT to be interrupted when switched. I'm assuming shorting the primary to the secondary of the isolation transformer will cause problems since the transformer is not ideal, and the voltages might not be 100% equal, there might be some phase shift, etc.

What could work for seamless switching the isolation of the DUT?

[MarkF]

My first question would be "why".

• Why would you care if switching between isolated and non-isolated was seamless?  I doubt any discontinuity in the power to any DUT would matter.  Most DUTs will have a diode bridge followed by a regulator.  A short power disconnect won't effect it.  What would your DUT do in a thunderstorm for instance?
  
  
• Why would you complicate your design with a relay when a simple switch will do?
  
  
• Why would you have banana jacks in a high voltage system?  Isn't the dim bulb there to prevent "sparks"?  Banana jacks seem dangerous.  If you want an emergency off switch, permanently wire one in.
  
  

It looks to me that you're making something more complex than it needs to be.  In a panic situation you want it to be as simple and straight forward as possible.  I would only have three switches:  Power on/off, isolated/non-isolated and a dim bulb bypass.  A fourth emergency shutoff if desired, but isn't that what the power switch does?

[aarons]

1) Because why not? The interruption might cause my voltage/current monitor to reset. A smooth switchover would bd more elegant.

2) Are you aware of a DPDT switch that mounts in a standard electrical box? Oh, and it needs to be a double switch because it’s a 1 gang box containing both switches for the dim bulb bypass and the isolation selection. Since I am not aware of such a thing the easiest solution was to use a relay with a 120V coil.

3) I’m comfortable with the safety of the banana jacks. And while I don’t consider 120VAC to be high voltage, the jacks are rated for 2.5kV and the plugs for 5kV.

There is already a power switch for the whole thing, but it is in a bank of small switches and an awkward reach. The e-stop box is an actual e-stop designed specifically to be easy to use in a panic.

[David Hess]

Over-complication leads to mistakes.  I have an on-off switch on my isolation transformer, and that is it.  Switching to non-isolated mode is an invitation to destruction.

[themadhippy]

Are you aware of a DPDT switch that mounts in a standard electrical box?

blanking plate and a drill.

[Stray Electron]

2) Are you aware of a DPDT switch that mounts in a standard electrical box?

   I'm pretty sure that they make DPDT electrical toggle switches for use in the ceiling light circuits that are controllable from multiple switch locations. 

   See https://www.electricaltechnology.org/2022/05/double-pole-double-throw-dpdt-switch.html

   But I agree with the others, I think this is an unnecessary, and possibly hazardous, complication.  I'm a firm believer in the K I S S rule. 

[aarons]

Those

2) Are you aware of a DPDT switch that mounts in a standard electrical box?

   I'm pretty sure that they make DPDT electrical toggle switches for use in the ceiling light circuits that are controllable from multiple switch locations. 

   See https://www.electricaltechnology.org/2022/05/double-pole-double-throw-dpdt-switch.html

   But I agree with the others, I think this is an unnecessary, and possibly hazardous, complication.  I'm a firm believer in the K I S S rule. 

That's interesting. I've somehow never noticed those before. But using that would require me running 4 additional wires between the box with my outlets and the box with the switches, and it's already finished and working perfectly with the relay.