Computing stuff tied to the physical world

Transformers – part 2

In Hardware on Nov 21, 2011 at 00:01

Yesterday’s post described a new test setup to be able to safely experiment with low- to medium-range AC voltages. As a first check when working on new AC mains powered circuits, and to allow me to poke around.

The main idea is to use a few small transformers to create relatively weak AC power sources with a range of different voltage outputs. Here’s one way to connect their six secondary windings together:

Transformers in series

The key safety feature is the connection to ground. Anchoring the middle of the output signal to ground effectively halves the voltage when you touch one of the outputs. It also (sort of) centers the sine wave around earth ground.

Note that touching both ends of the 6 series-connected secondaries will still cause a scary 148 VAC jolt.

To make this more convenient to test with, a double-pole 11-position rotary switch will be used (the best I could find for a reasonable price). This will allow me to switch between each of the above voltages, as well as 0V.

Every position up to 92V will have at most 56V to ground, the highest two positions will make that 74V and 84V, respectively (through secondary coils designed to supply at most 20 mA). I’ll consider up to 46V to be safe.

Here are the parts I got from DigiKey – and how I would like to arrange it all:

DSC 2797

Next step is to come up with a nice box for it – the work never ends!

  1. Neat concept.

    Adding a further 2 way, multipole switch can provide a “floating” or centre-tapped 220 Vac output for final testing at full mains potential while retaining the safety isolation. Basically, the second 28v transformer is fed in reverse from the first, providing a power limited, isolated copy of the mains input (a mini version of this)

    I’ll not post the circuit directly since choosing the switch needs special attention to “creep” distances to ensure full safety of the isolated output. Always treat mains potential with great respect !

    • I had originally planned to add a boost-up-to-230V-and-perhaps-a-bit-more option, but decided against it for safety reasons. One more switch to check each time. Now, all I need to do is decide how far to turn the knob. For dangerous stuff, I can use the isolation setup I already have.

      Btw, by popular request … added a comment edit option (via a plugin). Please let me know how it works out.

  2. Testing the comment edit option….

    Success !

  3. Great. I’ve disabled the countdown timer, it made me nervous. But there’s still a short time limit – editing is only possible right after posting.

  4. What happens if you serialize 2 windings, but in opposing directions (ie. phase swapped)? Would that add an (28-18=)10V to your system? If so, (18+18-28=) would make 8V. Too bad you have only 2 18V windings as 2×28-3×18 would make 2V! It would certainly mess up your diagram ;-)

    • You’re basically subtracting two sine waves with opposite and different amplitudes. I think it would work, but signal distortions might make the result even more distorted.

      Why would you want 2 VAC?

  5. Yes, by using more complicated switching, any combination of in-phase/out-phase windings is available. As JCW noted, some waveform distortion creeps in; the same current is flowing through all secondary windings, but the number of turns, hence the flux levels differ. That puts the transformers at different points on their B.H curves, hence non-canceling distortions.

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