Computing stuff tied to the physical world

Popping fuses

In Hardware on Dec 13, 2010 at 00:01

The power-line isolation saga continues…

Danger High Voltage alt 1 md

Ok, so the idea of adding an extra RCD device which trips on differences in current turns out to be a mistake. It adds no extra protection when used in the secondary winding of such a setup. I will leave it out.

Next step was to hook up the circuit as described in the above post. Soldered evertyhing in place, used heat-shrink tubing to isolate the loose ends, lots of hot-glue to fix things, and then:

Dsc 2398

POP, said the 5A “slow” fuse – and the next one too!

That’s not just a molten piece of metal, BTW, that’s a vaporized coating on the left. I suspect that the current was substantially over 5 amps. But no ill effect on the rest of the house, no lights dimming, and no other blown fuses (the main ones downstairs are all 16A).

Uh oh, something is wrong. Well, at least I now know the fuse works!

Here’s the plan again:

Ah, wait, not quite! … I have a transformer with dual secondary windings:

Screen Shot 2010 12 12 at 12.36.53

What I did was wire them up in parallel:

Screen Shot 2010 12 12 at 12.37.03

And that’s where I went wrong. Each winding generates an AC voltage. There are two ways to put them in parallel. Connect them right, and you get twice the power – connect them in reverse, and one will generate an AC voltage which is exactly opposite in phase w.r.t. the other winding. The result is not just a short circuit, but two power sources actively counter-acting each other:

Screen Shot 2010 12 12 at 12.42.00

Hence the blown fuses. I did not expect a 300 VA transformer to blow a 5A fuse, which implies its power draw was over 1100 watt – but that’s exactly what happened.

The solution is simple, once you step back and think about it. Instead of connecting the windings in parallel, I need to connect them in series:

Screen Shot 2010 12 12 at 12.37.11

(and the same on the other up-converting transformer too, of course)

Here’s the key: it’s still possible too hook them up the wrong way. But doing so will simply lead to no ouput, i.e. zero volts, instead of the desired double voltage output. But no blown fuse.

Once I made this change (which required a lot of undoing), I found out that the transformer secundaries were indeed wound the other way than their wires had initially led me to believe.

Problem solved!

I now have a working isolation setup which turns a 230.3V input voltage into a 230.9V output voltage under no load. An 80 watt test load worked fine.

P.S. I also gained more respect for AC “power”: such circuit mistakes are a different ball game from fiddling around with an AA Power board and getting all sorts of microcontroller- and sensor hookups messed up!

  1. That is a very impressive fuse blow indeed. You’ve effectively plated the inside of the glass tube with a thin layer of fuse wire metal.

    You’re lucky the glass stayed together, I’ve seen similar things shatter the glass due to the rapid expansion of the air inside the tube combined with the inside of the glass getting hit with very hot liquid/vapourised metal.

    It’s for the latter reason that a lot of fuses tubes are made of opaque ceramic. Which are rather boring ;-)

  2. RCD is only to be installed in noninsulating instalations. there are some “smart circuit breakers” that you can program in the tripping characteristics and this ‘might’ (depends on the case!!!) add some sort of protection, but price is rather prohivitive. What was the price for that RCD?

  3. If you use an isulation transoformer you can test if there is a leakage of current to earth with an earth insulation monitoring relay (but this will not prevent shock when touching both wires)

    • Does that add anything Mike? Given that a free floating isolated output doesn’t hurt when you only touch one wire?

      It’s the touching both wires situation which is the big risk you want to avoid.

      Depending on what load JC is testing, he could always use only a single 10.5 winding on the output of the live facing transformer, and the series pair as the input to the isolated side. This would give him an isolated 115v AC, which would happily power modern switch mode power supplies and illuminate lamps (all be it dimly), but with a greatly reduce risk of expletives!

      115v must be safe, even an army of American lawyers haven’t had it outlawed!

  4. @TankSlappa: if one of the two insulated wires leaks to earth, touching the other one is not safe anymore. I think that’s the situation where monitoring the earth insulation would help.

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