Computing stuff tied to the physical world

Isolation

In Hardware on Nov 7, 2010 at 00:01

There’s no escape… I have to get into mains voltage stuff and try some things out. All 220 volts of ‘em!

I want to experiment with power measurement, power control, dimming, an such – but, ehm… preferably in a way which will let me live to tell the story! (how’s that for dedication to this weblog, eh?)

Now the thing about electricity is that it’s not the voltage that kills you, it’s the current – after all, if this weren’t the case, all the birds on high-voltage wires would be dropping dead out of the sky…

Current means: electricity going from A to B. And the problem with us human beings is that we are conductors – which isn’t too surprising, given that we’re mostly made of water anyway (the rest is hot air, I’m told).

So the first thing to do is to isolate myself from AC mains, by not allowing a ground path to carry any current. This can be accomplished with an “isolation transformer”, i.e. a transformer from 220V to 220V which – like any normal transformer – creates a galvanic isolation barrier:

Screen Shot 2010 11 06 at 20.25.22

Trouble is, these transformers are relatively expensive. One reason for this could be that you want to get a fair amount of power across, so these are hefty units with a lot of copper, iron, etc. in them!

Fortunately, there’s a simpler way to accomplish exactly the same using two back-to-back normal transformers:

Screen Shot 2010 11 06 at 20.26.38

The first one is a normal step-down transformer, producing 10.5V – and then we simply take a second one of the same type, and connect it the “wrong” way around.

There will be losses, so the output voltage won’t be exactly 220V, but I’m fine with that. In fact, it’s a nice test to see what a mains circuit will do with a somewhat lower voltage.

I’m going to use a pair of beefy 300W transformers I have lying around here, which will let me test some control and measurement options up to at least 250 Watts or so. Here are the main ingredients, which I’ll put in a wooden box with slots for natural convection. This dual wine bottle unit looks like a good fit:

Dsc 2212

(I told you they were hefty tranformers – each of them weighs several kilo’s!)

What this gives me, is an isolated source of mains-like AC voltage. It is still just as lethal when touching both power lines. The only safety this adds, is that touching a single high-voltage output wire is now harmless.

The other benefit is that an accidental short will limit the amount of current that can flow through this setup, thus also limiting heat and damage somewhat. But 300W of power is still a huge amount, so I have no illusions that things won’t break. It’s mostly to prevent fires and burnt-out cabling.

Evidently, this thing needs a switch with indicator light, and fuses: a 5 Amp fuse on the input, and a 4 Amp resettable fuse on the output (lowest I could find):

Dsc 2213

Scary stuff. I could set up a pair of much lower-amperage transformers for loads up to 25 Watt or so, but it’d still be potentially lethal, so that would only lead to a false sense of security.

The low-voltage / high-amp intermediate circuit does provide an option to insert a fast-acting fuse. A 100 mA fuse @ 10.5 VAC would translate to 5 mA @ 220 VAC, which is no longer lethal, but I’m not willing to “prove” that! Trouble is that such a setup would blow its fuse with anything over 1 Watt of power, even if only a power-up spike.

Here’s the whole contraption (not wired up yet) – with thanks to Ikea for the “front panel”:

Dsc 2214

I’ll finish this isolation setup for some experiments, but it won’t alleviate much of my fear. Unfortunately, there’s simply no other way to get going…

  1. Please keep one hand in your pocket.

    Find a friend who is an electrician and ask his advice.

    Some ideas below — but I am not an electrician – just a hobbyist…

    Do test that those transformers are truly isolated from their metal cores. Also, please find a metal box. Also, consider putting a fuse in each leg. Perhaps the box could be grounded and the transformer frames also.

    • Please keep one hand in your pocket.

      Exactly.

      I’ll definitely ask for more advice (and thank you for yours), but I don’t quite understand the rest of your points. Both transformers I’m using are considered “safe” (and in good condition), i.e. isolated in all the ways you describe. Using one in reverse does not alter the way voltages are distributed across the different windings – the low-voltage side still carries low voltage, and the high-voltage side still carries a high voltage.

      I’ll definitely test all isolation paths you mention, and I’ll check that the fuses work when a higher-consumption appliance is plugged in.

      Thanks.

    • I’m not sure about the metal box William. The whole trick with the isolation transformer is that everything is floating. If you touch the live leg, you’ll just shift the potential difference of the whole circuit so that the live leg is at earth potential (through you). There’s no circuit, so no current.

      If you go putting things in a metal box you really need to earth it. Doing that will introduce a fixed point, and that will be the earth as obtained by the big metal spike driven into the earth near your fuse box. The earth obtained by the squishy water bag touching something will invariably be different. Having the “real” earth point available to being touched is asking for a bit of a tickle if you happen to be “live” at the time.

  2. PS: I’m concerned that the low-voltage winding (on the output tranny) will now be required to tolerate a high voltage. What is the rating of that winding’s insulation from the core/transformer frame?

  3. JC, if you are really that worried, get yourself an RCCD too. Connect that onto the output of your isolated outputs. That way if you do manage to make a circuit and ruin the isolation, the RCCD will trip out before you start saying rude words.

    Personally due to extensive exposure to mains, I’m pretty immune! I hardly say a rude word these days! Please note, this is not a recommended way of doing things! I have been very lucky! My preference for big clompy walking boots probably helps!

    My brother, an electrician, informs me that the same doesn’t apply to 3 phase. It appears you never really get used to 415v!

    • get yourself an RCCD

      Aha, an RCCD (“aardlekschakelaar” in Dutch) – I always thought they needed a ground connection, but they don’t! They really do compare the currents in both wires!

      You bet – I’m definitely going to add one that trips at 10 mA. Ok, so touching both wires is still as dangerous as ever, but all other failure modes will be covered. Note that with an isolation transformer, there is no difference between “live” and “ground”. As you say, it’s a floating potential difference.

  4. “aardlekschakelaar” – I’ll try to remember that, tad greedy on the vowels there, can you leave some for the rest of us please?!

    They were originally known as “Earth leakage trips” in English, but these days the correct term is RCCD – Residual Current Circuit Device. Which is a more accurate description of its function, even if it does sound like sales spin!

    For that to trip it will rely on some current finding an alternative route, which if the isolation is still intact, will not happen, but if you do happen to breach the isolation it will save you.

    The other option for testing purposes would be to run your AC circuit on reduced voltage. Not a problem for light bulbs, and a lot of switch mode power supplies, so maybe a 110v transformer used in construction would make you feel safer? After all, if 110v is safe enough for the army of lawyers who make up 90% of the population of the USA, it’s got to be pretty safe!

    • “aardlekschakelaar” – I’ll try to remember that, tad greedy on the vowels there, can you leave some for the rest of us please?!

      Haha – arceeceedee isn’t that much more modest, you just cheated a bit by using shorthand :)

  5. If there are more secondary windings in the transformer (like 2 times 12 volt) you van use one other trick. Use a secondary winding in serial connection with the primary winding and the secondary voltage will be 220+12v. This to compensate the transfomer loss.

  6. It is good that you are careful but do not worry too much. A small amount of 230V is not likely to kill you so and if you don’t fiddle with the wires while the power switch is on you should be safe. We do this frequently, without an isolation transformer (but with an RCCD). Tools with isolated handles also help.

    OTOH even a 300W of 12V is good at welding if you touch scope probes in the wrong places. (been there done that; unfortunately twice; once at 13,56 MHz and once at 50 Hz)

    That said: Kids, don’t try this at home. Adults, even if not in doubt, think twice.

    • Kids, don’t try this at home. Adults, even if not in doubt, think twice.

      Sound advice. The risk is real – all it takes is accidentally dropping a screwdriver in an unfortunate way!

  7. When I was 12 years old, I decided I needed to build a pulsed nitrogen laser as described in the Scientific American “Amateur Scientist” article (June 1974). Never got it to work, but I did make have a working 12 kV power supply (used a automobile spark coil and diode multiplier). I discovered that an 18″ wooden dowel is not much of an insulator at that voltage. Surprising that I survived, as I look back on it.

  8. The other rule is, never work on HV alone. Someone else needs to be home, to call the hospital just in case…

  9. One of my hobbies is Amateur Radio. Another is reading. I have a collection of old Amateur Radio magazines from the 1920s – 1950s. It is kind of sobering when you read parts 1 and 2 of an interesting project, only to find there is no ‘part 3′. Instead you find out the author is listed as a “Silent Key” (deceased).

    Thanks for pointing out my confusion about the voltage levels. But my recommendation for a metal box, multiple fuses and grounding the transformer frames remains. Should there ever be a failure, why not provide a safe path thru the ground wire rather than thru some part of your project, or yourself.

  10. School labs her in Sweden do not use transformers any longer but what you call RCCD (we call them earth failure switch or current comps) and nothing more. Ther is also a combination of adjustable ultra fast fuse and a RCCD in the market (called personal protector).

    But pleas observe that a connection from one of the wires to the other do NOT trip the RCCD. When I attended labs in high voltage constructions at CTH (http://www.chalmers.se/en/Pages/default.aspx) my lab instructor demonstrated this by barbecue a sausage by inserting the weirs to opposite ends of it!

    What you need is isolation gloves (boots do NOT help if the current passes both your hands (and therefore your hart), tools that are fully isolated, lab-wires that automatically cap them self when unplugged. Use goggles to protect you from exploding parts (caps, M-FET etc.) Never work alone (union rules forbids this in Sweden) remove anything made of metal from your workplace you do not absolutely need. Have a fire extinguisher ready. NEVER work on a live circuit if the work is possible to do on a dead one.

    And last do think twice!!! I once watch a surveillance movie from the Forsmark thermonuclear power plant of a electrician that forgot that he was working on bloc A (turned off) and put his hands in bloc B (all generators turned on). The reactor rapid switched to bloc C but he… I don’t now? Boiled, roasted? It was utterly terrible.

  11. All of this is beside the point – working with boots, gloves, etc…. YOU SHOULD NOT BE STICKING TOOLS INTO AN ENERGIZED SYSTEM, UNLESS YOU REALLY KNOW WHAT YOU’RE DOING!!! If you need to work on it, unplug it! Of course, with caps and lingering currents, that won’t be enough, and all the other elements apply.

    That said, don’t worry about it too much either. Yes, it’s dangerous, so show it a healthy respect, but it it not /hard/. Here in the US, we have both 120v and 240v in residences, and I’ve been hit by both. 240v is no fun, 120v isn’t either, but isn’t /so/ bad (I have 3-phase at work, and I never want to be hit by it). Always test circuits for energization, both to neutral and to ground. And, I’ve learned (when working on old buildings), test them even if you’re working on a branch where you’ve personally killed the breaker. I was hit by 240v, since I killed the water-heater breaker, but was unaware that the stove and the water-heater had been back-fed by someone previously – highly dangerous – and so the circuit was still very much alive.

    Oh yeah, and remember that not all safety measures are built with you in mind. That Forsmark vid – the reactor rapid-switching to block C probably wasn’t to save the electrician…

    Then again, I personally prefer electrical work to plumbing…

    • No it was definitely not! It is a computer controlled “action” to prevent the cooling (pumps) of the core from stoping. But it showed that high current kill at once (switch over takes less then a second). If it had been constructed as a human safety device the current had not exceeded safe levels (<30mA for less then 30ms @ 230VAC), I don’t know what it is @ KV levels (-; and if a RCCD is possible considering inertia in a switch of that size.

      Some times you have to work on a live circuit. To measure, to trim it, to find some types of failure etc. )-:

  12. isolation gloves

    Yes, I intend to use those. The tip on clearing my desk is also a good (and relevant!) one. As for never working alone – hm, yes. That’ll be harder, but I’ll heed that advice.

    Thanks again for all the wisdom and advice offered here. Much appreciated!

  13. I’ve been looking at building a power measurement plug for jeenode and I stumbled upon an interesting IC that might interest you- the Cirrus cs5463. The chip does all of the measuring and calculations to figure out energy usage. See http://www.cirrus.com/en/products/cs5463.html

    The reference design looks fairly straight forward. They interfaced with an atTiny2313: http://www.cirrus.com/en/pubs/rdDatasheet/CRD5463PM_RD2.pdf Add a relay and you’ve have a very interesting plug.

  14. Hmmm Van Dongen…

    Good taste in wines JcW :)

  15. Can we expect a AC Dimmer plug-in some time soon? If so, I’m buying a few ASAP :)

    • I’m still waiting for the RCCD trip switch to arrive. But yes, I’d like to get something going if I can figure out all the details.

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