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:
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:
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:
(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):
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”:
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…