Computing stuff tied to the physical world

It just blew apart

In Hardware on Sep 28, 2011 at 00:01

Got quite a scare the other day:


One moment I was cheerily plugging in and unplugging my 100 W light bulb for testing current measurements, and the next it came apart with a huge bang. No fuse tripped anywhere (as it would have in UK households).

It’s a good reminder of the amount of energy an AC power outlet can discharge at any time. In fact, the standard rating for individually-fused 220 V power groups is 16 Amps around here. That’s roughly 3600 W.

To get an idea of what that means – one Horsepower is the sustained power a horse can generate, which is about 740 Watt. So there are almost 5 horses in each power outlet, waiting to charge at you!

If you push that much energy into a liter of water, it’ll boil in 2 minutes flat. Doesn’t sound like much, eh? How about 1000 Amps at 3.3V? You’ll need copper wire with a diameter of 18 mm to handle that much current.

Compare that to a JeeNode in sleep mode drawing 10 µA, i.e. 33 microwatt. That’s 8 orders of magnitude less.

Anyway, I’ve gained some extra respect for 220V mains circuits.

Now, if only I could find a replacement … 100 W incandescent light bulbs are no longer sold in Europe.

  1. JC, see my comment to your previous post about a cheaper Hall-effect approach. It’ll take a custom-etched double sided PCB, and a little bit of calibration, but it’s dirt cheap, and the mains current flows through one PCB trace (and does not flow through any device!). The board itself gives you several kV of isolation.

  2. and the 100watters? Just go to any dutch supermarket: Philips has put halogen lights in bulbs – the 75watters actually are a bit brighter than your 100watt bulb :)

    • Trouble is JC wanted a 100watt load!

      I hate those energy saving bulbs, horrible colour spectrum. Which is why I have a stock of the old bulbs, quite a few actually! Unfortunately although I would happily send JC a nice new 100watt bulb, being the UK, we had to be different. Ours are bayonet, not screw.

  3. Does it have to be a bulb? Use an electric kettle, I did that when testing my powermeter.. It even has a nice on/off button on it :)

  4. How about 60W and 40W bulbs in series? And in eBay(.de?) is perhaps easy solution?

  5. Maybe the german company heatball ( can also deliver 100W heating elements. They sell heating elements that exactly fit the E27 fixture that you use to connect the light bulb. ;)

  6. Thanks for the tips :) – no worries, I’ve got plenty of options to create power loads.

    (@th: 60 + 40 W bulbs in parallel will make 100 W – in series they just glow dimmer)

  7. JC, you mix power and energy (or work) at some places (now I am getting picky, I know :-) ) and that makes it even ‘worse’ than what you already said. Only the sustained power is ~3600W in case of a 16A fuse or MCB. The momentary power you can draw from a power outlet is even much higher than you said, for small amounts of time it is only limited by the resistance of your cabling (and in case you have an MCB by its magnetic trip threshold).

    What you ‘experienced’ when shorting the two small wires without tripping the fuse was momentary power of much higher magnitude! This lets the wires virtually ‘explode’ (and as a nice side-effects removes the short-circuit :-) ).

    BR, Jörg.

    • Thanks for the clarification – I do appreciate it, always. Yes, I do get sloppy at times. I also understand that light bulbs can have up to 10x surges when turned on with a cold filament.

      This was a setup without my isolation transformer, btw. Which has a 4A resettable fuse on the secondary side.

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