Tag Archives: prototype

Avoiding electrocution while testing 240v circuits

3 Jan

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I needed to measure the output voltages on a bunch of unmarked transformers I’d accumulated. (No, I’m not a hoarder. I can stop any time I want to. Perhaps.)

I’ve done this before, with just a bunch of alligator clips, a mains cord, and a multi-meter. It’s very simple, but it’s *dangerous*. Reaching over live 240v wires to take measurements isn’t a good idea. Worse is that a bunch of wires in mid-air have a tendency to move around, generally shorting something wires together.

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I recently made up a number of little test boards to hold test circuits in place without having to worry about wires getting loose. Here, for example, is a test with an arduino (on a solderless breadboard), an L298 motor driver, and a worm geared motor. The boards are 85mm long, with two rows of M4 holes 10mm apart. The rows are 75mm apart. They’re mounted to a laser cut piece of 6mm mdf, with two long rows of M4 tapped holes. (MDF taps quite well with a tap in a cordless drill).

I really don’t know, yet, whether these ‘boards mounted on boards’ are a good idea or not. However, they’re cheap and worth trying. In particular, they hold wires securely for testing which was exactly what I wanted for testing my transformers.

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Here’s a test victim hooked up to a test setup. I grabbed a spare chunk of 12mm MDF (I didn’t even bother cutting it square). I marked a grid of cross marks at 10mm x 25mm spacing using the laser cutter, then drilled and tapped just the few holes I wanted to hold things down. Chocolate blocks, hot melt glued down, gave me a secure but adjustable fixing for wires.

Note that I used a female EAN mains plug rather than a standard power cord to supply power. I physically unplugged it every time I changed the circuit, which I probably wouldn’t have done if it was a plug into a wall socket.

This particular transformer made me glad I’d gone to the trouble, as it was very easy to power it off when I plugged it in and it started to hiss and smoke. Looks like I picked up a 110V primary transformer somewhere along the line and it *really* didn’t like running on 240V.

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Even when I ended up using crocodile clips, they were much more secure when clipped into the immobile terminal blocks.
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A quick test of a crude opto-coupled triac circuit felt a lot safer when wired up like this as well. I wouldn’t trust a solderless breadboard at 240V.

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