It was time for a dim bulb current limiter

4 months ago 8

One of my Minitels (the one that I modified a few years ago to run new firmware) started having power supply problems: the LED was on but nothing else was working. I suspected that the main power/CRT board needed new capacitors and so I recapped it:

Alas, despite some of the older capacitors measuring poorly compared to their specs, that wasn't the problem and so I need to go deeper. The only problem is there are a ton of nasty voltages on this board. Notably the 230V AC input and then the tens of kilovolts generated for the CRT.

Also the board seems to have a short somewhere.

So, it's time to a dim bulb current limiter. Which I didn't have. So I made one. Here it is:

It's a remarkably simple device which plugs into a standard power outlet (230V AC) and puts a lightbulb in series with one of the connections. The two wires coming in then go to a standard socket. Thus you plug your device (e.g. my Minitel) into the socket and the lightbulb limits the current.

These things were super common in the past for anyone working with mains voltages.

Works great if you have an incandescent bulb at, say, 100W. A 100W bulb on 230V AC would have a current running through it of I = V/P -> I = 100/230 (approximately 430mA). Limiting the current that way means you're less likely to damage the device you're working on if there's a short. Naturally, using a different bulb would give you a different current. 60W would give you around 230mA. 150W around 650mA.

As the Minitel's input fuse is 630mA, my current limiter will keep things well under its blowing current. Also, if there's a complete short then the bulb will glow brightly.

Here's a look inside:

Power comes in at the top, and goes to a DPST switch. I'm using DPST because it's not easy to know which input wire is live and which is neutral because the European Schuko/Type F plug/socket is not polarized and I like knowing that when I switch something off, it's off! And the DPST has an integrated lamp that gives me a visual reminder that it's on.

Because local power sockets aren't polarised, the power strip I am using for my bench gear uses DPST switches for each socket. It's relatively hard to find one that does this (and says it does). Thanks, Brennenstuhl!

But I digress.

To make my dim bulb current limiter do the right thing, I need a 100W incandescent bulb. These have been on their way to oblivion in the EU since 2009 and were, I believe, eliminated for home use in 2016. It is still possible to buy them, labelled for "industrial use", from some locations.

So, that limits the current and protects the device I'm working on, but what about protecting me. For that, I pair it with an isolating transformer. If you're not familiar with this then here's the basics. In a house the neutral line is connected to ground (typically at the point at which power comes into your house). This means that the 230V coming out the live wire is relative to ground. If I touch it the power will flow through me to ground.

The isolating transformer eliminates the link between live, neutral and ground. It's still outputting 230V but that voltage isn't relative to ground and so I won't get a shock if I touch one of the wires coming from the transformer's socket. Of course, touching both would send 230V through me.