Release Less Magic Smoke, With A Bulb Limiter

As electronics have moved lower in voltage, it’s perhaps less common to work on live-mains equipment. Thus particularly among younger hardware hackers it sometimes seems as though such work is viewed as so dangerous as to be only for the foolhardy. In practice it remains safe, so long as appropriate precautions are taken and a few pieces of useful safety equipment are present. One of those mains bench essentials is something less common in 2022, a mains current limiter using a set of switched incandescent light bulbs. [Donna LaRocco] shares a modern take on the idea, incorporating a digital mains voltmeter.

The idea is that a mains device under test is connected in series with a light bulb of a suitable wattage to let through enough current to run the device in normal operation, but to light up and bring down the voltage if the device draws too much. It’s an extremely simple but effective tool. Traditionally these are built using household electrical fittings on a board, and this one is no exception. The voltmeter comes from the RV market where voltage drop is an issue, no doubt giving European readers a chance to chuckle with their 230 V outlets.

If mains safety needs your attention, it’s a subject we’ve addressed in the past.

29 thoughts on “Release Less Magic Smoke, With A Bulb Limiter

  1. Handy circuit but I think I need it to disconnect faster than a relay. Can anyone suggest a link that will describe how to use a FET with it’s associated components instead of the relay?

  2. The question that occurs to me is, does this require tungsten filament lamps? And if so, where are you going to get them? Big Clive told a story a while ago about a piece of fairground equipment which used a tungsten bulb in this way. Someone innocently replaced it with an LED bulb, with disastrous results.

    1. Tungsten filament has the desirable property of a significant positive temperature coefficient. It has low resistance when cold, but once it heats up, the resistance goes way up. In normal (lightbulb) use, this provides the negative feedback to stabilize the current & temperature & power.
      When used as a protection device, the filament will quickly heat up if excessive current is drawn, causing significant voltage drop (and visual indication as a bonus:) This is often enough to protect the DUT from burning itself out.

      Neither LEDs nor fluorescent bulbs have the same characteristic. I’d guess that self-regulating heating wire could be a reasonable substitute, although it would probably have slower reaction time.

  3. When I was in 8th grade chem. learning about electrolysis, all of the benches had 25W lamps protecting the power supplies. Made it really hard to start fires (what most 8th grade kids were trying to do.)

    Another place you find lamp protection/fuses are speakers made by JBL, Peavey, etc. They are usually ~12.5v 12-25w…in series with the tweeter post crossover. They allow a soft compression until they finally open as a fuse.

  4. My first UV light to erase UVEPROMs back in 1975 while in college was an ozone bulb from an dead GE dryer that used a 20 Watt incandescent bulb as a ballast. It worked great. I think it’s still buried in my lab somewhere.

  5. You’re basically using an incandescent bulb as a cheap, ubiquitous (until recently) PTC device. Faster-acting (less thermal mass in the resistive element) and better controlled PTCs are available, but none so elegant and consistent with the hacker ethos than a light bulb.

  6. Over at Audiokarma the DBT is right up there with deoxit for repairs. Mine has saved me countless transistors and fuses. Installed mine in a metal desk lamp shade so it is protected from damage.

    FWIW DBTs often don’t work with SMPS devices.

  7. Yeah, incandessant light bulbs make great resistors. They change their resistance a certain amount with the temperature of the filament. This makes lightbulb resistors a bit non-linear, which in this case makes them even more desireable. Cool !

    1. Great idea. Those halogen bulbs used in calendar club displays years ago got wasted each year so finding a second use is both free and ecological. Not sure if they still are used these days but some people who did those Christmas season pop up mall sales events have hoards of those lights bc they had to buy new ones each year.

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