If you’re testing old stereo equipment, a dim bulb tester can really come in handy. It’s not for testing bulbs, though, it’s a tester that uses a dim bulb to test other things. [Nicholas Morganti] explains it all in his guide to making your own example of such a tool. Just be wary — you need to know what you’re doing with mains voltages to do this safely!
The dim bulb is a deceptively simple tool that nonetheless often proves useful in diagnostics. It normally just consists of a bulb connected in series with the equipment under test. The bulb is intended to be a similar wattage to the power draw of the equipment itself. Take for example, an amplifier. If the bulb glows brightly when the amp is under no load, it suggests there may be a short circuit somewhere. That’s because the glowing bulb indicates that plenty of current is being drawn under a condition when very little should be flowing. The bulb protects the equipment by essentially acting as a bit of a current limiting device. It’s a soft-start tool for a piece of vulnerable equipment.
Building one is usually as simple as gathering an enclosure, a plug receptacle, a bulb socket, and some other ancillary parts to lace everything together. [Nicholas] explains it all with clear diagrams and tells you how to follow along. It’s easy enough, but you really need to know what you’re doing to use one safely, as mains voltages are involved.
It’s a great tool to have if you’re getting into amplifier repair or similar work on old gear. If you’ve been whipping up your own must-have tools, don’t hesitate to let us know!
According to the NEC white is neutral and the black wire is hot.
Red is the other hot and 2 conductor wire means there are 3 individual copper wires. Fun times
Can’t. No incandescent bulbs of sufficient wattage available anymore.
Here in the UK they are readily available still if you know where to look. It’s only domestic lighting that they were withdrawn from sale for; you can still get them as ‘rough service’ bulbs for boats, inspection lamps etc. My lamp limiter (as we tend to call them here) currently has a standard 100W rough service bayonet fitting bulb that I bought in an automotive parts retailer for example.
IIRC an incandescent bulb has higher current draw when it’s cold, they current limit but don’t have what you might think of as typical soft-start behavior like slowly turning up a dimmer.
You could have a number of bulbs in parallel and some switches to switch them in, for soft start.
If you start with (say) 25W, 50W, 100W, then switch the higher ones in as needed.
That’s supposed to be the case, but I can’t find any other “rough service” bulbs than oven lamps, and those are barely 15 Watts. Everything else has already been replaced by LEDs.
I’ve been lucky here in Ontario. I’ve been able to get 100W bulbs as well as 60, 40, and 25, and I’ve made sure to buy lots of spares. I’m using them to make a relay-switched dim-bulb tester that allows for switching parallel combinations to reach higher wattages. I know this isn’t as good as using single higher-wattage bulbs, but it’s better than nothing.
On another note, dim-bulb testers are useful on modern non-audio-type equipment as well. I used to repair electronic boards and modules from commercial fitness equipment. Among the modules were motor drivers for treadmills. Putting an incandescent bulb in series with the mains – especially in conjunction with a variac – sometimes indicated trouble in the output devices soon enough to avoid a catastrophic release of magic smoke. And even if the output devices did blow and short out, the energy expended was limited to what was already in the reservoir caps, because the light bulb effectively took the line current out of the equation.
where do you live, here in the SE USA you have to be careful and be sure to hunt down the LED bulbs, there’s only a couple options in a entire shelf of incandescence bulbs
Halogen work/flood lamps, various heat lamps, or worst case, headlight bulbs (at 12V, needing a transformer or a series array).
Could always connect sufficient number of the 40W oven bulbs in parallel.
or connect many of the 12v automotive type in series. I built a similar one to the article, except mine is used for electrocuting rodents. The bulb works great to prevent the breaker tripping.
Suppliers for photographic/studio equiptment are another good place to look, especially if you need higher powered ones. You can still get 275w and 500w bulbs in screw or bayonet without needing specialist fittings.
Thanks for this. I just checked my local photo store and I can get 75W, 250W, and even 500W bulbs. I may have to wait for them to be shipped, and they range from $10 to $15 – a bit pricey, but not bad for something that may soon turn into unobtanium. I’ll probably pass on the 75W, and 500W is more than I’m likely to need, but having a couple of 250W bulbs will be sweet.
You can use the primary of a small transformer in series with the load for the same purpose. If you have a low voltage output such as 12VAC, a few switches and automotive lamps can be used to gradually lower the transformer’s impedance to allow more current through. A short on the secondary will let almost the total current pass.
Excellent idea! I may try that – it would make my project smaller. Then again, I already have a bunch of 120V bulbs and sockets…
Beware: dim bulb testers might damage modern equipment.
SMPSs are constant power *output* devices, delivering whatever power is required by the load. If you reduce the input voltage, you will obviously increase the input current.
If the current becomes too high, the input switching transistors and related components might be damaged.
Most of modern SMPSs are specified for 100-240V range, so at least outside of US and Japan there is a hefty voltage margin. This type of current limiter is also mostly used to prevent catastrophic failures, running only for a short period of time with minimal load.
The upper voltage is irrelevant.
Suppose it is specified to work with 100V and X input current. Use a dim bulb tester so the input voltage is 10V; the input current is then 10X! Whether or not that damages the input components depends on how over-specified they are.
To coin a phrase “how lucky are you feeling today, punk” :)
Usually the switching converter will not start up below 80 volts because of UVLO. It’s designed into the chip to avoid this scenario.
What chip, exactly?
Not all SMPSs are based around ICs.
Not all ICs are the same.
Don’t over-generalise
Exposed lightbulb sounds like a trouble. On a plus side it can be replaced with other wattage variants giving different current limits (I saw once variant with multiple switches lightbulbs inside), but I probably would accidentally break it in a week.
Here is similar tool reusing broken 250W Dell power supply (metal case + original IEC320 socket for input): http://tomeko.net/projects/current_limiter/.
This is a very old trick that has been well used for many years to keep the smoke in while fixing things.
If you cannot get mains lamps, another option is a couple of 12V, 5Amp transformers back to back with a headlight lamp in series between them. I have a number of lamps that the low beam has blown and have kept them for loads.
As a bonus, this way you also get mains isolation.
I don’t get the need for mains isolation. We have bulbs sitting in out lamps all over the house and we don’t worry about the bulb being isolated. Why would a bulb in a fitting lamp holder/fixture on the beck be any more of a risk than a bulb in a light fixture lighting up the house?
The isolation is for the device (amplifier or TV or whatever) you are testing, not the bulb.
Isolating the device lets you more easily make measurements. It also makes it safer to make measurements with the power on and the housing open.
I used to have a PSU for model trains, with a series-connected “dim bulb” rated for the train’s normal operating voltage (12v?). Under normal operation, the current drawn by the train didn’t heat the bulb and the voltage drop across the bulb was negligible. Under a short circuit (not hard to incur) the bulb would light up, increasing resistance and limiting current, while also giving visual indication.