r/AskElectronics • u/ConductiveInsulation • 14h ago
What is the modern equivalent of the Lightbulb as Current Limiter while troubleshooting?
As you know, lightbulbs are a great way to test circuits that won't run on a available bench supply. But what would replace them when they eventually become unavailable?
A Lightbulb is basically a PTC heater, barely any resistance when cold but can reach the max temp/current really fast. And it's not polarised. But most importantly, it's extremely cheap and easy to use. Could even add them in parallel for bigger loads.
Currently I only see two possible alternatives, both with their drawbacks.
1.) A bench supply with the drawbacks of the steep price and inflexibility of the power source. Lightbulbs can be in series with the existing power.
2.) Maybe a PTC heating Foil could work, but due to the low temperatures they reach I assume their resistance at room temperature will already be too high.
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u/merlet2 13h ago
You can use resetable fuses, PPTC, for moderate current circuits. They normally have low resistance, but when a high current is passed through the fuse, the temperature and the resistance increases quickly breaking the circuit path, providing circuit protection. When the temperature goes down after a few minutes they conduct again.
They have different trip currents and are cheap. You can combine it with overvoltage protection with zeners, etc. And you can put a led in parallel with a bigger resistor to indicate visually when you have an overcurrent event.
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u/ConductiveInsulation 12h ago
I've only seen those so far in a way that has a incredibly high resistance, barely letting any current pass. Not really comparable to the reaction of a lightbulb since it still lets a decent current through.
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u/merlet2 6h ago
Yes. Well, you could put several in parallel with different trip points and increasing power resistors.
With normal current the first one conducts without resistance. When it trips, the second one conducts restricted by the lower power resistor (and the other ones conduct a bit). When it trips the 3rd one takes over with a bigger resistor, and so on. You could even add leds.
I don't know if this would work :-) at least for moderate current circuits. Maybe would be too slow..
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u/Linker3000 Keep on decouplin' 13h ago
Filament oven lamps are still available for now. Grab a few + holder while you can and before they get even more expensive.
Post lamp, I'd go for a 15-25W equivalent load (big resistor or equivalent) and either measure in line current or the across-the-load voltage and do the math. I suppose there's also the opportunity to make an indicator circuit using the load voltage as a reference. I guess this might have been done before and there's circuits out there.
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u/ConductiveInsulation 13h ago
Still have a few 100W lights for that but honestly I was hoping that something better and similarly affordable got invented.
A while ago I had to sink like 200W of heat into to have something slowly getting slightly warmer, and this was after the heatsinks got removed.
Couldn't the resistor cause issues in case the stuff that gets tested is not broken? Certainly would always have a huge voltage drop.
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u/isaacladboy 13h ago
Shortish term, bulbs are available and I can’t see them going away for years, too meany specialist industries use them.
Solid state versions are available. Machines whose whole job is to make line voltage and measure real time characteristics, used a lot in compliance testing. Since these are synthesising a whole wave they can do current limiting to any arbitrary power you desire
Iv seen one by GW before, cleaver kit with cleaver kit price tag.
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u/ConductiveInsulation 13h ago
Would have thought they'll become hardly available soon outside of specialised stores with MOQs in truck loads.
Do you have more informations about those machines? Normal generators/inverters unfortunately don't have those features.
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u/isaacladboy 12h ago
LED don't really like being HOT for long periods of time, so the bulb in your oven will be a 50 watt long life filament bulb. Things like that I personally cant see being designed out.
A lot of high end applications involving light, and light spectrum stuff also seem to be using filament bulbs as the spectrum output is more uniform (IDK what exactly im an EE not physics)The device I saw a few years back was a source measure unit designed for line applications. It was either by GW or keysight. If you google "SMU" or source measure unit you'll find them.
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u/ConductiveInsulation 12h ago
With energy efficiency getting more and more relevant, I wouldn't be surprised if big ovens get kinda rare in the near future.
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u/isaacladboy 2h ago
I can’t see it myself, I don’t know anybody whose kitchen doesn’t have an oven. I think people will always want an oven to bake and sort the Sunday roast.
Most modern ovens are electric so the environment lot can’t complain, and heat from the bulb isn’t waste as it’s just added to the intentional heat from the heating elements
I could be wrong of course
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u/EndangeredPedals 12h ago
Would an electric stove element work?
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u/gadget73 12h ago
yes but it won't limit current very much. Same with a toaster or electric heater. Hundreds of watts vs tens.
A cheap soldering iron would work though. 30-40 watts, electrically similar enough to a light bulb for this purpose but it does lack the handy visual indicator factor.
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u/ConductiveInsulation 12h ago
Interesting idea, maybe a soldering iron with the circuitry mentioned in the comments here that uses an LED?
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u/1310smf 12h ago edited 12h ago
Go buy an oven bulb. Or a halogen bulb, depending on the load. 300W & 500W halogens are suitable for bigger loads, while a 15W oven bulb gives more protection for smaller loads.
Both are still commonly available. Oven bulbs will be available pretty much forever, as no other bulb type works well at 500°F
Depending on load voltage, filament bulbs are also still dirt common in 12V automotive supply chains. You could put 10 or 20 in series if you needed line voltage, but as noted, there are filament bulbs that are still available for that.
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u/ConductiveInsulation 12h ago
Really seems like I should make myself a stockpile with enough filament bulbs until I'm like 100..
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u/redacted54495 5h ago
Smaller incandescent bulbs are exempted from the ban. I use 60W E12 bulbs with a 2 prong socket adapter.
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u/ConductiveInsulation 5h ago
Fortunately, because there clearly is no alternative
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u/redacted54495 5h ago
I've thought about trying to make an electronic fuse with MOSFETs and a programmable trip current, and then relays as a backup in case the MOSFETs blow up due to over current.
There's also the $$$ option of buying a programmable AC source which would have cycle by cycle current limiting as well as other features.
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u/ConductiveInsulation 5h ago
Even with the shit ton of high current MOSFETs I have lying around that doesn't seem like a great thing.
It's quite funny that we're loving the lightbulbs for the things that got them cancelled.
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u/awshuck 13h ago edited 13h ago
You’ll typically see old school audio amp troubleshooting done with a dim bulb unit in series with the load. Filament bulbs will drop a lot of voltage across them leaving less current to flow through you. However it’s not linear, controllable and even a little unpredictable. The other issue is these the right bulb is no longer plentiful in your typical hardware or grocery stores as they typically now stock newer LED bulbs and other high efficiency lighting. Halogen bulbs work but best avoid it altogether for a more reliable way of controlling current.
If you’re looking for a constant DC load to test things like power supply builds, have a look around for adjustable constant loads. They can be set to deliver constant current across a range of voltages. You can even build one yourself by setting up an opamp with a feedback loop controlling a reasonably linear logic level MOSFET, reading a shunt resistor drop as its feedback loop. Basic 30v, 5A units can be built with stuff you’re likely to have in your junk bin.
Alternatively, if you’re only working with AC and need something to enhance safety, for around $20, you can buy a programmable circuit breaker and set it to trip at a set current if you’re only worried about the safety aspect of troubleshooting mains or other high voltage devices. If you’re going to do that, please consider an isolation transformer too, you can easily build both into a single unit. I’m not yet aware of any cool AC constant current designs but I’m sure they exist out there. Good luck and stay safe!