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Use Light Bulbs as Current Limiters?

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Plankspank
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Why yes, let's use this!
Plankspank   7/17/2017 8:30:54 AM
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I've used a 100W bulb on my test bench for years. We call it a "leak light" and works great when you suspect a unit has a short circuit someplace. It lights bright when a short is present, and briefly illuminates, then goes dim when current draw is minimal. Obviously, the light bulb is wired in series on the AC line.

 

I've also seen light bulbs used as fuses in speaker crossovers. 

vandamme0
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Old hat
vandamme0   7/16/2017 9:30:06 AM
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I keep a coffee can full of bulbs just for that. They're handy in battery chargers or anywhere you want to protect against short circuits without building a circuit to do the job.

The most uncommon application of an incandescent was when I used a couple car lights to simulate the heater of a large traveling wave tube; I was testing a heater power supply for the tube.

RichK
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Dim-Bulb tester
RichK   7/14/2017 3:02:29 PM
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I started repairing (1930's / 40's) Antique tube radios several years back and most repair sites suggest that you build what they call a Dim-Bulb tester.  Because the old paper and wax capacitors would dry out and eventually short in these units you should never just plug one in.  Using one these tests allows you to test your power supply safely without frying any other components.  Here is a good example of one of them.  https://antiqueradio.org/dimbulb.htm

 

anon3887601
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common in audio world
anon3887601   7/14/2017 2:31:48 PM
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This technique is ubiquitous in the audio world, commonly in speaker crossovers.  It is used to protect the tweeter or horn driver, which typically have much lower power rating than the low frequency transducers.

I once bought two Peavey speakers on EBay, cheap because they didn't work right.  Blown horn bulbs were all that was wrong with them.  Search on the web and you can find "Genuine Peavey speaker crossover fuse bulb" for sale.

W.

emulder
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bulb as stabilizer
emulder   7/14/2017 6:52:53 AM
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I still use the light bulb for amplitude stabilization, exactly as the original HP design. One half of a TL082 creates a virtual ground, the other half is the oscillator stage. My lamp is a 327, and I use a 1K trimmer, set roughly halfway. The circuit is used to generate a 60Hz sinewave to calibrate other equipment with. Neat detail: when you hook up a scope and power it up, you'll see the amplitude "bounce" a few seconds.

lakehermit
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Light Bulb Phase Indicator
lakehermit   7/13/2017 1:20:42 PM
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A mentor of mine who served in the military in WWII once told me that the way they phased portable single phase power generators for manual transfer was to series wire two 60 watt bulbs between the hot leads. Whenever the fuel tank on the operating generator was low, regs said the generator had to be powered down and cool before the tank could be filled. When it came time to fuel they would start the full generator and the bulbs would flicker while it came up to speed, then they slowly adjusted the throttle to bring the generators in phase by watching the bulbs. The bulbs would shine brightly and when the two generators were out of phase and when the bulbs were dark, it indicated that the generators were in phase and they could safely throw the manual transfer switch to change over to from the nearly empty generator to the full generator. After the change over they would shut down the empty generator, let it cool and refill it with fuel. This ping pong procedure went on for days, with someone always on duty to monitor operation and fuel supply and make the changeover when needed. 

alk
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Bulbs as Limiters
alk   7/13/2017 11:52:07 AM
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Years ago, I was installing an electromechainical telephone switch.  I noticed a light from inside to power supply whenever a phone was ringing.

The manufacturer (Stromberg-Carlson) had installed a 100W, 120V bulb in series with the output of the 90V ring voltage generator.

This protected the power supply in the case of a faulted subscrber line.

Quite clever.  Worked well.

I have since used this method when working on other projects.

Rcurl
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Strangest use of a limiter
Rcurl   7/13/2017 8:55:10 AM
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Since you've got me thinking about it,  I've got a weird one for you.  I had a friend who was an engineer /organist.  He had restored an old foot-pump organ at his house and decided that he was too lazy to pump the thing so he added a shop-vac in the basement with a hose the passed up through the living room floor to the bellows on the organ.  It worked OK, but the shop-vac ran faster than it needed to and you could hear it running while he was playing the organ.  He put a couple of incandescent bulbs in series with the shop-vac and it throttled down just right.  

That's not the end of the story, though.  He realized that the bulbs got brighter and dimmer as he played louder and softer, so he decided it would be a nice effect to move the bulbs to a couple of brass porch-lamp fixtures that he mounted on either side of the mirror on the ornate woodwork at the top of the organ.

Toccata and Fugue in D minor was really spectacular. 

 

David Ashton
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Bulbs as limiters
David Ashton   7/12/2017 7:09:57 PM
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I'm a big fan of this technique.  Back in the Rhodesian days (pre-1980) there was a need for cheap rugged battery chargers for radio sites.  Someone came up with the idea of a 13-14V transformer, a bridge rectifier and a 24V 20W truck headlamp bulb in series with the battery.  Worked like a charm.  Short circuit proof. current limited and even if you put it the wrong way round on a fully charged battery the lamp would just glow very brightly.  They were almost indestructible.  For the cost of a headlight bulb you'd need a lot of electronics to do all that - almost impossible I think.

Rcurl
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Thought I was the only one
Rcurl   7/12/2017 12:06:15 PM
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I'm glad you brought this up!

Yes- I have used incandescent bulbs as current limiters many times.  I think the first was when assembling some of the early Heathkits. The instructions for any of their products that operated from line voltage suggested connecting a 120 volt bulb in series with the input the first time the device was powered up - -and it's a good thing, because on more than one occasion I was surprised to see the bulb illuminate brightly while my newly-assmbled kit did nothing.  This allowed me to correct my assembly errors without a major catastrophe.

In later years I saw incandescent bulbs called "Barretters" used as current limiters on old step-by-step phone exchanges.  

Juist a few years ago I needed to knock together a quick circuit that would trickle-charge a 12 volt gel-cell battery that served as backup power for a microwave repeater. I used a "dome lamp" bulb between the battery and a 13.8 volt regulated supply and placed a schottky diode between the battery and the +12 feed to repeater.  As far as I know it's still out there somewhere working.

Come to think of it- back in high school I built a big strobe light as part of a "psychedelic" light show. I needed a way to quickly charge a 500uF capacitor to 500 volts.  Each time the strobe tube fired it caused a trememdous current surge through my transformer.  I put a 150 watt bulb in series with the primary and it solved the problem.  Charged the capacitor pretty fast too.

-Rick

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