Electronics > Beginners
Lightbulb Capacitance
codingwithethanol:
I just found a string of small lightbulbs used for a decorative wall thing, so I thought id hook em up and see if they still work and they do. Interesting thing is, when hooked to mains (120V here), one bulb draws under 200uA! I measured the resistance and found about 200ohms, which would allow about 600mA. I then thought maybe the filament was acting as an inductor, so I hooked it to my LCR meter and found that it saw it as a CAPACITOR! What gives here, and is this true for all bulbs?
Psi:
Did you measure it more than once?
Sounds more like you had a break in the wire somewhere and where measuring open circuit with the LCR.
Note: You can't measure a light bulb and say it is X ohms therefor Y amps will flow.
This is because the resistance of the bulb changes a lot when it gets white hot.
So if you measure it cold you wont get its operating resistance.
ArthurDent:
Some circuits like strings of light that draw low current actually use a capacitor to drop the line voltage. The string you have probably doesn't draw anywhere near 600ma and certainly doesn't draw 200ua. This is a really cheap (and 'potentially' dangerous) way to get a lower voltage from the A.C. line voltage. Be careful playing with or trying to modify these lights.
https://hackaday.com/2017/04/04/the-shocking-truth-about-transformerless-power-supplies/
radiolistener:
--- Quote from: codingwithethanol on July 16, 2019, 02:47:35 am ---I then thought maybe the filament was acting as an inductor, so I hooked it to my LCR meter and found that it saw it as a CAPACITOR! What gives here, and is this true for all bulbs?
--- End quote ---
Any capacitor turns into inductor and any inductor turns into capacitor above some frequency which is individual for each element. Such frequency is the resonant frequency of element.
This is because any capacitor has inductive wires and any inductor has capacitance between wires. So, the total result will depends which reactive component will dominate at specific frequency.
Here is the inductor equivalent circuit:
In your case it depends on frequency which you're used for measurement in your LCR.
Richard Crowley:
Capacitive dropping circuits are regularly discussed on BigClive's YouTube Channel.
You will find dozens of videos of the Bearded One tearing down dodgy lamps with capacitive dropping circuits.
https://www.youtube.com/user/bigclivedotcom/search?query=capacitive
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