Lightbulb Capacitance - Page 1
Forum section: electronics / beginners

author: codingwithethanol
author_rank: Regular Contributor
timestamp: July 16, 2019, 02:47:35 am
content: 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?

author: Psi
author_rank: Super Contributor
timestamp: July 16, 2019, 03:04:57 am
content: 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 16, 2019, 03:14:36 am
content: 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/

author: radiolistener
author_rank: Super Contributor
timestamp: July 16, 2019, 04:44:43 am
content: Quote from: codingwithethanol on July 16, 2019, 02:47:35 amAny 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.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 16, 2019, 04:54:20 am
content: 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

author: codingwithethanol
author_rank: Regular Contributor
timestamp: July 16, 2019, 01:33:44 pm
content: @PsiYeah I ran my LCR meter more than once, 32 and 36 pF respectively, The bulb sockets came in a string and so i cut one out and stripped the wire ends for testing.@ArthurDentThe sockets seemed pretty barebones, but ill rip one apart and see if there's something hidden in there.@radiolistenerYeah I mean its just 60hz mains. I looked at the filament, and it doesn't really appear coiled in any way. My LCR meter is a $7 Arduino special off Ebay, but still pretty useful.@Richard CrowleyHey, im subscribed to that guy! Yeah i've seen him teardown some bargain bin garabage, but its usually jank LED circuits. I didn't know they did the same for filament bulbs.

author: radiolistener
author_rank: Super Contributor
timestamp: July 16, 2019, 05:05:17 pm
content: Quote from: codingwithethanol on July 16, 2019, 01:33:44 pmany wire has inductance and capacitance. Which component will be dominant depends on frequency and wire geometry. The wire don't needs to be coiled in order to be inductive or to have capacitance between different part of wire.This equivalent circuit can be used to model piece of wire:

author: amyk
author_rank: Super Contributor
timestamp: July 17, 2019, 12:38:05 am
content: Are you sure they're actually incandescents and not neons? A few hundred uA is within the typical current range for a tiny indicator neon.Pictures please...

author: radiolistener
author_rank: Super Contributor
timestamp: July 17, 2019, 09:02:37 am
content: if you're using LC meter like this:https://www.aliexpress.com/item/32827008101.htmlit works at about 700-800 kHz, it's a bit higher than usual mains frequency.You can use long press on the button in right bottom corner, it will show you measurement frequency.So, in some case your LC meter may show capacitance at 700 kHz, while at 50 Hz it will be inductor.That's normal behavior for inductors with resonant frequency below 700 kHz.There is also possible capacitor ballast inside your light bulb. This capacitor may have very high leakage and it leads to 200 ohm resistance.