Light bulb as a choke? - Page 1
Forum section: electronics / beginners

author: Connecteur
author_rank: Frequent Contributor
timestamp: August 25, 2025, 05:36:17 pm
content: I am building a booster circuit from an auxiliary 12 volt battery to the main 12V battery. 12 gauge wire may not be adequate for the high current that may flow between the fully charged auxiliary battery to the dead main one. I have an isolator between the two batteries, so the auxiliary will not drain when the main one does. I plan to rig a bypass to "boost" the dead battery from the auxiliary.Is it a good idea to use a 12V incandescent bulb to limit the current flow?

author: CaptDon
author_rank: Super Contributor
timestamp: August 25, 2025, 05:43:56 pm
content: You won't get much current flow. A dead battery may show 10 volts while a fully charged battery may be 13.2 to 14 while being actively charged. How much current will pass through your light bulb with a 3 volt difference? How about when the difference is only 1.5 volts or less.

author: TimFox
author_rank: Super Contributor
timestamp: August 25, 2025, 05:57:20 pm
content: With only 3 V across the nominal 12 V incandescent light, the filament will be cold (probably just above ambient) and its resistance will be low, even more so at around 1 V.The resistance increases greatly at full voltage after the filament heats up to > 2000 K.If the current rises to approximately the normal operating current of the bulb, then you will drop 12 V across it, leaving little for the load.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 25, 2025, 06:42:13 pm
content: May be not a bad idea as it works similar to a constant current charger. When the battery voltage difference is high then the resistance of the bulb is high to limit the current. When the difference is low the resistance reduces to allow for more current. I have to do some experiment and see what it's like.

author: MrAl
author_rank: Super Contributor
timestamp: August 25, 2025, 06:47:19 pm
content: Quote from: Connecteur on August 25, 2025, 05:36:17 pmHi,A light bulb will not work very well as an inductor unless maybe at very high frequency. For 10kHz to 50kHz not too good.As current limit, it works OK as long as your looking for a variable current. For light loads, not much limit, for heavy loads, a lot more limit. You should look up the voltage and resistance changes for the particular light bulb and see if that fits your needs. The 12v DC bulbs vary a lot depending on what they were originally used for.

author: bdunham7
author_rank: Super Contributor
timestamp: August 25, 2025, 06:50:50 pm
content: Quote from: Connecteur on August 25, 2025, 05:36:17 pmIt's not a choke--inductance is minimal--it's a positive tempco resistor. Theoretically that works fine, but in reality you need a pretty large bulb. Multiple 12V 50W halogen bulbs in parallel might work, perhaps 6 of them.

author: Seekonk
author_rank: Super Contributor
timestamp: August 25, 2025, 10:11:41 pm
content: I've been thinking of using PTC fuses for battery to battery. You can get fifty 5A 16V versions for less than $10 on amazon. These are about 18milliohms cold and that increases to a calculated 28 at 5A. They start to trip at 7A. These are thermal devices so lead length and temperature shift everything. When they go into full protect mode it takes 1-2W to keep them hot. Parallel them up for higher currents.I sense they would never be able to go into full protect mode in normal battery to battery and stay in the sub ohm range. Operation just like a lamp only better.

author: inse
author_rank: Super Contributor
timestamp: August 26, 2025, 05:29:39 am
content: Afaik, those PTC fuses have a limited lifetime, repeated tripping will degrade them.PTCs also had been used to degauss CRT TVs back in the days, and guess what: replacing PTCs was one of the standard repair jobs.

author: MathWizard
author_rank: Super Contributor
timestamp: August 26, 2025, 05:30:37 pm
content: So what current levels are you dealing with and over what time period ? There's some nice power resistors of all sizes on ebay (maybe not compared to power plant resistors).I was thinking of inrush current limiting NTC's, but I'm guessing the high currents are sustained as the battery slowly charges.