Using Commom-Mode Chokes as (non-mains) Isolation Transformers? - Page 1
Forum section: electronics / beginners

author: Jebnor
author_rank: Regular Contributor
timestamp: June 20, 2013, 11:53:07 pm
content: Hello all, I have a thought. It might not be very good but here we go...I have noticed that all the projects for my bench need (yet another) wall wart, or built in power supply. I have a monster 12VAC transformer from a car battery charger. It puts out >30A @ 12VAC.If I used that 12V as a power rail, could I use common mode chokes (such as ) as small isolation transformers for all my bench-only gadgets? My understanding is that it should be theoretically possible. They will likely not be as efficient as proper mains transformers, but they are significantly cheaper.http://www.digikey.ca/product-detail/en/ELF-17N022A/PLK1180-ND/523281Related theory questions: How coupled are common mode chokes? I'm aware that it varies according to the core type, but would they be as coupled as standard mains transformers? How would inductance and/or impedance affect their coupling?If it is one oforcould you explain a little please.

author: c4757p
author_rank: Super Contributor
timestamp: June 21, 2013, 12:01:22 am
content: If you apply 12VAC at 60Hz (sorry, I'm American) to a 1.3mH "transformer primary", it will draw 12/(2*pi*1.3mH*60Hz) = 24.5 A (yep, 25 AMPS).

author: Jebnor
author_rank: Regular Contributor
timestamp: June 21, 2013, 05:26:00 am
content: So...

author: cellularmitosis
author_rank: Unknown
timestamp: September 28, 2020, 03:11:58 pm
content: Just to clarify, 24 amps would be the current drawn if you shorted the secondary? How would you calculate the "no-load" current?

author: magic
author_rank: Super Contributor
timestamp: September 28, 2020, 03:14:31 pm
content: No, the current when you simply apply AC to the primary. AKA magnetizing current.dI/dt = V / LIntegrate overone cyclehalf cycle, lol, of a 50Hz sinewave and you know why mains frequency transformers are huge.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2020, 04:28:52 pm
content: How good the coupling is depends on the design. Some CM chokes intentionally have reduced coupling to also provide some differential mode filtering - a little like the 50/60Hz transformers for microwave ovens.The usual CM chokes are ferrite type cores and would not work well (only very low voltage before they saturate) with low frequency. They may be still useful for a small DCDC converter running at some 50 kHz.There are high performance CM chokes with noncrystalline core material that is actually also used for special mains transformers. But is would still be something like a 0.5 V / 0.5 V transformer for low power (e.g. 0.5 VA).

author: TimNJ
author_rank: Super Contributor
timestamp: September 29, 2020, 05:20:55 am
content: A few notes:1. Although ferrite cores are usually used in power/RF applications above ~10KHz, theycanbe used at 50/60Hz, but in general does not make much sense.2. Laminated silicon steel transformers/chokes can operate at flux densities ("Bmax") between 1.5-2T. Meanwhile, ferrite cores max out at about 0.3-0.5T.3. High Bmax is preferred for low frequency applications because Bmax is related to the "energy" stored in the core's magnetic field, per cycle...or rather per half cycle. If you want to deliver 300W @ 60Hz, each cycle needs to contain a lot more energy when compared to a typical switch-mode converter's frequency of 100KHz. As fluxdensityis related to the cross-sectional size of the core, a higher Bmax allows a smaller core without saturating.If you had two identically sized cores, one ferrite, one laminated steel... the power throughput will be lower for the ferrite core (without saturation) @60Hz. So, youcanget power through a common-mode choke, but you'd probably reach saturation earlier.

author: Zero999
author_rank: Super Contributor
timestamp: September 29, 2020, 11:27:21 am
content: Quote from: c4757p on June 21, 2013, 12:01:22 amQuote from: Jebnor on June 21, 2013, 05:26:00 amThe result will be an arc, fire, magic smoke and hopefully the circuit breaker tripping, before it gets too bad.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2020, 11:43:38 am
content: Now this is some necromancy...For those still curious, they are fine to use, at frequencies where they are suitable. 10-100kHz typically. The downside is the leakage inductance is typically very high, so only a few watts can be transmitted, within limitations of typical output capacity (say 12V 100mA) and regulation (voltage drop with load).A resonant control with feedback could deliver much more power (10s, 100s W..?) but is more complicated. At this point you're more likely to shop for a proper pulse transformer (same sort of thing but with low leakage), SMPS transformer (flyback or forward type) or a fully integrated DC-DC module.At 60Hz, not only will large currents be drawn, but the core will be in saturation much of the time, so DCR dominates -- it would smoke and burn up nice and fast, while transmitting very little power.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: September 29, 2020, 11:54:58 am
content: Holy... I did not even see the thread’s original date.