Ferrite common mode choke - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: April 05, 2019, 11:11:36 am
content: Hi all,I'm building a small audio amp using an off the shelf TPA3116 board (2x50w + 1x100w). I'm powering it from a 12VDC SMPS.Should I loop the Mains AC power wires in a ferrite torrid? Or is the "noise" coming off the SMPS much larger than any other noise coming off the mains AC? Or should the choke be used somewhere else?The idea is to attenuate any stray signals from affecting the audio signal.Regards,

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2019, 11:17:12 am
content: The only reason for adding a choke on the mains side would be to stop the noise generated by the SMPS from going back into the mains. If it's probably designed and complies with all the relevant standards, it shouldn't need an additional input filter.It might be necessary to add a choke and filter capacitors to the 12VDC to prevent noise generated by the power supply from interfering with the amplifier.

author: made2hack
author_rank: Frequent Contributor
timestamp: April 05, 2019, 11:39:20 am
content: Ok cool, so better on the 12VDC output towards the amp then!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2019, 06:21:34 pm
content: Unless it's a too-small module intended to be placed within a system that's ultimately filtered -- it contains a filter (and CMC) already.Consider the PSU as a three-terminal device. The two mains leads are one, the two output leads are another, and the chassis ground (or ground screws for the loose PCB kind) is the third.If ground isn't present, or connected*, then the two leads (mains and output) are a through circuit, and a choke can be put on either one, equivalently. It's better to put it on the mains side, because the wire can be smaller = more turns on a given core.*Even if you don't physically connect it, notice there is still a stray capacitance to the surroundings.If it is connected, then there's probably some "Y" capacitance to it which improves filtering, and the noise level can be different on both ports. In that case, extra choking impedance in series, or bypass capacitance in parallel (to ground), will help, and you have a choice of which port to quiet down more.If you can't demonstrate you need it, though... what's the point in adding it at all? Just a "feel good" act? But it's just superstitious then. That doesn't makemefeel good, at least...You don't need expensive instruments to demonstrate some sort of effect. Is the amplifier susceptible to RF rectification? Is anything nearby or attached, susceptible? Mind, while this gives a testable hypothesis, it is only a tiny step above guessing in the dark. While technically possible to reason your way out, it requires an unnatural amount of observation and discipline to succeed. You may find it helpful to use a radio (an AM or shortwave radio would be best) to get some idea of efficacy.Tim

author: made2hack
author_rank: Frequent Contributor
timestamp: April 05, 2019, 07:16:32 pm
content: Thanks!What about interference from cell phones? I take it that a CMC won't have any effect and I should look to shroud the amp board in a metallic case separate from it's surroundings?

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2019, 10:19:14 pm
content: I've just realised, the title of this thread is about a common mode choke. When I initially replied, I was thinking about a series choke on the output to filter out the remaining noise on the DC side, which could interfere with the operation of the amplifier. If it's a common mode choke, then the previous poster is right: it can be either on the mains or DC side, but the former is lower current so will require fewer turns.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2019, 06:22:13 am
content: D'oh! Yeah, if you need (differential) ripple filtering, that's exactly what's needed.Cell phone noise is preferably blocked with a metallic enclosure, yeah, and some ferrite beads on the signals where they enter, followed by a little capacitance to ground.Had such a case when I brought my homemade audio amplifier to college; the college radio station (FM) got intoeverything(and probably GSM too, if I had had a phone that used it at the time). I put FB + Cs on my amp and no problem. Everyone else with powered speakers, the radio (and cellphone blips) came through easily... EMC fails on cheap products, who'd ever suspect?...Mind that all cables entering the enclosure are potential noise sources. If you can't keep the mains cable routed away from signals, you'll probably have to put ferrite beads on it, and maybe extra filtering beyond that, or a small module. Ditto the speaker terminals or what have you.If the enclosure is plastic or not fully enclosed, board layout becomes that much more critical, otherwise RF noise can couple into traces in the middle of the circuit, no matter what you do at the connectors. Usually, a poured and stitched 2-layer design, or middle ground plane 4-layer design, is adequate.Tim