Mains filter - dual schottky in series - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: July 05, 2025, 10:00:31 am
content: What is the purpose of this dual Schottky coupled to capacitors in a mains filter? It's in series with mains.Is it a DC blocker?

author: Gyro
author_rank: Super Contributor
timestamp: July 05, 2025, 12:20:55 pm
content: Yes, it is a DC blocker. These seem to be getting more popular in 'certain (audio) circles' to remove any minimal DC bias on transformer primaries.The diode pair, not in series in the conventional sense as their polarities are opposite, are there to prevent any significant reverse bias on the two electrolytic caps. There isn't anything to prevent excessive forward bias (in excess of their 25V rating) that probably won't the exceeded during normal steady state operation but could well be on a switch-on surge where the transformer core saturates, or in cases of overcurrent. It's likely to be difficult to find beefy enough Zeners to protect against this eventuality.Such circuits need to be treated with extreme caution, taking into account things like capacitor ripple current and voltage ratings, as the may blow up without warning!

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2025, 02:28:32 pm
content: 25V capacitors on the mains? There again, the impedance of 2.5mF at 50Hz is tiny, so the AC voltage across them will be no more than the ESR*I.It seems a bit pointless to me.

author: okw
author_rank: Frequent Contributor
timestamp: July 05, 2025, 03:54:41 pm
content: Quote from: Zero999 on July 05, 2025, 02:28:32 pmFrom an audiofile point of view, would it filter out some noise, even just marginally?This is part of a larger filter. I was just curious about this section.

author: TimFox
author_rank: Super Contributor
timestamp: July 05, 2025, 04:12:18 pm
content: Does that system use a toroidal power transformer? They can be very sensitive to the small DC component of the supply voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2025, 04:19:15 pm
content: Residual DC (like a few 100 mV or so) at mains can be an issue for toroidial transformers. It can be enough so that the transformer may come close to saturation at one side and than cause a little more audible hum from magnetostriction (50 or 60 Hz and harmonics). The magnetrostriction is nonlinear and tends to get worse when approaching saturation.One gets a similar type of noise with a bad rectifier (assymetry in the diodes) that adds DC from the secondary. With the DC block in place the transformer would react even more to DC load - so the filter can help, but also make things worse.The issue is indeed the possible turn on current spike from the same type of transformer. Already from a 100 VA toroid this may be enough to trip a 16 A breaker. A 1 ms 100 A pulse would aready give 40 V at a 2.5 mF capacitor.The impedance of the 2.5 mF capacitors is intended to be small, the point is getting rid of superimposed DC, not to reduce the mains voltage.Very low resistance in the transformer circuit comes with other isssues, like large current spikes from charing the filter capacitors. With some more resistance (e.g. an extra fuse and / or choke) the transformer would get more tolerance to DC and the charging spikes would be smaller.

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2025, 05:13:51 pm
content: The mains should already have a vert low impedance, especially at DC, so I doubt there's any measurable DC offset. The only time I would think it would happen is if a cheap inverter is being used to power the circuit, This smells like audiophoolery to me.

author: TimFox
author_rank: Super Contributor
timestamp: July 05, 2025, 05:38:11 pm
content: It depends on what other loads are connected to the mains.Some resistive loads (e.g. hair dryers) use a series diode for the low power setting, which results in high DC current drawn through the house wiring.

author: floobydust
author_rank: Super Contributor
timestamp: July 05, 2025, 06:30:19 pm
content: DC blockers I've made (telephony) I will add a resistor across each capacitor because they are never exactly the same value/leakage current. 100kohm kind of thing. To keep the midpoint at the midpoint.A quick search for audiophile DC blockers shows they usually forward-clamp to 2-4 diode dropsThat looks like a few volts DC block max. I don't see how flux walk in a toroid happens with a few 100mV DC offset on incoming mains.I get more growl and noise out of power transformers strictly due to mains harmonics when power quality is poor.OP's title - this is not a "mains filter" at all.

author: Gyro
author_rank: Super Contributor
timestamp: July 05, 2025, 07:00:07 pm
content: Good point, a couple of high current rectifiers in parallel with each capacitor would be much more practical solution for forward bias voltage limiting than trying to source power Zeners to limit the capacitors to their full rated voltage. I doubt any mains DC offset would exceed a volt or so.

author: floobydust
author_rank: Super Contributor
timestamp: July 05, 2025, 09:18:03 pm
content: What's wrong with something to simply measure it? Maybe an LPF that should read 0VDC.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2025, 09:24:02 pm
content: Many of the DC loads can be temporary. So one simple measurement would not show much. I would need monitoring over quite some time to really get an idea for what can come.

author: Seekonk
author_rank: Super Contributor
timestamp: July 05, 2025, 10:21:09 pm
content: There are reports in the solar world of some sine wave inverters having a DC offset and causing transformer buzzing.Here is another DC removerhttps://sound-au.com/articles/xfmr-dc.htm

author: jonpaul
author_rank: Super Contributor
timestamp: July 06, 2025, 06:37:37 am
content: just avoid CCP chinese junk inveertersMains power has ZERO DCj

author: MariuszD
author_rank: Regular Contributor
timestamp: July 06, 2025, 06:40:59 am
content: Quote from: Zero999 on July 05, 2025, 05:13:51 pmTry to measure it. It is real. It affects toroidal transformers. As the DC offset increases, both the magnetizing current and the transformer hum also increase. DC offset changes frequently, and transformer hum is never steady. In a quiet room I hear it. I don't like it because it feels like something is broken.Why is there DC in the mains? Some devices have a diode to reduce heater power. Several amps can make several hundreds millivolts DC.It's audiophoolery when someone thinks that it affects sound quality.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2025, 09:13:32 am
content: A decent quality meter set to measure DC should be able to measure the DC content of the mains. It's not something I've done intentionally, but I have mistakenly set my multimeter to DC and measured the mains, then double checked when it read 0V and changed it to AC. It's a beginner's mistake which can result in shock. Always double check.

author: MiroS
author_rank: Regular Contributor
timestamp: July 06, 2025, 10:40:53 am
content: Quote from: MariuszD on July 06, 2025, 06:40:59 amUp to 50mV in worst point, last flor, end point, no any load in socket at the moment of measuring DC component. 300VA transformer has resistance ca. 5Ohms, so it will ba ca. 10mA DC. It will go down to zero if I would attach low impedance load during taking the measurement.It will not saturate the core or result in sound efects. If you have more than that you should consider electrical installation inspection.

author: floobydust
author_rank: Super Contributor
timestamp: July 06, 2025, 05:09:46 pm
content: "Flux walking, also known as flux stair casing, is the successive accumulation of flux in a magnetic core ultimately leading to its saturation. In any circuit involving magnetic elements operating in steady state, the net (volt x time) product applied to the core must be exactly zero. If you have more than one piece of magnetics, the volt-second balance must hold for every one of them individually. Voltage polarity should be accounted for and summed over one switching cycle.Even the slightest unbalance will result in a bit of residual flux that will continue to pile up in each cycle, hence the staircase analogy.""There are several ways to avoid it, basic principle is to make sure that volt-seconds applied in one polarity are exactly equal to volt-seconds applied in the opposite direction in each switching cycle. AC coupled capacitor which manipulates the voltages to achieve balance, or current mode control which manipulates time to achieve the balance are two examples."sourceTI Section 4 – Power Transformer Design pdfhttps://www.eevblog.com/forum/beginners/why-does-flux-walking-in-a-magnetic-core-occur/