Filter Capacitor in emonVs-PSU

I’m very much enjoying learning about all the new things. Trystan, Glyn, Robert, everyone - amazing work!

I have a question about the emonVs-PSU BOM. It appears that the emonTx4 schematic is online now, but I haven’t been able to find the emonVs-PSU schematic. For the most part I can look at the pictures (Voltage sensors — OpenEnergyMonitor 0.0.1 documentation) and things are really clear. But I’m curious what particular component was used for (what I assume is) the filter capacitor. It appears to have a large “M” on it.


Hello @brandock thanks, glad to hear!, here’s the part it was a requirement alongside the choke for EMC

How did you get on with the safety approvals for the emonVs-PSU box? Was that difficult/expensive to certify?

It wasn’t too difficult, but we did have to add a number of additional components to pass conducted emissions and conducted immunity that complicated the layout, added both component and assembly cost. It’s something we would like to revisit again on a future revision. The supplier of the power supply did offer to help us using their test equipment to see if it would be possible to reduce the number and size of those additional components, we plan to take them up on that offer when we do our next round of development on this.

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I was thinking more of electrical safety than EMC. Approved components, spacing, surge voltage testing etc. Did they whack it with a few kV to ensure no leakage to the secondary side and no fires?

Yes that was all tested. We had to adjust the track spacing due to a small amount of neutral to earth leakage at very high voltage.

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Nice. So you went the whole hog and got an approval number? How much did that process cost? To which safety standard did you test?

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I noticed that emonVs has the .sch and .brd files up: GitHub - openenergymonitor/emonVoltageSense: AC Voltage Sensor and Power Supply for emonTx V4 / emonPi V2

I could not find a similar repository for the emonVS-mini. Any chance that could be posted? (Or point me to it if I am just missing it?)

I learned a lot about the emenVs theory by reading the voltage sensing article filed under the emonTx4 technical guide. Technical Guide — OpenEnergyMonitor 0.0.1 documentation

When I went looking for that guide a second time, I couldn’t find it because I was looking under the main Voltage Sensing article in the Learn area. This is where I thought I might find it: Learn | OpenEnergyMonitor

A few other questions:

I’m no electrical engineer, so I’m curious why there are current-limiting resistors on the Neutral leg of the Mains side of the AC:AC adapter. I would have guessed it sufficient to have these on the Line leg. I feel like there’s something to be learned here. (It feels like something Robert Wall might jump in and explain?)

I’m also curious about areas like the space between R27 and C10. A Line trace comes within about 2 mm of a Neutral trace. I’ve spent a little time trying to learn about creepage in IEC standards and such, and often run into 3.2 mm as a minimum distance for 240V. You’ve gone through all the design and testing - did you find a good guide for what distances are needed?

Also curious about the repetition of a choke and filter after the DC power supply? Why both before and after?


Yes, this is @glyn.hudson’s area of expertise and he is away at the moment so will have to defer to him with any detailed technical question on this. But here are the main summary points from the report and list of standards:

We use CASS industries in Manchester, they have been very helpful over the years


Hello @brandock Looks like I still havent pushed up the files for the emonVs-mini, I will get those up soon.

I will leave that one for @glyn.hudson, I do know the creepage was all checked over carefully.

This was required to pass conducted immunity when an attached raspberry Pi is connected via Ethernet and noise is injected along the ethernet cable. Without this the noise would travel up into the emonVs via the 5V usb power cable and then down into the emonTx4 affecting the readings.

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I’m impressed that you got electrical safety covered for just 204 quid! What did that involve/include? I’m keen to know for some projects I’m thinking about. Feel free to leave this one until @glyn.hudson returns if you like.

@glyn.hudson if you have a chance, I had some questions that Trystan thought you would be able to answer. This one in particular. Thanks!

These were the recommendations of the EMC test centre and were required to pass EMC testing to filter out the switching noise from the SMPS. After going through the manufacturing and procurement process we’ve now got a good relationship with the power supply manufacturer, they’ve suggested it should be possible to get the power supply to pass EMC without so many additional components, and they may be able to help with this and do some preliminary testing. However, for the moment we’re stuck with the current components unless we want to pay for another batch of EMC testing. This is something we may consider if we need to make other changes to the design.