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Meanwell PSU wiring question
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Author Meanwell PSU wiring question
NS4W


I'm trying to wire up this batboy to the mains and I am wondering what "COM" means on this thing?

Dead Banana Dead Banana Dead Banana
e-grad
COM = 0V of your power distribution board / modules.

Be careful when dealing with mains.

Dead Banana
NS4W
Ah ok, that's what I thought. But what is the difference between 0V and GND? I assumed that was the same.

The PSU in my monorockets does not have the COM wiring post.

And I am very careful, that is why this has been taking forever to finish up...
roglok
NS4W wrote:
Ah ok, that's what I thought. But what is the difference between 0V and GND? I assumed that was the same.

The PSU in my monorockets does not have the COM wiring post.

And I am very careful, that is why this has been taking forever to finish up...


What you labelled GND is actually the mains earth connection and is absolutely vital for safe operation. For security reasons COM/0V should also be tied to earth, preferably at the point of distribution.
NS4W
roglok wrote:

What you labelled GND is actually the mains earth connection and is absolutely vital for safe operation. For security reasons COM/0V should also be tied to earth, preferably at the point of distribution.


So what you are saying is that I should have a cable going from COM to earth(what I labelled GND)?
NS4W
I did all the wiring now with a rocket switch in between. Works as a charm! Outputting steady +/-12V and +5V. Yay! w00t

I am still unsure what you meant though Roglok. Should I also wire earth and COM together?
roglok
NS4W wrote:

I am still unsure what you meant though Roglok. Should I also wire earth and COM together?


yes, you must connect them together at one point, preferably at your busboard/distribution point. search the forum by posts by Graham Hinton, he knows a thing or two about power supplies and safety thumbs up
NS4W
Ok. I cannot find the thread you are referring to, but I will pass a wire from the COM post to the mains earth. thumbs up
roglok
NS4W wrote:
Ok. I cannot find the thread you are referring to, but I will pass a wire from the COM post to the mains earth. thumbs up


i wasn't referrring to a specific thread. graham posts occasionally about the subject of grounding and safety and it's always valuable advice. here's what i found doing a quick search:

https://www.muffwiggler.com/forum/viewtopic.php?p=1641203#1641203
https://www.muffwiggler.com/forum/viewtopic.php?p=621262#621262
https://www.muffwiggler.com/forum/viewtopic.php?p=1182246#1182246
https://www.muffwiggler.com/forum/viewtopic.php?p=1183620#1183620
https://www.muffwiggler.com/forum/viewtopic.php?p=1742938#1742938

this is the essence:

"The 0V of the PSU should be connected to mains Earth for both safety and common 0V reference. This is not a preference or optional. Many systems are not earthed properly, don't copy bad practise."
Graham Hinton
NS4W wrote:
Ok. I cannot find the thread you are referring to, but I will pass a wire from the COM post to the mains earth.


No, roglok just told you the correct way to do it and you missed the most important point: "preferably at your busboard/distribution point".

Connect your mains earth to the Gnd connection on the PSU, this is to earth the PSU chassis parts for safety.
Do not put a wire between Gnd and Com on the terminal strip, instead connect the PSU to your distribution board and the mains Earth to your distribution board and join the Com there.

There is a subtle, but important difference. The mains Earth is also your common 0V reference for your modules and also whatever other gear you are connecting them to. All these points will only be close to 0V when no current flows in the wires. If you earth the Com at the PSU end that will be relatively at 0V, but the distribution board won't be because of the PSU common current (roughly the difference between the +/-12V currents) because of the wire resistance. Anchoring the distribution 0V lets the PSU Com float slightly away from 0V rather than the 0V reference nearly all the op amps in your modules are using.

There will be currents flowing through the mains Earth wiring between equipment, you add some everytime you patch an unbalanced connection, but these are in the order of <1mA whereas the current flowing in the Com wire could easily be over 500mA. The only way to reduce all these offsets is to beef up the wiring for lower resistance. Your safety earth wiring should be capable of carrying 6 to 25A under fault conditions, for the PSU wiring don't use red crimp terminals, use the larger blue size and put as much wire in them as they will take. Wire is cheap, don't be mean with it.

When you look at schematics it is usual to think about power rails, or anything connected together, as ideal zero ohm paths, but when it comes to power wiring you have to start thinking in terms of very small resistances and every piece of cable, pcb trace and contact becomes significant.
NS4W
Thank you both for elaborating - I want to do this properly. In hindsight I see the distinction when I re-read Rogloks posts. (English is not my first language)
Siri
Interesting thread! I didn't do it on mine, as I was looking at the schematic in the data sheet and you can see a capacitor between COM et GND so I thought if I connect them together it will bypass the capacitor, right? I'm far from an expert, just a muffwiggler self-taught. w00t



I also read Oakleysound's PSU manual (here:http://www.oakleysound.com/psu3-um.pdf) and he has an elaborate system about having several grounds (to prevent earth loops?) so in my head it looked like COM/0V doesn't need to be connected to main's earth.

There should be a sticky about it, as a lot of people building synth here are self-taught without electrical background. I usually dig deeply in the forum using the search function but never saw this information.

It would be nice to have a detailed post about grounding, sharing ground, and ground loops, with nice illustrations! (illustrations are helpful for non native speakers). I wish I could do it but I don't yet grasp the logic, but I can help with the illustration thumbs up

Thanks Graham for taking the time to share!
Graham Hinton
Siri wrote:
I didn't do it on mine, as I was looking at the schematic in the data sheet and you can see a capacitor between COM et GND so I thought if I connect them together it will bypass the capacitor, right?


You will bypass it, but it is only there to bypass hf noise when the PSU is floating.

Quote:

I also read Oakleysound's PSU manual (here:http://www.oakleysound.com/psu3-um.pdf) and he has an elaborate system about having several grounds (to prevent earth loops?)


You can ask Tony what he thinks he is doing as he is on this forum. Do not worry about "earth loops", that is just voodoo talk. You can get mains hums and eddy currents, but not just by creating a loop and on a modular system you create loops every time you plug in a patchlead. It is better to ask why there are different voltages in 0Vs and why current is flowing.

Quote:

so in my head it looked like COM/0V doesn't need to be connected to main's earth.


Common 0V always needs to be connected to mains Earth at one place, but there is a finite limit to how far you can maintain this. A couple of adjacent rack units may be done with very heavy wiring or busbars, but wallpaper systems require a different approach.

Quote:

There should be a sticky about it, as a lot of people building synth here are self-taught without electrical background.


That is the problem. If you want to build something as complex as a synthesizer you need some background in electronics theory, otherwise you are just soldering. Everything that I have explained is a simple application of Ohms's Law and Kirchoff's Law which are the very first things you learn in electronics. You can read about them on web pages, but you really need some tutoring and to follow exercises to get the feel of them. They are the foundation that everything is built on so to it is worth investing some time in learning them properly.

Quote:

It would be nice to have a detailed post about grounding, sharing ground, and ground loops, with nice illustrations!


I am preparing some new reference pages covering this on my web site.
NS4W
Graham Hinton wrote:

That is the problem. If you want to build something as complex as a synthesizer you need some background in electronics theory, otherwise you are just soldering. Everything that I have explained is a simple application of Ohms's Law and Kirchoff's Law which are the very first things you learn in electronics. You can read about them on web pages, but you really need some tutoring and to follow exercises to get the feel of them. They are the foundation that everything is built on so to it is worth investing some time in learning them properly.


I guess 98% on these forums are naive amateurs like me knowing only the very basics and understanding even less about it. Still I have managed to build several DIY modules just by following instructions and getting help on this forum, maybe I'm just soldering as you put it but in the end no one is thinking about that when they hear the sounds coming out of it.

I'm very thankful for guys like you can chime in and correct/help us all from time to time, but I oppose making this an elitist (e.g., engineers only) hobby.

Don't get me wrong, I want to learn about electronics, and I have already learned a lot from DIY practice - but I don't think you have to "understand" everything in order to be creative and enjoying what a modular has to offer. I jumped right into it :-)

On a side note I guess it must be utterly frustrating for EEs to read a lot of the stuff coming up over and over on this forum... Will they ever learn??!!!
NS4W
Graham Hinton wrote:

No, roglok just told you the correct way to do it and you missed the most important point: "preferably at your busboard/distribution point".

Connect your mains earth to the Gnd connection on the PSU, this is to earth the PSU chassis parts for safety.
Do not put a wire between Gnd and Com on the terminal strip, instead connect the PSU to your distribution board and the mains Earth to your distribution board and join the Com there.


Like this?
Stewart Pye
Graham Hinton wrote:

Connect your mains earth to the Gnd connection on the PSU, this is to earth the PSU chassis parts for safety.
Do not put a wire between Gnd and Com on the terminal strip, instead connect the PSU to your distribution board and the mains Earth to your distribution board and join the Com there.


Graham,

Is this what you would recommend when using a linear power supply also?
Say you have a 3A linear power supply and 6 busboards. Do suggest not connecting mains GND to a 0V terminal on the PSU PCB, but directly to the chassis, and from that point to each of the busboards, and also connecting 0V of power supply to oV on busboards with thick wire???

Regards,
Stewart.
weedywhizz
Wow - always interesting what Graham says we're not worthy we're not worthy we're not worthy
Graham Hinton
NS4W wrote:

Like this?


Almost. Don't use the PSU terminal as a junction. The mains Earth should go to a big post where it enters your system, if you have a mains inlet on a chassis it should be right next to the inlet. From there wires should go out separately to each place that needs to be earthed.

I hope that those colours are just for the drawing and not the wires you will use. Always use the mains wire colours for your region for the mains.


Stewart Pye wrote:
Is this what you would recommend when using a linear power supply also?


Yes, there is no difference here between a digital and a linear PSU. Both have floating outputs that need to be anchored to a reference so that the 0V common is not altered.

Quote:

Say you have a 3A linear power supply and 6 busboards. Do suggest not connecting mains GND to a 0V terminal on the PSU PCB, but directly to the chassis,


Not to the PSU output terminals, but it may need Earth for its chassis and maybe mains filtering.

Quote:

and from that point to each of the busboards, and also connecting 0V of power supply to oV on busboards with thick wire???


Yes, but it becomes difficult to maintain a 0V common between separate bus cards when you have that sort of current flowing, especially if the rails have unequal loads because the difference will be flowing in the 0V. It becomes even more difficult when there are multiple PSUs. You can never have thick enough wire which is why I'm now building busbars and I'm using up to eight 32/0.2 wires to connect to them.

You can test your system by making some dummy loads that will take 1A, i.e. a 12Ω or 15Ω power resistor. Plug them on to different headers and measure the voltage drop along your wiring and buscards. If you have only one on the + rail the current will return via the 0V wiring, if you have one on each rail the 0V currents will cancel. Every milliVolt you measure will be a milliOhm of resistance in that path and if it is more than 10mV that path has room for improvement. You could just use your modules as the load, but then you won't know the exact draw for sure or be able to test the 0V wiring.
daverj
Siri wrote:
Interesting thread! I didn't do it on mine, as I was looking at the schematic in the data sheet and you can see a capacitor between COM et GND so I thought if I connect them together it will bypass the capacitor, right? I'm far from an expert, just a muffwiggler self-taught. w00t




Even with a wire going from the COM to the Earth pin of the power entry, which seemingly shorts out that capacitor, that cap will in fact still pass some high frequencies.

At RF frequencies, which the spikes from a switching supply contain, even a few inches of wire has enough inductance to change the characteristics of passing those RF frequencies. So not all of the RF energy will go through the wire. Some will still take the shorter path through the cap.

A piece of 18 AWG wire 1 foot long has about 0.4uH inductance. Not much, but at very high frequencies it can be noticeable.

Chances are you would never notice the difference if that cap was in the circuit or not. But sometimes tiny RF spikes in a switching circuit can effect their accuracy.
Siri
Thanks Dave and Graham!
acilator
I'm trying to figure out how to connect multiple PSU (Meanwell RD-3513) in one case (175 hp, 5 row). The drawing below shows how I think it needs to be done, after a lot of reading here on the forum and on de the Dutch synthforum, can somebody please check this and confirm or this is the wright way to go, or please give me some tips how it's better..
Graham Hinton
acilator wrote:
can somebody please check this and confirm or this is the wright way to go, or please give me some tips how it's better..


Your drawing is basically correct, but a lot depends on the physical layout and resistances of the wires involved.

The Chassis Ground connection on the PSUs should be short and go directly to your main Earthing point near the mains inlet. These wires would not be normally carrying any current, but may have to carry 25A if the PSU developed a fault for a short period until a fuse or trip blows. A single 24/0.2 or 32/0.2 would suffice.

The wire from the Earthing point to the 0V distribution (not Ground distribution) is similar except it may be a common path to other unbalanced gear so keep it short and/or increase the number of wires.

The wires from the 0V distribution to your distribution cards should be as low resistance as practically possible, as should the distribution block. Parts specified for 100A would be about right, even though they won't be carrying that sort of current. (The rating is indicative of the resistance and I^2.R temperature rise.)

The wires from the PSUs to the distribution should be heavy gauge or multiples to get the voltage drop between PSU and Distribution down below 10mV. The higher current you are drawing from your PSUs the more wire you will need here.
acilator
Your drawing is basically correct, but a lot depends on the physical layout and resistances of the wires involved.
Thanks for checking, nice to hear it's the wright way to go.

The Chassis Ground connection on the PSUs should be short and go directly to your main Earthing point near the mains inlet. These wires would not be normally carrying any current, but may have to carry 25A if the PSU developed a fault for a short period until a fuse or trip blows. A single 24/0.2 or 32/0.2 would suffice.
I wanted to use even a thicker wire for everything between the main power inlet and the PSU/main Earth, it's 1.5mm2 instead of your recommended 1.0mm2 (32/0.2). For the wiring between the PSU and the busboards I'm probably going to use the 24/0.2 (0.75mm2). Or is it better to use the thicker wire 32/0.2?
Graham Hinton
acilator wrote:

I wanted to use even a thicker wire for everything between the main power inlet and the PSU/main Earth, it's 1.5mm2 instead of your recommended 1.0mm2 (32/0.2). For the wiring between the PSU and the busboards I'm probably going to use the 24/0.2 (0.75mm2). Or is it better to use the thicker wire 32/0.2?


It can never be thick enough. Two 32/0.2s fit in a blue crimp terminal, five in a yellow. If you are carrying 2A or more you'll find that even eight or ten wires paralleled still produce a noticeable voltage drop.
acilator
Clear! Thanks!!
I'll show some pics in this threat when i'm finished building the power part.
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