Power Supply A Guide to 12V PSU

[REVOCCASES]

I've realized that while i'm pretty comfortable with 12v in general, I had a question I couldn't find an answer to yet.

3 prong vs 2 prong power connectors, for plugging into wall outlets.
I've seen sff builds and electrical devices done with 3 and 2 prong connectors.
Where is the line drawn that determines you need to go 3 prong instead of 2?

using an xt60/xt30 wouldl be a lot easier than an mr30 for me, but I recognize the importance of proper grounding.

It's a matter of safety. If you have a metal case it should be properly grounded so that in case of a fault (e.g. damaged insulation on L or N wire) the RCCB can trip and you don't get electrocuted.

I guess without any grounding something like this could have ended even worse:

https://smallformfactor.net/forum/t...-march-improvements-updates.15245/post-242857

Power adapters like the ones that come with a notebook don't need to be grounded because the case is made from plastic (insulated) and the AC input is galvanic separated from the DC output (e.g. using an optocoupler)

 

[Thehack]

I've realized that while i'm pretty comfortable with 12v in general, I had a question I couldn't find an answer to yet.

3 prong vs 2 prong power connectors, for plugging into wall outlets.
I've seen sff builds and electrical devices done with 3 and 2 prong connectors.
Where is the line drawn that determines you need to go 3 prong instead of 2?

using an xt60/xt30 wouldl be a lot easier than an mr30 for me, but I recognize the importance of proper grounding.

For computers, I would advise 3 prong unless necessitated by design. If using 2 prong, it needs to be double insulated. Usually anything over 200W requires a ground but I'm not sure where that comes from. Might be an international standard somewhere.

It's a matter of safety. If you have a metal case it should be properly grounded so that in case of a fault (e.g. damaged insulation on L or N wire) the RCCB can trip and you don't get electrocuted.

I guess without any grounding something like this could have ended even worse:

https://smallformfactor.net/forum/t...-march-improvements-updates.15245/post-242857

Power adapters like the ones that come with a notebook don't need to be grounded because the case is made from plastic (insulated) and the AC input is galvanic separated from the DC output (e.g. using an optocoupler)

Not sure what you mean by "galvanic," as a quick dictionary look comes back with: "relating to or involving electric currents produced by chemical action." Optocoupler has little to do with this situation, as optocoupler is used to electrically separate signals.

The technical answer is that power bricks are "double insulated" in American terms (not sure what they call that in EU). As you mentioned, it is insulated by the plastic enclosure, and the second part, to be exact, is electrically/physically separated. The output side is not connected electrically to mains (wall power) via the use of a *transformer*. Pretty much all modern AC/DC power supplies use a transformer to isolate from mains power.

And finally, a number of factors necessitates the use of a grounding prong. If it is commonly used near water, a grounding prong and GFCI is recommended. If it used nearby vulnerable groups, children or hospital patients it must be grounded or double insulated. I am sure there are many more factors that are outside my experience.

 

[REVOCCASES]

For computers, I would advise 3 prong unless necessitated by design. If using 2 prong, it needs to be double insulated. Usually anything over 200W requires a ground but I'm not sure where that comes from. Might be an international standard somewhere.



Not sure what you mean by "galvanic," as a quick dictionary look comes back with: "relating to or involving electric currents produced by chemical action." Optocoupler has little to do with this situation, as optocoupler is used to electrically separate signals.

The technical answer is that power bricks are "double insulated" in American terms (not sure what they call that in EU). As you mentioned, it is insulated by the plastic enclosure, and the second part, to be exact, is electrically/physically separated. The output side is not connected electrically to mains (wall power) via the use of a *transformer*. Pretty much all modern AC/DC power supplies use a transformer to isolate from mains power.

And finally, a number of factors necessitates the use of a grounding prong. If it is commonly used near water, a grounding prong and GFCI is recommended. If it used nearby vulnerable groups, children or hospital patients it must be grounded or double insulated. I am sure there are many more factors that are outside my experience.

Fyi:

Galvanic Isolation: Purpose and Methodologies - Technical Articles

Galvanic isolation is a way to isolate functional sections of electrical systems to prevent current flow. A brief explanation of differing methods to create galvanic isolation.

www.allaboutcircuits.com

Optokoppler – Wikipedia

de.m.wikipedia.org

 

[Thehack]

Fyi:

Galvanic Isolation: Purpose and Methodologies - Technical Articles

Galvanic isolation is a way to isolate functional sections of electrical systems to prevent current flow. A brief explanation of differing methods to create galvanic isolation.

www.allaboutcircuits.com

Optokoppler – Wikipedia

de.m.wikipedia.org

I see. That term is not used in my field. Today I learned.

Concerning optocoupler, when talking about DC Output, and AC mains, one wouldn't reference optocouplers since we're talking about power. Optocouplers aren't always necessary, but transformers are when isolating mains and DC output.

 

[REVOCCASES]

I see. That term is not used in my field. Today I learned.

Concerning optocoupler, when talking about DC Output, and AC mains, one wouldn't reference optocouplers since we're talking about power. Optocouplers aren't always necessary, but transformers are when isolating mains and DC output.

Galvanic insulation between AC and DC can be achieved by different methods. Depends on the design used for the specific SMPS. Usually a transformer is used together with optocouplers (transformer for the power, optocouplers for signals).

A short summary can be found e.g. here:

Isolation

What is power supply isolation? Isolation is the electrical or magnetic separation between two circuits and often used to separate two distinct sections of a power supply. The isolation provides a barrier across which dangerous voltages cannot pass in the event of a fault or component failure...

www.meanwelldirect.co.uk

 

[syin16]

Thank you for the info guyz. Learning so much. Noob reporting in.

Just a quick question, i notice that some of the psu mod use gx12-3 as ac in connector. But a quick look at the connector specs online give somewhat a limit current of 5A, is it safe using that kind of connector? For example, a 12V psu with power of 600W would require 50A through it. Is it safe?

 

[BaK]

Having you guys @Thehack and @REVOCCASES among us sorcerer's apprentices of the 12V and electricity in general is invaluable! Thanks a bunch guys for your active parts in our SFF community!

Not forgetting @Valantar for his always well thought advice on this subject!

 

[ermac318]

Thank you for the info guyz. Learning so much. Noob reporting in.

Just a quick question, i notice that some of the psu mod use gx12-3 as ac in connector. But a quick look at the connector specs online give somewhat a limit current of 5A, is it safe using that kind of connector? For example, a 12V psu with power of 600W would require 50A through it. Is it safe?

If they're using that as the AC Input connector, then the AC input is 120V or 240V, meaning at 5A you're pushing 600VA or 1200VA which is plenty. Normal Nema 5-15 plugs only can do 15A.

 

[Valantar]

Having you guys @Thehack and @REVOCCASES among us sorcerer's apprentices of the 12V and electricity in general is invaluable! Thanks a bunch guys for your active parts in our SFF community!

Not forgetting @Valantar for his always well thought advice on this subject!

In that company, if they are the wizards, I'm the back alley illusionist trying to get by with cheap card tricks and sleight of hand Not much skill, but a willingness to hack things together until they work! I'd be nowhere without the invaluable advice from the people on these forums though.

 

[REVOCCASES]

Thank you for the info guyz. Learning so much. Noob reporting in.

Just a quick question, i notice that some of the psu mod use gx12-3 as ac in connector. But a quick look at the connector specs online give somewhat a limit current of 5A, is it safe using that kind of connector? For example, a 12V psu with power of 600W would require 50A through it. Is it safe?

You more or less answered your own question: the voltage has to be considered when looking at those ratings. As we know: Watt = Voltage x Current

So if it is rated for 5A we get...

• 5A x 110VAC = 550W (used on the AC side, if you live in the US)
  
  
• 5A x 12VDC = 60W (used on the DC side)

In other words: using some connector like that for AC input would work but not for the DC output. If you need a connector that can handle 12VDC / 50A I would have a look for something like the AMASS XT90.

Also note that many of those aviation connectors selling on eBay, Amazon, etc are crap. I would try to get connectors always from electronics distributors like RS, Mouser, etc.

 

[syin16]

You more or less answered your own question: the voltage has to be considered when looking at those ratings. As we know: Watt = Voltage x Current

So if it is rated for 5A we get...

• 5A x 110VAC = 550W (used on the AC side, if you live in the US)
  
  
• 5A x 12VDC = 60W (used on the DC side)

In other words: using some connector like that for AC input would work but not for the DC output. If you need a connector that can handle 12VDC / 50A I would have a look for something like the AMASS XT90.

Also note that many of those aviation connectors selling on eBay, Amazon, etc are crap. I would try to get connectors always from electronics distributors like RS, Mouser, etc.

Noted, thank you.

 

[Curiosity]

@REVOCCASES @Thehack
Roger, thank you my dudes. I figured it was a matter of safety whic is why I had ordered MR30 connectors previously, but then started wondering if I really needed a proper ground. (I do, I don't know why that thought even crossed my mind, ground everything ALWAYS)



I'd like to note for the thread that XT30/60/90 connectors have panel mount variants, or available PCBs to allow panel mounting.
The MR30 I'm planning to use can be found with a pcb that would allow panel mounting, but I've only found one source.

 

[BaK]

There is also the tiny Amass MT30:

Amass MT30 2mm Banana Plug Three-hole Connector Black Male & Female 1 Pair

Only US$7.50, buy best amass mt30 2mm banana plug three-hole connector black male & female 1 pair sale online store at wholesale price.

www.banggood.com

But I never found a panel mount, so I went with these CnLinko 3pin male + female socket:

5.07US $ 24% OFF|Cnlinko 2 3 5 8 Pin Ip67 Waterproof Connector Industrial Circular Copper Gold Pating Plug Socket M12 Deutsch Automotive Electric - Connectors - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

Also note that many of those aviation connectors selling on eBay, Amazon, etc are crap. I would try to get connectors always from electronics distributors like RS, Mouser, etc.

From what I see these CnLinko connectors are top quality, specs in the above link, hopefully I'm right as I am about to test them soon!

 

[REVOCCASES]

But I never found a panel mount, so I went with these CnLinko 3pin male + female socket:

5.07US $ 24% OFF|Cnlinko 2 3 5 8 Pin Ip67 Waterproof Connector Industrial Circular Copper Gold Pating Plug Socket M12 Deutsch Automotive Electric - Connectors - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

From what I see these CnLinko connectors are top quality, specs in the above link, hopefully I'm right as I am about to test them soon!

Those look interesting. Looking forward to your test once you received them.

 

[Valantar]

I'm no electrician, but I would be very wary of using something like the XT/MT/MR connectors with their close-mounted, uninsulated terminals for AC power inputs. I saw the MR30 was rated for 500V, but I still don't think I'd trust it. A single layer of heatshrink isn't much of a match for mains voltage AC, and if that heatshrink gets damaged or pulled off ...

Oh, I just took a look at the LP-12s @BaK linked, and they're rated for 125V ... That's a no-go, even for 110VAC - peak-to-peak voltage in a 110V AC circuit is far above 125V after all, let alone what it would see in case of a power surge or similar. Seems like most of these connectors are designed solely for low-voltage DC applications.

 

[REVOCCASES]

Oh, I just took a look at the LP-12s @BaK linked, and they're rated for 125V ... That's a no-go, even for 110VAC - peak-to-peak voltage in a 110V AC circuit is far above 125V after all, let alone what it would see in case of a power surge or similar. Seems like most of these connectors are designed solely for low-voltage DC applications.

Good point, seems I didn't check the link carefully.

You are right, many of those connectors are meant for low voltage applications. E.g. for signal cables between PLC and frequency inverters in machinery.

Just one note: the rated voltage usually refers to RMS Voltage, not Peak-to-Peak. So if you choose one which is rated 250V (like common IEC connectors) you are good in 110V and 220V countries.

PS: Mouser, RS, etc. let you search and filter for Current and Voltage ratings. For example here I found one rated for 250V

https://www.mouser.com/ProductDetail/Amphenol-Tuchel/T-3274-501?qs=3KHpEbifJUAaidR5LoI82g%3D%3D

 

[BaK]

Oh, I just took a look at the LP-12s @BaK linked, and they're rated for 125V ... That's a no-go, even for 110VAC - peak-to-peak voltage in a 110V AC circuit is far above 125V after all, let alone what it would see in case of a power surge or similar. Seems like most of these connectors are designed solely for low-voltage DC applications.

Oh! Is that another occasion from you to save me?
I am indeed at 230V AC here in Switzerland.
Thanks a lot for having checked!

Here is the calculation I did:
The specs says 5A / 125V, which means the connector can handle up to 625W.
It will be attached to a MW PSU LSP-160-12T, max power drawn will be 160W.
And 160W at 230V correspond to around 0.7A.
Finally, as 0.7A is very far from the 5A max, I thought I was all good.

And further more, looking at another LP12 product page from CnLinko, we see:

So what do you guys think, is it safe to use that connector @230V?

 

[DwarfLord]

It will be attached to a MW PSU LSP-160-12T, max power drawn will be 160W.

If I may, just a small correction here.
As seen here, the LSP-160-12T has a rated power of 162W (this is what it can deliver on its output) with a typical efficiency of 92.5% (7.5% is lost during the transformation from 230/125V to 12V).
So the max power drawn from the wall can go up to 175W (92.5% of 175W = 162W).

It has no impact in your case, but just to let you know for your future projects

 

[Valantar]

Oh! Is that another occasion from you to save me?
I am indeed at 230V AC here in Switzerland.
Thanks a lot for having checked!

Here is the calculation I did:
The specs says 5A / 125V, which means the connector can handle up to 625W.
It will be attached to a MW PSU LSP-160-12T, max power drawn will be 160W.
And 160W at 230V correspond to around 0.7A.
Finally, as 0.7A is very far from the 5A max, I thought I was all good.

And further more, looking at another LP12 product page from CnLinko, we see:

So what do you guys think, is it safe to use that connector @230V?

It might be, but current draw isn't the only concern - a given voltage spec typically means insulation is designed around that voltage, and arcing and shorts can happen even with zero current being drawn (even if it is more likely as things heat up). It depends entirely on how overbuilt the insulation is. I don't like to take any chances whatsoever with AC power (12VDC on the other hand, that I trust enough to mess with), so I wouldn't use anything with uninsulated terminals as close together as the ones seen on the photos for those connectors. At the very least I would pot the terminals in nonconductive epoxy or similar after soldering - there's barely room for heatshrink on those by the look of them.

 

[REVOCCASES]

Oh! Is that another occasion from you to save me?
I am indeed at 230V AC here in Switzerland.
Thanks a lot for having checked!

Here is the calculation I did:
The specs says 5A / 125V, which means the connector can handle up to 625W.
It will be attached to a MW PSU LSP-160-12T, max power drawn will be 160W.
And 160W at 230V correspond to around 0.7A.
Finally, as 0.7A is very far from the 5A max, I thought I was all good.

And further more, looking at another LP12 product page from CnLinko, we see:

So what do you guys think, is it safe to use that connector @230V?

In general I would recommend to always stay within ratings given by the manufacturer.

My personal experience is: I had (no-name / generic) connectors failing way below their claimed ratings but I also had connectors which would work just fine at almost double of their official ratings. Sometimes it depends on the quality and if the manufacturer considered safety margins. Luckily I have access to HiPot and Load Testing equipment so I can check connectors before using them.

The two product pages you linked look pretty similar to me. Maybe one of those are better insulated. Or there could just be an error in one of the descriptions. Not sure.

As @Valantar pointed out, the solder terminals look pretty close to each other. For higher voltage applications where safe creepage distance cannot be maintained there should be some separators in between. If there there is not enough space for separators, @Valantar 's idea of using epoxy is also sometimes used.

I have no experience with CnLinko products so it would be a wild guess to say it's safe to use. If in doubt, I would always ask the supplier / manufacturer if it's safe to use for your application. Or just go with a connector from a well known brand where you can be sure that ratings match with your application.