Arcing in Mega STe PSU

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Pacman
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Re: Arcing in Mega STe PSU

Post by Pacman » Sat Oct 21, 2017 11:44 pm

guus.assmann wrote:
Sat Oct 21, 2017 1:36 pm
Hello Pacman,

Please don't overdo.
Just trying to be on the safe side ;)
Household currents aren't to be played around with.
Yes, AC-Live voltage can be dangerous.
That's why you need to have the ground wire connected. (Yellow/Green)
And than it's no problem.
Unfortunately not all AC wall-sockets are grounded as in the past this wasn't mandatory in all rooms. I believe it is now, but this could pose a problem in such situations....
Overhead sheet for insulation is just fine.
Arcing or sparks from the switch normall cannot get out of the switch.
Some incident may have caused the marks. But usually not one of the standard components.
And the insulation sheet should prevent the metal from being blackned.
The original sheet is Polyesther, quite flame retardant and sturdy. (Will not schrink when moderately heated)

And if still in doubt, buy a power supply from EXXOS.
The transformer prevents (almost all) dangerous events, as there's no "live mains" on the bottom side of the PCB.
BR/
Guus
I haven't decided on which PSU to get when I finalize my modified and 19" recased STe, but will look further into that when I figure out the total power requirements, physical size available etc. From what I've read those PSUs have really good specs. How do they compare with a standard PC type PSU?

I've just had a look at my STe PSU (a Mitsumi SR98) and noticed it did indeed have a thick (almost 1mm) stiff transparent plastic sheet underneath the PCB. I can get hold of acrylic sheets (plexiglass) which may or may not be too thick (2mm) -are they the same thing as polyester sheets?
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guus.assmann
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Re: Arcing in Mega STe PSU

Post by guus.assmann » Sun Oct 22, 2017 11:30 am

Hello,
Plexiglass will also do just fine.

It is very important to have the power supply connected to an outlet with the ground pin.
This is the case with the Atari supply, but also with a PC-Supply.
As for specs, the PC-supply is usually better than the Atari ones.

Back to the grounding.
In a switching PSU and in many filter inputs, there are two capacitors mounted for filtering.
One is from Fase to GND, the second from Neutral to GND.
If the GND is not connected, both capacitors form a voltage devider.
End result is half of the supply-voltage on the GND = computer shield and minus.
Now if you have the computer connected and you connect a printer cable to a printer, there could be (half supply voltage) as a difference between the 2 devices. Current is high enough to do damage.
For us, the current can be felt, but is far below the danger level for our body.
Yet it's not without risk !
Specially if you recase the computer, make sure to only use grounded connections between the devices (AC side) and preferably also use an outlet with GND on it.

BR/
Guus

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exxos
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Re: Arcing in Mega STe PSU

Post by exxos » Sun Oct 22, 2017 11:37 am

:goodpost:

Equipment which is "double insulated", aka "no ground" is awful stuff, The secondary core of a transformer can actually "float" up to 1,00's if not 1,000's of volts! So if you connected a "double insulated" device to a grounded device, you can blow some stuff up. Its also the reason for audio hums appearing in audio equipment when no cause is really apparent.

Happened to my poor Falcon and fried its audio :( Also my portal TV.. Used to get shocks between my tape deck and video player due to similar issues. how this is considered "safe" I have no idea :lol:
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Pacman
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Re: Arcing in Mega STe PSU

Post by Pacman » Mon Oct 23, 2017 11:48 pm

Having an electrician to rewire and add grounded outlets is probably a smart move.
What you both say doesn't seem very pleasant, safe or good for the computer(s) and other gear.

Good to hear plexiglass is OK.
I noticed that the thickness of the SR98 plastic sheet determines how low/tight the mounting screws for the PSU circuit board goes, so it should probably not be too thick. Perhaps the same goes for the Mega STe PSU in which case I should get something in the same thickness. I'll look closer into it when I get the power switch for replacing the old one.
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Pacman
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Re: Arcing in Mega STe PSU

Post by Pacman » Thu Nov 02, 2017 1:29 pm

A new AC power switch (as suggested by IngoQ) has been received from Exxos, and will no doubt become yet another improvement to this PSU when installed.
I still haven't had any luck in finding any thinner plexiglass or plastic sheets at around 1mm, so I haven't quite decided what to do with that. In the meantime I've taken a couple of pictures of the Mega STe power supply PCB (both sides) as there was a request for that.
20171102-134035__MG_2337.jpg
20171102-134035__MG_2337.jpg (437.96 KiB) Viewed 975 times
20171102-133010__MG_2321.jpg
20171102-133010__MG_2321.jpg (502.63 KiB) Viewed 975 times
Oh, that's the new (Noctua) fan you can see in the bottom photo. I've included a photo of in my "how to" guide on recapping the Mega STe PSU.
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Re: Arcing in Mega STe PSU

Post by exxos » Thu Nov 02, 2017 1:49 pm

Pacman wrote:
Thu Nov 02, 2017 1:29 pm
I still haven't had any luck in finding any thinner plexiglass or plastic sheets at around 1mm, so I haven't quite decided what to do with that.
sheet bakelite ?
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Pacman
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Re: Arcing in Mega STe PSU

Post by Pacman » Thu Nov 02, 2017 2:14 pm

That's something I hadn't thought of! Yes, should be fireproof as far as I know.

I took a closer look at the Mega STe PSU and it appears not to hold the plastic sheet in the same way as the SR98 (STe PSU) does. I'm guessing that because of the standoff-posts, it's cut to fit in between and won't fall out anyway (see illustration below), so maybe the 2mm plexiglass will work (they sell them nearby), or else the bakelite sheets (don't know of anyone locally off hand).
MSTe PSU plastic board.png
MSTe PSU plastic board.png (10.01 KiB) Viewed 969 times
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Re: Arcing in Mega STe PSU

Post by exxos » Thu Nov 02, 2017 2:21 pm

Yes the shape is probably what you will need for this.

If you Google for the sheet it should be up to find somewhere which sells them anyway. But practically anything is better than air ;) you could always add a bit of glue on the bottom to glue it to the metal as a bit of extra security anyway.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Pacman
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Re: Arcing in Mega STe PSU

Post by Pacman » Thu Nov 02, 2017 2:38 pm

Indeed, air is good too, but the problem is that there's probably not enough of it between the PCB and the metal enclosure :lol:

I'll google around a bit or else I'm going for the Plexiglass. I'm betting it won't melt unless there's an actual fire :D
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Re: Arcing in Mega STe PSU

Post by IngoQ » Thu Nov 02, 2017 3:05 pm

Actually Plexi and Polyester have roughly the same dielectric breakdown voltage of around 30-50 kV/mm. If you take a sheet of Polyester OHP (Zweckform 3566 for example), which is 0.13mm thick, you will get a breakdown resistance of at least 3.9kV. And generally OHP is temperature resistant up to 130°C.

I'd say this is more than enough ;) If I was really paranoid, I would take two sheets ;)

Fun fact: One thick layer of Polyester has a lower dielectric breakdown voltage compared to multiple layers with the same overall thickness. The electric field is not linear to the thickness. That's why HV film capacitors consist of multiple layers separated with unconnected metal sheets.

So if you want to really get over the top: Use multiple layers of OHP with layers of aluminum foil in between. Just make sure, that the aluminium foil is not connected anywhere :mrgreen:

Air btw (dry air) has a breakdown voltage of only 1kV(AC)/mm... So if you measure the distance of your soldering point and the metal shield, you can guesstimate the voltage. If you then replace 0.13mm of the air with polyester, calculate the resulting breakdown voltage (air+OHP) it should be higher than your estimated arcing voltage.
Ingo :geek:

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