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General Category => Technical Support => Hardware related => Topic started by: Skunkfish on 15:25, 11 February 17

Title: PSU question
Post by: Skunkfish on 15:25, 11 February 17
Realised that more juice is required for my 6128 Plus to use both X-Mem & M4 boards connected via a Mother X4, so I've bought myself a new 4A PSU.

What is the best way to connect this up to provide adequate power to the CPC + expansions from the options below? (I still have the monitor power available as well)

1. Connect the new 4A PSU to the CPC and leave at that, let the expansions draw the 5V from the expansion port.

2. Connect the new 4A PSU to the CPC, and connect the monitor 5V output to the power connector on the Mother X4.

3. Connect the monitor power output to the CPC, and connect the new PSU directly to the power connector on the Mother X4.


Advice would be greatly appreciated, along with a quick explanation of why that's the best way! Thanks!
Title: Re: PSU question
Post by: ||C|-|E|| on 16:04, 11 February 17
I think that you should not draw power from two different sources to the same circuit simultaneously  :) However, 4A is enough for your setup and you could, from the same PSU, take a lead to the computer and another one for the MX4 board. The alternative is just to connect the computer to the PSU and let the expansions draw the current from the port. With two boards it should be fine and it is easy enough to see if the computer is stable or not :) .
Title: Re: PSU question
Post by: Duke on 16:21, 11 February 17
With your M4 board (earlier ones, didn't like the voltage drop on the MX4), I would use option 1.
Title: Re: PSU question
Post by: Bryce on 23:02, 12 February 17
Definitely option 1. The other options will cause issues and with 4A there's no reason to use the 5V from the monitor at all.

Bryce.
Title: Re: PSU question
Post by: m_dr_m on 22:15, 27 April 21
The other options will cause issues
Oh, why is it so? I guess that's why I'm having so much instability with any other combinaison than X-MEM + X-MASS on MX4. Or maybe that's London power grid.

Would the answer be the same with 3A?

My CPC 6218 is alimented by an Amstrad MP-2F module, and the MX4 by a PSU I thought strong enough (to cry). (See figure 1, 5V 3A)

Well, a Symbiface III only connected on MX4, it complains about the voltage (4.3V) being too low (which is pretty cool).

I guess the most important question is:
* What setup and which PSU would you recommend, to power at least Symbiface III (or M4) + X-Mass + eventually 2 other cards?

I trying to dig in ancient threads, but the links were dead.

(http://orgams.wikidot.com/local--files/notes/psu.jpeg)
Title: Re: PSU question
Post by: eto on 08:54, 28 April 21
I am also interested why option 1. When I asked a similar question I was strongly advised to go for option 2 as option 1 could cause issues. I am using a small PC power supply.

There is so much contradictory information around the power supplies for the CPC. E.g. the Wiki says that a PC power supply is ideal and when I mentioned that somewhere (I don't remember where) somebody said this is dangerous and could put too much current through the CPC.
Title: Re: PSU question
Post by: pelrun on 09:20, 28 April 21
It's not contradictory, it's talking about different issues.
ATX power supplies can easily power *working* CPC's without breaking a sweat. But they're hardly ergonomic, and if any sort of low-impedance fault appears in the CPC the PSU will happily keep pumping power into it until everything is on fire. A less capable PSU will run out of puff much much earlier, potentially causing less total damage.

Also, instability is usually due to unwanted voltage drop in the system, but voltage drop has multiple causes. Only one of them is "voltage drop due to insufficiently rated PSU", so once you've got a big enough PSU and your system is still unstable, going bigger is going to do precisely nothing. My 6128 with an MX4 and various boards has enough voltage drop *inherent in it's 35-year old guts* that a rock solid 5V at it's input was only 4.5V or less when measured at various points internally, and it crashed constantly for *years*. Using a 5.1V PSU magically and conclusively fixed all my problems, but in the end I adjusted my CTM644 to provide about 5.3V to give a bit of headroom. You really don't want to go much higher than that, although I still measure a bit less than 5V at any of the IC's.
Title: Re: PSU question
Post by: eto on 11:16, 28 April 21
the PSU will happily keep pumping power into it until everything

OK, understood. But then, the recommendation on the Wiki page should at least contain a warning or a recommendation for a different solution. Maybe after the discussion here, we can update the Wiki page with the information. (I can do, but it would be good if someone can check if I did it right)

Would this one be a good fit? https://www.meanwell-web.com/en-gb/ac-dc-dual-output-enclosed-power-supply-output-rd--35a
5V/4A and 12V/1A and just about 16€

I quickly checked my (very tiny) PC-PSU and it has 5V/6A and 12V/2A. Would you say this is still too much?
Title: Re: PSU question
Post by: Bryce on 12:47, 28 April 21
If the PSU only supplies 6A on the 5V rail then it's perfect for the CPC. The PC PSU's that are dangerous are those that can pump out 25A or more on the 5V rail - One accidental short in the system and you'll have fried the traces of the PCB.

Can you give me a link to the dodgy information on the Wiki?

Bryce.
Title: Re: PSU question
Post by: eto on 13:00, 28 April 21
If the PSU only supplies 6A on the 5V rail then it's perfect for the CPC. The PC PSU's that are dangerous are those that can pump out 25A or more on the 5V rail - One accidental short in the system and you'll have fried the traces of the PCB.

Can you give me a link to the dodgy information on the Wiki?

Awesome, thanks.

Here is the link: https://www.cpcwiki.eu/index.php/Power_Supply_for_CPC_and_CPC_plus#Power_pack_project

It's bullet point 4 of the "Notes" section.
Title: Re: PSU question
Post by: Bryce on 13:17, 28 April 21
The article is very badly written and is useless in its current form. It does actually mention that you need inline fuses, but it never mentions where you need them or what value they should be. The entire article should be removed.

Bryce.
 
Title: Re: PSU question
Post by: eto on 13:45, 28 April 21
The article is very badly written and is useless in its current form. It does actually mention that you need inline fuses, but it never mentions where you need them or what value they should be. The entire article should be removed.

Bryce.

I think it is broken. I remember that I stumbled across exactly this article somewhere else, but I think it was longer and had more content. Maybe something got lost. Or I don't remember it right.

But as it's the only resource that actually explains the connectors and power requirements, it should definitely not be removed. Maybe the parts can be removed that can be harmful. There's already so much information that is only available on the forum and not organised that it would be a pity to remove another resource that can easily be found.


Edit: Would such a ATX breakout board do the trick? It already has fuses. That would be an easy change to the article.
https://www.ebay.de/itm/324435392960
Title: Re: PSU question
Post by: Bryce on 14:57, 28 April 21
Edit: Would such a ATX breakout board do the trick? It already has fuses. That would be an easy change to the article.
https://www.ebay.de/itm/324435392960


Yes.


Bryce.
Title: Re: PSU question
Post by: m_dr_m on 15:49, 28 April 21
Thank you all for your input!


I'm still confused, though:
If you can point to a good power supply (plug and play, with nothing to solder!), I would be very grateful.
Title: Re: PSU question
Post by: pelrun on 16:42, 28 April 21
Power supplies are active devices - they monitor their output and change how much energy they inject in order to keep the voltage constant. If you connect two supplies together that aren't explicitly designed to work in parallel, they will fight, each with a slightly different idea of what the voltage should be and each making changes without knowing there's another entity trying to do the exact same thing.

Basically, it's a bad idea, and even if the supplies don't fight it's not necessarily going to prevent any crashes for reasons I've already explained.
Title: Re: PSU question
Post by: Bryce on 16:49, 28 April 21
Power supplies are active devices - they monitor their output and change how much energy they inject in order to keep the voltage constant. If you connect two supplies together that aren't explicitly designed to work in parallel, they will fight, each with a slightly different idea of what the voltage should be and each making changes without knowing there's another entity trying to do the exact same thing.

Basically, it's a bad idea, and even if the supplies don't fight it's not necessarily going to prevent any crashes for reasons I've already explained.

The MX4 uses double diodes to isolate the CPC power from the MX4 power source. This stops the power rail contention but also has the effect that it drops the voltage of the power rail.

Bryce.
Title: Re: PSU question
Post by: TotO on 18:51, 28 April 21
Exactly.

The M4 and the SF3 are using voltage regulator (3V3) to power their modern circuits. Using a regulated 5V PSU not work.
If you have no choice, you can remove the two diodes and add only one wire link to restore the PSU line.

Since 2020 I provide a board with a power switch, 3x MX4, 1X Edge slot instead.
Title: Re: PSU question
Post by: pelrun on 09:10, 29 April 21
The MX4 uses double diodes to isolate the CPC power from the MX4 power source. This stops the power rail contention but also has the effect that it drops the voltage of the power rail.
I forget about that because I bypassed the diodes long ago - another of my failed attempts to stop the crashes. :D
Title: Re: PSU question
Post by: Bryce on 09:50, 29 April 21
If you've bypassed the diodes you definitely shouldn't be using two PSU's in parallel. That's asking for trouble.

Bryce.
Title: Re: PSU question
Post by: pelrun on 11:55, 29 April 21
I've already been saying not to do it, so I hardly need the warning.  :P
Title: Re: PSU question
Post by: m_dr_m on 14:23, 29 April 21
Would this one be enough for CPC + SF3 + X-Mass?
It's just 5V 3A, but regulated.


[/size]
[/size][size=78%] [/size][/size]https://uk.rs-online.com/web/p/ac-dc-adapters/1753307[size=78%]
Title: Re: PSU question
Post by: Ast on 16:10, 29 April 21
warning : using an external psu for xmass with 512mo will make your xmass undetected !!

Title: Re: PSU question
Post by: TMTLOGIC on 20:16, 02 May 21
With the AMSDAP42 I experimented a bit with a 5 VDC relay
When the CPC 5v is present, the coil of the relay switches, then the external voltage is activated
Title: Re: PSU question
Post by: m_dr_m on 23:14, 03 May 21
Ok, according to @Duke (https://www.cpcwiki.eu/forum/index.php?action=profile;u=1624) doc for the M4:


Quote
The M4 Board can be powered directly from the CPC or via USB power supply (5V). There is one single jumper on the board, to set this. If set at EXT it will use the CPC's power supply and if set at USB it will use the power from the USB socket. Notice: If using MX4 motherboard with M4 plugged in, you must use external powersupply, either directly to the M4 or to MX4.


At first I was utterly confused, because 'EXT' means... *no* external power supply.
Now, I am just bemused. This recommandations also seems to go against option 1!
Title: Re: PSU question
Post by: pelrun on 03:23, 04 May 21
EXT means "powered from the EXTension port".  ;D