Author Topic: PSU question  (Read 1759 times)

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Offline Skunkfish

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PSU question
« 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!
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Offline ||C|-|E||

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Re: PSU question
« Reply #1 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 :) .
« Last Edit: 23:56, 12 February 17 by ||C|-|E|| »

Online Duke

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Re: PSU question
« Reply #2 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.

Offline Bryce

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Re: PSU question
« Reply #3 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.

Offline m_dr_m

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Re: PSU question
« Reply #4 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.



Offline eto

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Re: PSU question
« Reply #5 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.

Offline pelrun

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Re: PSU question
« Reply #6 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.
« Last Edit: 09:23, 28 April 21 by pelrun »

Offline eto

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Re: PSU question
« Reply #7 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?

Offline Bryce

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Re: PSU question
« Reply #8 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.

Offline eto

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Re: PSU question
« Reply #9 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.

Offline Bryce

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Re: PSU question
« Reply #10 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.
 

Offline eto

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Re: PSU question
« Reply #11 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
« Last Edit: 13:55, 28 April 21 by eto »

Offline Bryce

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Re: PSU question
« Reply #12 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.

Offline m_dr_m

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Re: PSU question
« Reply #13 on: 15:49, 28 April 21 »
Thank you all for your input!


I'm still confused, though:
  • For symbiface III @TMTLOGIC recommends to power the MX4 (IIRC), that is, solution 2.
  • Why 2 distinct power supplies (for CPC and MX4 respectively) would cause issues?
If you can point to a good power supply (plug and play, with nothing to solder!), I would be very grateful.

Offline pelrun

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Re: PSU question
« Reply #14 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.

Offline Bryce

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Re: PSU question
« Reply #15 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.

Online TotO

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Re: PSU question
« Reply #16 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.
« Last Edit: 18:53, 28 April 21 by TotO »
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Offline pelrun

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Re: PSU question
« Reply #17 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

Offline Bryce

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Re: PSU question
« Reply #18 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.

Offline pelrun

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Re: PSU question
« Reply #19 on: 11:55, 29 April 21 »
I've already been saying not to do it, so I hardly need the warning.  :P

Offline m_dr_m

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Re: PSU question
« Reply #20 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%]

Offline Ast

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Re: PSU question
« Reply #21 on: 16:10, 29 April 21 »
warning : using an external psu for xmass with 512mo will make your xmass undetected !!

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Offline TMTLOGIC

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Re: PSU question
« Reply #22 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
« Last Edit: 20:17, 02 May 21 by TMTLOGIC »

Offline m_dr_m

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Re: PSU question
« Reply #23 on: 23:14, 03 May 21 »
Ok, according to @Duke 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!

Offline pelrun

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Re: PSU question
« Reply #24 on: 03:23, 04 May 21 »
EXT means "powered from the EXTension port".  ;D