Why do Atari ST power supplies need load to work correctly?

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curlypaul
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Why do Atari ST power supplies need load to work correctly?

Post by curlypaul » Sat Jul 17, 2021 10:43 pm

Should have asked this the other day, but what are they missing they makes them unreliable to test without load? Is it actually dangerous to not have them loaded?

What about newer transforners? If I get a new Megadrive PSU for example, I'd put the multimeter on it before using it, is this a valid test?

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Re: Why do Atari ST power supplies need load to work correctly?

Post by stephen_usher » Sat Jul 17, 2021 10:49 pm

Basically, any switch mode power supply will not work correctly without enough load.
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Re: Why do Atari ST power supplies need load to work correctly?

Post by rubber_jonnie » Sat Jul 17, 2021 11:44 pm

As @stephen_usher said, these PSUs are designed with a load in mind, and as such won't behave as expected without one. Unreliable is not the word I'd use, because technically you are expecting the PSU to work without the load it was designed for.

You may see higher voltages without a load for example, which might lead you to believe the PSU was not performing correctly, but when correctly loaded with either the real or an artificial load, such as resistors, everything behaves as expected.

The PSU is intended to drive a circuit, in this case the ST, but without that there is no circuit, in fact it's an open circuit, so no current draw, and not the behaviour you'd expect under normal circumstances.
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Re: Why do Atari ST power supplies need load to work correctly?

Post by curlypaul » Sun Jul 18, 2021 12:17 am

So is this limited to switching supplies?

Just thinking about my to do pile, I think the resistors I got from here will be fine for testing the ones in my Amstrad, but I might need to collect more :)

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Re: Why do Atari ST power supplies need load to work correctly?

Post by exxos » Sun Jul 18, 2021 12:38 am

Switch modes work by pulsing the transformer with high voltage on the primary. While there are turns ratios involved, its down to the total flux on the core which transfers energy from primary to secondary. Energy on the secondary translates to your voltage and current on the output, 5volts etc.

Now when you pulse the primary, you pulse the secondary which its circuit is designed to output 5 volts. With no load on the secondary this charge has to turn into something, which is generally more than 5 volts. When a magnetic flux collapses it just inherently turns to voltage. This could be 50volts or more. But we cannot output 50+ volts as we only want 5 volts.. So the PSU shuts down . When this charge has dissipated ( normally on a 5 watt resistor on the output ) the PSU will turn on again.. And the whole cycle beings again. So what you see on your meter is at best, a average voltage of maybe 2 volts. The PSU turns on and off due to its protection circuits. It simply cannot function this way.

When you load a PSU, when you pulse the primary, which pulses the secondary, on the first cycle the PSU cannot maintain the required flux in the core, so on its next cycle it pulses the primary with twice as much energy, which then gets dumped into the secondary.. This repeats very fast until the PSU is able to come to a stable state where it can regulate its 5volt output , which also depends on current drawn. The more amps you pull on the secondary, the harder the primary has to work to maintain the output .

Of course I try to explain as simply as I can think. The whole subject can get rather in depth. There are many operating modes of PSUs . but generally switch modes will not function correctly without a load. Some PSUs can even explode when not under a load.
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Re: Why do Atari ST power supplies need load to work correctly?

Post by sporniket » Sun Jul 18, 2021 9:32 pm

Personnally, in order to work anything psu related, I plan to get one kit like this at one point :

https://www.sparkfun.com/products/14449

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Re: Why do Atari ST power supplies need load to work correctly?

Post by curlypaul » Mon Jul 19, 2021 1:15 am

That's a cool device! I didn't know they existed.
sporniket wrote:
Sun Jul 18, 2021 9:32 pm
Personnally, in order to work anything psu related, I plan to get one kit like this at one point :

https://www.sparkfun.com/products/14449
Appreciate the explanation @exxos, it triggers vague memories of my physics course.

Seeing what a difference shiny solder made to my PSU (it's barely warm after a couple of hours use), I've just been over the motherboard again. I knew dull old joints were a problem, but my standards were not high enough. Removed that horrible lead free solder I was duped with as well. Not fall for that again either.

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Re: Why do Atari ST power supplies need load to work correctly?

Post by Tomswork » Mon Jul 19, 2021 1:19 am

I have found all regulated power supplys need a load to test properly switch mode and linear being cheap I keep a old 5.25 spinner HD for that it puts a good load on the psu 5v and 12v

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Re: Why do Atari ST power supplies need load to work correctly?

Post by terriblefire » Mon Jul 19, 2021 1:06 pm

Even modern PC Power supplies give you a rubbish 5V line unless the 12V line is loaded up.
———
"It is not necessarily a supply voltage at no load, but the amount of current it can provide when touched that
indicates how much hurting you shall receive."

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