exxos's Atari, Amiga & retro forum

How much power can the Atari STe video port (DIN-13) output?
I'm working on a VGA video adapter board (similar to Ubeswitch and others like it) except mine will also have an Atari DIN-13 jack for allowing standard Atari monitors to be attached.

It'll have a couple of multiplexer chips and likely a 5V regulator, so I'm wondering what the limits for the +12V output on the DIN-13 jack of the STe are?

If you look here: ST PSU you can see that for a standard SR98 PSU is rated for 3A @ 5V and 0.9A @12V, so 15W and 10.8W respectively.

12V isn't used for a lot, I believe serial, FDD and possibly the AY sound chip, but you'd have to trawl through the schematics to be 100% sure. Later FDD's and Goteks don't use 12V, which helps. I can't imagine the few components you're going to be using will pull much power.

Pacman was asking specifically for the video port, though. 12 V on the video port has a 1.2 kΩ series resistor. The maximum allowable current can be derived from the minimum required voltage by applying Ohm's law.

czietz wrote: 18 Sep 2024 11:14 Pacman was asking specifically for the video port, though. 12 V on the video port has a 1.2 kΩ series resistor. The maximum allowable current can be derived from the minimum required voltage by applying Ohm's law.

I was simply providing the information I had about the PSU so he had a starting point and understood the PSU output. He can't use more than the PSU can supply.

In any case with 12v and a 1.2kΩ series resistor it's 0.01 a and 0.12 watts.

I was never any good at electrical theory :?
I'll be using two MUX chips that take around 0.3 mA each at 5V. And I need to convert the 12V line to 5V meaning a voltage regulator of some sort is needed too.

What's the +12V line in the DIN13 connector for anyway? Do the Atari monitors use it for something?
Trying to find an answer to this I just looked up the Info-coach Atari ST interfaces and connectors -Video connector page which doesn't say anything about what it's for but does say "12V/10mA". Does this sound about right?
EDIT: oops! That's just what your calculations showed, Rubber_jonnie: 0.01A :D
... which of course is 10mA

I certainly don't want to cause any potential trouble if for instance an SM-124 or SC-1224 is selected and plugged into my output board. Would it be better to take the power directly from the computer's power supply? I'll most likely get myself a new and better power supply anyway. Depending on how much power each line supplies I might just be able to connect it to the +5V and GND lines directly (instead of using the PSU's +12V line and GND, then needing a voltage regulator to bring it down to +5V).

Pacman wrote: 18 Sep 2024 23:27 What's the +12V line in the DIN13 connector for anyway?

https://en.wikipedia.org/wiki/SCART

Automatic Switch Voltage to AV ...

frank.lukas wrote: 19 Sep 2024 13:09

Pacman wrote: 18 Sep 2024 23:27 What's the +12V line in the DIN13 connector for anyway?

https://en.wikipedia.org/wiki/SCART

Automatic Switch Voltage to AV ...

I see.
So Atari actually added this 12V line (not present in previous ST models as far as I know, before the STe) with SCART inputs for TV in mind?
I wonder if they issued a DIN13 to SCART cable at some point.

So, when +12V is inserted into the SCART connector's pin 8 the TV automatically selects the SCART input so the user doesn't need to do anything.
Perhaps the best idea for my video output board would be for power (+5V) to be taken directly from the power supply, just to be on the safe side?
I could use multiple power supply 'Y' splitter cables from the power supply to the STe along with the various add-on boards I'm having. Good idea?

As recently discussed the Exxos ST2VGA adapter (https://www.exxosforum.co.uk/atari/store2/#0170) uses the 12V line to power its circuit, so if you keep your current requirements low enough it's a perfectly feasible power source.

BW

Is the Exxos ST2VGA a variation of the Ubeswitch but without the programmable logic?
It's hard to tell from the photos here (and elsewhere) but if it's anything like the Ubeswitch it'll use a TS5V330 multiplexer (which, according to the datasheet draws maximum 3uA (0.003mA). There's also a 78L05F voltage regulator which I'm not sure how much current it draws.

The two MAX4885 multiplexers I plan to use in my board appear to be drawing a lot more (300uA, or 0.3mA) each which is strange when compared to the MUX in the Ubeswitch, but that's what the datasheet says. Then there's the 5V regulator (unless I power it directly from the computer's power supply 5V line as suggested earlier.). In that case, would I need to insert a current limiting resistor between the power supply's +5V and the +5V inputs of the MUX chips?