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   Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:39:02 JST Studio 8502 :verified:

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   • Graham Sutherland / Polynomial

   @gsuberland Looking at an El Cheapo $30 power supply off Amazon and it claims to have 22A on the 3.3V rail.

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     Graham Sutherland / Polynomial (gsuberland@chaos.social)'s status on Sunday, 30-Jul-2023 04:39:05 JST Graham Sutherland / Polynomial

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     @mos_8502 that'd do it.

     another option would be to use the 3.3V rail off the ATX supply (assuming it's rated for 8A or so) and stabilise it by sticking a 100Ω 2W resistor over the 5V rail and a 330Ω 2W resistor over the 12V rail as a method of applying a minimum rail load. wastes a little power but keeps the design simpler and the BOM cheaper.

     but if you do this, make sure the ATX voltage tolerance spec is ok for the types of parts you'd expect to see here (e.g. the Z80 chips)

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     Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:39:07 JST Studio 8502 :verified:

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     • Graham Sutherland / Polynomial

     @gsuberland So, something like this https://www.mouser.ca/datasheet/2/1458/DS6228ABC_03-3104914.pdf for the 12V->3.3V, and then a small ~100mA LDO running off of that switching reg's output for the 1.2V line?

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     Graham Sutherland / Polynomial (gsuberland@chaos.social)'s status on Sunday, 30-Jul-2023 04:39:08 JST Graham Sutherland / Polynomial

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     @mos_8502 I was mostly just working back from the 10A figure. 500mA per card is fine, so that's a budget of 6A for 8 cards. I'd go with around 7.5A or 8A on the buck for headroom.

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     Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:39:10 JST Studio 8502 :verified:

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     • Graham Sutherland / Polynomial

     @gsuberland Currently it's set at 500mA peak per expansion card -- do you think it should be a full 1000mA?

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     Graham Sutherland / Polynomial (gsuberland@chaos.social)'s status on Sunday, 30-Jul-2023 04:39:11 JST Graham Sutherland / Polynomial

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     @mos_8502 depends what the current limits are per card. if we're budgeting 2A for drive current on whichever card is actively driving the bus, and 1A per card in general, that's 10A total so would make sense.

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     Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:39:12 JST Studio 8502 :verified:

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     • Graham Sutherland / Polynomial

     @gsuberland So what do you think for the switching DC-DC converter, 12V nominal input, 3.3V output, rated for what, 10A?

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     Graham Sutherland / Polynomial (gsuberland@chaos.social)'s status on Sunday, 30-Jul-2023 04:39:14 JST Graham Sutherland / Polynomial

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     @mos_8502 I'd 100% go with switching regardless, particularly because the spec for termination is currently gonna pull about 2A in the drivers anyway at 50MHz, so even at 5V to 3.3V you're gonna have a very toasty linear reg!

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     Graham Sutherland / Polynomial (gsuberland@chaos.social)'s status on Sunday, 30-Jul-2023 04:39:16 JST Graham Sutherland / Polynomial

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     @mos_8502 or, even better, 12V and regulate, since most ATX supplies are single-rail and regulate the 5V/3.3V down, so you're wasting efficiency by going from the 5V rail. also the 12V rail will be able to give you all the current you need and then some. (make sure you put a fuse inline!)

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     Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:39:16 JST Studio 8502 :verified:

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     • Graham Sutherland / Polynomial

     @gsuberland I always fuse my power supply systems, yeah. Going from 12V to 3.3V and 1.2V, that's going to drop a lot of heat with a linear regulator. Switching is better here, yeah?

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     Graham Sutherland / Polynomial (gsuberland@chaos.social)'s status on Sunday, 30-Jul-2023 04:39:18 JST Graham Sutherland / Polynomial

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     @mos_8502 5V and regulate. ATX rails aren't guaranteed to be voltage-stable unless you have load on all rails.

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     Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:39:19 JST Studio 8502 :verified:

     Which do you think is smarter when using an ATX power supply to power a 3.3V system: Using the 3.3V rail off the PSU directly, or using the 5V rail and a regulator to power it?

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     Studio 8502 :verified: (mos_8502@oldbytes.space)'s status on Sunday, 30-Jul-2023 04:43:48 JST Studio 8502 :verified:

     • Thomas 🔭🕹️
     • Graham Sutherland / Polynomial

     @thomasfuchs @gsuberland I mean, that's what, 72.6 watts?