MDLR case - why 85W?

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guigui
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MDLR case - why 85W?

Post by guigui » Thu Nov 07, 2019 9:50 pm

Hi everyone,

I'm about to purchase a 14U 126hp MDLR case and I'm struggling to decide between 85W and 45W power supply. Importante note: it comes with two power supplies, one for each 7U half.

I currently have a 7U 104hp Intellijel case, which has a 80W power supply. I don't know if that's an overkill (in general).

I'd like to know if there's a certain condition due to which I need a 85W power supply. I mean, something like "if your rack has a lot of X, then you need 85W"

I'm sorry if that's a very stupid doubt.

Cheers!

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Yes Powder
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Post by Yes Powder » Thu Nov 07, 2019 11:30 pm

Tube modules— and to a lesser extent, digital modules— have high inrush currents that can cause problems upon startup. If your system has a lot of digital modules— or you’re even remotely considering any tube modules, go for 85W..
In fact, just go for more power regardless. If you dig around you can find countless threads started by users who cheaped out on power supplies and ended up bumping their heads way sooner than they thought because thay didn’t factor in how much inrush current some of their modules might take. (I’ve yet to see power specs for a digital module that mentions inrush current, though would love to be proven wrong on that.)

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Re: MDLR case - why 85W?

Post by leterell » Fri Nov 08, 2019 12:32 am

guigui wrote:Hi everyone,

I'm about to purchase a 14U 126hp MDLR case and I'm struggling to decide between 85W and 45W power supply. Importante note: it comes with two power supplies, one for each 7U half.

I currently have a 7U 104hp Intellijel case, which has a 80W power supply. I don't know if that's an overkill (in general).

I'd like to know if there's a certain condition due to which I need a 85W power supply. I mean, something like "if your rack has a lot of X, then you need 85W"

I'm sorry if that's a very stupid doubt.

Cheers!
Yes Powder is right. More power is better, and in this case the smaller one is too small if you ask me. When you've stuffed that thing to the brim, might be that you run into strange noise issues, instability on v/oct, and so on in addition to what YP said. Been there, you don't wanna go there. Troubleshooting a eurorack case's problems takes time and nerves, not worth the savings here.

You shouldn't push that PSU beyond 90% of spec anyway. Or beyond 85%.

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guigui
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Post by guigui » Fri Nov 08, 2019 2:40 am

I just realized I was not thinking about the 22hp difference on each rows. Should I worry about not having enough juice because of that?

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Post by butter » Fri Nov 08, 2019 8:49 am

guigui wrote:I just realized I was not thinking about the 22hp difference on each rows. Should I worry about not having enough juice because of that?
How long is a piece of string?

It depends what those 22HP are made up of - it could be 11 skinny modules wanting 30mA each, or one large one that takes up more than just that 22HP making the power usage on that section of rail effectively 0. You could also have a single module sucking more power on its own than the 11 would!

In general you are thinking a bit in the wrong frame of reference - the wattage is not as interesting a figure as how that is broken down into +12V,-12V and possibly 5V too.

Then compare those figures to what you calculate you will need by adding up the required power of each module you plan to put in the case. The most sensible way to do this is to go build your fantasy rack on modulargrid.net and it will add up the requirements per power rail (they are not equal!) for you.

Doing this will also just give you a rough sense of how much overhead you have in your intellijel case - you might be right up against you "80W" already.

"I wish my case had less power" said no-one ever is the short version.

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Post by starthief » Fri Nov 08, 2019 12:58 pm

You specifically want to look at:

+12V current (amps) -- you want some overhead, don't max it out.

-12V: modules vary a LOT. Some modules are lightweight here, but a few are extra hungry. Your specific module choices can make a huge difference in your power needs. And some PSU designs are a little weak in this area.

+5V: you might not actually need it at all, but there are a few modules that do, and some with a jumper or switch to draw from +5V and reduce the +12V load.



When I bought my MDLRCASE 12ux114, the PSU options weren't as good as the current one. My case has two PSUs, each with only 500mA on the -12V rail. I have to use a spreadsheet to balance the load, deciding whether to plug each module into the top or bottom sections. When the E520 ships next year, I may go over the limit on them both and have to let go of something, or upgrade to another PSU.

But I don't know or care how many watts those PSUs are. If I were running it off battery power, that might matter.
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Post by Fog Door » Fri Nov 08, 2019 6:46 pm

Having your module choices limited or dictated by your power supply is a pretty crap feeling which I know well from my first case. So when I bought my new case, MDLR 12Ux114 , I went with the 85w. I no longer know the cold sweat while seeing if I can juggle my planned modules to fit my power profile. BUT my current modular plan for this case sees me sitting at an estimated 56% capacity on the 12v and 50% on the -12v, so maybe 85w is overkill and I'm just a dick ;)

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