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Am I doing this Power Supply Math correctly? Noob Question
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abelovesfun



Joined: Dec 17, 2012
Posts: 18
Location: Portland, OR

PostPosted: Mon Dec 17, 2012 8:04 pm    Post subject: Am I doing this Power Supply Math correctly? Noob Question
Subject description: Calculating power supply needs
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Hello,

I just bought my first modular. It's a very large collection of MFOS modules.
About 3/4 of them are racked up in a project cabinet, and running off of a 12VAC 1000 mA MFOS wall wart power supply.

I am hoping to re rack all of the units in a slightly nicer set of cabinets.
I went through the list of modules, and the MFOS site, and added up the mA for each module (I assume I add up the -12V mA draw and the + mA draw for a modules total mA draw?). When I could not find the mA draw - I just assumed 70mA - seems like a safe bet.

The total mA draw, according to my math, for all modules, is 861 mA.
Does this mean I can power all off the modules from the existing power supply?

Thank you for your help!
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Uncle Krunkus
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PostPosted: Tue Dec 18, 2012 3:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I know it looks feasible on paper, but unfortunately, I think you would run into trouble.
You probably noticed when you were adding up the mA draws that the amount of current drawn from the -ve rail would be about 1/10th what is drawn from the +ve rail, when you add them all up. Whereas the 1000mA MFOS PSU will probably be good for about 450mA per rail. (half each)
So if all the modules draw about 600mA from +ve and the other 260mA from the -ve,.... need I say more?
If it were me, I'd build an extra PSU which could deliver 1A per rail and use the MFOS one for, say, just your VCOs. Make sure your star connection for the earths goes directly from one PSU to the other.

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abelovesfun



Joined: Dec 17, 2012
Posts: 18
Location: Portland, OR

PostPosted: Tue Dec 18, 2012 12:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you so much for the quick reply.
I did note that the negative mA draws were less, but they actually tended to be closer to the positive draw.
Regardless, I take your point, and I'm actually worried that the original builder might have had them wired wrong.
I've seen others post about using Power One and Condor Power supplies from ebay. Are there any existing forum posts you are aware of that show how to wire these up? I assume you just grab the +, -, and ground terminals, wire them up to the buss-board, and then plug the sucker in, but is there anything else involved?
I see these on ebay for around $30-40, so it seems like a great way to go.
Thoughts?
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Uncle Krunkus
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PostPosted: Tue Dec 18, 2012 1:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

As long as it's a linear power suppy, (ie not switch mode, it should be heavy due to the transformer) and can deliver 1A per rail, you should be fine. You can use switch mode transformers at a pinch, but they have minimum draw requirements, and/or issues with noise, so stick with a linear for your main supply. Yeah they should be easy to wire.
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