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Old 28th February 2014, 02:08 AM   #1
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Default robbing from the positive rail

So lets keep this generic..if I need +/- dc voltage for one circuit pulling a heavier load, and a positive only dc regulated voltage of another part of the device pulling a light load, and have only one transformer with dual secondaries, can I take the light load from only the one rail without upsetting things too much? I have a (poor) diagram below to help me explain. I know there are no fuses, etc.. this diagram is only to quickly illustrate my question and is not a final design.

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Old 28th February 2014, 02:46 PM   #2
polsol is offline polsol  South Africa
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If you have dual secondaries why not use them separately?
In other words you use each secondary with it's own rectifier rather than as a dual output as you've shown.
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Old 28th February 2014, 02:53 PM   #3
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If you read his post again he needs the +/- split PSU.

Unbalancing the PSU does have an affect, allbeit small. Can you not just add a 1A load to the -ve supply to keep it all in balance ?
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Old 28th February 2014, 03:04 PM   #4
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What about using a positive and negative 12v regulator and take the 24v across them?
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Old 28th February 2014, 05:41 PM   #5
polsol is offline polsol  South Africa
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Quote:
Originally Posted by KatieandDad View Post
If you read his post again he needs the +/- split PSU.

Unbalancing the PSU does have an affect, allbeit small. Can you not just add a 1A load to the -ve supply to keep it all in balance ?
Sorry, you're right. My bad. Misread the 24V 1A as the 'main voltage requirement'.
One could add a separate, second bridge rectifier across the secondary series transformer output but looks like the OP would then have to 'tame' a high voltage to meet requirements (assuming the second supply is required for driving relays or similar).
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Old 28th February 2014, 06:10 PM   #6
infinia is offline infinia  United States
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three terminal (3T) regulators need 3V headroom (min) . more than that heat sinking and power waste become a concern. check v span unloaded to max load from the transformer.
BTW > Better grounding is thru 2 bridge rectifiers grounded after the Capacitor banks. instead of grounded at the transformer using a single bridge ( good threads on this )
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Old 28th February 2014, 06:15 PM   #7
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Just thinking aloud. If it is a toroidal transformer, why not wind an auxiliary winding on the toroid for the 12V supply.
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Old 28th February 2014, 06:28 PM   #8
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by dutchess View Post
So lets keep this generic..if I need +/- dc voltage for one circuit pulling a heavier load

... and a positive only dc regulated voltage of another part of the device pulling a light load ...

a heavy pulling amp can cause voltage drop
and even if only small, the little headroom you have have for the low power voltage reg might be gone
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Old 28th February 2014, 06:31 PM   #9
infinia is offline infinia  United States
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Quote:
Originally Posted by KatieandDad View Post
Just thinking aloud. If it is a toroidal transformer, why not wind an auxiliary winding on the toroid for the 12V supply.
can be done, usually only for low voltage and low power , other than that it becomes impractical, fitting more copper on a limited area.
wind 10 or so turns using a gauge properly sized for the current. lightly loaded, measure the voltage > calculate volts / turn . to decide.
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Old 1st March 2014, 02:12 AM   #10
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Originally Posted by mcandmar View Post
What about using a positive and negative 12v regulator and take the 24v across them?

I'd use both rails if I knew how and if it would work, then they could share the load. How would I do that. I don't mind using a couple regulators if need be.
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