How to split single 40v into +/-15v ? - diyAudio
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Old 18th September 2012, 09:15 PM   #1
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Default How to split single 40v into +/-15v ?

I need to get +/-15v 300mA max varying / non-symmetric load currents on both +/-.
No need for high precision though.
There is only a single 40v 1amp available inside the box.
I first thought about just using a pair of 7815/7915 but don't think it would work now ... or does it ?
The only other idea I have so far is to first regulate to 30v and then split via zeners.
Will #1 and/or #2 work ?
Are there alternatives ?
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Old 19th September 2012, 01:12 AM   #2
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They will both work.

But beware of the temperature effects on Zener regulation points.

If you are concerned about the regulation headroom on LM7x15 regulator take a look at this cct using 21-22V zeners.
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Old 19th September 2012, 07:13 PM   #3
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My concern with #1 is as follows:
what if I load the +15v rail but not the -15v (or much less)
there's no return path from the virtual ground to the actual power supply ... !?
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Old 20th September 2012, 02:28 AM   #4
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Through the npn (or pnp) transistors.

They (with the zeners) will put the ground point within a volt or two of half.

This will make sure each regulator has headroom independent of load imbalance.

I've used this type of circuit of dozens (yes, dozens) of times with a 30V isolated supply to get -4V and +26V supplies. It was only running a couple of instrumentation amplifiers so I did not need the transistors but the idea is the same.

In your case if there is a load imbalance there will be a shift in regulator supply potential. This is what the "line regulation" part of the regulator spec is for.
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Old 20th September 2012, 07:57 AM   #5
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you cannot do this if u have sigle power supply....put two diodes one side cathode and anothr one anode on one terminal and use other terminal as gnd....and then regulate it....
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Old 20th September 2012, 08:13 AM   #6
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I have been using this schematic with good result.
http://hem.bredband.net/hififorum/schema/Power.png

Last edited by bjaris; 20th September 2012 at 08:26 AM.
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Old 20th September 2012, 11:36 AM   #7
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Works like a champ
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Old 20th September 2012, 01:18 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by payloadde View Post
Will #1 and/or #2 work ?
Are there alternatives ?
#2 will work, but will be touchy about the adjustment: you could easily fry the zeners, or end up with a "dead-band" of high impedance in your ground.
You could increase the 1 ohm resistors, but of course this will reduce the available current

Quote:
Originally Posted by DUG View Post
If you are concerned about the regulation headroom on LM7x15 regulator take a look at this cct using 21-22V zeners.
The circuit will fry the zeners if the 1A supply is not limited in some way

Quote:
Originally Posted by bjaris View Post
I have been using this schematic with good result.
http://hem.bredband.net/hififorum/schema/Power.png
This circuit is not properly compensated and will exhibit a huge impedance peak somewhere in the KHz range.
Depending on the amplifier and the 470µ's parameters, it could even oscillate under certain conditions.
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Old 20th September 2012, 06:16 PM   #9
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Quote:
Originally Posted by zigzagflux View Post
Works like a champ
2x TL431 for small loads, ok, but with 300 mA loads?
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Old 21st September 2012, 12:13 AM   #10
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Originally Posted by DUG View Post #2
If you are concerned about the regulation headroom on LM7x15 regulator take a look at this cct using 21-22V zeners.

Originally Posted by Elvee View Post #8
The circuit will fry the zeners if the 1A supply is not limited in some way

A 21V zener (example DDZ21) has a tolerance of 20.64V to 21.71V at 5 mA of test current and is rated for 500mW.

At 20V the current will be less than the 5 mA of test current and therefore less than the 105mW that would be dissipated under test conditions.

I don't believe they will fry at 20% of rating.
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