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Old 14th November 2005, 07:25 AM   #1
leander is offline leander  Malta
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Default Would this work

I have a 34v dc power supply, can I do this to achive +/- 15v
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Old 14th November 2005, 07:37 AM   #2
djk is offline djk
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No!
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Old 14th November 2005, 07:42 AM   #3
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Hi Leander,

Put two R's and C's in parallel and in series with another two to define the divide, before the regs.

Cheers,
greg
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Old 14th November 2005, 08:51 AM   #4
leander is offline leander  Malta
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Like this ?

What values should I use ?
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Old 14th November 2005, 09:09 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi Leander,
starting from the bottom your outputs will be 0V, +15V, +30V. You have only 2V overhead before each reg. Look up the datasheet to find what a 7915 needs. Then add on the ripple.
I think you need a new transformer 2 times 15Vac output to generate your +-15Vdc.
If your current requirement is very low you can generate a -dc using a chip.
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Old 14th November 2005, 09:16 AM   #6
djk is offline djk
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It still won't work.

If you really need this supply to work, and just a few mA, do the following:

Change the regulators to 12V types.

Make a voltage divider on the input out of a pair of 10K resistors.

Feed this ground reference to an opamp wired as a buffer, the output is your ground reference.

If you need more than a few mA: wire the base of an NPN transistor to the output of the opamp, the collector to the positive supply, loop the feedback from the emitter to the inverting input, and the emitter becomes your high power ground.
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Old 14th November 2005, 09:18 AM   #7
leander is offline leander  Malta
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I think you need a new transformer 2 times 15Vac output to generate your +-15Vdc.
If your current requirement is very low you can generate a -dc using a chip. [/B][/QUOTE]

the input 34v are already dc, there is no ripple
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Old 14th November 2005, 09:28 AM   #8
djk is offline djk
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Then just use an LM1875 as the opamp as described above (skip the transistor), but you will still need to use 12V regulators unless you buy LDO types.
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Old 14th November 2005, 10:49 AM   #9
djk is offline djk
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Figured it out.

Regulate the 34V to 30V with whatever regulator you have.

Derive the ground with a resistive divider on the 30V output with an opamp big enough to handle whatever current you need. An LM1875 will deliver a bunch of current.
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Old 14th November 2005, 03:23 PM   #10
sss is offline sss  Israel
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if your 35V supply is transformer that is not connected to ground this is what u can do (the battery representd your floating 35Vsupply)



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