Would this work

leander · 14th November 2005, 06:25 AM

I have a 34v dc power supply, can I do this to achive +/- 15v

djk · 14th November 2005, 06:37 AM

No!

amplifierguru · 14th November 2005, 06:42 AM

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

leander · 14th November 2005, 07:51 AM

Like this ?

What values should I use ?

AndrewT · 14th November 2005, 08:09 AM

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.

djk · 14th November 2005, 08:16 AM

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.

leander · 14th November 2005, 08:18 AM

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

djk · 14th November 2005, 08:28 AM

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.

djk · 14th November 2005, 09:49 AM

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.

sss · 14th November 2005, 02:23 PM

if your 35V supply is transformer that is not connected to ground this is what u can do

(the battery representd your floating 35Vsupply)

14th November 2005, 08:09 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. __________________ regards Andrew T.

14th November 2005, 02:23 PM	#10
sss diyAudio Member Join Date: Jul 2003 Location: Israel , haifa	if your 35V supply is transformer that is not connected to ground this is what u can do (the battery representd your floating 35Vsupply) __________________ if you are not living on the edge you are taking too much space

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14th November 2005, 06:37 AM	#2
djk diyAudio Member Join Date: Feb 2001 Location: USA	No!

14th November 2005, 06:42 AM	#3
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	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

14th November 2005, 08:16 AM	#6
djk diyAudio Member Join Date: Feb 2001 Location: USA	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.

14th November 2005, 08:18 AM	#7
leander diyAudio Member Join Date: Aug 2005	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

14th November 2005, 08:28 AM	#8
djk diyAudio Member Join Date: Feb 2001 Location: USA	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.

14th November 2005, 09:49 AM	#9
djk diyAudio Member Join Date: Feb 2001 Location: USA	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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