Facing a problem with Dave's power supply design

[Asim]

LM317 here I come , though i will need to generate a negative rail from the battery to get to 0V output.

It costs a couple of bucks, but I once used an LM385-1.2 and a 7660. I think used a voltage regulator too, I'd have to dig up the schematic to be sure. It might not be a great solution for battery power though. A CMOS 555 inverter may be better. The LM385 is pretty low power though.

I already have some lm317, if things worked out with it i will look for a better regulator.

[Asim]

generic 3.3/5V isolated switcher, you hook the positive side of the output to ground and away you go with -5V,

I can't find such thing where I live( Dubai) and ordering it will take time

[rdl]

I took the power for the negative side from the unregulated input, which was about 22 volts. So I had to use a little TO-92 LM337 to regulate the input voltage to the 7660 down below 10 volts ( I think I set it to 8 volts). 

Then an LM385-1.2 generated the the -1.25 volt reference for the LM317. The reference was a little bit too negative though. When turned all the way down, the LM317 actually put out about -0.05 volts.

[facumedica]

Remember that Rail-to-Rail Op-Amps are ideal in theory, but as you may know they are made from transistors and resistors inside, and the transistors are like diodes and diodes are PN junctions of semiconductor materials and these materials aren't "perfect" they obviously will have a loss (just like every material, even copper, silver, aluminium).
So, in brief, Op-Amps aren't perfect because transistors aren't perfect because diodes aren't perfect because the PN junction isn't perfect because the materials and methods of construction aren't perfect. (Nobody's perfect  )
I am not an engineer or anything (just a student  ) but something I learnt is that you will need to use some "practical tricks" to get what you expected to do in theory. So, test for yourself every option.
And the voltage supply does not need to be symmetrical (I asked that question few weeks ago  ) I'm working in a microcontrolled power supply so I'm struggling with the same problems as you.
Good luck! Keep the good work!

[Asim]

Remember that Rail-to-Rail Op-Amps are ideal in theory, but as you may know they are made from transistors and resistors inside, and the transistors are like diodes and diodes are PN junctions of semiconductor materials and these materials aren't "perfect" they obviously will have a loss (just like every material, even copper, silver, aluminium).
So, in brief, Op-Amps aren't perfect because transistors aren't perfect because diodes aren't perfect because the PN junction isn't perfect because the materials and methods of construction aren't perfect. (Nobody's perfect  )
I am not an engineer or anything (just a student  ) but something I learnt is that you will need to use some "practical tricks" to get what you expected to do in theory. So, test for yourself every option.
And the voltage supply does not need to be symmetrical (I asked that question few weeks ago  ) I'm working in a microcontrolled power supply so I'm struggling with the same problems as you.
Good luck! Keep the good work!

I know it doesn't need to be symmetrical, what I am asking is, is their something I am missing on the idea I posted earlier? it seems to me as valid solution 

[Rerouter]

guys, I have an idea feel free to correct me if i am wrong. I want the power supply to be portable that's why I want it to be battery powered. I will be using portable(3 or 4)  5V phone chargers( it has a lithium ion with a charging  and step up circuit ).

here is the idea: connect two batteries in series to get 10V and the 3rd one connect it backward so I will have 10V_ground_ -5V . i will be using the -5V to power the op amps. I will generate a -1.25V from the +2.5V reference with a inverting amplifier  and use that as a reference voltage to the set pin, when the current limit kicks in i will short the set pin with a transistor to -1.25V instead of the ground.

 any problems with this ?

You dont connect the last one up backwards, else you will just get +5V where you where expecting -5V,

The other issue you face is dissimilar rates of discharge, as the -5V only effects the low current control circuitry, it will last much longer than the other 2, resulting in you replacing the top 2 much more often, not actually a big issue but something to be aware of,

other than that, it should work, just keep track of where the current path is flowing, as with -5V being your op amps negative, any current they sink will discharge the battery, if you minimize this you could well get away with a CR2032 coin cell, and 2 honking big batteries for the main supply,

Edit: thinking further i should warn you that a battery can be discharged to a negative potential, if there is a greater positive potential (the other 2 batteries) driving current through it while flat, so it would probably be smart to make up some form of low battery indicator as most chemistry get very unhappy when they go over a volt in the negative,

[Asim]

guys, I have an idea feel free to correct me if i am wrong. I want the power supply to be portable that's why I want it to be battery powered. I will be using portable(3 or 4)  5V phone chargers( it has a lithium ion with a charging  and step up circuit ).

here is the idea: connect two batteries in series to get 10V and the 3rd one connect it backward so I will have 10V_ground_ -5V . i will be using the -5V to power the op amps. I will generate a -1.25V from the +2.5V reference with a inverting amplifier  and use that as a reference voltage to the set pin, when the current limit kicks in i will short the set pin with a transistor to -1.25V instead of the ground.

 any problems with this ?

You dont connect the last one up backwards, else you will just get +5V where you where expecting -5V,

The other issue you face is dissimilar rates of discharge, as the -5V only effects the low current control circuitry, it will last much longer than the other 2, resulting in you replacing the top 2 much more often, not actually a big issue but something to be aware of,

other than that, it should work, just keep track of where the current path is flowing, as with -5V being your op amps negative, any current they sink will discharge the battery, if you minimize this you could well get away with a CR2032 coin cell, and 2 honking big batteries for the main supply,

Edit: thinking further i should warn you that a battery can be discharged to a negative potential, if there is a greater positive potential (the other 2 batteries) driving current through it while flat, so it would probably be smart to make up some form of low battery indicator as most chemistry get very unhappy when they go over a volt in the negative,

I had that in mind, i will need to recharge the main batteries more often than the other one(recharging two is better than 3  )

about the batteries running flat, I will be using 2 additional op amps for the battery indicators.

here is the schematic( I draw it in 5 min i am sure something wrong will turn up xD)





EDIT: I noticed the battery, I will fix it

[edavid]

I don't think this design is a good starting point for a battery powered supply.  I think you would be much better off with an adjustable buck-boost converter module... maybe something like this: http://www.ebay.com/itm/Buck-Boost-Voltage-Converter-Voltmeter-Control-Start-up-Display-Blue-Color-/261373900003

[facumedica]

I don't think this design is a good starting point for a battery powered supply.  I think you would be much better off with an adjustable buck-boost converter module... maybe something like this: http://www.ebay.com/itm/Buck-Boost-Voltage-Converter-Voltmeter-Control-Start-up-Display-Blue-Color-/261373900003

Let him experiment with this kind of stuff! Let him suffer the steps of a design!   
Good luck!