Dual voltage regulated power supply

[eng1]

I need a variable dual voltage regulated power supply. I have a LM317 and a LM337 and I wonder if I can change the voltage with one pot instead of two. Is there any configuation with those ICs and one pot??
Thanks

 

[audioguru]

You can't use a single pot when each IC needs its own pot with different polarities.
Use a dual pot which is two pots with a single shaft for a single knob.

 

[Hero999]

A dual pot is no good since each track probably has a tollerance of 20% which would make the + and - output totally different.

The only real way to do this is by using an op-amp with an p-channel FET for the positive and an n-chanel for the negitive, use a zenner reference and a pot attached to an inverting amp with a unity gain for the negitive side, I'll draw you a diagrem if you don't undersand.

 

[Roff]

I designed a solution for this and posted it here.
EDIT:
After posting this, I realized it was set up for digital control. Below is a pot-adjusted version.

DISCLAIMER:
I haven't built this or simulated it.

 

[eng1]

thanks for all replies.

To Ron H: thank you for the schematic. With R1, R2, R9 you're going to sense (Vpos-Vneg)/2 that would be equal to 0 V ideally... Are all those parts necessary? I had this idea.. I post the schematic..
The feed-back loop around the LM337 set Vy so that Vn = -Vp. Ra and Rb must be precision resistors or you can make Ra variable and calibrate. The supply voltage of the op amp can be regulated with LM7815/7915.

 

[Roff]

thanks for all replies.

To Ron H: thank you for the schematic. With R1, R2, R9 you're going to sense (Vpos-Vneg)/2 that would be equal to 0 V ideally... Are all those parts necessary? I had this idea.. I post the schematic..
The feed-back loop around the LM337 set Vy so that Vn = -Vp. Ra and Rb must be precision resistors or you can make Ra variable and calibrate. The supply voltage of the op amp can be regulated with LM7815/7915.

Draw a finished, working schematic that accounts for loop stability (no oscillation), op amp output range, and minimum current (10mA) for the LM337, and see if it looks much different from my schematic.
BTW, the PNP is there because the op amp I chose will only handle 10mA output current. The op amp model, in turn, was chosen for high voltage (36V max) VCC-VEE, and rail-to-rail output swing. For a slightly reduced output range, you can probably find an op amp that will not require the PNP. However, if you want the output of your supplies to work all the way down to 1.25V, you still need the small negative regulator (or a positive one, in you circuit) for the op amp.
I said that I have not simulated this, but I did simulate the one I originally posted, including for load transient response. It's easy to draw a half-baked schematic. It's not so easy to get it to work correctly. The devil is in the details.

 

[eng1]

It's not so easy to get it to work correctly. The devil is in the details.

Yes Thanks again for your suggestions.

I'll check for stability for sure... that was not intended to be a final design
I am going to set the minimum output voltage to 4 V with a resistor and the pot in series. And I'm going to supply the op amp with LM78xx/LM79xx with xx > |Vout max + 3|... In my schematic Vy will never be greater than the output voltage (it will be equal to Vx) and the output range of the op amp will be ok.

Another question: you worry about the current... the adj pin current is 100 micro ampere max, what about that 10 mA spec?

 

[Roff]

Another question: you worry about the current... the adj pin current is 100 micro ampere max, what about that 10 mA spec?

LM317 and LM337 require that the output current be at least 10mA to ensure regulation. I'm not sure if this is still required if you only want to use the regulator as a bullet-proof, high-power voltage follower, so I decided to be safe and assume it was required. You may find that it is not. Also, many op amps have no problem sourcing/sinking 10mA.