Designing a variable dual rail supply (tracking)

[Googler]

Hi,
I want to build a power supply (basically) for powering up op-amps.
Specs :
variable voltage from +/-1.5 to +/-15 V.
current -> 1 A
(i also want to power other ckts when op-amps are out of "fashion" ;-)).

my intention is to "gang" the positive and negetive supply and to be able to
control both +ve and -ve supply with a *single* pot( no ganged pots pls).

can we rig up a power supply with this requirement with the LM317 type of
regulators ?
is there any type of "digital pot" ( not rotary switches) instead of usual
analog ones?( i hope digital pots would be repeatable and less drifty)
i dont mind putting a microprocessor to control the digital pot if that is
required.

any ideas are welcome.

Regards

 

[martin griffith]

Hi,
I want to build a power supply (basically) for powering up op-amps.
Specs :
variable voltage from +/-1.5 to +/-15 V.
current -> 1 A
(i also want to power other ckts when op-amps are out of "fashion" ;-)).

my intention is to "gang" the positive and negetive supply and to be able to
control both +ve and -ve supply with a *single* pot( no ganged pots pls).

can we rig up a power supply with this requirement with the LM317 type of
regulators ?
is there any type of "digital pot" ( not rotary switches) instead of usual
analog ones?( i hope digital pots would be repeatable and less drifty)
i dont mind putting a microprocessor to control the digital pot if that is
required.

any ideas are welcome.

Regards

www.circuitcellar.com Issue 144 July 2002

an 8031 controlled psu




martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

 

[John Popelish]

Hi,
I want to build a power supply (basically) for powering up op-amps.
Specs :
variable voltage from +/-1.5 to +/-15 V.
current -> 1 A
(i also want to power other ckts when op-amps are out of "fashion" ;-)).

my intention is to "gang" the positive and negetive supply and to be able to
control both +ve and -ve supply with a *single* pot( no ganged pots pls).

can we rig up a power supply with this requirement with the LM317 type of
regulators ?
is there any type of "digital pot" ( not rotary switches) instead of usual
analog ones?( i hope digital pots would be repeatable and less drifty)
i dont mind putting a microprocessor to control the digital pot if that is
required.

any ideas are welcome.

Regards

I think you can make an ordinary 317 type variable regulator and make
a negative one track it with the addition of a single opamp. The
trick is to sum the two outputs and force that sum to equal zero.

So you need a pair of equal resistors, one to each of the supply
outputs, with the other end of each going to the - input of an opamp
(powered by the unregulated + and - supplies). The + input goes to
ground (the equals zero reference point). The output goes to the
reference input of the negative regulator. You will also probably
need a series RC network from opamp output to - input to stabilize the
control of the negative regulator (cause the opamp output to move no
faster than the negative regulator can respond).

 

[CFoley1064]

Subject: Re: Designing a variable dual rail supply (tracking)

From: John Popelish [email protected]
Date: 9/9/2004 10:32 AM Central Daylight Time
Message-id: <[email protected]>


I think you can make an ordinary 317 type variable regulator and make
a negative one track it with the addition of a single opamp. The
trick is to sum the two outputs and force that sum to equal zero.

So you need a pair of equal resistors, one to each of the supply
outputs, with the other end of each going to the - input of an opamp
(powered by the unregulated + and - supplies). The + input goes to
ground (the equals zero reference point). The output goes to the
reference input of the negative regulator. You will also probably
need a series RC network from opamp output to - input to stabilize the
control of the negative regulator (cause the opamp output to move no
faster than the negative regulator can respond).

Life sure is easy with Google. I punched in

lm317 +lm337 +"Dual tracking regulator"

and the first result is:

http://members.shaw.ca/roma/supply-2.html

At the bottom of the page, it gives the circuit Mr. Popelish is talking about.
Gosh, it even gives you a PC board layout. Can life get any better?

Chris

 

[mike]

Hi,
I want to build a power supply (basically) for powering up op-amps.
Specs :
variable voltage from +/-1.5 to +/-15 V.
current -> 1 A
(i also want to power other ckts when op-amps are out of "fashion" ;-)).

my intention is to "gang" the positive and negetive supply and to be able to
control both +ve and -ve supply with a *single* pot( no ganged pots pls).

can we rig up a power supply with this requirement with the LM317 type of
regulators ?
is there any type of "digital pot" ( not rotary switches) instead of usual
analog ones?( i hope digital pots would be repeatable and less drifty)
i dont mind putting a microprocessor to control the digital pot if that is
required.

any ideas are welcome.

Regards

The answer is yes, but do you really want to?

If you do much breadboarding, you'll soon find many places where you
need a supply that can be adjusted to 0V. 1.5Vmin is just too high.

About the third time your probe slips and you smoke the circuit,
you'll be wishing you had a variable current limit. You can't charge
a (small) battery with a 1A limit or a 1.5V minimum.

You're also gonna be wanting floating outputs. You'll find two floating
supplies much more useful than one dual tracking one. I've been doing
this for 40 years and I've NEVER needed a tracking supply. Simultaneous
+/-15V yes, simultaneous +5V +15V yes, tracking no.

If you really want a useful supply, build one that can source or sink
current from +20V to -20V.

The hardest part of a power supply is the mechanical part putting
together all the transformers, heat sinks, caps, pots, meters,
connectors etc.
Add another 10% to the project effort and build a really useful one.

mike

--
Return address is VALID.
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
Compaq Aero floppy,ram,battery.
MINT HP-41CV, 2-METER AMPS, 200CH SCANNER
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[John Popelish]

Life sure is easy with Google.

Love it.

I punched in

lm317 +lm337 +"Dual tracking regulator"

and the first result is:

http://members.shaw.ca/roma/supply-2.html

At the bottom of the page, it gives the circuit Mr. Popelish is talking about.
Gosh, it even gives you a PC board layout. Can life get any better?

I would either eliminate the 10 uf between the opamp output and ground
or put a 10 to 47 ohm resistor in series with it, to be sure of not
driving the opamp crazy.

 

[Googler]

Thanks all ,
i will give a try to the ckt from ckt cellar.
did google before posting here though.
looks like i need some help in creating proper google query.

Thanks & Rgs.

 

[CFoley1064]

Subject: Re: Designing a variable dual rail supply (tracking)

From: [email protected] (Googler)
Date: 9/10/2004 9:38 AM Central Daylight Time
Message-id: <[email protected]>

Thanks all ,
i will give a try to the ckt from ckt cellar.
did google before posting here though.
looks like i need some help in creating proper google query.

Thanks & Rgs.

Sorrry -- I didn't mean to sound too harsh. It must be the season or
something. We're getting a lot of that on s.e.d. these days. After the
"Housewife" in a prior post, you would think we eat our dead.

Mr. Popelish and I have seen that circuit before, and knew what to look for.
Unless you had known, you would not have added "+LM337" for the negative
adjustable regulator, which was the key to getting the answer fast. Proper
google queries come with knowledge, which is what you're looking for.

I hope we were of help.

Good luck
Chris