DIY dual tracking power supply for the benchtop

[Paul Perry (Frostwave)]

http://members.shaw.ca/roma/supply-2.html
At the bottom of the page, there is a dual tracking variable power supply.
I just built it on a bit of stripboard (used an OP27 for the op amp, happened to have a few at hand) and, it works fine.
note it doesn't go to +-15 - it goes to +-13 OK though. But I used a 16V AC plugpack, half wave rectifier for the raw+ and -, then a LM7815 + reg and a LM7915 - reg for the +-15 inputs to the tracking supply.
That might seem long winded - and it is - but, that way I have the +-15 available directly anyhow.
And the adjustable supply region I need, is in the +-12 zone anyhow.
I was a bit suprised that nobody -  so far as i can see - makes a reasonably priced tracking power supply, or even a kit (no, $150 isn't 'reasonable' for a kit thanks!!)

Before trying this, i built  the one from ESP (normally a very reliable source of circuits) http://sound.westhost.com/project44.htm (the one lower down). well, the trouble was, the tracking was completely shitful. Now, maybe it was the dual pot that itself didn't track... but since it was a new pot from an OK manufacturer, I'm inclined to think, one is better off using a single pot & the extra electronics.

[remmelt]

What does tracking mean in this context? Where's the input on this schematic? What is the input?

/confused

Please forgive the questions!

[Paul Perry (Frostwave)]

There is no input, the device is not an effect, it is a power supply.
The kind of adjustable power supply that is nice to have on the bench when testing and developing things.
The "tracking" refers to the fact that there are two voltages output, a positive and a negative, and as you adjust the controls they keep track of each other.
Why I need it at the moment, is that I have an analog filter operating on + and-5 V rails, and I am wonderign whether it will behave differently with a +-8V supply.

Yeah, I could get a secondhand B&K 1651 for $295 - if I was in the USA, and if I had $295 plus postage. Thank God for veroboard.

[remmelt]

Haha, I did get the power supply part. I was just wondering how much V it wants to see on its in-line and where that in-line would be. The power doesn't just appear out of thin air (regretfully. The world would be a different place) so I wondered if I should feed it 9 Volts?

[Paul Perry (Frostwave)]

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

You attach a 15V DC supply to A and B on the left side of the circuit (+ to A)
and a -15V supply to B and C (-to C).

Of course, you can just use one side of the output eg if you have a + earth circuit to test, just take the middle wire (ground) and the - output. Because a lot of my stuff is based on traditional +- rail analog circuits, I need a tracking supply like this.

[MKB]

Nice circuit; unfortunately the maximum output voltage is limited by the max power supply limitations of the op amp. The LM317 is good to a 37V Vi/Vo differential IIRC, if you could find an op amp that would run on a higher supply voltage you could get more voltage range out of the circuit.

[grapefruit]

A circuit like this would be handy. i've been adjusting + and - rails separately and it gets annoying.

The voltage limitation is a bit of a downer. I was looking in "The Art of Electroncs" 2nd edition, and Fig 6.34 dual tracking regulator.
Sorry to anyone who doesn't have said book...

It looks like you could replace the positive regulator with an LM317 and just use the bottom section for the negative regulator. As the bottom op amp Is powered from V- and GND you could get more voltage out of it, but I can't see how it would work because the op amp is comparing a voltage near GND and powered by GND and V-.

If anyone has the book handy and knows I'd be keen to hear.

Cheers,
Stew.

[Paul Perry (Frostwave)]

If one wanted to go to higher voltages, then I would be thinking switchmode. Well, TWO switchmodes

But if I REALLY needed a high voltage tracking supply - and I'm hard pressed to think of an application, except working on solid state amps - I would use a variac to change the input to a standard kind of power supply. Simplicity is a major virtue.