Building a +15 -0- -15 DC PSU

Thread Starter

SamR

Joined Mar 19, 2019
2,812
Playing around with Op Amps and really need a rail supply instead of tying up my bench PSUs. 120VAC XFMR, full-bridge rectifier, and actually plan to use LM317 & LM337 adjustable voltage regulators. Would like the output from the regulators to be ~20V max. So it comes down to the XFMR. Powering Op Amps for breadboarding I don't need a lot of VA. Even 1VA should be gracious plenty. Digikey has lots of transformer building stuff but don't think I want to go that route. Where can I find a center-tapped secondary XFMR like this? Maybe even surplus 24VDC SMPSs? Any ideas?
 

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Alec_t

Joined Sep 17, 2013
11,755
Since the current requirement is small, you could use heavily-smoothed half-wave rectified DC from an untapped single secondary winding if you can't source a centre-tapped one.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,812
@OBW0549 Yep, I must have been looking in the wrong place on Digikey.

I did think about using a single secondary XFMR and found a 24VDC SMPS 2.5A for $20 that I'm leaning towards. Since it is already enclosed and rectified, I can put the actual dual regulators in a small "Module" enclosure that plugs in. Just throwing ideas around for now but need to do this. I assume the regulators would tame the 24VDC SMPS?
 

OBW0549

Joined Mar 2, 2015
3,566
If you use a 24 VDC SMPS, how are you going to establish a "virtual center tap" to get +/- 12V?

I don't know what sort of op amp circuits you intend to be working with, but for anything involving low-level signals I would never, EVER use a SMPS to power op amps because of all the noise.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,812
That was what I suspected. I'd have to use a pair of them or a voltage inverter and now it's getting complicated... I've used Triad XFMRs before for lower voltage and it is looking like the way to go. 28VAC on the secondary gives me plenty of headroom and can use for other things. Looked at the selection on DK and lots to choose from in my range.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,812
@bertus I do like that schematic having a single Vadj pot. Now thinking about adding a digital meter to make things even easier...
 

Delta prime

Joined Nov 15, 2019
584
Good morning sir. :)I don't like stirring a pot this early in the morning before I had my coffee,buto_O Well...This helped me when I started swinging from rail to rail. For me Burr Browns got rid of the bad juju. I know it will for you too as well. Keep your powder dry Sir.
 

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dl324

Joined Mar 30, 2015
11,926
Where can I find a center-tapped secondary XFMR like this?
Have you considered putting two independent secondaries in series? Or using a switching regulator to generate the negative voltage?

My go to switching regulator is MC34063 because I bought hundreds of them from a company liquidation.

From On Semi:
clipimage.jpg
The voltage divider of R1 and R2 set the voltage, so it can be variable. If you want to make a tracking regulator, you could set it at 25V and use your preferred circuit to implement tracking. The datasheet says it can handle 1.5A, but you could add an external pass transistor for more current.

You could also generate bipolar voltages from a single winding if you're willing to have lower power efficiency using this circuit. Just ground the node between the capacitors.
 
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Thread Starter

SamR

Joined Mar 19, 2019
2,812
Here is what I have so far. XFMR, rectifier, tantalums, and meter ordered. Already have the regulators and other parts except for the enclosure. I'll see what I need after I put the parts together and bench test it. May have something already.

Edit: Arghh... Those primary windings need to be parallel! Also concerned the meter will unbalance the output.

IMG_0731.jpg
 
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dl324

Joined Mar 30, 2015
11,926
Edit: Arghh... Those primary windings need to be parallel! Also concerned the meter will unbalance the output.
I wasn't sure what you were trying to indicate on the primary side. If you're using two transformers, you need to determine winding polarity.

The imbalance caused by the meter will depend on it's shunt resistance and the current. Those meters usually have 3 connections because the voltage measurement needs to have a ground reference.

My preference is to put the current sense resistor on the unregulated voltage. You'll include the regulator adjust current, but the meter resistance won't affect voltage regulation.
 

dl324

Joined Mar 30, 2015
11,926
Which raises a question for me... Just how balanced do Vcc & Vee need to be?
It depends on the circuit and the opamp. If you're using rail to rail inputs and/or outputs and you want to get close to the rails, then absolute rail voltage matters.
 

sparky 1

Joined Nov 3, 2018
384
The filter section. There are different approaches to ripple that are cost effective.
I see a few power supply boards on Ebay for high fidelity PS.
They have decent rectifier diodes and 4 wide diameter low ESR 50V 10,000 Farad ECaps.
I think audio people like linear power supplies. The older lab supplies list the specs including maximum ripple.
Some of those audio rectifier filter boards look nice, have great specs and are well suited for dual polarity supply.
 
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crutschow

Joined Mar 14, 2008
25,686
Note that only one of the 100 ohm resistors needs to be a trimmer (if wanted).
Just use a 50ohm resistor in series with a 100 ohm pot for trim.
 

Thread Starter

SamR

Joined Mar 19, 2019
2,812
Let me ask another way. Are +Vmax and -Vmin being equal but opposite values dependant on Vcc & Vee being balanced? If so how close, 0.1V, 0.01V, 0.001V?
 
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