OK, I got it. I was trying to power the opamp from the output of the negative regulator instead of directly from the negative rectifier output. Dumb mistake but now works very nicely and tracks spot on the voltage of the positive regulator output. Less than 0.02V across the span! Definitely going into the final build! Thx Guys!

 

Pinout. Your schematic indicates that the pinouts of the 317 and 337 are the same. they are not.

In the protoboard image, which are the ins and outs, and which TO-220 part is the 337?

ak

 

Already corrected my sketch. Caught that when troubleshooting but thx. In the photo I have some org/yel jumpers from the rectifier to the 317 top/337 bottom inputs and then the top rail is +output from the regulator and bottom rail the - output. Ground rail top and bottom tied together. So from top to bottom, it is +, gnd, -, gnd, for the rails.

EDIT: Also using a single 5k multiturn trim pot for V adjust. Temporarily put a single turn pot in testing the span for a quick response but it is way too touchy. The multiturn 5k works nicely. It has a bit of backlash if you overshoot but much easier to use especially the ones I have on order with the knob on them for adjusting.

Also, there are 1kΩ load resistors on the rails so @ 15V it has a 150mA load.

 

Already corrected my sketch. Caught that when troubleshooting but thx. In the photo I have some org/yel jumpers from the rectifier to the 317 top/337 bottom inputs and then the top rail is +output from the regulator and bottom rail the - output. Ground rail top and bottom tied together. So from top to bottom, it is +, gnd, -, gnd, for the rails.

EDIT: Also using a single 5k multiturn trim pot for V adjust. Temporarily put a single turn pot in testing the span for a quick response but it is way too touchy. The multiturn 5k works nicely. It has a bit of backlash if you overshoot but much easier to use especially the ones I have on order with the knob on them for adjusting.

Also, there are 1kΩ load resistors on the rails.

Next step would be adding a digital pot... and then a direct to WiFi connection to your smartphone... that would be gilding the lily ...

 

I played around a bit with Arduino and Encoders with Digipots but never quite got to the smartphone/wifi stage!

 

Also, there are 1kΩ load resistors on the rails so @ 15V it has a 150mA load.

? ? ?

 

Yeah, I know, overkill! Still using the Triad 2A XFMR which will get placed on the shelf when the 180 mA 5VA gets here. Very pleased with this addition of a 741 voltage follower opamp. It seriously simplified the design problem of setting and matching voltages. Eliminated more costly precision components which I now no longer require and matching of the regulators. What is running now is not matched at all and using whatever resistors I grabbed out of storage without trying to match anything and performing far better. Very pleased at this point and looking forward to the final build. Which I can start now instead of waiting on the precision resistors I have on order. I do want a secondary fuse but may just solder a temporary one in while waiting on the PCB fuseholders I have on order.

 

No, reread #106.

 

OOPS! Off a digit, 15 mA and on each rail so 30mA total load... I'll put more load on it later and see what it does to the output voltage.

EDIT: Put 100Ω 1W (it's pulling over 2W) @ 15V across both rails and the output dropped ~0.3V but opamps are never going to draw 300mA. Kinda dumb to even try it, especially since this XFMR won't be the one used. Oh well... On to finishing the perfboard layout and soldering. Still waiting on the 5kΩ trim pot and there is a shipping snafu. Package made it from CA. to FL. and apparently got lost a week ago and has to be resent... I'll start on it in the morning.

 

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?

Vout=-21.92V @ the rectifier and & Regulator Vout(max)=-20.13. So I've got ~1.45V overhead from Vin to Vout(max) on the regulator so there may be dropout. At that delta V @ room temp it would be a bit shy. The curve looks @ room temp to be ~2V minimum. Preferable 2.5-3.0V. The 317 can handle it but not the 337 even though they have the same curve. Odd but it is what it is.

Next time look at these transformers in stock at DigiKey. Toroidal are much better when building low noise circuits.

https://talema.com/wp-content/uploads/datasheets/Transformer-Catalog.pdf

 

Toroidal are much better when building low noise circuits.

I have never bought and used a toroidal. I know there are some in some of my gear but I guess I never really paid much attention to their advantages or didn't really know there was any real advantage other than weight. Something I should probably look into.

 

Next step would be adding a digital pot...

Be very, VERY careful when using digital pots to ensure they aren't subjected to excessive voltage. Most digital pots that operate off a +5V supply cannot tolerate having any of the pot terminals-- either of the ends, or the wiper-- subjected to any voltage outside the 0-5 volt range, or they emit magic smoke. If I recall correctly Analog Devices makes some with 5V supply and interface connections and the ability of the pot terminals to withstand up to +/- 15 volts (the AD7376 is one example), but I don't recall any that go beyond that.

People sometimes assume that you can simply take a digital pot and plunk it down anywhere in a circuit you'd use an electromechanical pot. Big, big mistake.

 

I had forgotten about that limit since I always used them with an Arduino they were limited to 5V by the requirement for the Arduino.

 

Straightened the bench and then messed it up again with the build. Not quite done but over 90%. 180mA XFMR came in so need to fuse and cord it. Shipping snafu on the 5kΩ trimmer w/ handle so will go ahead for now with the standard Bourne trimmer until it shows. Hopefully finished, tested, and ready tomorrow.

 

Be very, VERY careful when using digital pots to ensure they aren't subjected to excessive voltage. Most digital pots that operate off a +5V supply cannot tolerate having any of the pot terminals-- either of the ends, or the wiper-- subjected to any voltage outside the 0-5 volt range, or they emit magic smoke. If I recall correctly Analog Devices makes some with 5V supply and interface connections and the ability of the pot terminals to withstand up to +/- 15 volts (the AD7376 is one example), but I don't recall any that go beyond that.

People sometimes assume that you can simply take a digital pot and plunk it down anywhere in a circuit you'd use an electromechanical pot. Big, big mistake.

Thanks for the warning OB. I've worked with digital pots before and used them successfully for a couple of projects... And yes, of course I read the datasheet thoroughly before including them in my circuits. Specifically, I've used the AD5245 which has an 8-bit resolution. One of its limitations is that the device has to be powered respecting a specific sequence, which demanded a bit of ingenuity from my part both hardware and firmware-wise.

 

In some cases, a multiplying DAC, which can have 12-bit or better resolution, can be used in place of a lower resolution digital pot.

 

Wired cord and fuse to the 180mA XFMR and terminated to the perf board. Wired it all up and started it up on the Variac. Ran into a couple of problems that required desoldering. Working on that, I noticed a problem with my vacuum desoldering gun and ended up spending a few hours working on it. Power supply still not working quite right so I sketched up the entire circuit with all the changes included. I'll start going through it in the morning. The 270Ω resistor on the 317 adjust wants to burn up. Still reads 270Ω but charred so I swapped it out and may need a higher W resistor. It shouldn't be getting hot so I suspect a wiring problem. Anyway, more to do after my morning coffee.

 

Here is what I am working with now. And it is not adjusting. The 1/2W 270Ω resistor between pins 1 & 2 of the LM317 gets hot. Nothing else on the circuit is warm. Worked perfectly on breadboard. The chip is a LM741 and the resistor on it is 4.7kΩ.

From the PDF:

I don't understand the Iadj. Is that the adjust current x set value of the variable pot? or...

LM337 at top and LM317 just below it.

Red +V
Yel -V
Blu 317 Adj
Wht 337 Adj
Grn GND
1kΩ resistor next to trimmer to GND to try and cool the 270Ω resistor

 

The 1/2W 270Ω resistor between pins 1 & 2 of the LM317 gets hot.

The first datasheet I looked (NatSemi) didn't assign pin numbers to the leads. When looking from the front, pins are Adjust, Output, Input. Since there's a fixed (nominal) 1.25V between the adjust and output pins, 270 ohms will give an adjust current of 4.6mA and dissipate 6mW. If you don't have other load on the regulator, it isn't guaranteed to regulate. Worst case minimum load current is 10mA.

 

Yeah Dennis that's what I thought. No way this thing should be getting hot. I started over this morning. The first design was a bit crowded around the regulators so spacing them out a bit especially if they need to be swapped. Will test regulators before soldering in. I already had to replace a cap and regulator that I had in backwards. My brain doesn't translate top to bottom sometimes. Anyway, no real rush and I am still having problems getting the 5kΩ trimmer with the handle on it. The ones I ordered for the twin power supplies came right in but the second order has been sitting in J'ville for over a week now and the vendor hasn't agreed to reship yet. Ah well...