Voltage regulator Question ?

[DarkSky]

The transformer is a Block VB 1,5/2/24

https://www.block.eu/en_EN/productversion/vb-15224

They do not mention outout voltage without load.


edit...

Interesting, I ordered two of these same transformers, so I took the one that was still in the box, put some wire clips on it to connect to mains and took a AC reading... weird ! 35V at both secundaries... seems perfectly normal.

Why do I get 44V DC ?

Even 35V seems a little on the high side, but perhaps the transformer design is consistent with 50% over voltage at no load. Something closer to 25% would be more in line with (general) expectations.

Still the question as to what on earth is going on with the other trafo that's measuring 2 x 44V. My take is that's far enough out of range to qualify as faulty. YMMV.

 

[C H]

I couldn't find a spec on that exact transformer, but their 3.2VA offering quotes an off-load voltage of 150%. So it doesn't seem unlikely that a 1.5VA trafo, with its proportionately smaller wire, has even poorer regulation.

They look very much like Vigortronix branded parts from Rapid in UK, or Rapid own-brand. Rebadged or from the same factory, who knows.

 

[Khron]

Even 35V seems a little on the high side, but perhaps the transformer design is consistent with 50% over voltage at no load. Something closer to 25% would be more in line with (general) expectations.

Still the question as to what on earth is going on with the other trafo that's measuring 2 x 44V. My take is that's far enough out of range to qualify as faulty. YMMV.

I couldn't find a spec on that exact transformer, but their 3.2VA offering quotes an off-load voltage of 150%. So it doesn't seem unlikely that a 1.5VA trafo, with its proportionately smaller wire, has even poorer regulation.

That more or less checks out, if one can expect ~18% regulation on a 15VA transformer.

https://sound-au.com/xfmr2.htm#s113

 

[user 133392]

The absolute maximum input voltage for a 7918 is -35V. (ON/Fairchild data sheet.)
I recall +40V is the absolute maximum for the 7824 only.

As others have said you need to find out why the regulator input voltage is -44V (and +44V.)

@warpie Why two bridge rectifiers? There is a reason for that in some circumstances but as the OP said in this case it was not for the reason I'm about to give.

Two independent bridges fed from isolated windings (as drawn earlier) are a better solution when the load currents between the plus and minus rails are heavily imbalanced. A heavy relay or illumination load on the positive rail, returning to ground, are two examples. A single "fullwave bridge" providing split rails are actually two "full-wave center-tapped" rectifiers. When a load current imbalance occurs, DC is setup in the center tap which, if large enough, will saturate the transformer core and increase even-order hum, even-order radiated hum fields and heat. Using two full-wave bridges avoids DC being developed in the windings and core.

I had never given this serious consideration until I found it in the Texar Audio Prism power supply which puts about 50 LEDs on the positive rail. Though not the ideal solution, and counter-intuitive, increasing the load on the negative rail, as a test, reduced the even-order hum by pulling the transformer core back into its linear region. https://www.proaudiodesignforum.com/forum/php/viewtopic.php?p=15586#p15586 The final solution was to use a switcher.

Edit. Added LM79XX absolute maximum input voltage.

 

[Matt Syson]

There are several factors at play here I would expect. Your mains might not be 230 Volts at the time you measured your circuit then the transformer nominal voltage if what is expected for full load Then the factor for regulation(impedance of it's winding essentially) and yopu probably don't lose as much across the diodes in the bridge at low current. 18 to 22 Volts AC is the preferred range to produce a regulated 18 Volt DC output so 24 is a bit high. Don't forget to use Ohms law (or at least an approximation because you are dealing with AC and ripple voltages) to give the amount of heat your regulators will give, and of course all other components involved!

 

[AngryAudio]

Because you have used two rectifier bridges, you can actually build this as two positive 18V regulators. Get rid of the 7918 part. Just use 7815 in both places. Both regulator circuits are absolutely identical. The output of the positive one is as you expect. But the negative one gets connected a little differently at the output. You connect the positive output to your signal ground. The "ground" side of that regulator circuit becomes your -18V output. Here's a schematic of a 15V version.

 

[C H]

That more or less checks out, if one can expect ~18% regulation on a 15VA transformer.

https://sound-au.com/xfmr2.htm#s113

Without wishing to derail the thread, does this link work for everyone? I've been unable to access Rod's site the last few days. "could not connect to server..." same on phone and PC

 

[PermO]

Link works here...

Never mind the off topic, some very usefull info has been given.

I think I'll just order two 18V versions, exact same size, slightly higher power rating so slightly thicker wire maybe...
10,- euro mistake, **** happens, and I will just keep the 24V in my drawer, with sec windings in series these could become phantom power transformers one day...

I'll trow in some 7918 as well, maybe less prone to quiting as the ones I have now.

 

[warpie]

The absolute maximum input voltage for a 7918 is -35V. (ON/Fairchild data sheet.)
I recall +40V is the absolute maximum for the 7824 only.

As others have said you need to find out why the regulator input voltage is -44V (and +44V.)

@warpie Why two bridge rectifiers? There is a reason for that in some circumstances but as the OP said in this case it was not for the reason I'm about to give.

Two independent bridges fed from isolated windings (as drawn earlier) are a better solution when the load currents between the plus and minus rails are heavily imbalanced. A heavy relay or illumination load on the positive rail, returning to ground, are two examples. A single "fullwave bridge" providing split rails are actually two "full-wave center-tapped" rectifiers. When a load current imbalance occurs, DC is setup in the center tap which, if large enough, will saturate the transformer core and increase even-order hum, even-order radiated hum fields and heat. Using two full-wave bridges avoids DC being developed in the windings and core.

I had never given this serious consideration until I found it in the Texar Audio Prism power supply which puts about 50 LEDs on the positive rail. Though not the ideal solution, and counter-intuitive, increasing the load on the negative rail, as a test, reduced the even-order hum by pulling the transformer core back into its linear region. https://www.proaudiodesignforum.com/forum/php/viewtopic.php?p=15586#p15586 The final solution was to use a switcher.

That's very interesting. I have some circuit where is heavily unbalanced (positive rail is around 250mA while the negative rail is 30ish mA) and I do get a bit of 2nd harmonic (still quite low though but much higher comparing to the fundamental). So, this might be the reason? Hm...

 

[user 133392]

That's very interesting. I have some circuit where is heavily unbalanced (positive rail is around 250mA while the negative rail is 30ish mA) and I do get a bit of 2nd harmonic (still quite low though but much higher comparing to the fundamental). So, this might be the reason? Hm...

The quickest and easiest way to find out is add 220 mA of load to the negative rail.

The current waveform in the center tap that develops is very high in harmonic content as the Texar's FFTs show. The band splitting filters in the audio path used smaller value film and silver mica caps and are high impedance as a result making it particularly susceptible to hum field ingress from itself and the channel mounted above or below it.

 

[ccaudle]

what on earth is going on with the other trafo that's measuring 2 x 44V

The transformer doesn't measure 44V! The rectified no-load DC measured 44V. I went through the quick calculations in post #11, and I assumed that was common knowledge, but maybe it would help to expound on the basics here.

When you measure the AC output of a transformer you are measuring the RMS voltage, which is the equivalent DC voltage which would dissipate the same average power in a resistive load. Historically it references the heating potential of an AC voltage so you can compare the DC voltage required to produce the same heat from a resistive heater.

Because you have diodes between the AC transformer and the capacitors, the capacitors charge up to the peak voltage of the AC waveform. The peak voltage of a pure sine wave is approximately 1.414x the RMS value. The voltage on the capacitors will exhibit a sawtooth pattern based on the current draw and the total capacitance, but with no load there will be very low current draw, just the quiescent current of the regulator circuit, so effectively a constant DC value same as the peak voltage. That is the 44V value that is measured on the DC side of the bridge rectifiers.

This is the first online link I found, full bridge rectification with no capacitor is at the top of the page. Scroll down to the "Full Wave Rectifier with Smoothing Capacitor" header to see the equivalent of the supply under discussion. The peak of the waveform (highlighted with the thick black line) is the 44V in this case.
In the top part of the page (the waveforms without the smoothing capacitors) the Vmax designation would be the +44V, and the Vdc would be the RMS equivalent, which I calculated as approximately 32V, but which PermO measured in post #18 as 35V unloaded. That could be partly caused by the quiescent power of the regulators, or could be variation in AC line voltage at different times of day (for best accuracy the line voltage on the primary side should be noted as well so that can be cancelled from the calculations).

Full bridge power supply explanation at electronics-tutorials.ws

A resistive load of 775 Ohms would draw 31mA from 24V AC, so it would be instructive to put a pair of 1K or 750 Ohm resistors across the secondaries and measure the voltage then. I would expect that much closer to 24V would be the measured result in that condition.

 

[jokeramik]

I did mind the pinout, it is connected the right way.

Also, they are not attached to anything (yet) as I dont expect them to run hot with max 31mA load.

The 44V is with no load, not even a LED

1. The IC´s need 100nF on input and output against oscillation.

2. The voltage difference is 26 volts. There is around 1,1 Watt power dissipation. The regulator ic´s need some cooling.

3. max. input voltage is around 35 Volts on the inputs

Best regards

 

[user 133392]

Not sure DarkSky was actually saying the secondary RMS was 44V but that the resulting peak DC voltage was 44V. Could be interpreted either way...

But 44 VDC/1.414 puts the unloaded secondary voltage at 31V RMS which is +30% over its rated presumably fully-loaded voltage of 24V RMS. The unloaded voltage seems high and the transformer full/no load regulation poor as other have pointed out.

 

[warpie]

The quickest and easiest way to find out is add 220 mA of load to the negative rail.

How would I do this? Is there any faster/ better/ easier way than connecting a bunch of LEDs?

Also, would the imbalance matter in an external power supply (i.e. further away from the audio circuit) scenario?

 

[PermO]

I did do an experiment last night strapping a resistor to ground before the regulator on the working possitive half of the PSU.

At 1K I got it back to 28V, so that would work, though these resistors ran very hot !
I don't want to create a bunch of exes heat in a small enclosure.

Because the intended circuit only requires a few mA at +18 and -18V my thought was that a simple linear regulated supply would be the most economic.

If I tweak this PSU the right way, it will be small, efficient and will not require any heatsinking with the regulators running at 3% of their max power rating.

In the past I've had this happen the other way around, got a 15V transformer for a 15V PSU, all seemed fine, untill I hooked up a good load and the voltage dropped below the requirement of the regulators wich resulted in a bunch of nasty hash.

But I understand now as transformers get smaller the difference between unloaded and loaded gets bigger ?

The smallest transformer in this series is only 2x 7mA !

The reason you don't see these in the US is probably due to the single primary that will only take 230V ?
The larger 1A versions come with dual primary for 115 / 230 V

 

[user 133392]

How would I do this? Is there any faster/ better/ easier way than connecting a bunch of LEDs?

Also, would the imbalance matter in an external power supply (i.e. further away from the audio circuit) scenario?

Just add a power resistor across the negative rail.
Not sure what your negative rail voltage is but if it were -15V a 220 mA load would require 68Ω >1W.
Not sure how distance would affect it overall - since there's a lot about your external supply we don't know - but radiated/induced hum would be reduced with increased distance.

 

[JohnRoberts]

How would I do this? Is there any faster/ better/ easier way than connecting a bunch of LEDs?

Back last century when I was designing rack mount SKUs for Peavey I/we tried to make every SKU work with the same 1A/16VAC wall wart. This sometimes required gymnastics to generate useful power supplies. I recall one SKU that had a relatively high current 5V digital rail while requiring bipolar +/-V rails. The simple wall wart was not center tapped so it was not easy to generate similar voltage split supplies. The 5V rail loaded down the + rail resulting in imbalanced +/- rails. This was before switching PS were widely used (especially by me). I used an op amp driving a power resistor to load down the - rail to bring the two bipolar rails into balance. IIRC I only realized +/- 8V regulated rails for the audio path but this SKU was intended for use in a -10dBV bedroom recording environment so it was adequate.

JR

Also, would the imbalance matter in an external power supply (i.e. further away from the audio circuit) scenario?

 

[Newmarket]

I am not sure measuring the output voltages without a load is valid. It does not need to be a heavy load, just a 10K resistor maybe, but both regulators need a load in order to work properly.

Cheers

Ian

Do they need a minimum load ? I've never considered that with linear regulators (unlike switching devices). The internal circuitry uses a small current. And measuring with a voltmeter/ DMM pulls a finite current, albeit very small in power terms.

 

[user 133392]

Do they need a minimum load ? I've never considered that with linear regulators (unlike switching devices). The internal circuitry uses a small current. And measuring with a voltmeter/ DMM pulls a finite current, albeit very small in power terms.

Yes. The output voltage specification specifies a 5 mA minimum output load for the 78XX-series. This is in addition to the 3.5 mA typical quiescent current measured at the input terminal. They typically will measure within range without a minimum load but its not guaranteed. IIRC ST regulators will go out of regulation with no load. I have a foggy memory that we dealt with an ST regulator having that problem here a decade or two ago.

 

[Philbo King]

Because you have used two rectifier bridges, you can actually build this as two positive 18V regulators. Get rid of the 7918 part. Just use 7815 in both places. Both regulator circuits are absolutely identical. The output of the positive one is as you expect. But the negative one gets connected a little differently at the output. You connect the positive output to your signal ground. The "ground" side of that regulator circuit becomes your -18V output. Here's a schematic of a 15V version.

I've done this before and it worked great. It would be desirable to have less voltage on both inputs. And to use silpads under both regulators before you bolt them to a heatsink.