apzx

Split & Single Rail PSU Question
« on: February 27, 2020, 08:58:03 PM »
So, I've done some reasonable searching and did not find anything particularly relevant to what I was looking for unfortunately.

I get the general gist of making a split power supply from a CT transformer. That is not the hard part for me at the moment. The issue I am encountering is trying to get a third, single ended power rail off the same transformer if possible. Essentially, what I'm thinking here is to use an appropriately rated bridge rectifier and followed by the filter capacitors. After the capacitors is where I want tap the voltages for this. For example the common LM317/337 configuration. However, what I was really wondering is can I also use the voltage across the filter capacitors if I ignore the center tap ground and leave the single supply just purely across the transformer? Example schematic below.

More than likely I'll end up using some sort of dual primary & dual secondary toroid transformer for the job and as long as I select an appropriate VA on the transformer would this work? Now, I feel I should mention that the reason I'm asking this question is two fold. First, while I am not sure on the split rails current needs (yet) I know that the single ended supply needs to be able to provide nearly 4A. Second, I have a couple of projects that I am working on that will require something like this anyway so just trying to figure out what my options going forward are.

Some other options that I can think of. Use two transformers with the second transformer being probably the same or maybe a little lower in voltage and use a second bridge rectifier except that instead of wired as a CT, just make it one series winding. The other real option I can think of is to use some sort of SMPS for the single ended rail.

So, would I fry the transformer or would things work about how I'd expect them to?



Rocinante

Re: Split & Single Rail PSU Question
« Reply #1 on: February 28, 2020, 12:14:23 AM »
Teekay from TK audio has a good three rail design using 317, 337, and 783 (or lm7506). It was used for various Neve projects. Another thing to consider is the typical dual rail and 48v phantom supplies out there that can easily be converted so that the 'phantom' supply rail can be whatever you want. I've done this for LED's and digital power lines.
If there's a harder way to do this, I haven't found it yet.

moamps

Re: Split & Single Rail PSU Question
« Reply #2 on: February 28, 2020, 03:58:16 AM »
..... Essentially, what I'm thinking here is to use an appropriately rated bridge rectifier and followed by the filter capacitors.....
You forgot a rectifier in your schematic.

PRR

Re: Split & Single Rail PSU Question
« Reply #3 on: February 28, 2020, 02:05:05 PM »
> a third, single ended power rail

Where does it want to be grounded?

These things often work in isolation but smoke when you run audio interconnects.

apzx

Re: Split & Single Rail PSU Question
« Reply #4 on: February 28, 2020, 03:18:32 PM »
You forgot a rectifier in your schematic.



Mea culpa. Basically, that is what I'm aiming for if it is possible.

> a third, single ended power rail

Where does it want to be grounded?

These things often work in isolation but smoke when you run audio interconnects.

The third power rail would simply be across the bridge rectifier (please see corrected schematic above). In order to get the signals out I would AC couple them and use a pull down resistor on one side of the capacitor to keep that side referenced to everything else that would be running on the split rail. I know a short between the grounds would let the magic smoke out things. To my understanding of electronics, this should be okay as long as I keep the circuit being powered from the single ended rail floating. If the AC coupling capacitor were to fail then some magic would escape. To "limit" the potential damage I am still debating if I want to use fuses or resettable fuses to prevent gross runaway of things.

Essentially, what I'm trying to do with this particular power supply is provide power for a drone synth built around a bunch of oscillators like the following. Sorry, that I may have missed a few details that I am doing with the actual oscillator design (moving right now and my main computer is currently packed up along with the schematic for it). But the general concept is there. I've built and tested a slightly simplified version of this, basically without the opamp in circuit and it draws ~10-11mA. So, my guesstimate for each oscillator would be in the region of 20-30mA all said and done, and I'm looking to do 128 oscillators in total. If memory serves with a 2N3904 they have about a 12VDC offset and the actual voltage movement is ~2.5V. There are some other things that I'm looking to implement in the design, but as I really got to thinking on it I would need at least these three power rails in order for it to work properly.


PRR

Re: Split & Single Rail PSU Question
« Reply #5 on: February 28, 2020, 11:10:14 PM »
That doesn't look like an oscillator, except by finding transistors with a rare imperfection.

I sure don't see why it can't be done with single 15V. Or really why it needs many mA.

apzx

Re: Split & Single Rail PSU Question
« Reply #6 on: February 29, 2020, 07:28:32 AM »
That doesn't look like an oscillator, except by finding transistors with a rare imperfection.

I sure don't see why it can't be done with single 15V. Or really why it needs many mA.

The transistor is used in a reverse avalanche condition. Kerry D. Wong did a short little article on it. The oscillation time is set by the the RC time constant set by (in my simple schematic) R1 & R2 and C1. This makes a sawtooth-ish oscillator. I've seen this particular configuration be called a "Negistor" as well, at least it seems to pull more results on the Googles.

And yeah not every single NPN transistor will actually work, but in my quick test board I made I just grabbed a random 2N3904 I had lying around, 10k pot, 1k resistor, 10uF elctro, and a white led. It worked without a hitch, but I did cut the transistor's base lead off (apparently that can help in making the transistor work in this configuration more readily). I had to supply it with about a minimum of ~+12.5VDC for it to start oscillating. Any lower and the circuit would not begin and as I increased the supply voltage it started going faster and lowering the voltage made it a bit slower even to the point where it would not oscillate at all. I believe the main reason for the current being so high is because with say a supply voltage of about 12V and a 1k resistor gives ~10mA or so flowing through the led. Then adding some support circuitry to the mix and the current goes up a bit.

JohnRoberts

Re: Split & Single Rail PSU Question
« Reply #7 on: March 01, 2020, 10:09:43 AM »
The transistor is used in a reverse avalanche condition. Kerry D. Wong did a short little article on it. The oscillation time is set by the the RC time constant set by (in my simple schematic) R1 & R2 and C1. This makes a sawtooth-ish oscillator. I've seen this particular configuration be called a "Negistor" as well, at least it seems to pull more results on the Googles.

And yeah not every single NPN transistor will actually work, but in my quick test board I made I just grabbed a random 2N3904 I had lying around, 10k pot, 1k resistor, 10uF elctro, and a white led. It worked without a hitch, but I did cut the transistor's base lead off (apparently that can help in making the transistor work in this configuration more readily). I had to supply it with about a minimum of ~+12.5VDC for it to start oscillating. Any lower and the circuit would not begin and as I increased the supply voltage it started going faster and lowering the voltage made it a bit slower even to the point where it would not oscillate at all. I believe the main reason for the current being so high is because with say a supply voltage of about 12V and a 1k resistor gives ~10mA or so flowing through the led. Then adding some support circuitry to the mix and the current goes up a bit.
I have never heard of using that, and I like discrete transistor tricks. I found the Wong references but that's about it. It seems too process dependent to rely upon for production unless a manufacturer sells selected parts for that attribute.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

apzx

Re: Split & Single Rail PSU Question
« Reply #8 on: March 01, 2020, 01:16:18 PM »
I have never heard of using that, and I like discrete transistor tricks. I found the Wong references but that's about it. It seems too process dependent to rely upon for production unless a manufacturer sells selected parts for that attribute.

JR

Without a doubt the parts would have to be selected because it relies on the B-E breakdown which is probably not very controlled in the manufacture of the actual devices. Though enough transistors seem to work just fine in the actual circuit. In trying to understand some more about how these things actually work (just for curiosity sake more than anything) I found a paper from Raymond Arthur Kjar at Iowa State University titled Avalanche mode operation of transistors from 1964. In my searches the most common thing I've found about this particular configuration is in making a circuit with extremely fast fall time and making something with an extremely fast rise time is also possible. It is an interesting circuit if all sorts of flawed.

JohnRoberts

Re: Split & Single Rail PSU Question
« Reply #9 on: March 01, 2020, 02:43:16 PM »
Without a doubt the parts would have to be selected because it relies on the B-E breakdown which is probably not very controlled in the manufacture of the actual devices. Though enough transistors seem to work just fine in the actual circuit. In trying to understand some more about how these things actually work (just for curiosity sake more than anything) I found a paper from Raymond Arthur Kjar at Iowa State University titled Avalanche mode operation of transistors from 1964. In my searches the most common thing I've found about this particular configuration is in making a circuit with extremely fast fall time and making something with an extremely fast rise time is also possible. It is an interesting circuit if all sorts of flawed.

forward avalanche is well known (I think) and a typical consideration in power amp design... (secondary breakdown, etc).

Reverse avalanche is far less common, while some bipolar transistors are designed to be used backwards for lower saturation voltage when used in audio mutes, that application is nowadays done with JFETs.

JR

PS: A diac is another semiconductor device targeted to take advantage of it's breakdown threshold.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


apzx

Re: Split & Single Rail PSU Question
« Reply #10 on: March 01, 2020, 09:47:42 PM »
forward avalanche is well known (I think) and a typical consideration in power amp design... (secondary breakdown, etc).

Reverse avalanche is far less common, while some bipolar transistors are designed to be used backwards for lower saturation voltage when used in audio mutes, that application is nowadays done with JFETs.

JR

PS: A diac is another semiconductor device targeted to take advantage of it's breakdown threshold.

Indeed, anything I've read on power amp design forward avalanche is taken into consideration or should be at least.

Now, before I saw this I was not aware of a BJT being able to be used in this condition. Frankly, just looking at the schematic and I I would just think that the capacitor would charge up and cause the led to illuminate briefly and that is about it. But having built it and trying to get a better understanding of things it seems to behave more like a relaxation oscillator.

Circuits like this remind me of the sheer ingenuity of some folks. A couple more that I've scratching my brain with are VCFs in particular the Steiner Parker filter wherein he used 8 diodes and their dynamic resistance with respect to their forward voltage. The other is the ARP 4072 filter and its uses a LM3900 (I believe in another thread you mentioned something about Norton Opamps) plus a bunch of differential pairs to operate. Still trying to wrap my head around how that one works.

JohnRoberts

Re: Split & Single Rail PSU Question
« Reply #11 on: March 02, 2020, 10:06:42 AM »
Indeed, anything I've read on power amp design forward avalanche is taken into consideration or should be at least.

Now, before I saw this I was not aware of a BJT being able to be used in this condition. Frankly, just looking at the schematic and I I would just think that the capacitor would charge up and cause the led to illuminate briefly and that is about it. But having built it and trying to get a better understanding of things it seems to behave more like a relaxation oscillator.

Circuits like this remind me of the sheer ingenuity of some folks. A couple more that I've scratching my brain with are VCFs in particular the Steiner Parker filter wherein he used 8 diodes and their dynamic resistance with respect to their forward voltage. The other is the ARP 4072 filter and its uses a LM3900 (I believe in another thread you mentioned something about Norton Opamps) plus a bunch of differential pairs to operate. Still trying to wrap my head around how that one works.
The Norton amp is pretty much the back half of an op amp with the input long tail pair bypassed. They do somethings well mostly due to the shorter path (with less delay) but fell out of favor when real op amps are so cheap and widely available.

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: Split & Single Rail PSU Question
« Reply #12 on: March 02, 2020, 10:30:12 AM »
Are UJT's (unijunction transistors) still in production?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: Split & Single Rail PSU Question
« Reply #13 on: March 02, 2020, 10:50:04 AM »
Are UJT's (unijunction transistors) still in production?
digikey shows many thru holes are obsolete, but has some SMD parts.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: Split & Single Rail PSU Question
« Reply #14 on: March 02, 2020, 11:08:12 AM »
digikey shows many thru holes are obsolete, but has some SMD parts.

JR
Really? It's ages I haven't seen any in actual use...
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Newmarket

Re: Split & Single Rail PSU Question
« Reply #15 on: March 03, 2020, 04:18:08 AM »
Really? It's ages I haven't seen any in actual use...

Looking from the UK Farnell have a few types in stock - thru hole only.
But I've never used one or remember seeing one used !

abbey road d enfer

Re: Split & Single Rail PSU Question
« Reply #16 on: March 03, 2020, 04:29:40 AM »
I used one in the 60's; I made a foot-operated bass organ. Sounded awful (lots of clicks).
A little later, as I was an intern, my supervisor asked me to investigate the possibility to make a clock for an arctic event recorder. He fancied that an UJT would better behave than a crystal at -40°C. Of course it just didn't...
And I never crossed path with any since...
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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