Zeroing out a LM317 reference voltage with a zener - PSU build

[chopnhack]

Hi all - continuing on the bench psu build, I have started to play with the design in spice to get an idea of how things are working and what purpose they serve in the schematic. According to the nuts and volts author whose plan I am loosely following, using a zener of 1.225v from the adjustment pin of the LM317 IC can bring the output down to zero. Clever. I cobbled it up in LTSpice to see how it would react and with zener installed I get 2.5v at the output, despite R4,R1 being set low to 1 Ω each.

Am I doing something wrong, or will spice simply not recognize the zener?

The schematic is below as well as an attached file of the LTSpice model if anyone cares to toy with it.
Thanks in advance!

 

[Bluejets]

http://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/

 

[davenn]

I would replace D10 with a 1N4007, the 1N4148 is probably a bit underrated for that position

and out of curiosity, what is the purpose of the lower winding and rectifier circuit ??


Dave

 

[BobK]

The Zener is upside down.

Bob

 

[73's de Edd]

Sir chopnhack . . . . . . .

Ahhhhhh . . .yes . . .one of friend . . . 'ole Robert-o Reeds contributed N&V tidbits . . .but being from sometimes back . . .circa 2007.
BUT . . . . it just seems like you did not follow the letter of the law. . . . in its design concept, if utilizing the N&V unit.
Note where D3 grabs raw AC and filters it with C2 and then introduces it, via R4, to the anode of that Z1 Reference zener diode . . . . .(or if you are a 'po boy, you could use two "series backwards 1N40xx diodes)
That counter voltage referencing then lets you get DOWN below the normal lower limit voltage threshold of a conventional LM317 circuit design.

Can we assume that the otherwise, load simulated , other bottom L3 AC winding of your transformer, is just for a followup negative voltage twin ?

I read back now and see that Sir Dave was also curious:
"and out of curiosity, what is the purpose of the lower winding and rectifier circuit ??"

N&V Power Supply Schematic Referencing :

Thasssit . . . .

73's de Edd

 

[Herschel Peeler]

Hi all - continuing on the bench psu build, I have started to play with the design in spice to get an idea of how things are working and what purpose they serve in the schematic. According to the nuts and volts author whose plan I am loosely following, using a zener of 1.225v from the adjustment pin of the LM317 IC can bring the output down to zero. Clever. I cobbled it up in LTSpice to see how it would react and with zener installed I get 2.5v at the output, despite R4,R1 being set low to 1 Ω each.

Am I doing something wrong, or will spice simply not recognize the zener?

The schematic is below as well as an attached file of the LTSpice model if anyone cares to toy with it.
Thanks in advance!

View attachment 26178

Is the circuit missing a wire from the Zener to the negative supply? this is the option I used. No extra supply was needed. I used a 7660 to generate the -V.

 

[chopnhack]

http://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/

Ah!!! Many thanks Blue - now I have to chase down that rabbit hole. Talk about design creep... I must say that the op-amp supply has me intrigued. I will start researching that later today.

I would replace D10 with a 1N4007, the 1N4148 is probably a bit underrated for that position and out of curiosity, what is the purpose of the lower winding and rectifier circuit ??
Dave

Hi Dave, many parts of my schematic are not of correct spec due to the limited varieties available in LTSpice. The lower winding is going to be a twin supply to the upper - just a dual channel supply.

The Zener is upside down.
Bob

Thanks Bob, apparently as Edd has mentioned I am approaching this in a half backsided fashion. I had tried the zener in both positions to cover my bases when I ran the simulation.

BUT . . . . it just seems like you did not follow the letter of the law. . . . in its design concept, if utilizing the N&V unit.
Note where D3 grabs raw AC and filters it with C2 and then introduces it, via R4, to the anode of that Z1 Reference zener diode . . . . .(or if you are a 'po boy, you could use two "series backwards 1N40xx diodes)
That counter voltage referencing then lets you get DOWN below the normal lower limit voltage threshold of a conventional LM317 circuit design.
Can we assume that the otherwise, load simulated , other bottom L3 AC winding of your transformer, is just for a followup negative voltage twin ?

Hi Edd, thanks for clearing this up! I did not understand the original schematic so well. I thought the extra diodes in the N&V schematic were being used because of the center tapped xfmr. I thought it required him to have a different kind of 'bridge' rectifier. I researched and found that a full wave rectifier can be used on a center tapped xfmr because the center tap acts as the ground for the circuit. On further reading, I saw that in my case using a split bobbin I would have to use a bridge rectifier and thus omitted the other diodes in my circuit. That is how I got to where I am at. I figured I could then simply use the zener to pull the adj pin 'down' by 1.25v.

Bear with me now, if I understand correctly, one of the AC cycle passes through D3 (negative half - I believe) charges the plate at C4, thus filtering it, then this more stable waveform is passed through R4, limiting the current, this more negative than common does what exactly at the zener? I am abit confused here as I don't fully understand the forward/reverse bias of the zener as I should. I will ponder and hopefully return with a response before you do, LOL.

Thanks again all.

 

[Alec_t]

Here's your asc file with a few mods.

 

[hevans1944]

A zener diode behaves exactly the same as an ordinary diode except the reverse voltage rating of a zener diode is the zener voltage. The reverse voltage rating of ordinary diodes can be just about anything, but heavy doping of the PN junction produces a specific reverse breakdown voltage for zener diodes. That transition from non-conduction to conduction is typically "sharper" for a zener diode, occurring at a well-defined and stable voltage. The conductance after crossing the zener threshold voltage is typically greater too. In any case, some means of current limiting is required when the applied reverse voltage exceeds the zener voltage. Unlike ordinary diodes, which are not intended for conduction in the reverse breakdown region, zener diodes are designed to work in this region.

Your circuit does not provide for a negative voltage to be applied to the adjustment terminal via a zener diode, as was done with an auxiliary diode rectifier, capacitor, and current limiting resistor applied to the zener in the original Nuts and Volts schematic. Your zener is NOT installed up-side down, but its anode needs a negative voltage applied with respect to circuit common. The zener voltage will appear across the reverse biased zener diode, and since it is negative with respect to circuit common, that voltage will be applied to the adjustment terminal of the LM317 when R4 and R5 are adjusted to nearly zero resistance.

Unfortunately, there is no simple way to obtain the negative voltage you need from an untapped secondary winding connected to a bridge rectifier because you are already using the "negative" output of the bridge as your circuit common! To get a negative supply for the zener will require a small transformer (a 6.3 VAC filament transformer works fine), a rectifier, a filter capacitor, and a current-limiting resistor for the zener. Sorry 'bout that, but that's the way the current flows.

Edit: changed cathode to anode.

 

[Herschel Peeler]

A zener diode behaves exactly the same as an ordinary diode except the reverse voltage rating of a zener diode is the zener voltage. The reverse voltage rating of ordinary diodes can be just about anything, but heavy doping of the PN junction produces a specific reverse breakdown voltage for zener diodes. That transition from non-conduction to conduction is typically "sharper" for a zener diode, occurring at a well-defined and stable voltage. The conductance after crossing the zener threshold voltage is typically greater too. In any case, some means of current limiting is required when the applied reverse voltage exceeds the zener voltage. Unlike ordinary diodes, which are not intended for conduction in the reverse breakdown region, zener diodes are designed to work in this region.

Your circuit does not provide for a negative voltage to be applied to the adjustment terminal via a zener diode, as was done with an auxiliary diode rectifier, capacitor, and current limiting resistor applied to the zener in the original Nuts and Volts schematic. Your zener is NOT installed up-side down, but its cathode needs a negative voltage applied with respect to circuit common. The zener voltage will appear across the reverse biased zener diode, and since it is negative with respect to circuit common, that voltage will be applied to the adjustment terminal of the LM317 when R4 and R5 are adjusted to nearly zero resistance.

Unfortunately, there is no simple way to obtain the negative voltage you need from an untapped secondary winding connected to a bridge rectifier because you are already using the "negative" output of the bridge as your circuit common! To get a negative supply for the zener will require a small transformer (a 6.3 VAC filament transformer works fine), a rectifier, a filter capacitor, and a current-limiting resistor for the zener. Sorry 'bout that, but that's the way the current flows.

Re: "Unfortunately, there is no simple way to obtain the negative voltage..."

The original post shows a second power supply doing nothing. think the intention was to use this as the negative supply. My method was to was a 7660 voltage inverter but it throws off the 1.25 V reference circuit of the317. Some adjustments are necessary.

 

[davenn]

The original post shows a second power supply doing nothing. think the intention was to use this as the negative supply.

chopnhack has already stated that this will be a second channel the same as the top supply
so just ignore it at the moment and concentrate on any possible issues with the top section

Dave

 

[Herschel Peeler]

chopnhack has already stated that this will be a second channel the same as the top supply
so just ignore it at the moment and concentrate on any possible issues with the top section

Dave

Ah!

 

[Herschel Peeler]

chopnhack has already stated that this will be a second channel the same as the top supply
so just ignore it at the moment and concentrate on any possible issues with the top section

Dave

Is this what the original post was trying to accomplish? I haven't tried it yet. Might it work?

 

[davenn]

not sure what you are trying to achieve there ?
Does it allow the voltage range to go between 0 and max available volts ?


PS I edited you post and uploaded an actual image .... so much easier than everyone having to open a pdf doc

 

[Herschel Peeler]

not sure what you are trying to achieve there ?
Does it allow the voltage range to go between 0 and max available volts ?


PS I edited you post and uploaded an actual image .... so much easier than everyone having to open a pdf doc

Thanks for the editing. I didn't think pdf documents would be a problem. And yes, output down to zero volts. Trying to move the reference down to -1.2 Volts instead of ground.

 

[davenn]

I didn't think pdf documents would be a problem.

they are not a problem per sae ... it's just much better to post an inline image so people don't have to mess around opening documents

 

[hevans1944]

@chopnhack: Maybe you don't need an auxiliary transformer after all. I found this "voltage doubler" schematic on another web site forum that @Bluejets linked in his post #2. It was posted by "Hero999" and you have to scroll way down the page to find it:

Notice this derives -9V from a transformer that is NOT center-tapped and is connected to provide +20 V raw DC output to a regulator from a full-wave bridge rectifier, which is your power supply configuration.

You don't need -9V to bias your zener diode, so D3, C3, and 9.1V zener diode D4 can be eliminated. R1, connected to the anode of D2 and capacitor C2, becomes the current-limiting resistor for your zener. C4 smooths the half-wave rectified voltage appearing across D2. The "-9V" output becomes your -1.25 V zener voltage at the anode of your zener diode. Cathode of your zener goes to your circuit common as shown in your original diagram.

You might want to model or breadboard this circuit. I haven't built or used it before, and have a "bad" feeling about it for some unknown reason. There will be a lot of ripple voltage across D2, but I think R1 and C4 will adequately smooth that out. Be sure to measure current through R1, maybe starting with 1000 ohms and working your way down to a suitable value to deliver about 20 mA to your "zener look-alike".

Is this what the original post was trying to accomplish? I haven't tried it yet. Might it work?

View attachment 26218

It depends for its operation on current from an external load creating a forward voltage drop across D2 and D3, said voltage drop being a function of the current and the temperature of the diodes. The precision "zener look-alike" integrated circuit that @chopnhack will use is temperature compensated and has a much more stable output.

 

[chopnhack]

Thanks Hop. I did look at the schematic when I visited the page and thought, I wonder if I could simply just swap in a 1.225 zener in place of the 9.1v zener in the schematic.

I haven't quite decided what I will do - continue along this line of thinking and produce the simple linear regulator design or go with an op amp design.
What are your collect thoughts all?

 

[AnalogKid]

I wonder if I could simply just swap in a 1.225 zener in place of the 9.1v zener in the schematic.

Yes. Remember to adjust R1 to keep the zener current within its range.

ak

 

[chopnhack]

Thanks ak ;-)