TL431 + LM317 + PNP = confuzion

LM317 out has no controlled sinking capability, it can't drive the PNP, respectively the schematic makes no sense. Particularly there's no feedback path from the output, why do you think it's a linear regular?

Exactly...

I do not understand this device. I am asking here because its really strange. *Supposedly* the mfgr used SPICE tools and models for the parts and found a unusual implementation. Or is BS..

WHy would they make something like this, when it would be cheaper and more regulated to do something more sane tho ?

I refer to the term "linear" as a name VS what it implies hehehe..

Its a very odd circuit. It does work. They are hot sellers. It does seem to hold regulation, sorta, as you would expect from no feedback. Of course it selling means nothing.. Maybe feedback has some downside ? Transit response times ? I dont know.. Its really odd.. A TL431 is really odd. One of the datasheets on ON semi shows a application as a phono preamplifer that drives a speaker, with just a TL431 as the active part. Its a odd one, from another era.

The seller makes a lot of supplies.

This supply is selling well. They seem to sell about 40 a week resetting this when it gets to 0 twice a week.

The TL431 has a really high resistivity divider. This is odd. Thats 70K. I can't find any use of the TL431 with values like that. It can be operated nearly open loop with gain.

I have been trying to break this down section by section and trying to wrap my brain around it. I am still in trouble..

I have a thread going here with more discussion.

Strange behavior.

Here it is physically..

SPICE models.. The TL431 is not easy to model..

Stability constraints for 431 with C load -



Regards, Dana.

i agree. This circuit makes zero sense.

The LM317 can’t drive the PNP.

Where did you find this circuit? Have you tried simulating it.?

Normally one thinks of using PNP to boost current -



Regards, Dana.

Below is the LTspice simulation of a circuit properly using a TL431 to provide a very stable output voltage using an LM317.
It has the advantage that remote sensing from both the output and the ground connections can be used, unlike an LM317 by itself, which can only remove sense the ground.

barry said:

i agree. This circuit makes zero sense.

The LM317 can’t drive the PNP.

Where did you find this circuit? Have you tried simulating it.?

Click to expand...

I bought the linear supply listed above and discovered this weird circuit inside.

I am not smart enough and dont have the tools to simulate it.

Its does mostly work, but has the weird behavior I show in the above vid. Its popular and sellling.

crutschow said:

Below is the LTspice simulation of a circuit properly using a TL431 to provide a very stable output voltage using an LM317.
It has the advantage that remote sensing from both the output and the ground connections can be used, unlike an LM317 by itself, which can only remove sense the ground.

View attachment 176797

Click to expand...

Yep. This is a well established topology and is what I THOUGHT was going on until I looked closer because of the weird instability ringing and high noise.

That 70K of restive divider is weird. Well,, the whole thing is weird. It really makes no sense to me at all.

Xymox said:

I bought the linear supply listed above and discovered this weird circuit inside.

I am not smart enough and dont have the tools to simulate it.

Its does mostly work, but has the weird behavior I show in the above vid. Its popular and sellling.

Click to expand...

Simulators, e.g. LTspice, are free.

barry said:

Simulators, e.g. LTspice, are free.

Click to expand...

But they take experence to use.,. And in this case the model for the TL431 looks tricky to get right..

I was hoping there might be a really experenced SPICE user here maybe with TL431 experence and power supply experence who could sort this out and maybe help me in the right direction.

Honestly, I am good with the device. Strangely its working better for a use I have better then a Lambda LCS-A15 which I recapped and put in a better Vref.. I get 15uV pk-pk noise and +-0.0001Vdc drift with awesome transit response and really low impeadance across a widebandwidth... I DO NOT understand why this $100 ebay linear is working better for my use. That was why I dug into it and discovered this weird circuit. Well, I dug into the circuit to add sensing..

I am good with the supply. Its just really odd and I would like to understand it better. So far no one has been able to answer even the basic question of why use a 10k > 50k > 10k on the adj of the TL431. This is gain right ? Why would you operate a TL431 with gain in a Vreg circuit ? I think the LM317 + PNP is obvious - higher current and lower output impeadance ? But why not just use a power FET or something, why use a LM317 + NJW0302. Why no neg feedback ? It seems like someone either had ZERO idea what they were doing, made a mistake OR maybe they came up with something interesting..

What would happen if I moved that + and - sides of the resistor divider out past the 317 and PNP ? So sensed things out there ? I assume it would self distruct in a fiery burst of ringing..

the main o/p xtor should be an NPN, not pnp, with b-e resistor as shown - on the new b-e positions

Easy peasy said:

the main o/p xtor should be an NPN, not pnp, with b-e resistor as shown - on the new b-e positions

Click to expand...

But this is what is in the circuit ? I am trying to understand what is already working.

Xymox said:

But this is what is in the circuit ?

Click to expand...

The circuit can't work as you posted, so there must be an error in the schematic if it works in reality.

or the circuit has not been copied exactly from the actual hardware ....

Xymox said:

But they take experence to use.,.

Click to expand...

did you try it on your own? It´s not as difficult as one might think. to get used to it.
Give it a try, but start with simple things, like an R-C ...

and difficult "to use" or " to interprete the results"?

Xymox said:

I was hoping there might be a really experenced SPICE user here

Click to expand...

I fully understand that.
And if you are lucky you find someone.
But that´s not the idea of a forum. "helping" does not mean others do your job, but others explain how to do it .. and point to mistakes. So in best case the next time you can do it on your own. A gain for the future.

Klaus

Easy peasy said:

or the circuit has not been copied exactly from the actual hardware ....

Click to expand...

in the photo:
* PNP_pin1 (base) goes to LM317_pin3 (input)

but in the schematic:
* PNP_base goes to LM317_OUT

or am I mistaken?

KlausST said:

did you try it on your own? It´s not as difficult as one might think. to get used to it.
Give it a try, but start with simple things, like an R-C ...

and difficult "to use" or " to interprete the results"?


I fully understand that.
And if you are lucky you find someone.
But that´s not the idea of a forum. "helping" does not mean others do your job, but others explain how to do it .. and point to mistakes. So in best case the next time you can do it on your own. A gain for the future.

Klaus

in the photo:
* PNP_pin1 (base) goes to LM317_pin3 (input)

but in the schematic:
* PNP_base goes to LM317_OUT

or am I mistaken?

Click to expand...

100% correct....

i have redone the schematic... This makes way more sense.. i *think* this is correct...

That makes MUCH more sense, it uses the transistor as a 'wrap around' current booster for the LM317 and the output voltage is sensed to regulate the voltage.

Brian.

Um, will that ckt work ... (?) - I have a few concerns ....

this from the data sheet:


and for completeness ( min load etc ) :