TL431 + LM317 + PNP = reg confusion

Thread Starter

Xymox

Joined Jun 12, 2022
11
I recently purchased a linear supply for use with a product i make. I have been looking thru a lot of supplies.. This was off ebay.. Yes, I know,, bad idea..

But its sexy, has a R-Core and I figured I could change things around for better performance.

LinReg3.gif

I do NOT understand the circuit it uses. It makes no sense. YET it seems to have been all picked for some reason. A more traditional circuit would have been cheaper and better performance. So this makes no sense to me..

With no load the lineaer has a normal TL431 instability sawtooh noise on the output. 100mV on a 14V rail Any load over 75mA it abruptly switches to a 3 mV wideband zener like random noise. Which is odd as that could have been avoided easily.

The TL431 is a odd part. The resistive divider feeding the ADJ pin seems to be higher then any values I see for this part. It implies it has gain ? Why ?

Why use a LM317 here ? Why not just use a resistor to load the TL431 and a output transistor.

Why pick the NJW0302 PNP as the output ?

WHy no feedback ?

I have posted in a lot of places attempting to find out why this circuit was used. Its a really weird topology. Maybe there is some SPICE modeling that shows this maxes out some

The weird behavior

The circuit on the board..

IMG_6207.jpgIMG_6221.jpg
 

dl324

Joined Mar 30, 2015
14,686
Welcome to AAC!
I do NOT understand the circuit it uses. It makes no sense.
You're right. It makes no sense.

LM317 won't regulate with no load current. Having active components between the regulator output and the load is dumb.
 

Papabravo

Joined Feb 24, 2006
18,793
The tendency to criticize what you don't understand is powerful. The PNP transistor is referred to as a "series pass element". The purpose of the circuit is to get a higher current output than is possible with the LM317 all by itself. Would a SPICE simulation help you understand what is going on and put your mind at ease?

You do not specify the input voltage or the output voltage. I kind of need to know what I'm working with. The purpose of the TL-431 is to make the supply adjustable.

1655093788620.png
I was partially right. The POT U1 is able to adjust, not the output, but the dropout level. This is a low dropout high current regulator. The 2N5884 is good for Vce of 80 volts and 25 Amperes. The dropout is less than 250 mV.
 
Last edited:

Thread Starter

Xymox

Joined Jun 12, 2022
11
The tendency to criticize what you don't understand is powerful. The PNP transistor is referred to as a "series pass element". The purpose of the circuit is to get a higher current output than is possible with the LM317 all by itself. Would a SPICE simulation help you understand what is going on and put your mind at ease?

You do not specify the input voltage or the output voltage. I kind of need to know what I'm working with. The purpose of the TL-431 is to make the supply adjustable.
YES. A SPICE simulation would be great ! I am not experenced with SPICE and someone experenced with it would be great. From my other postings tho i have learned a really complete SPICE model for the TL431 is a bit tricky.. BUT I have no idea.. https://www.audio-perfection.com/vo...formance-simulation-of-tl431-shunt-regulator/

Its like 25 V in on the cap after rect and it was designed for 12V out but I am doing 14.1...

Its 120V AC

This is it..

https://www.ebay.com/itm/164538779109?var=464117043926

Thanks :)
 

Papabravo

Joined Feb 24, 2006
18,793
YES. A SPICE simulation would be great ! I am not experenced with SPICE and someone experenced with it would be great. From my other postings tho i have learned a really complete SPICE model for the TL431 is a bit tricky.. BUT I have no idea.. https://www.audio-perfection.com/vo...formance-simulation-of-tl431-shunt-regulator/

Its like 25 V in on the cap after rect and it was designed for 12V out but I am doing 14.1...

Its 120V AC

This is it..

https://www.ebay.com/itm/164538779109?var=464117043926

Thanks :)
See post #3. Now it appears we have a conflict between reality, simulation, and theory. How delicious.
I have several different TL431 models as well as a decent collection PMP transistors. I think I just might have a tendency to trust the LM317 model from LT/ADI. I'm interested in alternate opinions.

I can add the input side circuit if I know the details of the transformer and the bridge rectifier as well as the rest of the components of the pi-filter on the output. Since you referenced an audio site and the last time I saw a pi-filter it was for the unregulated power supply of a vacuum tube amplifier, I'm wondering if that is your main application interest.
 

Thread Starter

Xymox

Joined Jun 12, 2022
11
See post #3. Now it appears we have a conflict between reality, simulation, and theory. How delicious.
I have several different TL431 models as well as a decent collection PMP transistors. I think I just might have a tendency to trust the LM317 model from LT/ADI. I'm interested in alternate opinions.

I can add the input side circuit if I know the details of the transformer and the bridge rectifier as well as the rest of the components of the pi-filter on the output. Since you referenced an audio site and the last time I saw a pi-filter it was for the unregulated power supply of a vacuum tube amplifier, I'm wondering if that is your main application interest.
No. I am feeding a digital device with a trio of PWM regs.. Not audio related. BUT it does appear a lot of people buying it might be using it for audio.

The audio site was because there was a lot of discussion of the TL431 that comes up from audio sites on google.

The actual part is marked F KB31 LM431ACZ as i know there are a number of flavors.

I dont have the details of anything more then what I have posted and I have pics. I can work on getting more info. I can pull one apart and take some pics, measuring the transformer and other things might be more work then its worth for me. I think we can figure out this stupid little reg without all that ?

I will get some more pics tonight if that helps..
 

Papabravo

Joined Feb 24, 2006
18,793
In your original post you indicated that you were puzzled by the lack of feedback. As you can see from the drawing and the simulation that may explain why the results are not consistent. Normally there would be a resistor from OUT to ADJ and it would try to maintain that voltage constant at like 1.25V
 

Thread Starter

Xymox

Joined Jun 12, 2022
11
YEP... Schematic wrong.. Gimme a bit and I will correct it. Never trust strange schematics that dont make sense... Oopsie... Embarrassing...

It still has weird behavior that would be fun to understand...

I will get this update but might be a hour.. So it was good to beat me up on this. Stupid me not fully verifying EVERYTHING..
 

Thread Starter

Xymox

Joined Jun 12, 2022
11
That still look all wrong to me. I did just ohm out the 220 R to the output.. That really should be on input side of the 317 right ? The top of that 220 ohm is for sure connected to the linear output. That seems wrong..
 

ronsimpson

Joined Oct 7, 2019
2,161
Looks good to me. R220 will have 1.25V across it when working.
About 3.5V is as low as it will work, because of the voltage loss in the LM317 + TL431.
 

Papabravo

Joined Feb 24, 2006
18,793
I still don't think we have the correct circuit. the pot will now adjust the output voltage, but the output looks horrible!!
1655142890597.png
200 mV of output ripple is not a supply I would use. I still can't figure out how to get 12V of output.
 
Top