Electronics > Beginners
TL431 linear power supply
spec:
This is a genuine question asked out of interest.
Has anybody ever built a variable power supply that has actually worked using a TL431, not simulated but in the flesh. My experience with the TL431, and family, is that unless they are in a nice benign position, like the reference for a switch mode PSU, they are not happy.
spec:
mike-mike
I have done a sketch for a PSU that replaces the TL431 with an opamp, but otherwise uses the architecture and components of the TL431 circuit. Hopefully I will be able to post the schematic in about 17 hours from now.
By the way, there seems to be an uneasy relationship between the voltage regulation and the current limiting function.
If you like, just remove the current limiting LED indicator from the circuit and see if the PSU settles down (the current limiting function will be completely disconnected and not involved in the circuit).
David Hess:
The only reason I have not used the TL431 that way is why would I bother when better performing and easier to use operational amplifiers are plentiful? It is routine in isolated switching power supplies however.
The problem is a little like using the 723 or most switching regulator controllers where stabilizing feedback to the inverting input comes from the transconductance output instead of the low impedance buffered output. The TL431 has a transconductance output, despite what the datasheet specifications show, so gain varies considerably from device to device and with load. If you build a series regulator with it, then the variable biasing conditions are altering the transconductance unless steps are taken to prevent it so the frequency compensation becomes difficult to control.
A better question might be *why* the TL431 operates so benignly when used as a simple shunt regulator. It will normally have a relatively large load capacitance which rolls off the gain of its transconductance output for dominant pole compensation which is exactly how a transconductance amplifier is compensated. But in a circuit where it controls a series regulator, that cannot be done if fast response is desired and instead feedback from the squirrely output to the input implements the frequency compensation because people are used to working with voltage feedback amplifiers. It can work as the 723 shows but can also be hit or miss.
This brings up an alternate compensation scheme; place a series RC network from the TL431 output to ground.
not1xor1:
--- Quote from: xavier60 on December 12, 2018, 10:19:33 pm ---
--- Quote from: mike_mike on December 12, 2018, 05:58:49 pm ---Thank you.
But I need a power supply made by me. I am a kind of maniac in electronics.
I need that the power supply to be variable via potentiometer and stabilized.
--- End quote ---
You have not mention if you have tried adding compensation to the TL341 or not.
That first power supply circuit has the best chance of working properly than most others you are likely to find.
There are a few more small improvements that can be made. For example the overload LED can be moved to a better spot.
The BD140 isn't necessary.
--- End quote ---
The BD140 BE resistor sets a coarsely constant current through the TL431.
BTW 1kΩ is too much. 330-470Ω would be more appropriate and might even help mitigate the self-oscillation trouble as 1mA is the rated minimum current for TL431.
The BD140 at the maximum voltage allowed for the TL431 would dissipate approximately 250mW. It might be replaced with a BC327 if the ambient temperature (inside the PSU case) is not too high.
not1xor1:
--- Quote from: spec on December 12, 2018, 05:21:36 pm ---No probs :)
I will have a look for a suitable PSU, but I am quite busy at the moment. A number of members are experienced in PSUs, perhaps they could recommend some suitable designs as well.
But I can recommend a complete lab PSU that will do your job for around £47UK, including shipping (the transformer for a DIY PSU would cost more than that). The lab PSU provides 0V to 30V and 0A to 10A, with digital readouts for both voltage and current. There are controls for coarse and fine voltage output and the same for current. I have three of these and use them for electronics, battery charging, you name it.
https://www.ebay.co.uk/p/Adjustable-DC-Power-Supply-30v-10a-Variable-Precision-Digital-Lab-PSU-Clip-Cable/23025566378
You can probably get this generic lab PSU delivered to Romania.
--- End quote ---
From the pictures (if you look through the case aeration slots it looks empty inside) I guess it is a switching PSU.
The linear ones are better as they can be easily improved by replacing the electrolytic caps (they are low quality and too low voltage) and adding a proper TO3 heatsink (as minimal cure).
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