Electronics > Beginners
TL431 linear power supply
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mike_mike:
There should be a NPN transistor where it writes "vdd" (with the base on vdd) ?
spec:

--- Quote from: not1xor1 on December 15, 2018, 09:28:39 am ---some notes:
- The common emitter stage inverts the phase so you have to swap inverting and non-inverting inputs.

--- End quote ---
Thanks- standard error ;D


--- Quote from: not1xor1 on December 15, 2018, 09:28:39 am ---- I've not made any calculations yet, but I think it would be safer to replace some transistors with BD139/40.
--- End quote ---
Good observation. I did look at the BC546/BC556 dissipation which will be 300mW worst case. I haven't done a thermal budget, but the BC546/BC556 limit on the datasheet is 500mW. I am not too keen on the BD139/140 for those circuit functions for various reasons. I was intending to recommend that mike_mike checks how hot the transistor cases are (by the finger test), and if necessary, add small heatsinks- no big deal.


--- Quote from: not1xor1 on December 15, 2018, 09:28:39 am ---- Probably it would be hard to compensate.
--- End quote ---
Not sure why you think this.


--- Quote from: not1xor1 on December 15, 2018, 09:28:39 am ---I'll see if I can simulate it.
--- End quote ---
Be great if you did do a bode plot of the circuit. :)
spec:
mike-mike

I have been away from your PSU thread for a few days, but have now caught up with the latest posts.

Attached is a revised schematic for the 25V, 5A Version 3 PSU with the opamp phase error corrected and other modifications as follows.


--- Quote from: mike_mike on December 15, 2018, 05:46:04 am ---C2, C17 and C15 are 10uF non polarized capacitors ?
--- End quote ---
Yes, they are X7R ceramic types for good high frequency performance.


--- Quote from: mike_mike on December 15, 2018, 05:46:04 am ---Does they really need to have such a big value and they really need to be non polarized ?
--- End quote ---
  Yes, as above, but I have changed the schematic in view of your comments so that all solid capacitors are 100nF except one 1uF.


--- Quote from: mike_mike on December 15, 2018, 05:46:04 am ---R11, R16, R6, R13 could have a more usual value ?

--- End quote ---
R11 was 330k, revised to 220k (330k is a normal value though),
R16 was 24k and is now 240k and must be this value.
R6 is 56R and must be this value (56R is a standard value)
R14 is 14k and must be this value. But you will notice that it is annotated AOT (Adjust On Test). The actual value should be adjusted to give exactly 25V output voltage with the output voltage setting potentiometer set to maximum, but this is optional and up to you. I could have put another zener diode in the circuit to eliminate the need for AOT and then R14 could be a standard value. If you would like this option just shout and I will modify the schematic accordingly.

About odd/unavailable values, you do know that you can make any value capacitor and resistor by connecting them in parallel/series.

mike_mike:
@spec Thank you.
D5 should be a simple zener diode or it is better to use a zener diode and a npn transistor ? Or there can be used an LM317 ?
Kleinstein:

--- Quote from: spec on December 18, 2018, 02:50:41 am ---mike-mike

I have been away from your PSU thread for a few days, but have now caught up with the latest posts.

Attached is a revised schematic for the 25V, 5A Version 3 PSU with the opamp phase error corrected and other modifications as follows.

...

--- End quote ---
The regulator shown looks like it is rather slow with 1 µF at the OP and 100 µF after the transistor.  The more normal values are about a factor of 1000 smaller.  Is the stability (e.g. loop gain) check in a simulation ? With 2 low pass filtering stages and no phase boost there might be a stability issue, at least with a capacitive load.

Even if the BC546/BC556 are specified at 500 mW, a TO92 case still gets rather hot at 300 mW, if there is no fan.
So for the 10 mA current sink at the output the BD139 is a good idea. For the current source for the regulator one should get a way with a TO92 case transistor if the current is a little lower (e.g. 5 mA). This should be sufficient even for 5 A output.
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