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TL431 linear power supply

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T3sl4co1l:
Just increase the resistor value, it doesn't need to be that beefy.

C4, C7, D7 and D11 also aren't needed (at least not for a -- so far unstated -- ripple spec).

Tim

not1xor1:

--- Quote from: Cliff Matthews on December 15, 2018, 10:04:26 pm ---Dumb question since 30v 2-watt zener could be $$... Would jelly bean LM317 and small heat sink be just as stable?

--- End quote ---

It depends on the transformer characteristics since the rectified voltage might drop a lot with a "heavy" load (I set the inner resistance of the voltage source to 0.3Ω to coarsely simulate that).
I used just a zener in order to simplify the circuit.

Nothing forbids to replace it with a zener/LT431 + BJT for a shunt regulator or a classical 2BJTs + zener series regulator which behaves as a capacitance multiplier providing low ripple even when the input voltage is too low (I've no idea of the behaviour of LM317 in that case).

BTW I forgot to add a bleeding resistor to discharge the input capacitors.
The capacitor in the constant current source should mitigate a bit the power-on overshot.
Later I'll post an updated circuit.

not1xor1:

--- Quote from: Kleinstein on December 15, 2018, 01:21:07 pm ---D11 is still very useful to allow a lower minimum output voltage. Due to the LED, the OP can not pull the voltage down so far.
One could get away without if D2 and R14 are also left out. The LED D10 current would than be only limited by the OP internal current limit - should be OK with the LM358, but could be too much with another OP.

If worried about the dropout, one could use a separate rectifier and filter stage for the current source around Q7 and Q8. This could give some 1 V extra, as there can be less drop due to ripple.

--- End quote ---
You are right about D11.

A resistor to limit the LED current is mandatory as LM358 can source up to 60mA according to the datasheet (ST) so it will blow a classical indicator led (and hurt your eyes  8) ).
Besides that overloading the output of the LM358 would degrade further its poor characteristics due to the inner thermal gradient.

A charge pump might be used to get an higher voltage rail. In that case the constant current source might even be replaced by a resistor since the current variations through it would be much reduced, that is approximately a range of 55V...32.5V / R vs 25V...2.5V / R (to provide 5...27.5V to the base of the darlington). That would also allow to use a low power zener to supply the LM358 thanks to reduced zener current variations.

David Hess:

--- Quote from: not1xor1 on December 15, 2018, 10:19:21 am ---
--- Quote from: xavier60 on December 15, 2018, 09:51:06 am ---When the op-amp's inputs are reversed, its minimum gain becomes unity.
This can be a problem if there is too much gain elsewhere in the loop.
I know from recent experience.
--- End quote ---

What do you mean?
In the classical floating PSU design, where the opamp inputs are swapped, the overall gain can be set to 0.
--- End quote ---

I think what he is getting at is that the non-inverting configuration with a gain of 1 is less stable than the inverting configuration with a gain of -1 which for noise gain purposes is actually a non-inverting gain of 2.  This is why some operational amplifiers are not stable as voltage followers and must be used at a minimum non-inverting gain of 2 or inverting gain of -1.

A common solution to this is to raise the noise gain by placing a series RC network directly between the inverting and non-inverting inputs.  By doing this for instance an OP37 or LT1037 can be used at lower gains even through they are specified for a minimum gain of 5.

I have occasionally seen this crop up in power supplies where a specific operational amplifier could not be made stable no matter what was done unless the circuit was changed to raise the noise gain.

not1xor1:
Here is a more complete circuit.
Probably you do not need C12 in the real world as the instability of TL431 on startup would last fraction of seconds, but it was slowing the simulation (since Vdd rises slowly the TL431 doesn't get enough bias and oscillates, that agrees with the datasheet).

BTW since the supply voltage of the LM358 is about 5V higher than the maximum output voltage one might even use the second opamp to implement current regulation (worst case max in voltage is Vdd - 2).

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