LM324 Power Supply with variable voltage and current

[iMo]

CC LED test in V8.

PS: V8.1 is not good.

[mike_mike]

Something started to work bad, because the simulation blocks without giving any error messages at Transient analysis 50% done.
What should I do ?

Later Edit: a few minutes ago it was working. Now it sais Defcon1, I will google it, but I have no idea how to fix it.

Sometimes it works, and sometimes not.

[iMo]

Post your .asc file.

[mike_mike]

Please find attached the .asc file.

Later Edit: I forgot to add the feedback capacitor on U5.

[iMo]

Here is the V9.
It has problems with 10k input resistors in CC_LED opamp comparator. Why?
100k resistors there work.

[iMo]

Added a Soft Start
V10

[mike_mike]

It is safe now to built the practical circuit and test it with the oscilloscope ?

Later Edit: C15 needs to be non polarized ? The local electronics shop does not have in the stock such big values for non polarized capacitors. This is why I am asking.

[iMo]

C15 4u7/50V is polarized. Could be 1u-10u (based on its value the start of Vout will be delayed).

[xavier60]

Why is the softstart needed?
A better use of the transistor would be as an Under Voltage Lock Out between R25 and ground.

[mike_mike]

Some tests in LT Spice. You can find the pots settings in the image.

I replaced D4 by BZX384B3V6, because I did not found the model for the original diode.

Are the results good ?

[xavier60]

All looks fine to me. The small amount of voltage overshoot after a shallow drop is because the output current  can't be turned off instantly.

[xavier60]

What is D4 for? It could cause under voltage stability problems. It is better if the op-amps stay in control  for as long as possible as the unregulated voltage drops.
Kleinstein's idea from reply #218 might help if there is a problem.

[mike_mike]

What is D4 for?

D4 is for lowering the input voltage for 7812, if the voltage goes above 35V.
I will delete D4, if it can cause problems.

[xavier60]

Keeping the 7812 safe is important also. What input voltage do you expect?

[xavier60]

I used an LM317. It is working near its maximum rated voltage so I have a 10V zenner crowbar protecting the 8V rail and a 100mA polyswitch at the output of the LM317.
When I tested one of my ST branded LM317's, it didn't break over until 70V anyway.

[iMo]

D4 is there in order to drop the 7812 input voltage (as his input is around 35V), afaik.

@MM: you may show the I(R_Shunt) with the peak upwards such you right click on the "I(R_Shunt)" label (trace window), and edit it to "-I(R_Shunt)".

@MM: the slow CC_limit is

I_cc = V_wiper_cc / (R_Shunt * R3/R5) = V_wiper_cc / 2.2

For example 6.908V gives you 3.14A slow CC limit.

The output

Vout = (R12 + R13) / R13 * V_wiper_cv = 2.833 * V_wiper_cv

For example 5.67V gives you 16.06V Vout.

When you want to see the correct voltages in the schematics after the simulation use ".tran 0.2", the "startup" there causes the simulation starts at 0V and it shows some weird uVs then.

[mike_mike]

What input voltage do you expect?

I wanted to use a 24Vac transformer, which has a measured output voltage of about 26-27Vac.
I also have some 30Vac transformers which I can use if necessary.

Using the .tran 0.2 gives me something like the attached screenshot. Probably, I am missing something.

[iMo]

 
You miss the soft start..
Either disable the sstart or use longer tran.

[mike_mike]

I will use longer tran.

It is better to use the 30Vac transformer and an LM317 ?
If I will use the 30Vac transformer, then the output voltage will go higher than with the 24Vac transformer...

[iMo]

You may use higher voltage zener for a larger drop.
The max current through the zener I see is 12mA (except a 250mA 30ns peak which is probably coming from ideal simulation).

The "soft start" is experimental. Some ripples or fluctuation on the input voltage rail may trigger the soft start such it lowers Vout. Thus it still needs some tuning.

[mike_mike]

Ok, then I will not use the soft start circuit.
I think that it is not really necessary...

[mike_mike]

Are there any other modifications needed or I can build and test the practical montage ?

[Kleinstein]

In stead of the soft (delayed) start circuit with the capacitor, reacting on rising voltage, I would prefer the zener / LED in series to D1. It should disable the output if the voltage from the 7812 is too low (e.g. below some 5-8 V, the exact limit depends on the zener and R7). This would work and turn on and turn off.

If really worries about peak current to the zener in series to the 7812, one could also have some series resistance in addition to the zener to limit peak currents and ripple currents.

The buffer for the current limit setting is no really needed - it does not hurt, but just a wire is lower noise (though it does not matter). One could use the 4 th OP for something like temperature supervision (turn off if the heat sink gets too hot, or enable a fan).

For the actual build one should watch out where the shund voltage is sensed - make sure there is no other drop included.

[xavier60]

The LM7812 maximum input voltage rating typically is 35V. The LM317 has a typical maximum input to output voltage difference rating of typically 40V.
An LM317 set for 12V would then have a maximum input voltage of 52V so long as the output is never shorted to ground.
There is also the option of getting the high voltage version, the LM317HVT like I did and got sent blacktopped fakes anyway.

[iMo]

In stead of the soft (delayed) start circuit with the capacitor, reacting on rising voltage, I would prefer the zener / LED in series to D1.

You do mean R1, don't you?

That would work with a zener (ie 5.1V) in the Q1's emitter (and say 3k3 in the collector), it seems.