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You may well be right, but the 10 ohms and the two 2mF caps might suggest otherwise.? Anyway, the circuit works without change if a FET with much lower Rdson is used.

I presume the regulator is not designed for high current outputs, but for small signal low noise circuits. Just a guess, as it's not my circuit.

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On 03/03/2021 10:24, John Woodgate wrote:

Anything that's disabled by? 100 megohms in parallel with 1 k is surely unacceptable. I still say that the 2N7002 is not a good choice of FET because of its Rcson being rather high.

That's all obfuscation. I think the OP was trying weird things to figure out the startup issues, which are correctly addressed by a proper startup circuit, as demonstrated in the example.

The 2N7002 Rdson does not impact the circuit because it's operating in a linear mode (source follower) and has 6V across it anyway. I presume the regulator is not designed for high current outputs, but for small signal low noise circuits. Just a guess, as it's not my circuit.

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Anything that's disabled by? 100 megohms in parallel with 1 k is surely unacceptable. I still say that the 2N7002 is not a good choice of FET because of its Rcson being rather high.

On 03/03/2021 00:07, John Woodgate wrote:

I can't imagine what this is supposed to do, even if it would run. Did you mean R7 to be 100 megohms?

For anyone that's interested, I've uploaded a fixed version of boid_twitty's regulator circuit with a proper start-up circuit. This kind of regulator should always include one.

With added .measurements that demonstrate that indeed it does provide 110dB rejection of the input ripple.
45216607d4_puzzle_fixed.zip

--
Regards,
Tony

On 03/03/2021 10:15, LV wrote:

There are lot of strange peoples...
I knew one who draw circuit schematics in MS Word.

This is true. I worked with guy many years ago that used MacDraw for schematics and PCB layout. Almost everything else he did was top class. I learned a lot from him except that.

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Regards,
Tony

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On 03/03/2021 00:07, John Woodgate wrote:

I can't imagine what this is supposed to do, even if it would run. Did you mean R7 to be 100 megohms?

For anyone that's interested, I've uploaded a fixed version of boid_twitty's regulator circuit with a proper start-up circuit. This kind of regulator should always include one.

With added .measurements that demonstrate that indeed it does provide 110dB rejection of the input ripple.
45216607d4_puzzle_fixed.zip

There are lot of strange peoples...
I knew one who draw circuit schematics in MS Word.

P.S.
Sorry for offtopic.
--
Best Regards
Victor

On 03/03/2021 10:01, LV wrote:

LTSpice (analog simulation) and ModelSim (digital simulation, for example) is a different tools for different task.
Like spoon and fork. Normal people will use spoon for soup and fork for steak.

Not when the only tool they have is a hammer.

--
Regards,
Tony

LTSpice (analog simulation) and ModelSim (digital simulation, for example) is a different tools for different task.
Like spoon and fork. Normal people will use spoon for soup and fork for steak.
--
Best Regards
Victor

RL, remember that all replies have no "context", unless you provide one.? When you wrote, "That's the interesting thing", we don't know WHAT is the interesting thing, and we don't know which message you replied to.

(I think your reply is about why the capacitor made a difference here.? I don't think there is any really compelling reason; just that the circuit might go either way.? Adding the capacitor changes the network equations which can have a subtle effect.)

Andy

That's the interesting thing.

In this sim schematic, 1pF or a megohm output contact stops
the output at start-up.

RL

Switched load circuit is part of the issue. Disconnect it - it goes.

RL

Can't help you there. Works out of box with no alterations on this older
and one completely up to date LTspice install. Same behavior for normal
and alt solver.

RL

The non-starting state is also avoided if the switched load circuit is
disconnected.

I recognize that biasing the reference from the output is a bit iffy,
but it's not my circuit. I'm just? interested in the sim's behavior around
these apparently trifling differences and wondering what relationship
it has to reality.

RL

Yes, I do see that.? Connect R5 to VCC instead of the source of the MOSFET and it will work fine.? The circuit isn't "starting up" because it is in a stable state where the output of the op-amp is zero and both inputs are zero,? I understand that you want to provide current for the reference diode, D1, from a regulated source to minimize ripple, but you need to provide some startup current into D1 to get the whole circuit off dead zero,??

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I just found it. I don't know why I couldn't before. Its Rdson is much too high to drive those big caps The circuit works with the? AO6408

John,

2N7002 is a small N-channel MOSFET.

Andy

RL,

It would probably help to get rid of all the switching stuff (V3, R7, S1, V2) and concentrate on just the core circuit without the other stuff dirtying the signals.

Andy

John,

2N7002 is a small N-channel MOSFET.

Andy

I think it is a start-up problem, and that the feedback loop may have more than one stable operating point.? Without the capacitor, it comes up driving the MOSFET ON.? With the capacitor C4, it seems to start up with the MOSFET OFF (I don't know why the cap should make any difference for DC), and finds a stable operating point, with nothing to bring the MOSFET ON.? The op-amp's inverting input pin has the extra 10K resistor, and the input bias current of the op-amp seems to go up quite a lot when its inputs are both low, and this drives the op-amp's output low so it never turns ON the MOSFET.

I can't think of an obvious reason why C4 would affect that.? Maybe things are so very close to teetering either one way or the other, that the slightest change alters which way it goes.

You can change which "state" it comes up in, by adding an .IC statement.? That's why I think the circuit has multiple stable operating points.

Andy

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I can't imagine what this is supposed to do, even if it would run. Did you mean R7 to be 100 megohms?

A filter circuit simulation was offered in ABSE, uploaded as 45216607d4_puzzle.zip

If an output transient load section, or a component C4 is connected, the circuit
won't start. Gives miraculous ac filtering, when off, but curious to know the
simulator mechanics behind this odd effect in sim schematic.

RL

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Where is the 2N7002 in LTspice? I can't find it.

RL,

Please make sure to upload complete schematics including custom symbols.? Yours is missing a special custom symbol for the 2N7002.? (Why, I don't know.)

Andy