Resistors purpose.

[sureshot]

I've like the idea of little in and larger out, quite fascinating the a regulator the size of a T092 package regulator, with the help of a Darlington power transistor can delivery up to 5 Amps of usable current. Or idle on mA or even uA. I need to learn more circuit analysis really. My maths in an engineering application is not to bad, I just struggle with some equations.

[David Hess]

I've like the idea of little in and larger out, quite fascinating the a regulator the size of a T092 package regulator, with the help of a Darlington power transistor can delivery up to 5 Amps of usable current. Or idle on mA or even uA. I need to learn more circuit analysis really. My maths in an engineering application is not to bad, I just struggle with some equations.

The TO-92 parts are pretty limited in power dissipation and the 78L05 and LM317LZ have a pretty high quiescent current in milliamps.  For the LM317LZ this current flows out the output pin so it can at least power the load but for the 78L05, it travels out the common pin so for lower power applications, a different regulator is needed.  It has been a while but I used to use the LP2950 (75 microamps instead of a couple milliamps) in place of the 78L05 where low power was needed.

[JS]

And you are likely to have a very unstable circuit,

This is true for capacitive loads, and the easiest is to use external compensation to stabilize an op amp based regulator, but then it's no longer a beginner project.

It's pretty common to load a supply with a board with plenty decoupling capacitance, so you want a stable circuit for that condition.
I'm struggling on the simulator to get a good one, not hard for voltage only but quite hard for voltage and current for a lab psu. I could share one for voltage only, with three feedback paths gets quite stable in for almost any load and keep precission, once you introduce current it gets really tricky to get the best performance, I'm considering using independent power devices for current and voltage to get the best performance on both cases, but I'm trying to avoid that. I did got to a really nice dummy load circuit, fast and stable in CC and CV modes.

JS

[David Hess]

It's pretty common to load a supply with a board with plenty decoupling capacitance, so you want a stable circuit for that condition.

The wiring inductance and resistance between the output of the regulator and decoupling capacitors is usually high enough that decoupling capacitors should have no effect on stability.  The bulk decoupling capacitor close to the regulator is another matter.

Integrated regulators can cheat to provide stability for low ESR output capacitors close to the regulator by tapping AC feedback from the output transistor structure.  This is the equivalent of adding a series resistor to the output, tapping AC feedback before it, and then tapping DC feedback after it; now the resistor is in series with the low ESR output capacitor raising its ESR as far as stability is concerned.  They cheat!

Note that it is often bad idea or at best useless to put a ceramic decoupling capacitor directly at the output of a regulator.

[JS]

It's pretty common to load a supply with a board with plenty decoupling capacitance, so you want a stable circuit for that condition.

The wiring inductance and resistance between the output of the regulator and decoupling capacitors is usually high enough that decoupling capacitors should have no effect on stability.  The bulk decoupling capacitor close to the regulator is another matter.

Integrated regulators can cheat to provide stability for low ESR output capacitors close to the regulator by tapping AC feedback from the output transistor structure.  This is the equivalent of adding a series resistor to the output, tapping AC feedback before it, and then tapping DC feedback after it; now the resistor is in series with the low ESR output capacitor raising its ESR as far as stability is concerned.  They cheat!

Note that it is often bad idea or at best useless to put a ceramic decoupling capacitor directly at the output of a regulator.

LT3080 suggest a MLCC at the output...

Adding an output resistor and inductor, maybe parallel, maybe series is one of the things I'm considering for mine.

JS

[sureshot]

I thought about this, and to get good results I was considering a multi tap transformer and relay switching windings in and out for different voltage ranges. The benefit being no large voltage differential between high and low voltage ranges. Sound good in theory, but more tricky in practice.

[JS]

I thought about this, and to get good results I was considering a multi tap transformer and relay switching windings in and out for different voltage ranges. The benefit being no large voltage differential between high and low voltage ranges. Sound good in theory, but more tricky in practice.

The tricky part is to go back up fast and smooth when going from CC to CV for a shorted output and a high set voltage, but not really a deal stopper, many done  using just a few parts, usually a center tap transformer is good enough and easy to get. SMPS pre regulator is ideal keeping constant differential voltage and smooth transition between levels.

JS

[sureshot]

I would like to try building a smps, but think its above my skill level at the moment. When i get to grips with circuit analysis in its entirety then i might be ready for that.

[David Hess]

It's pretty common to load a supply with a board with plenty decoupling capacitance, so you want a stable circuit for that condition.

The wiring inductance and resistance between the output of the regulator and decoupling capacitors is usually high enough that decoupling capacitors should have no effect on stability.  The bulk decoupling capacitor close to the regulator is another matter.

Integrated regulators can cheat to provide stability for low ESR output capacitors close to the regulator by tapping AC feedback from the output transistor structure.  This is the equivalent of adding a series resistor to the output, tapping AC feedback before it, and then tapping DC feedback after it; now the resistor is in series with the low ESR output capacitor raising its ESR as far as stability is concerned.  They cheat!

Note that it is often bad idea or at best useless to put a ceramic decoupling capacitor directly at the output of a regulator.

LT3080 suggest a MLCC at the output...

The LT3080 being a modern integrated regulator likely cheats as I described.  Good datasheets say what the acceptable output capacitor ESR is.

Adding an output resistor and inductor, maybe parallel, maybe series is one of the things I'm considering for mine.

That works outside of the feedback loop.

Audio power amplifiers often include a parallel RL network in series with the output (after the point of feedback) and a controlled series RC network in parallel for the same reasons that regulators use them even if they are not explicitly shown.

[JS]

Jensen has an output isolator for audio line drivers, it's basucally a 33Ω with something like 30 turns around it, so it's los impedance for audio but capacitive load of long lines doesn't make it oscillate

JS

[bson]

It's pretty common to load a supply with a board with plenty decoupling capacitance, so you want a stable circuit for that condition.
I'm struggling on the simulator to get a good one, not hard for voltage only but quite hard for voltage and current for a lab psu.

Really?  Here's a quick simulation using an ideal opamp with ro=20ohm (but no internal poles and ridiculous bandwidth - hence one prone to oscillating like mad)...





At lower currents (e.g 2-3A) the op amp can drive the 2SC5242 directly, but at >3A or so the base currents get a little overwhelming.  The 2N3904 can be replaced with something with higher max Ic and good hfe for higher load currents.  Even a BD135.  (The 2SC5242 will dissipate > 100W in TO-3P with a suitable heatsink.)

I've attached the  LTspice .asc... if you want to try to experiment without Ccomp or see how far out you dare move the compensation pole.

The forum software doesn't like the extension .raw, so here are URLs for the files:
http://www.rockgarden.net/download/eevblog/opamp_reg.asc
http://www.rockgarden.net/download/eevblog/opamp_reg.raw

By the way, I think the lack of a flat Vout is a simulation artifact.  I see no reason why it should happen since the ideal opamp has no input bias currents.  It shouldn't be possible in that circuit.
Edit: I think it's a rounding error in LTspice; it should just show spikes during the load switching.  If KiCAD's simulation UI were mature enough to have multiple plot panes I'd use that and ngspice-28 instead...

But anyway, it's just a simulation.

[sureshot]

The only time i had a linear regulator oscillate was when i put two smps in series to get 24 Volts to power the circuit. The voltage was jumping up a volt down a volt. As soon as i powered it from a transformer it was fine. I'm yet to use an op amp in a power supply circuit.

[JS]

@bson: that looks voltage only, as I said that isn't a problem, voltage and current at the same time is harder. Also, changing the load from very inductive to a very capacitive load can change dramatically the conditions. While switching from CC to CV or back, for a load change like a short circuit getting in and out, avoiding overshoot and keeping a fast response is hard, and if you want to add remote voltage sensing you are facing a harder and harder problem. All those should be specs of a reliable lab psu and that's what I'm struggling with, not just a stable voltage or current regulator for a particular load.

I've made a dummy load and then improve it in simulations so a second itteration should be good enough. They are very similar but dealing with PSRR, line regulation and swapping the voltage feedback to regulate the voltage in the load not in the supply makes the device as it is not a straight forward conversion.

I haven't touch it in a while, when I get some time I might as I do want to build one but rock solid and being able to adapt the design for different power levels and precission would be nice, once you get one scaling might be fine if it's carefully designed.

JS

[ayman adel]

how do you know that after rectify it to get about 50 VDC ??

[wasedadoc]

how do you know that after rectify it to get about 50 VDC ??

Hint: Peak of a sine wave is sqrt(2) times the RMS value.