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#61 |
Banned
Join Date: Jun 2012
Location: NSW, Australia
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Well, that was quick off the mark, Karl! However, what may make it more useful for those want to understand how real supplies do actually work, is to add parasitics for the smoothing cap, and a transformer with real behaviours, rather than an ideal voltage source ... I appreciate that this is "advanced" modelling
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#62 | |
diyAudio Moderator Emeritus
Join Date: Jan 2011
Location: In Transient
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#63 |
diyAudio Moderator
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Hi Frank,
That is advanced modelling indeed and I'll be honest, its an area I haven't looked at (or even seen on any such simulations around here ![]() ![]() Its easy to add a little series impedance to the supply to replicate sagging under load but modelling a transformer and then driving it such that it behaves as a real one ![]() Stick with where its going... we're not done yet. (To model a transformer and its driving voltage source accurately I imagine would be a monumental task... leakage inductance, inter winding capacitance, the combined interactions, saturation behaviour... so that the output reflects accurately for example mains hash and noise. And then model a driving voltage source with all that noise and hash and spikes present. Is that do-able ? Any examples ?)
__________________
Installing and using LTspice IV. From beginner to advanced. |
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#64 |
Banned
Join Date: Jun 2012
Location: NSW, Australia
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Very much so. The busy people at the LTspice user group have done many examples of transformers behaving as closely to the real thing as one would typically want, and then the mains voltage on the primary side can be made as dirty as one likes by adding voltage sources in series with the nominal sine wave, representing the harmonics, etc.
In this thread, Power Supply Resevoir Size, all these considerations were discussed in excrutiating detail, and gootee in particular contributed great reems of useful data, and a spreadsheet ... |
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#65 |
diyAudio Moderator
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Thanks Frank. That's a long thread and I do appreciate there are some extremely experienced users out there.
I'm going to stick with my original plan for this thread and keep building on working examples because that is (from the questions I have been asked) where the beginners are struggling. The more you learn, and the more you find your own ways of doing stuff, and the stuff you are talking about is right at the top level of modelling.
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Installing and using LTspice IV. From beginner to advanced. |
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#66 | |
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders
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Quote:
I think the sim set up is in his web site.
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regards Andrew T. |
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#67 |
diyAudio Moderator
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We can begin to eleborate on the simple PSU. Here is the design brief. We are making a simple amplifier but would like to run the VAS stages and front end from a stabilised and smoothed rail. The main rail is around 28 volts DC (off load) and our target for the auxiliary rail is +40 volts DC. How do we do it ? and can we simulate it ?
Taking our simple PSU above as a base we can now add to it. I've moved the load resistor to the bottom to clear screen space and tidy things up. The regulator is going to be a classic text book two transistor design. For those that want to try and create this, here is the circuit diagram. For those that want to skip that and just simulate it, the file is attached to this post. Notice there are four "net" labels attached to relevant points in the diagram. These will be used to make looking at the 'scope traces more understandable. The 1N750 zener is in LT's library. To place a zener on the diagram open the "component" library as we have done before and look for "zener". Right click the finally placed device and select 1N750 from the list. The blue text saying "4.7 Volts Zener" is a user added note. You can add any text by using the .Aa option. The other options and drop downs in the Window are self explanatory. Make sure your simulation time is set to say 2 seconds and run the simulation. Probe in sequence the four points of interest. You should see this. The traces show the ripple on the various points and the reasonably clean +40 volts (this isn't as much an exercise in best circuit design as showing how to build a circuit up and simulate it). As before, this is a dynamic simulation and all the voltages and currents must be probed rather than seeing them as steady state values (remember... the .op or DCop pnt command is no good for dynamic simulations like this). You can zoom in on any part of the traces... so look at just the ripple of the regulated output. It looks like this. There is much you can do to test the effect of component changes. Try increasing and decreasing the caps. Try other transistors. A low gain 2N3055 for example. Notice how the 2N5550 that we are using doesn't melt or fail ![]() Next, and we will attempt to test the regulator dynamically by attaching a varying load.
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Installing and using LTspice IV. From beginner to advanced. |
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#68 |
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders
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Mooly,
I can just see a little glitch in the regulated output. 3rd pic. The 4th pic looks very different. Is this simply a limit of the regulator? Or is it due to drop out from the ripple on the input?
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regards Andrew T. |
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#69 |
diyAudio Moderator
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Hi Andrew,
I think you are seeing more of a limitation of the way LT (and the PC ?) handles the graphics. Pictures 3 and 4 are from the same run. Picture 4 just zooms in on a specific part of the trace to highlight the ripple more accurately. If you run the attached file in that post and probe the four marked output tags in sequence and then zoom into the regulated output you will see the same. Nothing changes, only the effect of the way LT handles zooming in.
__________________
Installing and using LTspice IV. From beginner to advanced. |
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#70 |
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders
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Looks like you are confirming that the remaining ripple is just as good as this version of the regulator can achieve with this level of ripple on the input.
Not a glitch due to drop out. I think this does show that voltage doubling and even more so with tripling and quadrupling, creates a very ripply supply. Here with rCRC on the input, ripple still comes through on the output. Use the sim to model and compare non doubled to doubled and tripled and quadrupled rectifiers.
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regards Andrew T. Last edited by AndrewT; 25th August 2014 at 11:57 AM. |
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