Diodes and capacitors

In some, but certainly not all power supply circuits, capacitors can be found in parallel with the rectifier diodes.

Just search like this

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To find some examples

For AC transformer secondary rectification the capacitors are usually in the range 10nF to 100nF.

And for diodes on the secondary side of a switching power supply, 220pf or so is not uncommon.

What's the purpose of these capacitors?

My guess is that they are either part of an EMC filter or they are expected to reduce noise caused by diode switching. Or is there some other reason? And can the optimum capacitor value be determined by calculation or simulation? Or is it done by either guesswork or experiment?

Reply to
John Smith
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It's to keep the power wiring leading to and from those rectifiers from turning into a variable-impednce antenna (and modulating any coupled RF signals).

The converse of the idea is useful: one can couple to an antenna for WiFi frequencies, and instead of driving it, just change its impedance in a modulated way: the power requirement for such a gizmo can be low enough for decades of operation on a battery. Its central controller just needs to sense automatic gain control to read it out, given a little excitation from an oscillator in the vicinity.

Reply to
whit3rd

I don't see it done too often in mass manufactured equipment though, when the purpose is to reduce parasitic LC ringing caused by diode switching it seems like one of those cargo-cult things that only audiophiles can "hear."

Reply to
bitrex

Power diodes can have reverse-recovery spikes, like step-recovery diodes, that can shoot ringy EMI into other circuits. A parallel capacitor can reduce the dv/dt and radiation. Soft-recovery diodes don't need this.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

the kind of components Muntz would make you sleep in a cardboard refrigerator box for two weeks for having the temerity to include in a mass manufacture design

Reply to
bitrex

At least in 50/60 Hz power supplies with a 4 diode bridge for AM receivers, such capacitors were quite common.

Many radios had an antenna connector for an unbalanced random length wire antenna. Such an unbalanced antenna system really requires a ground connection. Some receivers did not have a ground connector or the user did not provide a ground connection wire.

The antenna coil cold side was connected into the internal signal ground (=negative DC voltage). On older receivers with a CT secondary and 2 diodes, the DC/signal ground was connected directly to center tap and there is a constant capacitance between the secondary CT and primary. From the primary there is a direct connection to mains neutral and ultimately to a grounding electrode. Thus the antenna had a constant low impedance (capacitive) connection to ground all the time.

Look at a 4 diode bridge with a large storage capacitor. The diodes conduct only at peaks of both half cycle and there is a low impedance connection to the secondary and from there a capacitive connection to primary and finally real ground. The antenna has a good ground connection as long as the diode conducts.

However, when the diodes do not connect, there is no path from DC ground to transformer secondary and further to the real ground. At this time, the unbalanced antenna doesn't work well and the switching between good and bad grounding is heard as buzz.

Putting capacitors across all 4 rectifiers, there is a good grounding path, regardless if the rectifier is conducting or not.

Reply to
upsidedown

John Smith wrote

That is what I learned way back, forward step top mains sinewave distortion when charging big electrolytic caps. So indeed a measure against EMC. That is why they called it 'rattle caps' here, Supposed to eliminate 2 x fmains rattling noise in radio reception.

Simulations are not reality, I rarely use those. Understanding is the issue, the human neural net can do a lot better than some lines of code. Experience is what counts.

Building and testing real circuits is always required.

Reply to
<698839253X6D445TD

You can solve a nonlinear system of discrete time partial differential equations in your head? I'm impressed.

Assuming you want to have customers I suppose most of them will appreciate being provided with a physical product at some point.

Reply to
bitrex

that's just handwaving. If you can't explain to another engineer by what methodology you came up with your "optimum value" it probably isn't. It wouldn't past muster in any other engineering discipline.

One of the main reasons computer simulations were developed is because many physical problems, e.g. nuclear weapon implosion chain timing/lens topologies are non-optimizable either on paper or simply by thinking about them. There are many variables that interact in a complex non-linear way, change one of them and the problem becomes radically different, and no amount of "understanding" or experience with some different set of parameters will necessarily help you come up with a new solution any better than chance.

The human neural net is great at coming up with topologies but really terrible at figuring out if the optimum snubber is 100n or 150. How the f*ck should I know? If it's exactly the same circuit with the same other components as I've seen before then that's not designing that's copy-pasting.

If it's not then all bets are off. If the strategy is "use 100n in all circuits that look sort of like this" then that's ballparking, not optimization.

Reply to
bitrex

Makes no sense. It is possible to make a simulation for say some RF circuit to be as exact as you like. It is also possible however once you build it it won't work. I am not questioning the validity of math, merely the application of it with incomplete knowledge of reality.

In that last case it is religion, like you religiously hug your spice.

Same thing as earth was flat, and obviously you fall of when you go to far.

In physics, almost ALL discoveries were made measuring things / effects that had NOT be predicted by the known models. Then you can improve your model. But you should never turn things around and take your models as reality.

There are a thousand things that come into play when designing circuits and these days the software that goes with it, the software that makes it work.

Do not make me laugh, the A bomb was developed when there were no computers. The steam engine worked and changed the world, that math came later. The wheel came first, and pi came later. (see how string theory can put you astray).

Less computer power then my raspberry pi took US to the moon and back. It was, for the Apollo engines trial and error and some spectacular failures.

It will always be that way, or the species is dead.

Reply to
<698839253X6D445TD

His TVs worked in big cities close to transmitters, but he skimped on IF gain so they didn't work well in the suburbs.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

I'd like to add something to that.

A while back I was watching a documentary about birds, special birds, some crows and some sort of parrots. It seems these have 'intelligence' in the sense of making and using tools to solve [their] problems (getting food basically).

There were test with those crows in a local setup, and observations in the wild. The crows were presented with a bit of food in a transparent box with all sorts of trapdoors, moving sliders, what not And given a stick and stone ..

Crow walks around the box, peeks into it from several angles, takes stick, disables trap, throws stone on trapdoor, food plunges out.

Apparently it 'understood' the mechanics of the system by just looking at it. The documentary calls this 'intelligence', I think it is something inherent to everybody [especially] in engineering. Really amazing, and BTW the animals did not do any math.

Then they showed the same crows in the wild, cutting of a piece of a leave that had tooth like a saw (one way cutting), using that to dig larvae out of old tree wood. Fascinating. So, THAT neural net configuration, is in my view the same problem solving part of the brain we use, and no math required, it transcends that, I am retired, spend much of my life trouble shooting extremely complex systems, am good in that, but same thing I recognized that.

Reply to
<698839253X6D445TD

Power diode reverse recovery isn't well controlled or generally even mentioned on a data sheet.

As an engineer, if another engineer says "that value works" that's fine with me. Don't civil and aeronautical engineers do what works? And avoid things that fail?

I don't have to understand it; I only have to make it work.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

They're held to a higher standard because in civil and aeronautical engineering sometimes things just work until the day they don't and the only reason you know it's the day they don't is some people are dead.

The 2018 indictment against Schlitterbahn wrote that Henry and Schooley "lacked technical expertise to design a properly functioning water slide" and did not perform standard engineering procedures or calculations on how the slide would operate. Instead they used "crude trial-and-error methods" to test its performance, out of haste to launch the ride. According to court documents, Schooley conceded that ?If we actually knew how to do this, and it could be done that easily, it wouldn?t be that spectacular.?

It was a spectacular, innovative waterslide with the slight downside that it killed people from time to time.

IMO the logical error in this siltation isn't that computers aren't extremely useful for optimizing circuit parameters it's that something like a snubber capacitor even needs to be well-optimized for the majority of applications.

All engineering decisions are a three-legged stool tradeoff between cost, quality, and time, and while experience can't really help in picking precise component values it likely is pretty good at giving a general idea of what any particular stool should look like. That is to say experience helps you not waste time.

Reply to
bitrex

Put a different way as they like to say in the software industry - "premature optimization is the root of all evil."

Reply to
bitrex

Hmm, If you read Feynman about his work in the manhattan project. Some of what he did was coordinate a bunch of people (mostly women) crunching numbers on adding machines.. Early human powered computer. Slow, but got the job done.

George H.

Reply to
George Herold

I really believe that any non-trivial design should linger for a while in a state of high confusion. Firming up a design too soon tends to miss interesting opportunities and lock in clumsy architectures. But most engineers and all managers hate uncertainty and zip their way through a defined design phase.

--
John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Engineers have self-esteem too and want to feel like they're making progress. States of ambiguity often doesn't feel much like "actual work."

Reply to
bitrex

+1 exactly correct m
Reply to
makolber

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The same thing tends to happen in trouble shooting. You figure you know what's wrong, and head in that direction, where I've learned to just relax, and get more data. That can help you not make the wrong assumption.

George H.

Reply to
George Herold

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