Low ripple, low schmipple - Page 1
Forum section: electronics / beginners

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 23, 2014, 12:00:09 am
content: I was thinking about PSs and I watched a video about bridge recitifiers and ripple and I thought to myself okay it's good to have stable, known baselines from which to work. But in the real world isn't the power messy and unclean? So do you have a way of introducing uncleanliness into your power supply? i.e. maybe a clever circuit between PS and project that produces random noise.Or do you take this into account and design accordingly?Or maybe there's yet another instrument that's tailor made to do this?I was thinking about this because I realized that PS have a lot of thought put into them (well the good ones do,some do not, I won't mention names).p.s. came across a funny youtube video, search for olivia and dirty power, (they're selling an analyzer)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2014, 02:31:02 am
content: That's the point of a good power supply, it rejects outside noise.The only reason you would inject noise into a supply is to test the tolerance of a circuit to that noise.Do you mean to ask, how do you know how much noise you can tolerate? This is calculated from the components in the circuit, in particular the PSRR (power supply rejection ratio), of op-amps for instance. You could also measure it, but this is more of a pain.Typically, PSRR tests are performed by coupling a signal generator into the respective supply line, using voltage dividers, coupling capacitors, transformers, etc. The method may vary between parts and manufacturers (for instance, a high bandwidth op-amp might oscillate without bypass capacitors; the test circuit has to accommodate that), but ultimately all that matters is the amount of ripple being injected can be measured, and compared to the output level, the ratio of which is the PSRR of the device under test.These tests are probably normally done with a continuous sine wave, swept in frequency to test the full range required, rather than using a pulse, random (noise), or other kind of source. If the response is linear, the results will be the same in any case, however they aren't always, which is one reason they would keep things simple, by using a single tone test.Tim

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: February 25, 2014, 10:08:39 pm
content: Quote from: T3sl4co1l on February 23, 2014, 02:31:02 amI think you've answered the question.Great pains are taken to have some kind of clean power supply. But in the real world the power that is supplied to the product/circuit/widget might not be so clean. I have been thinking about power supplies and was contemplating how obsessed should I really get over a power supply.Analogies can suck but here goes, you build your race car and you test it on a flat oval, on 75 degree, sunny days and you only make left turns. All's well, then you find your car has problems at Laguna Seca in the cold rain and you notice right turns under heavy braking, with vertical drops are causing performance issues.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 25, 2014, 11:08:31 pm
content: I would consider that lab power supplies are often used to power prototypes and individual circuit elements. A finished circuit should be able to withstand a somewhat dirty power supply, but power filtering is generally not the first part of a circuit you design.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: February 26, 2014, 02:56:57 am
content: On a somewhat related note; the Linear Technology Application Note 101 by Jim Williams; "Minimizing Switching Regulator Residue in Linear Regulator Outputs" (Banishing Those Accursed Spikes) might make for some interesting reading.Here is the caption under figure 5 of the pdf file which shows a noise injecting circuit..."Circuit Simulates Switching Regulator Output. DC, Ripple Amplitude, Frequency and Spike Duration/Height are Independantly Settable. Split Path Scheme Sums Wideband Spikes with DC and Ripple, Presenting Linear Regulator with Simulated Switching Regulator Output. Function Generator Sources Waveforms to Both Paths."This capability of this circuit is beyond any present need that I have personally, but the app-note is also full of interesting, practical info. I've included the pdf below for anyone that may have a need for such a circuit.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 05:18:12 am
content: Quote from: Nerull on February 25, 2014, 11:08:31 pmThis makes sense but for one leap "A finished circuit should be able to withstand a somewhat dirty power supply,"But that's like faith or hope. As a naive enthusiast, I guess everything's literal but I am learning that there's wiggle room and fudge factors (maybe other better terms in your neck of the woods). So where does the wiggle room come from? I'm not asking you, it's more rhetorical. But maybe you understand what I'm pondering. I'm not struggling with it. Just curious about the gray areas.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: March 01, 2014, 05:20:26 am
content: Quote from: fpliuzzi on February 26, 2014, 02:56:57 amI will need to find a good dictionary just to decipher the quote, but I'll have a look for sure.