Electronics > Projects, Designs, and Technical Stuff
Is this really a 100uF cap or marketing BS
JustMeHere:
--- Quote from: trobbins on March 28, 2020, 03:28:50 am ---Perhaps it's a good lesson to learn - 'know' what you are buying.
Are you making up a pcb yourself, or wanting to replace an existing decoupling part on a pcb, and what is the operating voltage?
--- End quote ---
I'm making the PCB myself. Everything works. I accidentally switched my power supply from 5v to 30v and made some magic smoke.
I was testing these caps to see if they were damaged and read them to be 75 uF at most frequency ranges on my LCR. I then measured one fresh out of the packaging. I saw the same thing. That's when I went to the spec sheet to see what the C vs Hz was supposed to be. That's when I saw the graph suggesting the cap only gets to 100uF in a very limited environment. SMPS at 500k might work very well with these.
I got these to help with the sudden power demand the ESP32 can place. There is also a 100nF cap with them to be the high frequency filter.
magic:
--- Quote from: T3sl4co1l on March 28, 2020, 06:43:42 pm ---Well, it does; but if you restrict your thinking to instantaneous charge, you're going to have a hard time agreeing.
--- End quote ---
I straight out refuse to agree on the grounds of pedantry ;D
It seems you just demonstrated that it is indeed a measurement artifact resulting from ignoring ESL and trying to blame its effects on capacitance and ESR. Perhaps it's a convenient simplification for some steady state AC applications, but that's it. It's not even a convenient way of modeling transient conditions in said "steady state" AC applications.
I suppose if you add that L into the equivalent circuit, most if not all of the apparent frequency dependence in C and R will go away.
T3sl4co1l:
--- Quote from: magic on March 29, 2020, 07:20:41 am ---
--- Quote from: T3sl4co1l on March 28, 2020, 06:43:42 pm ---Well, it does; but if you restrict your thinking to instantaneous charge, you're going to have a hard time agreeing.
--- End quote ---
I straight out refuse to agree on the grounds of pedantry ;D
--- End quote ---
That is, a hard time in the sense of: you can do it in the time domain with differential equations, or you can use transforms as I implicitly did above. The agreement is inevitable, as the results are equivalent; unless you're not much of a pedant after all and refuse on non-technical grounds. ;D
--- Quote ---It seems you just demonstrated that it is indeed a measurement artifact resulting from ignoring ESL and trying to blame its effects on capacitance and ESR. Perhaps it's a convenient simplification for some steady state AC applications, but that's it. It's not even a convenient way of modeling transient conditions in said "steady state" AC applications.
I suppose if you add that L into the equivalent circuit, most if not all of the apparent frequency dependence in C and R will go away.
--- End quote ---
Well no, L remains in the expressions, as you can plainly see. :)
What's inconvenient about modeling it? Also, what's being modeled?
For transient applications like SMPS, the simple fact that the impedance is low suffices. Its angle doesn't matter at all. I'm quite happy to use whatever values of capacitors, as long as they do what they do!
Put another way: a ladder network of inductors, if they have alternating large and small values, is still just as good a filter as for any other angle of impedance.
Tim
magic:
L shows up in your expression because you use the RLC model with constant R, L and C to derive a C or RC model with frequency-dependent R and C, which mimics the way naive capacitance/ESR meters work.
The C in your C model is not the same as the C in your RLC model.
Philosophically, we can argue which C is the "true" C of the capacitor ;) I'm going to say it's the RLC C because the RLC model more closely reflects the physical reality, which is plain magnetic inductance existing in the capacitor rather than some magical property of the dielectric or frequency-dependent variation in electrode spacing.
By transient conditions I mean startup, shutdown or abrupt frequency change, when resonance gets out of whack and sudden peaks or dips are possible due to the L. Yes, the models are theoretically equivalent so you could model it in frequency domain by integrating the variable capacitance over all frequencies present in the transient, but in practice it would be a rather masochistic approach.
rsjsouza:
For a few additional insights about construction and a few prscticsl aspects.. Check free_electron's posts in this almost eight year old discussion
https://www.eevblog.com/forum/beginners/esr-meter/
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