Electronics > Beginners
Polarized caps in lieu of non-polarized?
Zero999:
I take your point: at least a ceramic capacitor won't blow up, but it would be easier if they just specified the rated voltage, at the rated capacitance or at least 90% of it.
eamoex:
Okay thanks. Kind of beyond my understanding.
To get down to facts... I have this 12V switching power supply and I would like to power another device in parallel which takes 6V (means I could carry one power supply unit to power both devices). I know the 12V supply can deliver up to 10A. The device using 6V draws between .5 and 1.5 Amps. Is it even a good idea to use this transformer like that? And if the configuration is viable, are electrolytic caps good for filtering?
Siwastaja:
--- Quote from: Zero999 on July 02, 2019, 07:26:44 pm ---I take your point: at least a ceramic capacitor won't blow up, but it would be easier if they just specified the rated voltage, at the rated capacitance or at least 90% of it.
--- End quote ---
Look at the actual data: almost no X7R cap can provide 90% of rated capacitance in almost any practical condition. At least 90% is available at nearly 0V DC bias, at room temp. No one uses the capacitors in such conditions.
The reality is dire, you can read on the forums how X7R is good and Y5V is bad, then you design in an X7R without looking at the curves just to find out it loses 80% of the capacitance as well, and well before the rated voltage!
I've been thinking about it, how to put it in one number. For example, they could define rated voltage so that the capacitance is at least 50% of the zero-bias capacitance. But then they would likely want to have a surge voltage rating because it could be much much higher.
Or, they could rate the capacitance at, say, 80% of the rated voltage, and then rate a capacitance separately at no bias. But they do exactly that (well, at least the decent guys do; some won't, just don't use their products), in the form of the curve set.
In any case, I can't figure out any practical and usable way to turn this into a single capacitance and a single rated voltage number. A bare minimum would require two voltage ratings and two capacitance ratings, and it would suck because a Digikey product engineer wouldn't figure this thing out until maybe year 2030. They could be doing it right now, from the curves, but they don't.
So, like with any complex device, we are stuck with a set of curves.
Completely different to tantalums, which can be simply rated with capacitance, voltage, current and temperature rating, and the simplicity is proven by the fact that the manufacturers give you a fixed voltage derating factor (typically 0.5 or 0.6), which they basically admit is just a factor between their measurement standards, and the expected reality.
Siwastaja:
--- Quote from: eamoex on July 02, 2019, 07:31:35 pm ---To get down to facts... I have this 12V switching power supply and I would like to power another device in parallel which takes 6V (means I could carry one power supply unit to power both devices). I know the 12V supply can deliver up to 10A. The device using 6V draws between .5 and 1.5 Amps. Is it even a good idea to use this transformer like that? And if the configuration is viable, are electrolytic caps good for filtering?
--- End quote ---
I wouldn't bother adding the extra filtration unless you actually have identified a noise problem.
If you do end up needing filtration, there is no easy answer, as filtering SMPS noise is a fairly complex subject. The appnote you posted doesn't give enough information IMHO.
Often a simple, cheap electrolytic cap does fine. It has many positive aspects, like you can't accidentally create massive voltage spikes with it like you can do with large MLCC's, or it won't accidentally crack and turn into a red hot heater element, like an MLCC can. So it's almost positive that the cheap elcap won't make things worse.
It may not be enough as is, but worth trying if you have identified an issue. The biggest minus is the instability of the ESR, so even if it seems to be filtering properly, then it might stop filtering at cold weather.
eamoex:
This post I understand :)
Thanks everyone
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