What is the result of wiring an electrolytic cap in series with a ceramic when parallel was intended?

Question

FINAL UPDATE A reassessment of my power consumption has led me to the solution to all of my problems. I picked up a 5V, 5A power supply (giving me plenty of overhead), reassembled my project, and it is now working as intended!! No flickering or dropouts. I've been testing for nearly fifteen minutes with the new power supply, and with my old setup the voltage drop normally happened in under two minutes if not just seconds. Many thanks again to all who replied.

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Update Thanks to all who responded. As I go more deeply into this issue, it looks like my real problem goes all the way back to bad power consumption calculations. Without going into detail, I went back to basics and realized I had a minimum 3A draw and there was no way I was providing that through a 7805. The voltage drop was the big clue to the bigger problem. For the specific issue about series caps that first answer was on point even if it wasn't my real problem, so I'll pick that as the answer. The rest is purely dummy in me, it appears. Thanks again.

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I been have been trying to diagnose a problem with a series of 7-segment LEDs driven by a MAX7221. My problems have ranged from random flickering to total shutdown. I blamed (variously) bad solder jointd, bad wires, bad power, and bad grounds, but none of those have solved the issue.

In finally looking at my circuit board up close, I think I spotted the problem: The 7221 is supposed to be wired with a 10uF and .47 uF cap across V and GND in parallel. I believe I have discovered my board has them wired in series, which I believe results in a net decrease in capacitance.

Other components not on this board but part of the same project (same power/ground) are working correctly. Could this dumb mistake cause hard-to-find electrical issues and inconsistent behaviors?

EDIT: I rewired the caps as suggested, and the component no longer functions at all. More importantly, though, I'm noticing that after the project starts up, after a couple of minutes or so, my 5V source on my power board drops to about 3.6V - and that's with my problem child component disconnected.

I have soldered up a board to take a 9V supply and power a Nano, then use a 7805 VR to provide 5V to my other components. Testing, I get a rock-solid 4.98V to my components, and 9.1 to my Nano. I run the 5V/Ground wires to a simple strip of headers so I can connect the other components. Is there something fundamentally wrong with what seems to me a pretty basic setup? If I see a sudden voltage drop after the project runs for a few minutes, is that indicative of ground bounce or back-current somewhere?

With everything hooked up, the project draws just over 400mA, which shouldn't be taxing a 9V/500mA supply powering my VR setup.

Something totally crazy is going on and I'm just not wise enough to spot it. Worse still is I'm sure it's a dumb mistake on my part.

Answer 1

If you have a 10µF capacitor in series with a 0.47µF cap, the capacitance would be a little less than that of the smallest (about 0.45µF). In addition, the 10µF cap is likely a polarized electrolytic capacitor, which does not last long when subjected to alternating current -- and being in series with that 0.47µF, likely a ceramic with negligible DC leakage, would subject the electrolytic to reverse voltage if any AC voltage is present.

Yep, it seems like you found the culprit, and need to scrape off the series PCB trace and use jumpers to parallel them to correct the issue.

Answer 2

Is mentioned on other answers, the total parallel capacitance would be 0.45uF. I would clip off the 0.45 uF cap and run a short piece of wire across the two leads of the ceramic cap (short it/bridge the pads together). Then try it. It is highly likely that everything will work fine and you have an easy peasy answer. The 0.47uF ceramic cap is like an insurance policy ("just in case...") component. It will very likely work without it. If not, you can run add back a ceramic cap in parallel across the two legs of the electrolytic cap. Good luck.

Answer 3

It could be but it could also be something like ground bounce if the layout is bad or if there are wires involved.

The capacitance will still be almost half a uF and the impedance relatively low. I'm not as optimistic as the other answers. Fast switching and relatively high currents can cause a plethora of interesting problems when you add a smidge of inductance.