Capacitors in series

Thread Starter

Standisher

Joined Jan 16, 2015
156
Just a general question. Are 2 capacitors in series (e.g 2 x 10nf to produce 5nf) as stable as a single capacitor in terms of capacitance value or would it be better to use a single 4.7nf capacitor for a stable outcome?
 

Hypatia's Protege

Joined Mar 1, 2015
3,228
Just a general question. Are 2 capacitors in series (e.g 2 x 10nf to produce 5nf) as stable as a single capacitor in terms of capacitance value or would it be better to use a single 4.7nf capacitor for a stable outcome?
The principal 'annoyance' attending series connection of high impedance/quasi high impedance components (e.g. capacitors) is the requirement of EMF equalization (aka 'balancing')... While obviation of said requirement may be achieved via use of components specified at or above the (total) applied EMF, such is not always practical (especially as increased operating EMF is generally the objective of such connection)...

Best regards
HP:)
 

Thread Starter

Standisher

Joined Jan 16, 2015
156
Mmmm, that's interesting @Hypatia's Protege. I'm having an issue with higher frequencies generated by a function generator chip (XR2206) where I used two 10nf capacitors in series between two of the ic's pins to give me the 5nf I needed and wonder if a varying EMF could have something to do with the stability issue I'm experiencing when this series capacitor arrangement is hooked into circuit (there is no issue when a higher value capacitor is switched in, in parallel to this arrangement, for lower frequency production.
 

MrAl

Joined Jun 17, 2014
11,342
Just a general question. Are 2 capacitors in series (e.g 2 x 10nf to produce 5nf) as stable as a single capacitor in terms of capacitance value or would it be better to use a single 4.7nf capacitor for a stable outcome?
Hi,

This question probably demands an associated application because the view we have to take in a given app will be innate to the application and therefore some aspects of the connection will be more important than others. Let me try to point out a few.

First, the electric field for both caps is 1/2 of what it is for one cap. That means we may see less DC leakage if we have two caps in series if the leakage is not proportional to the capacitance. This could help in certain applications.

Then we have the issue of creating a third terminal which is subject to more noise pickup.
With that third terminal we also reduce the reliability of the end product by a small amount (which may or may not be negligible) because the reliability is related to the number of external connections.

We may find that the initial capacitance is more accurate if the capacitances of the capacitors being used vary in capacitance around a fixed mean value both positive and negative.

Lastly, if we select two capacitors one with a positive temperature coefficient and one with a negative temp co, we can partially cancel out the temperature drift of the newly created capacitor, although they will have to be hand selected in order to ensure that the two cancel to the desired degree of accuracy over the range of temperature.

So in short depending on what we are looking to get out of this, it may not make much difference or it may may a lot of difference.
 

atferrari

Joined Jan 6, 2004
4,763
Mmmm, that's interesting @Hypatia's Protege. I'm having an issue with higher frequencies generated by a function generator chip (XR2206) where I used two 10nf capacitors in series between two of the ic's pins to give me the 5nf I needed and wonder if a varying EMF could have something to do with the stability issue I'm experiencing when this series capacitor arrangement is hooked into circuit (there is no issue when a higher value capacitor is switched in, in parallel to this arrangement, for lower frequency production.
Have you tried another cap? Tweaking designs with that chip is not simple.
 

Thread Starter

Standisher

Joined Jan 16, 2015
156
Hi All,

This circuit 'seemed' to work perfectly on breadboard but I only tested the series capacitors very briefly to verify the frequency range I desired so I may have missed the fact that the two 'in-series' capacitors I used later on the soldered prototype (incidentally, they were the same values but a different pair) were causing the issue I now see. For example a frequency will rise to a set frequency of, say, 175kHz (verified with oscilloscope) but slowly falls and continues to fall at a rate of a few Hz per second. When I switch a second, higher vale capacitor (470nf) into the circuit to achieve the lower frequency range of 20Hz to 2kHz everything is just fine and there is no frequency 'drift' whatsoever. I've covered all the other bases I can think of (such as ensuing good grounding, good connections/wiring, correct components etc, etc) and the only area of doubt I am left with is this series capacitor arrangement between XR2206 pins 5 & 6 in the attachment. I've ordered some 4.7nf capacitors so that I can swap the two 10nf series capacitors with a single 4.7nf but if I'm barking up the wrong tree. From what has been said so far it certainly seems a possibility worth investigating.I guess it is OK to attach someone else's base circuit without their permission as it was in a YouTube video so is already in the public domain .

The only change I've made is to use different value capacitors to achieve different frequency ranges and, as I say, everything 'seemed' to work fine on breadboard with this capacitor selection .
 

Attachments

WBahn

Joined Mar 31, 2012
29,932
I'm a bit skeptical that the drift you are seeing is due to the series capacitors as dissimilar leakage is usually more a factor at very low frequencies, particularly DC.

I would be more suspicious that the behavior you are seeing is more related to the circuit just behaving differently at 10x to 100x the frequency output.

What happens if you just use one of the 10 nF caps? Do you still see the drift? This should put you at a frequency range that is close enough to the original one that the circuit behavior should be pretty similar.
 

Thread Starter

Standisher

Joined Jan 16, 2015
156
I haven't desoldered anything yet @WBahn as I'm waiting for the 5nf capacitors to arrive so I don't know if a 10nf would produce different behavior. The thing that bothers me is that everything worked perfectly on the breadboard (certainly until I tried out the series capacitors briefly). I expected, if anything, for the circuit to more robust when soldered on a pcb (albeit a prototype board). It's almost as if there is some frequency 'leakage' for want of a better word. :confused:
 
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