Capacitor basics question(s)

Question

Hey again folks, just when I think I am getting somewhere I find something that makes me reevaluate if I have been learning ! So I am back with yet another question!

I was looking at making a "Simon" type toy from scratch using an Arduino and other components. I got the idea reading an early Ciarcia's Circuit Cellar circa 1980-81 where he made one of these and was thinking of doing it with more recent components.

Anyway, in the article he presents a schematic of the hardware tone generator he built for the game and I thought "Cool I can understand this!"... or so I thought lol! What I am lost on is the charging of the capacitor in the oscillator circuit (below, and then a detailed view below that):

I studied this quite a while and realized I had seen it before in Forrest Mims' Digital Logic Projects:

and his explanation:

Where I am lost is how C1 (C in Ciarcia's circuit) is charging. So two questions:

1) I would think the positive side of C1 would be accumulating positive charge, but if pin 4 is low then nothing is going to the positive side of C1?

2) The first answer in this discussion I found of this very circuit says "C1 will get charged through R1.By "charged" i mean lower terminal of C1 will get increasingly positive." However, the "lower terminal" is the negative polarity side of the capacitor - so how is it accumulating a positive charge? Isn't putting a positive charge on the negative side of a capacitor a Very Bad Thing?

I know I am (again) missing something very basic here! I have looked at this for 3 days now and not made progress...Thanks for any help on this!

Robert Opalko

michaelwylie · Answer

The two extremes of the circuit are shown below. For this capacitor, it must be non-polarized, that is, able to hold electrons or holes on either plate. Look at the top circuit, when the output of the leftmost 4011 toggles, the output of the rightmost 4011 will want to go to 0 volts as shown in the bottom picture. Essentially the current flow alternates charging and discharging the capacitor.

jc2048 · Answer

Here are a few examples of capacitor symbols from old books. This is from Troubleshooting Analog circuits by Robert Pease [1991]. The circuit is a simple voltage regulator. He uses a curved plate to denote the 'earthy end' for both of the capacitors. For the polarised electrolytic [100uF], he's also added a plus sign. You might say that this diagram could have been the style of the draughtsperson who drew the schematic or the 'house style' of the publisher, but there are reproductions of circuits hand-drawn by him, so it was also his own personal style. This next is from Practical Techniques of Electronic Circuit Design by Robert Bonebreak. Here we have straight plates and an assumption that the reader would understand what kind of capacitor each was and how to connect them. This is from a decade before the Pease book, even though, to my eye, it looks a more modern style. In this case, we can't know if it was his personal style as his schematics would have been redrawn for publication. Finally, this is the traditional British style. This is from A Beginner's Guide to Radio published in 1964 (6th edition). I still draw my capacitors like that, though I don't bother with the plus and minus on the electrolytic (for blogging here, I try to remember to add a plus sign) and usually reduce the plates of the nonpolarised part to lines just out of laziness. Though a bit off-topic, that last book has the most wonderful illustrations. Here's one of the simple radio receiver that starts the book off

gdstew · Answer

Hi, First these are non-polarized capacitors so they don't have positive or negative terminals. Polarized capacitors like electrolytic and tantalum do have positive and negative terminals and applying reverse polarity voltage to these terminals will cause a very bad (and usually noisy) thing to happen. Schematically, polarized caps should have a + on the straight "line" terminal to indicate polarity in the circuit. So with pin 3 high, pin 4 will be low and C1 will charge at a rate dependent on R1 and the current supplied by the pin 3 output..

colporteur · Answer

I think I can see where the explanation and schematic could be confusing. Is the indication that capacitor C1 is a polarized introducing the confusion? However, the "lower terminal" is the negative polarity side of the capacitor - so how is it accumulating a positive charge? Are you interpreting the capacitor polarity as you would a battery? If a non-polarized capacitor symbol was used would you feel more at ease? If you view the polarization of a capacitor as a manufactures means to and ends would that help? The ratio of physical size to capacity is different in polarized and non-polarized capacitors. Polarized capacitors provide more capacity in a smaller size. MW's michaelwylie explanation of the function of the capacitor in the circuit is more appropriate. It ignores the capacitors polarization and views it just as a charge device. First the capacitor charges in one direction from a positive source and then in the alternate direction for a different positive source. That charge time is dependent on the capacitance and resistance in the circuit. You have a polarized capacitor here because it provides the charge capacity in a small size. That is not to say you ignore polarity on a polarized capacitor. Install them backwards in a power supply for example and they can explode launching them across a room [ I have done this].

michaelwylie · Answer

When I went through school both symbols were acceptable for a non-polarized cap, but if there was a plus sign marked on the symbol with the curved line it was deemed polarized.

gdstew · Answer

Yeah, I can understand your confusion. However I disagree with the books author about just being smart enough to connect a polarized capacitor correctly. Making any polarity related assumptions in electronic circuits leads to lots of "magic smoke" and loud bangs! I don't think I've ever seen a schematic with a polarized capacitor that did not have the polarity clearly marked.

jc2048 · Answer

If you wanted to investigate your questions further, one way might be to use a simulator. The simulator physically models the components and the really nice thing is that you can put voltmeters and ammeters anywhere you want to quickly see what's happening. Here's a quick simulation of the CMOS astable that I've knocked up. The simulator didn't have an inverter, so I threw a crude one together from a couple of MOSFETs [arbitarily chosen from the list]. It won't be a very good inverter [I had to push the supply up to 8V to get it to run sensibly like the real circuit would], but it really doesn't matter for this because you are just interested in the capacitor and how it behaves. Here are the waveforms. You can see how the voltage across the capacitor [VM2] swings positive and negative, so it confirms what Michael W. explained in the comments above about not using a polarized device. The pertinent traces to help further with your question are the two current waveforms. I've shown the currents in both leads of the capacitor for a reason. Have a go at thinking about what that's saying about what happens on each plate of the capacitor as it charges and discharges and see how you get on.

Capacitor basics question(s)

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