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In my recent study of electronics, I chance upon a guide on building a line-following robot. In one of the diagram shown, I realized that the capacitors, namely, C1, C2, C4 and C5, all marked with red arrows, are connected with the wrong polarity. This is from my recent understanding of capacitors. I have listed my observations and reasoning below.

Kindly help verifies if my understanding is correct. I have tried contacting the author but to no avail ;(

My observations:

  1. The symbol indicates electrolytic capacitors in use, which are polarised capacitor. The flat side of the symbol should be the positive terminal. However, the curve side(-ve) is connected to the power source(+ve) instead.

  2. C3(marked with green arrow) is connected correctly, at least IMO(beginner).

  3. I think this is important as the reverse voltage in electrolytic capacitor will cause self-destruction of the oxide layer and fire.

Here is the link to the guide: http://www.circuitstoday.com/line-follower-robot-using-8051-microcontroller

Diagram from [circuitstoday.com][1]

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The capacitors marked red are too small to be electrolytic. They must be ceramic. They just used same symbol for all capacitors except without the plus sign.

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It's actually a common misconception that the curved side means negative or that the curve signifies an electrolytic. The + sign is supposed to be used to indicate polarity. Any specific meaning of the curved side, if there is one, depends on the situation.

IEEE Std 315-1975:

2.2.1.1B - For style 2, if it is necessary to identify the capacitor electrodes, the curved element shall represent:

a) The outside electrode in fixed paper-dielectric and ceramic-dielectric capacitors;

b) The moving element in adjustable and variable capacitors;

c) The low-potential element in feed-through capacitors. (IEC Preferred)

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  • \$\begingroup\$ Interesting -- Just to be a nit-picker, is this quote from the "reaffirmed" 1993 edition? \$\endgroup\$ Commented May 30, 2019 at 12:58
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    \$\begingroup\$ @CarlWitthoft Yes, same in the original and reaffirmed edition. \$\endgroup\$ Commented May 30, 2019 at 15:13
  • \$\begingroup\$ +1 for technical correctness, but I would find it difficult to identify a polarized capacitor for which the curved side (according to this spec) is not the negative. Electrolytic capacitors are usually (always?) made such that the cathode is the outer electrode. Variable caps aren't (usually?) polarized. \$\endgroup\$ Commented May 31, 2019 at 14:34
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It looks like lazy symbol use. Those motor noise suppression caps and the crystal caps should be ceramic caps which are non-polarized. You'll have a hard time finding 33pF or 0.1uF electrolytic caps anyways. I don't think they exist.

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  • \$\begingroup\$ thanks for your speedy comment. Ceramic cap is what I thought too! \$\endgroup\$ Commented May 29, 2019 at 5:18
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    \$\begingroup\$ Not so much "lazy" as just "old fashioned". IIRC all capacitors were drawn that way in the 1950s. Polarization was shown with a + added to the symbol. There are still some web sites showing this, e.g. rapidtables.com/electric/Capacitor_Symbols.html (but there are no references on that site for the source of any of the information!) \$\endgroup\$ Commented May 29, 2019 at 15:34
  • \$\begingroup\$ @alephzero Interesting \$\endgroup\$ Commented May 29, 2019 at 15:35
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    \$\begingroup\$ 0.1μF electrolytic capacitors are easy enough to find. \$\endgroup\$ Commented May 30, 2019 at 0:22
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    \$\begingroup\$ The surface of the GND is curved -- just ask Buzz Aldrin :) \$\endgroup\$ Commented May 30, 2019 at 10:25

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