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I was looking for some reference and I found this resistor symbol:

enter image description here

All the resistors in the schematic (several sheets) have the same symbol, so I concluded that it means nothing, but I'm not sure. What does the dot mean?

Jonas Stein
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rfariasc
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    Can't say I've seen it before, is it from a circuit likely to use resistors in DIP packages or similar? Just wondering if it's so when drawing the schematic you can make sure all of one side is connected to a particular net etc without having to do pin swaps later. – PeterJ Jun 13 '14 at 06:34
  • Did all the other resistors with dot they have one (or more) digit(s) after comma? Have you observe a non-standard value? I'm asking this because I have seen some military and avionics schematic diagrams with similar symbol and actually there is a trimmer not for adjustment purpose, but to obtain a particular resistance i.e. a fix resistor. Or may be indicate a precision resistance. – GR Tech Jun 13 '14 at 11:14
  • I concur with @PeterJ- he should put this into an answer. It's an alternative to forcing pin numbers to show on parts where the orientation may matter physically, but not matter (much, or at all) electrically. – Spehro Pefhany Jun 14 '14 at 14:25
  • Looks like someone climbing up a rope to me... Maybe a COPD way to make sure all the colour bands end up the right way on the board... – Trevor_G Jan 04 '18 at 03:14

3 Answers3

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The dot is not standard. It is a marker on the resistor symbol to indicate pin 1. The only reason I can think of when this is useful is when using the circuit for spice simulations so that you know what side to probe for pin 1 or the orientation for current.

Update: Asked around some more and found someone who does this on schematics! The answer they gave me as to why: "it helps when debugging the board". They place the dot on the printed circuit board as well as in the schematic. That way when they are debugging they can quickly determine which side of the resistor they are probing without having to trace the line.

Avoid this style. You will confuse people!

Paul
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  • If the schematic in the question above is from a simulation tool, this is the correct answer. I believe some flavors of spice have the dots on non-polar components like resistors, and IIRC, Ansys Simplorer (or whatever it's called today) has them, too. – zebonaut Dec 29 '15 at 22:18
  • The only practical reason I can see for a dotted resistor, is if it is part of a mult-element device. Only to make it easier to wire later (tie all "dotted" ends to Vcc, then when laying out the PCB, routing is a lot less messy.) – rdtsc Dec 29 '15 at 22:48
  • Many decades ago it was sometimes used to indicate a NTC thermister sometimes used in bias stabilisation of Ge output stages. – Autistic Dec 30 '15 at 08:07
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It may be an old thermistor symbol (see below): -

enter image description here

That was my initial thought then I noticed a device called a barretter in the picture above so maybe it's a version of that. I googled barretter and it seems to be similar to a thermistor.

On the other hand it could be sloppy drawing of a winding of a transformer showing the coil polarity.

Andy aka
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  • I can recall a barretter was used in Strowger telephone equipment in the UK. It was iirc like a small lightbulb, about 2.5 in long and 3/4 in dia, with two filaments in one glass envelope. Its function was to compensate for different line lengths in the 'local loop' - the circuit from the local exchange to the subscriber's premises - by means of its resistance having a positive temperature coefficient – peterG Jun 14 '14 at 02:43
  • @peterG - that makes sense because I read yesterday they were used as AM demodulators - the carrier-wave cyclic heating and very low thermal mass made them into a demodulator. Maybe the phone line current controlled some kind of AGC ??? – Andy aka Jun 14 '14 at 08:12
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    The barretta was the AGC. Kept the line current approximately constant, and hence the voltage across the carbon-granule microphone in the telephone handset. That was as sophisticated as it got in the Strowger days! @Madmanguruman Thanks - good catch - I intended it to be a comment not an answer, of course. Finger trouble. – peterG Jun 14 '14 at 13:47
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    WOW!! i know i have been working outside of circuits for a few years (or decades), but i didn't know at all that the zig-zag symbol for a regular fixed-value resistor was deprecated officially. who is saying that? some IEEE standards committee? it will take me a little getting used to before i am comfortable with the long rectangle symbol for a resistor. i would reserve the rectangle symbol for a *"generic two-terminal device"*. – robert bristow-johnson Dec 29 '15 at 22:06
  • @robertbristow-johnson - I was taught the rectangle as the new correct symbol for a resistor while in high school about 25 years ago ... but now working in the USA I find that the zig-zag is still the most commonly used. – brhans Dec 29 '15 at 22:11
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I use the resistor dot notation to designate pin 1 of the resistor. Since all pins need to have a unique number this allows you to tell which side of the resistor a particular net is connected to.

Furthermore, pin 1 / the dot, is placed on the net with either:

  1. The higher voltage or normally higher voltage
  2. The transmitting side of the resistor - for example: pin 1 would be on the master side of a MOSI SPI bus

Typically this means the dot is at the top or left side of the resistor.

user315440
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