Tuesday, 29 September 2009

Three step(per)s forward

I "fried" the last stepper motor driver board last night, and touched it up in a couple of places and removed solder balls before hand-soldering the rest. I cleaned it and checked it this morning, plugged it in and it all works. Now I have a triplet of boards at last. Yay!

Although I can understand the routing of the circuit on the boards themselves, I still haven't learnt much about the circuit diagrams. I know that the white dots on the board denote the negative ends of the LEDs, because I worked out the negative end of the circuit diagram, and I now know that VC+ is the 12 V supply to the board and that VCC, the voltage common collector, is the adjusted voltage, which is 5 V on this board. I came to realise that the arrow heads on the diagram don't have anything to do with direction of electron flow
or what is thought of as current; they just mean this part of the circuit carries on somewhere else! Oh, and I now know the symbols for LEDs, resistors, variable resistors, electrolytic and ordinary capacitors. And that's about it.

But I don't know how to count the pin numbers on the chip, for example. And I am confused by the power LED on this circuit - on my boards it is connected straight from the 5 V leg of the voltage regulator, and yet the diagram places a capacitor before it, either an electrolytic or ceramic, depending on which route you take.

Hmm, that really doesn't look like what's actually in front of me.

........................................unless this (below).......................really means this
And if so, why wasn't it drawn like that for clarity?

Finally, as promised, a costings update:

Farnell sub-total from separate orders = £54.88
  • A3982 driver chip, part number 1521717, £4.60 and
  • chip-leg cutter, part number 7256449, £39.90, combined VAT of £6.68
  • IDC header, straight, 10-way, £1.61 each (x 2), VAT of £0.48
No P&P.

Running total (excluding set of washers and screws from B&Q) = £487.51


  1. The up arrow symbol means a power connection, just like the flat bar means ground. All the grounds are connected together. All the up arrows with the same label are connected together.

    The reason for using those symbols it to avoid a rats nest of connections across the schematic. In this case it is pointless as the 5V line from the regulator could have just continued to the LED.

    The order that components are connected to each other on the schematic has no relation to the order on the PCB. I.e. the schematic shows which things are to be connected. At the PCB design stage the routing is done to get the best physical layout.

    In general the order of connections does not have any effect. Everything on the same net (track) is at the same potential (voltage).

    When dealing with high speed signals (say > 1 MHz) or high current (say > 1A) then the inductance, capacitance and resistance of tracks becomes significant. In that case the order they are routed on the PCB can matter. But even then it is rarely indicated on the schematic.

    Decoupling capacitors should be close to the chip they are decoupling. This is often hinted at by placing them close on the schematic, but often they are all shown together in one corner of the page. There are no fixed convensions.

  2. Chip pin numbers are anti-clockwise when viewed from above. Pin 1 can be indicated by dots and chamfers, etc. it varies. If in doubt consult the datasheet.

  3. Ah, that makes a lot more sense. I had assumed things were in order, like the stations on an Underground map, even if the tracks aren't in that position.
    So much learnt, and so much to learn...