Author Topic: Super newbie diode polarity question  (Read 1641 times)

vacourtoy

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Super newbie diode polarity question
« on: April 19, 2014, 01:56:18 PM »
Hey all, I'm working on a first build of a Bumblebee, and am building the Road Rage, to reverse the ground polarity. My understanding is that the - lead of a diode is the side with the line drawn on it, and the + pad of a pcb is the square one. However, as the Madbean pcbs are drawn with little rectangles to indicate the diodes, but with little lines next the square pad, making it look like the diode lead coming from the side with the line (the negative lead) should go to the square pad (the positive pad).

You can see in the stock Bumblebee image below, at the bottom of the board. D1 clearly has a line on the side leading to the positive pad.

You can also see it in the Road Rage stock image below that. D3 and D4 on teh far left show a sort of line on the + pad side.

I'm a bit stumped. Anyone able to clear this up for me? Thanks!




RobA

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Re: Super newbie diode polarity question
« Reply #1 on: April 19, 2014, 03:24:46 PM »
The square pad isn't always a positive indicator. It's typically just a way of designating a "special" pin, like pin 1 of an IC. On a diode, the convention I usually see is the square pad is the diode's cathode. It indicates the direction of forward current. That makes the lines on the PCB markings consistent with those on the devices themselves.

Edited to note that when looking at LED's, it isn't always consistent. The usage I see most has the square pad going to the anode of an LED which corresponds to the long leg of the device.
« Last Edit: April 19, 2014, 03:37:45 PM by RobA »
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alanp

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Re: Super newbie diode polarity question
« Reply #2 on: April 19, 2014, 06:18:16 PM »
If you're not sure, best way to check is to trace the board and see which parts connect to the diode pad in question, then check it against the schematic.
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vacourtoy

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Re: Super newbie diode polarity question
« Reply #3 on: April 20, 2014, 11:08:15 AM »
The square pad isn't always a positive indicator. It's typically just a way of designating a "special" pin, like pin 1 of an IC. On a diode, the convention I usually see is the square pad is the diode's cathode. It indicates the direction of forward current. That makes the lines on the PCB markings consistent with those on the devices themselves.

Edited to note that when looking at LED's, it isn't always consistent. The usage I see most has the square pad going to the anode of an LED which corresponds to the long leg of the device.

Thanks, Rob! That makes sense. I'll treat these boards as such.