Chapter 1: Beginnings
Chapter 2: Further Beginnings
Chapter 3: Hello World
Chapter 4: More On GPIO
Chapter 5: Interrupts
Chapter 6: More On Interrupts
Chapter 7: Timers
Chapter 8: Adding Some Real-World Hardware
Chapter 9: More Timers and Displays
Chapter 10: Buttons and Bouncing
Chapter 11: Button Matrix & Auto Repeating
Chapter 12: Driving WS2812 RGB LEDs
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In circuit board design you often need flexibility. It can cost hundreds or thousands of dollars to respin a circuit board, so I need flexibility for two main reasons:
And so we have jumpers and DIP switches and zero-ohm resistors:
Jumpers and DIP switches are quick, but if I want something more permanent and reliable, I use a zero-ohm resistor — sometimes called zero-ohm jumpers. Plenty of companies sell them.
Here's the deal: you have to read the fine print. How close to zero do you need in your circuit? If you're working with milliamp signals it probably doesn't matter. If you have hundreds of milliamps or a few amps, well, be on your guard. Here's what a few common manufacturers promise:
And then there are the specialty high-current zero-ohm jumpers:
The problem with these is that they're pricey. Digikey carries the Rohm high-current jumpers, but even at 100 quantity they're still in the 30 cent range. The Susumu YJP jumpers are 20 cents each at 100 quantity but are only available in the 0603 size. Vishay's CRCW-HP series seems to be the least expensive at 14c each in 100 quantity from Mouser.
I recently needed a bunch of 1206 high-current jumpers (16 per board) and ran into this dilemma. And then I had an epiphany. "You know what, this is stupid," I thought. "There is no such thing as a zero ohm resistor. What I need to look for is 1206 resistors below 5 milliohms." (At the time, I hadn't found the CRCW-HP series yet, and they're 5 milliohms max, which is just on the hairy edge of what I needed for my application.)
So I found the TT Welwyn LRMA series 1 milliohm resistors, 20c each in qty 100. Problem solved!
I still think there ought to be a more inexpensive and reliable way to add or remove an electrical connection without having to incur a significant voltage drop — but I haven't found any.
The same type of issue comes into play when you're considering fuses, or connectors, or wires, or even circuit board traces, for that matter! When you are working with more than about 100 mA, read the fine print, and make sure the current-carrying capacity and series resistance are adequate for your application.
© 2013 Jason M. Sachs, all rights reserved.
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