Quoting from the print version, “a 1/4W Zener diode should work fine; a 1/2W Zener should work, but is a bit on the borderline.” It seems that while 3.6V 1/2W through-hole Zeners are easy to obtain, there are no 3.6V 1/4W through-hole Zeners, just surface-mount (the largest I could find was in a TO-236AB package.) While the 1/2W ones apparently do work, I’d prefer to use the 1/4W ones…
It would be nice to have a little USB female breakout board with surface-mount components on it for people who want to make USB devices without soldering the tiny 1/4W diodes. This could be similar to SparkFun’s existing USB breakout board but with two SMT Zeners and three SMT resistors.
If I understand correctly, Zener wattage ratings aren’t like resistor wattage ratings – with resistors, a higher wattage means the resistor can tolerate more power, but with Zeners, a higher wattage means it uses more power.
It’s probably a typo. There is no reason I can see why 1/4W Zeners won’t work fine in that circuit assuming the expected 5V is all that will be supplied to the input. There will only be 70mW power used by the zener and that’s well within 1/4 watt rating.
Power dissipation ratings of any component is the amount of power it is capable of handling. Power consumption is the amount of power it uses. The power specs of Zener diodes, resistors, transistors, etc. are all power rating - ability to dissipate the energy safely.
I had it wrong above. According to the book, the Zeners with a higher power rating also have a higher capacitance, and as this is a USB data line, we don’t want to mess up the signal that way. Here’s the section from page 55 of “Practical Arduino”:
One final word about Zener diodes: power rating is critical, but not in the way you might expect. […] most of the time it’s perfectly safe to overrate your parts and use a component with a higher rating than required for this particular circuit. However, in this case that approach can actually prevent the circuit from working because the trade-off in Zener diode design is that as its power rating increases it also exhibits more capacitance […] Capacitance on a high-speed data line is very bad and needs to be avoided or the circuit simply won’t work. In practice, a 1/4W Zener should work fine; a 1/2W Zener should work, but is a bit on the borderline; and a 1W Zener almost certainly won’t work – it will have too much capacitance.
It’s a ridiculously simple circuit – a USB socket gets its +5V power line from the usual place, and the 3.3V data lines use three resistors and two 3.6V 1/4W Zeners to reduce the Arduino’s 5V to 3.3V. This is the circuit diagram:
However, in practice, 3.6V 1/2W through-hole Zeners are hard to find, and 3.6V 1/4W through-hole Zeners do not seem to be available at all. (However, 3.6V 1/4W surface-mount ones are common.) This is why I was thinking it would be nice to have a little breakout board with the USB socket, three resistors, and two surface-mount Zeners, and hey presto, instant USB device. It could be tiny.
There must be other ways to build a simple USB port without such fussy Zeners – I’m open to suggestions for alternative circuits. (For my application, I can’t just use the built-in USB port on the Arduino. I’m using the Arduino to send keystrokes to a different host.)
The ideal solution would be to use an actual USB transceiver chip running at 3.3V, and whatever level conversion is needed. Zeners work, but are still kind of a kludge.