Hi, lurked for a while, finally registered to ask.

Every time I open a book on basic electronics projects, I'll see a pile of projects that depend on using diodes for a variety of reasons. Sometimes they want to exploit the forward voltage, sometimes they are just protecting the circuit from back voltage. And I'm not even talking about Zener diodes.

Here's the thing. There is usually a part number in the textbook that bears no resemblance to the part numbers on my local store shelf. As a rank beginner, I'm invariably put into this improvisation situation. I walk down the aisle, and I see dozens of diode values. For example, the store displays three items next to each other: 1N4001 (1A to 50Vpk), 1N4004 (1A to 400Vpk), 1N4007 (1A to 1000Vpk). All the same price, all the same as well.

Assuming my needs are always in the 12V or TTL range, less than 0.7A from the +12V pole, and I only care about digital signalling, is there a reason these are not functionally equivalent? As a digital guy, it seems like I would just want a yes/no world, but I guess diodes aren't as simple as all that. Is it better to go with the 50Vpk than the preposterous 1000Vpk for some reason I don't understand?

Don't worry, I'll ask the same dumb question about digital switching transistors and power transistors next! : )

 

At low voltages, the whole 1N4000 series will act about the same. It's when you get into things like higher voltage doublers/triplers/etc that the PIV really comes into play.

But even in low voltage applications, let's say you built a power supply and used a couple of rectifier diodes with low PIV, and you get a transient through the transformer due to a lightning strike - a low PIV diode might get "zapped" by the spike.

1N4004's will take care of most of your low-voltage low-power rectification needs.
If you want to use diodes for signal switching, use 1N4148 or 1N914 diodes instead; they're limited to much less current than the 1N4000 series, but switch on and off far more quickly, and have comparatively low capacitance.

If you need really low forward voltages, look to Schottky diodes. These generally have much lower PIV's than silicon diodes, but the forward voltages are 1/2 to 1/3 that of the silicon diodes. Also, they are quite fast.

 

Thanks, SgtWookie. I totally forgot about the "time to switch" consideration. I'll look for 1N914 for general digital stuff. I'm not TOO worried about lightning damage, just boneheaded newbie mistakes.

 

A 1N914 and 1N4148 fast signal diode diode switches hundreds of times quicker than a 1N400x slow rectifier.

 

For example, the store displays three items next to each other: 1N4001 (1A to 50Vpk), 1N4004 (1A to 400Vpk), 1N4007 (1A to 1000Vpk). All the same price, all the same as well.

Years ago, price of 1N4007 > 1N4004 > 1N4001.

Why not mark up the price on 1N4001 & 1N4004 to that of 1N4007 so people who have no clue on which diode to use would simply follow the schematic and go for the 1N4001 and pays the same as 1N4007.