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Posted by David Sherman on Monday, December 29, 2008 at 22:01:03 :
In Reply to: sorry dave but posted by copey on Monday, December 29, 2008 at 21:31:41 :
You're right about the connections. The catch is that in the simplified electrical books, they show diodes as simple one-way valves, but they're not. But don't believe me. Try it for yourself. Run some direct current through a diode and measure the voltage. For example, hook up a tail light bulb to a car battery and put a diode in series with it. If the bulb doesn't light, turn the diode around. (If the bulb lights no matter which way you put the diode, the diode is bad) Then measure the voltage across the diode. It'll be roughly .7 volts for a silicon diode and .3 volts for a germanium diode. They don't use germanium diodes much any more because they don't hold up like silicon ones do. They drop less voltage but they cost more, won't stand much heat, and get destroyed by power glitches easier. If you want to go way back in time, measure the voltage drop across an old mercury arc rectifier like they had in pre-war battery chargers. As I remember, they come in at something like 15-20 volts! Vacuum tube diodes are more of a resistance than a fixed voltage drop, so the voltage drop depends on the current more than with solid-state diodes, but they drop A LOT of voltage.
Think of it like this. In a plumbing check valve, there's a spring that holds the valve closed. Until you get enough pressure in the line to overcome the spring, no water goes through. It's sort of the same with the electrical "check valves".
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