Can anyone help an idiot with info on diodes?

[compedgemarine]

I need to put diodes in line on a circuit in a boat and am confused. the ones I have are a 1N5408 which are hard to find unless I buy hundreds. I found a 1N5404 at Radio Shack but they show a difference in the Peak Repetitive Reverse Voltage. the one I have is for 1000 V and total capacitance of 25, the one at RS is 200 V with total capacitance of 50. this has me completely confused. the diode is in a 12 volt system working a relay. is the difference going to be a problem or will it work fine. someone out there hopefully understands this stuff.
thanks
steve

[Len Silva]

Steve,

I'm confused myself as capacitance is not normally a part of a diode spec.  If you are just using it to operate a small relay, then 1 amp/ 50 volts should be fine.  RS has a larger 6 amp/ 200 volt (don't know the number) that will work for most anything you need.  It kind of chunky, about 1/4" diameter and about 1/2" long.  If you have the room for it, it should do the job.

Not sure if you need a diode to operate the relay or a protection diode to go across the coil. The 6 amp should work for either.

Len

[compedgemarine]

all the diode does is allow for a single switch to override the three drive switches. it lets you hit one switch to move all three drives and lets you have the three switches to make individual adjustments. the diode keeps the individual switch from cross feeding into the other two. the diode I have-1N5408- is about 1/4" around and 3/8" long so the size is not that big an issue. the whole "Peak Repetitive Reverse Voltage" kind of threw me and I dont know what it does or if it is even a factor.
thanks
steve

[Stan]

Steve: You are using the diode as a steering diode so 200 volts Peak Reverse Voltage is OK. Relay coils can generate high voltages when switched off so higher voltage diodes are used across the coil to suppress the voltage spike. Diodes like that only cost pennies when bought in quantity so a manufacturer just buys the high voltage ones and use them in all locations.

[boogiethecat]

Actually Stan, for  spike supression, high voltage diodes are not necessary.  The spike causes the diode to conduct before it attains any voltage, and never gets above half a volt or so (as long as the diode is fast enough, and for most practial purposes "garden variety" doides are) because the diode goes into a conductive state when a spike occurs, so it's forward volage is all that occurs. (.7 volt or so for a silicon diode, less for a shottky)
So for sipke protection across a coil even a 50 volt diode is more than enough. 
For Steve's purposes it might be better to use the 200 volt diode since it's not known if his coil is supressed,  so there may be spikes afoot...but in general anything you need to do on a vehicle can be done with 50 volt diodes.

Steve, as Stan said, manufacturers tend to buy higher voltage diodes because they work in many circuits that need higher voltage ratings, as well as working fine in lower voltage circuits.  Low voltage diodes on the other hand, only work on lower voltage circuits.  So it's cheaper to just inventory the high voltage ones.
For your purposes, anything rated over 50 volts will work fine.

Diodes do have a capacitance spec, but it is in picofarads and only something you need to worry about if you're designing high frequency or very "fast" circuitry.   Busnuts can ignore that it's there for all practical purposes....

Cheers
gary

[David Anderson]

Steve,

Here is a short primer on diodes.  Actually, it is quite long.

http://www.busconversions.com/bbs/index.php?topic=6186.0

Perhaps it will help, some.

David

[compedgemarine]

thanks guys
I knew someone here would understand this better than me. looks like the one at radio shack will work and I can just get those rather than have to order something.
steve

[Ncbob]

What in blazes were they talking about?

Bob

[boogiethecat]

Just the little guys behind the curtains, pulling the levers that make your world tick, talking shop...  You'd never believe how many diodes work for you in your daily life...