# Thread: Diode Polarity (This is REALLY dumbass :) )

1. ## Diode Polarity (This is REALLY dumbass :) )

Ok, I've just got my pcb's from mastero and was kinda trying to finish the supplies. However, I've come to soldering the diodes on and i've got a bit stuck... How do you tell the polarity of a diode without building a tester circuit (I'm too lazy ) The diodes have a white band on one end, but no triangle markings indicating the flow of conventional (positive to negative) current. If someone can tell me what the band is (positive or negative), and if i'm on the right track, I'd be very greatful!

Cheers

Andrew

2. the band/line is the cathode (negative)

cheers
pcman

3. Doh! Time to unsolder all the diodes i've just wired in the wrong way round...

Cheers - I would have been able to look it up in my physics textbook, but we haven't been given them yet - the year above nicked most of them!

Andrew

4. Just to confirm, does the black border round the solder point on the sproggy diagram represent the +'ve pin? I'm a bit confused about the mbr340 next to the big caps - am i right in thinking this is a reverse diode (thats what the R stands for)??

Cheers

Andrew

5. Think of the current flowing from plus to minus for a device that uses power. For a device that gives power, like a battery the current flows out of the plus into the minus. +--->|--- -

A diode can only let the current flow from plus to minus, if you try to hook it up the other way current won't flow until the voltage gets really high. A zener diode lets current flow in the positive direction easily but has a breakdown voltage that is precise. So you use a zener diode backwards..

Jeff

6. Cheers Jeff. Sorted.

7. Eehhh... Now time for a really dumbass follow-up to the really dumbass question

I thought I had it all figured out *until* I read this thread, now I'm confused. So, on the Sproggy layout guide one hole for the mbr340 has a black circle, and the other does not. I put all the other diodes in with the anode facing the circled hole and the cathode facing the uncircled hole. This matches my interpretation of the schematic symbols which show the triangle pointing away from the circled hole. But for the mbr340, the triangle points towards the circled hole, so which should I follow, the symbol, or the circle?

8. Hehe the only diode i haven't done yet. This is kinda what was confusing me about this diode! Perhaps it is some funny reverse diode or something. Or maybe not ??

Cheers

Andrew

9. Originally posted by Andrew Chappell
Hehe the only diode i haven't done yet. This is kinda what was confusing me about this diode! Perhaps it is some funny reverse diode or something. Or maybe not ??

Andrew
There is no such thing as a reverse diode. Look at the schematic, look at the datasheet. Don't look at some funky interpertation on the layout diagram.

Jeff

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