Diodes in series vs parallel

What is the difference between placing diodes in series vs parallel? Does series increase the current capability? Does parallel do anything at all? What about with zener diodes how does series/parallel effect them?

Parallel increases current capacity, series increases voltage rating - BUT - special action is required to make them work and not fail.

Zeners are even worse!.

Something to think about:

Components in a serial connect must have the same current flowing through them. There is no other path to take. So putting 3 diodes or 3 resistors or whatever in series can't improve the current rating over a single component, as they will all see the same current.

If components are in parallel, they will all have the same voltage across them. So putting components in parallel can't change the voltage rating.

Beyond that, depending on components, there will be complications as Nigel said. Let's consider a simple case. Three 10k resistors in parallel. They will have the same voltage across them, and each should carry ⅓ of the current, right? It's not quite that simple, as 10k resistors aren't exactly 10k ohms. The one with the lowest resistance is going to carry more than its share of the current. But that's going to cause it to get hotter, and increase in resistance.... eventually, they'll reach equilibrium.

With diodes, the situation is much more complicated.

Visitor said:

Something to think about:

Components in a serial connect must have the same current flowing through them. There is no other path to take. So putting 3 diodes or 3 resistors or whatever in series can't improve the current rating over a single component, as they will all see the same current.

If components are in parallel, they will all have the same voltage across them. So putting components in parallel can't change the voltage rating.

Beyond that, depending on components, there will be complications as Nigel said. Let's consider a simple case. Three 10k resistors in parallel. They will have the same voltage across them, and each should carry ⅓ of the current, right? It's not quite that simple, as 10k resistors aren't exactly 10k ohms. The one with the lowest resistance is going to carry more than its share of the current. But that's going to cause it to get hotter, and increase in resistance.... eventually, they'll reach equilibrium.

With diodes, the situation is much more complicated.

Click to expand...

So basically diodes can set off a chain reaction while in parallel because of the slight difference in breakdown current?

Also paralleling diodes will bring higher capacitence...

billybob said:

So basically diodes can set off a chain reaction while in parallel because of the slight difference in breakdown current?

Click to expand...

It's commonly done in poorly designed equipment, and they WILL fail. The solution is to add low value resistors in series with each diode.

Likewise, in series diodes won't share the voltage evenly, and WILL fail - the solution in this case is high value resistors in parallel with each diode.

The added resistors balance out the current and voltage respectively.

i've seen low-end AV receivers that use parallel bridge rectifiers in the amp power supply.... they fail often because of differences in Vf causing one of them to have more current... as silicon heats up, Vf reduces by 2.2mV/degC, so the diode drawing more current will heat up faster, further reducing it's Vf, and further increasing it's share of the current... you can see where this is going...

unclejed613 said:

i've seen low-end AV receivers that use parallel bridge rectifiers in the amp power supply.... they fail often because of differences in Vf causing one of them to have more current... as silicon heats up, Vf reduces by 2.2mV/degC, so the diode drawing more current will heat up faster, further reducing it's Vf, and further increasing it's share of the current... you can see where this is going...

Click to expand...

We used to see it a LOT in switch-mode output rectifiers, often paralleled in cheap TV's, and a very common source of failure.

It's really depressing, you can study the circuit of a new TV model and spot common design flaws that you know will fail a couple of years in the future.

I've always presumed it's down to one of two reasons:

1) The designers are fresh out of University, and have never actually worked in Electronics, and have zero practical experience, so no idea what fails or why.

2) The designs are altered on the production line, to make them cheaper and easier to manufacture.

Unfortunately it's impossible to find out from the manufacturers, who won't let you talk to the designers or the production team.

Personally I've always presumed option 2) - mainly because of a Sony amplifier that had multiple mains transformer failures - the transformer was FAR too small for the job (60W odd transformer in a 200W amplifier). But there was plenty of space for a MUCH larger transformer, so always presumed if was a production change. This was an example where I pushed the manufacturer a LOT, over a good few years - and got nowhere - other than a pathetic excuse that they "hadn't had many failures".

The was a manufacturer of early cheap B&W TVs in Los Angeles (Muntz TV) whose owner called himself Madman Muntz (he was the first to coin the word "TV").
From Wikipedia:
He often carried a pair of wire clippers, and when he thought that one of his employees was "overengineering" a circuit, he would begin snipping components out until the picture or sound stopped working. At that point, he would tell the engineer "Well, I guess you have to put that last part back in" and walk away.

Perhaps there still a version of him working in some of our factories today.

crutschow said:

The was a manufacturer of early cheap B&W TVs in Los Angeles (Muntz TV) whose owner called himself Madman Muntz (he was the first to coin the word "TV").
From Wikipedia:
He often carried a pair of wire clippers, and when he thought that one of his employees was "overengineering" a circuit, he would begin snipping components out until the picture or sound stopped working. At that point, he would tell the engineer "Well, I guess you have to put that last part back in" and walk away.

Perhaps there still a version of him working in some of our factories today.

Click to expand...

=)

A classic example of a non engineer making a change is the walkway collapse at the Hyatt Regency in Kansas city.

The builder thought what he had done was as strong as the original design.

He couldn't get long enough threaded rods so used multiple shorter ones as so,

The problem is that the lower nut carries the weight of all floors rather than just one.
It cost 114 people their lives.
Often there's a very specific reason something is done a particular way.

Mike.
Actually, having read the wiki page it wasn't the builder but the manufacturer of the threaded rods that made the change.
[/thread drift]

crutschow said:

The was a manufacturer of early cheap B&W TVs in Los Angeles (Muntz TV) whose owner called himself Madman Muntz (he was the first to coin the word "TV").
From Wikipedia:
He often carried a pair of wire clippers, and when he thought that one of his employees was "overengineering" a circuit, he would begin snipping components out until the picture or sound stopped working. At that point, he would tell the engineer "Well, I guess you have to put that last part back in" and walk away.

Perhaps there still a version of him working in some of our factories today.

Click to expand...

Decades ago Ferguson supposedly employed a woman to try and find parts that could be removed without issue from their TV's.