MOV - Before or after?

Gaz · Post by **Gaz** » Sat Mar 31, 2012 5:04 am

I'm curious whether an MOV across the primary winding of the PT should be placed before or after a) the primary fuse and b) the mains switch. "Before" meaning towards the power inlet.

I have my theories, but they are stupid.

VacuumVoodoo · Post by **VacuumVoodoo** » Sat Mar 31, 2012 9:55 am

Think what happens when MOV shorts placed directly across mains intake without a fuse in between.

Gaz · Post by **Gaz** » Sat Mar 31, 2012 4:49 pm

VacuumVoodoo wrote:Think what happens when MOV shorts placed directly across mains intake without a fuse in between.

Bad things?

Cliff Schecht · Post by **Cliff Schecht** » Sat Mar 31, 2012 5:58 pm

Gaz wrote:
VacuumVoodoo wrote:Think what happens when MOV shorts placed directly across mains intake without a fuse in between.
Bad things?

You'll pop the breaker that your amp is on if it shorts. Imagine sticking two wires in the wall and shorting them together. Same trick, just inside your amp

.

Gaz · Post by **Gaz** » Sat Mar 31, 2012 7:23 pm

I used to do that in middle school by making a paper clip into a U-shape and taping it to a plastic ruler. Once we lit the carpet in the music room on fire. Those were the days. I'm just happy no one was stupid enough to use a steel ruler!

But what about the standby? Should it go inbetween the fuse and MOV to protect it from damage from surges?

VacuumVoodoo · Post by **VacuumVoodoo** » Sun Apr 01, 2012 7:35 am

Standby is on the secondary side of transformer isn't it?
MOV is there to protect the amp against high energy transients of too short duration for your fuse to react. The fuse is not meant to protect the MOV.

Structo · Post by **Structo** » Sun Apr 01, 2012 2:39 pm

I just stick a butter knife into the outlet, same difference!

ToneMerc · Post by **ToneMerc** » Sun Apr 01, 2012 2:45 pm

VacuumVoodoo wrote: MOV is there to protect the amp against high energy transients of too short duration for your fuse to react. The fuse is not meant to protect the MOV.

Yep, nothing more, nothing less.

@Gaz, you are making this far too complicated.

TM

NickC · Post by **NickC** » Sun Apr 01, 2012 3:05 pm

http://www.eeel.nist.gov/817/pubs/spd-a ... e%20BW.pdf

Gaz · Post by **Gaz** » Sun Apr 01, 2012 5:10 pm

VacuumVoodoo wrote:Standby is on the secondary side of transformer isn't it?
MOV is there to protect the amp against high energy transients of too short duration for your fuse to react. The fuse is not meant to protect the MOV.

Sorry, I meant the Mains switch. I suppose I'm "making it too complicated", but I just don't want to naively place the MOV.

VacuumVoodoo · Post by **VacuumVoodoo** » Sun Apr 01, 2012 5:46 pm

On the other hand, tube amps damaged by mains transients can be counted with on one hand fingers I think.

Gaz · Post by **Gaz** » Sun Apr 01, 2012 5:53 pm

The amp is 200W, and I read that the MOV is "to suppress dI/dt induced voltage from stressing the mains switch (whether in the amp, or if further upstream) - bigger transformers have higher operating currents, and higher leakage inductances."

I honestly just don't understand what is meant by "upstream"

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