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Overvoltage protection for 115 V transformer primary against 230 V mains
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ampwizard:
I am designing a portable musical instrument amplifier for a touring musician. It has a dual primary 2 x 0 - 115 V mains transformer with a switch to go between 115 V and 230 V operation. Now, I would like to include some kind of protection against having the switch set to 115 V and plugging the unit into 230 V which would be very destructive. The primary fuses are time-delay (slow blow) types: 1.6 A per primary. Would I be able to use a pair of thyristors (one per primary) to blow the fuses or even trip the venue's circuit breaker?

Since the thyristor resets on every half cycle of the 50 Hz mains, I'm not sure the average AC current will be high enough to blow the fuse fast enough to offer any protection. Has anyone had any experience with this or is there a better way of protecting against this scenario?
Zero999:
It might be better to monitor the voltage on the DC side and switch a relay on to short the fuses on the primary side.

How about designing out the problem? Switched mode power supplies with a universal 100V to 250VAC input are widely available. Noise and interference can be mitigated by careful filtering an additional linear regulators on the secondary side.
Ian.M:
A rough calculation based on the specific heat capacity and latent heat of fusion of copper, assuming no heat is lost from the wire, (i.e. quick heating) suggests that as long as the primary wire diameter is at least 3.2 time the fuse wire diameter, the fuse will blow before the max temperature of 180 deg C rated magnet wire can be exceeded.

For a more accurate answer and further advice, why not talk to your transformer manufacturer?
Jeroen3:
How much room is there? Can you fit a voltage monitor relay?
Zero999:

--- Quote from: Ian.M on January 21, 2019, 10:44:12 am ---A rough calculation based on the specific heat capacity and latent heat of fusion of copper, assuming no heat is lost from the wire, (i.e. quick heating) suggests that as long as the primary wire diameter is at least 3.2 time the fuse wire diameter, the fuse will blow before the max temperature of 180 deg C rated magnet wire can be exceeded.

For a more accurate answer and further advice, why not talk to your transformer manufacturer?

--- End quote ---
Just to clarify I was talking about putting the relay contacts on the primary side, as it might not be easy to ensure the fuse blows, before the transformer.
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