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Overvoltage protection for 115 V transformer primary against 230 V mains

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mikeselectricstuff:
Another approach : use a relay, which in its NC position selects 230V. Have a circuit on the secondary, which, after a delay to allow settling only turns on the relay when the supply is below a fixed threshold.

One caveat for using relays to do series-parallel voltage switching though, you can get issues with arcing during switching, which effectively links both the NC and NO contacts together briefly during switching. At the very least this will cause rapid contact damage, if not blow a fuse.
The solution is to use two relays, one for 110v and 0ne for 230v, configured in a way that only one can be on at a time, both using only NO contacts.

Another approach could be to have a "soft" power switch, with a zero-volt dropout, so when plugged in, a button needs to be pushed to turn it on. Some logic can be added to the button-to-relay drive to prevent it turning on if the wrong voltage is selected.
 

GeorgeOfTheJungle:
Can also put a beefy 140V varistor after the fuse.

ampwizard:
One detail I've omitted is I actually have two dual primary transformers. I was thinking of switching them both between 115 and 230 V with a single DPDT switch (or relay) which looks OK on paper.


--- Quote from: Wolfgang on January 21, 2019, 06:17:11 pm ---I use a part called a SIOV from TDK, like here:

https://en.tdk.eu/inf/70/db/var/SIOV_Leaded_StandarD.pdf

They will reliable blow a fuse, must faster than the overcurrent caused by a saturated core.


--- End quote ---

I'll take a look at that, thanks.

ampwizard:
The more I think about relays the less keen I am on using them since they must presumably be rated for the transformer inrush currents (which are high).

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