Surge on power on causing major headache

[Hairy McClary]

Hi there I have a small problem with my power on surge for my power supply, it's nice and high and I need a good cheap and reliable way of suppressing it.
I can quite easily bypass it and it runs fine, but that's not really a permanent solution.

I have a 750VA transformer 120v to 24-0-24 which i am running at 240v and have split the cetre cwinding to form two 48v secondaries(run slightly higher).

The mains comes in through the iec socket, then onto a 5 amp switch, followed by(what used to be a 6.3amp fuse) into the transformer onto a NTC thermister (5r012) and them back out to neg.

from the transformer; this is fed into my rectifiers with resistor and cap in parallel(0.01uf 250 mkt and 18k 5w) that then runs to two 10000uf 50v caps in parallel with 2 0.1uf 250 mkt caps, then creates a electrical ground here between the two sets of caps which is the continued onto the amp.(ground all wired fine too)


even though i have put in the ntc, i believe this has no chance of creating the required resistance for the transformer, any help would be greatly appreiciated.

Oh and incase you wondering why I split the secondaries of a 120v transformer, it was because i could get that transformer really cheap, brand new from a good place i found(they only had one left)

[Steve Dunlap]

I have a 750VA transformer 120v to 24-0-24 which i am running at 240v and have split the cetre cwinding to form two 48v secondaries(run slightly higher).

You can't run a 120V transformer on 240V. The primary will draw too much power and burn up.

[event horizon]

Originally Posted by Hairy McClary

even though i have put in the ntc, i believe this has no chance of creating the required resistance for the transformer, any help would be greatly appreiciated.

Too right. I suggest you try running the transformer on 120V AC as it was designed to do

Originally Posted by Steve Dunlap

You can't run a 120V transformer on 240V. The primary will draw too much power and burn up.

Indeed fire.gif explode-04.gif

[Hairy McClary]

hmm, ok...
Is there anyway of running it on 240v?

Or would i need to run at 120v because it's impossible to run at 240v without rewinding the transformer with a more suitable gauge?

Why would the primary draw too much power and burn up, a detailed explanation here please :-)

I want to increase my knowledge, I'm not very good with power transformer theory, so in this instance I am a fool amoung kings.

[Hairy McClary]

I guess I won't be able to get this transformer to work on 240v! :-( - have to get a new one!

Anyone know of a cheap 240v to 120v converter that i can just stick on the power supply side and cheat that way (It'll save me alot of money if it's below the cost of an equivilent transformer in 240v)

I'll have to do a bit more research and teach myself more before I continue on this project!

[event horizon]

I'd suggest a look on ebay as you'll find a heck of a lot of "site" transformers that convert 240V to 120V to power equipment.

I'm not a transformer guru by any means, but the magnetising current will be massive with twice the voltage & you'll likely saturate the core with twice the input voltage by drawing even a little current. Saturate the core & the transformer no longer has any reactance & will blow fuses left right & centre as it might as well be a dead short.

[Hairy McClary]

I thought that it might be to do with the saturation on the core.
Anyway the 750VA is way overated for my needs, so i can probably rewire back to original 120v and use a site transformer like you suggested.

Fortunately I would only need 240w continuous power, so it shouldn't be too difficult to find a transformer for that rating, assuming that I can underpower it or would initial surge kill the site transformer and force me to get a transformer rated for the same power

It's just a shame about it as this was a bargin for what it is, should of check up on it first lol.

[Hairy McClary]

actually you're right, you can pick up a site transformer for 20-40 pound that will suit that down to a tee!

Cheers