Toroidal Transformer blowing fuses

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#1
Hi guys, i have a Toroidal tranny which is 35v-35v. It is running on 240 volts, (UK). i am building an Amp, and only yesterday I wired it up, brown and blue, to a mains IEC socket, to do a test.

i wired the grey and violet wires together, left hanging, And i had two secondarys, black/red , orange/yellow, that were giving me both 35 volts.

Now when i do the test, it blows the fuse on the IEC socket to the mains. And I am guessing, as im rather new to electronics, that i should not cut the unused violet and grey wires ?

Would that be right?

As at the same length, yesterday, it was perefectly allright.

Any input would be appreciated
 
#3
Please elaborate on the transformer and the load.

Pics would be good.


There was a problem with my original description on Apex.jr transformers and BrianGT's PCB's... That was cleaned up and correct instructions on Steve's (apex.jr) website.
 
#4
Hi Andrew

rating is 35 - 35volts, its a 5amp fuse to the 240volt plug, and the IEC sockets fuse is a fast blow, kind, that it not blowing at all, its the mains plug fuse.

and no, i have not built the lightbulb tester.

I purely was checking the 35v secondarys yesterday, i cut the 115 volt violet and grey wires down, soldered them together and today this happens.

thanks for the input
 
#7
thankyou Andrew and Troy.

I have now resoldered the violet and grey wires to their original size, and its all working fine.

The transformer i have requires that when you wire up the primarys in series, to get 240 volts, so the Blue, neutral , and brown ,live, go to the power switch, and the Violet and grey wires, have to be put together and left, floating.

i made the mistake of cutting the wires which must have ,messed with the windings. the only reason i say that as After doing a few searches online for cutting the grey and violet wires.

One Amplifiers instructions i cam across said that the Violet and grey wires should not be cut. Smacks own hand. Im so glad it was'nt in the finished Amp.

:smash: :hot:
 
#10
Violet and Grey are just random colours and will vary from toroid to toroid, you can't apply anything on the web.

The toroid itself should have a label on depicting connection scheme. Usually though for 230V operation the two primaries are connected in series which sounds like what you did.

A 500VA toroid needs a soft start. There is a very simple one on my website. Once you fit a soft start then you can size the fuse normally using

I = VA / Volts

or in your case 500 / 230 = 2.17 or a 2.5A rated time delay
 
#11
kevinkr said:
just wondering where you came up with that equation? Typically I would use/recommend about a 3A or 4A slow blow with a 500Va transformer on 230V.
Click to expand...
Hi,
typical inductive loads require a fuse rating roughly three times the VA rating of the equipment to survive the high starting current that these non resistive loads draw.

Motors and transformers are inductive loads. Expect nuisance blowing of the fuse if you try to close rate the fusing of the equipment.

The solution for all, is to install a soft start. Large fuses do not offer much protection for the supply wiring.
 
#12
AndrewT said:
Hi,
typical inductive loads require a fuse rating roughly three times the VA rating of the equipment to survive the high starting current that these non resistive loads draw.

Motors and transformers are inductive loads. Expect nuisance blowing of the fuse if you try to close rate the fusing of the equipment.

The solution for all, is to install a soft start. Large fuses do not offer much protection for the supply wiring.
Click to expand...

Yes I agree, particularly about the comment regarding wiring protection - I have seen this all too graphically demonstrated more than once, and so oversized in some cases provides little to no protection for any fault other than a dead short. (Nothing funny about a toroid meltdown.)

I use time lag or slow blow fuses and generally do not size them much more than 30% higher than the VA rating of the transformer, for toroids though this can be problematic and a soft start circuit is a very good idea. Being tube oriented I mostly use EI core power transformers which generally have somewhat better controlled inrush characteristics compared to toroids. (Not always, big EI can also be a headache.) I also suspect this may be more of an issue in countries with 230V mains - I've not had much trouble recently in this regard, although I have had instances where 500VA transformers needed an 8A slow blow fuse on 120V.
 
#13
My transformer EL keeps blowing fuses too.

Hi, I have a similar problem to the one above my large EL type transformer keeps
blowing fuses.

Its rated at 180VA 18-0-18 and has only 2 wires for the 240v primary. So far ive tried 2 x 3amp fuses and 1 x 5 amp and it blown then all.

I hope to use this transformer for an LM3886 type amplifier and get 27-0-27 after its gone though the bridge rectifier and the filter capacitors.

Can any one help me?

I have a picture but I don't think it will be much help.

https://skydrive.live.com/?cid=4BA427A8D2338779&id=4BA427A8D2338779%21246&sc=documents
 
#15
For higher power toroidal power supply transformers socalled NTC resistors are strongly recommended by manufactureres like Amplimo for instance (sometimes even delivered along with the transformer); they function as kind of slow start, cheap and effective.
 
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