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Old 2nd July 2008, 08:00 AM   #11
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by kevinkr
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.
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.
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Old 2nd July 2008, 02:50 PM   #12
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by AndrewT
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.
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.
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Old 5th November 2011, 11:24 AM   #13
Haz is offline Haz  United Kingdom
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Default 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=4BA42...6&sc=documents
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Old 6th November 2011, 02:33 PM   #14
AndrewT is offline AndrewT  Scotland
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You need to build a bulb tester and use it to save the expense of blown fuses.
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Old 6th November 2011, 02:42 PM   #15
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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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Old 6th November 2011, 05:30 PM   #16
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More on light bulb testers:

Light Bulb Tester
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