blown fuses in dual fuse iec inlet - diyAudio
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Old 31st March 2016, 03:31 PM   #1
JAY X is offline JAY X  Spain
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Exclamation blown fuses in dual fuse iec inlet

Hi All!,

Just finished building my psu with pcb transformer. (Talema Nuvotem 10Va).

I calculated the fuse: (10VA/230V) X 1.414 = 61,47 ma; roughly 63maT FUSE.

That's what i put in the filtered IEC AC inlet (SHAFFNER FN9260-10-06) rated at 10 amps.... big filter... It features DUAL FUSE holder, so two fuses to protect LIVE and NEUTRAL. Ok so far.

Before this psu i already built other one with the same trafo, and 63maT fuse (1 unit in the LIVE wire), and had no problems...

After blowing 4 fuses.... i re-checked the trafo specs sheet, and they recommend 80maT fuse in the primary. I suppose to hold the initial inrush current. But, i already have only 100maT and 125maT fuses right now...i don't know if they will be ok or a bit too big...

Thanks for your help!
JAY X

Last edited by JAY X; 31st March 2016 at 05:13 PM.
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Old 1st April 2016, 10:36 AM   #2
DF96 is offline DF96  England
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100mA fuse should be fine. If I were building it I might use 250mA; the aim is to stop it from catching fire under fault conditions.
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Old 1st April 2016, 11:58 AM   #3
Mooly is offline Mooly  United Kingdom
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Its not just initial switch on surge that can be a problem when using such low value fuses, mains drop outs and glitches can have the same effect.

I'd probably go with DF96's suggestion as it would prevent nuisance blowing.
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Old 1st April 2016, 01:14 PM   #4
AndrewT is offline AndrewT  Scotland
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The fuse rating does not change the start up current surge.
It's the resistance in the primary circuit that affects the start up current while the transformer core is partly saturated.

Very low VA transformer use extremely long thin wire for the primary winding. These inherently have very high primary resistance. This resistance fairly effectively limits the start up current and usually there is no advantage to adding any more current limiting resistance.

This is confirmed by the manufacturer's recommended fuse rating of T100mA for a maximum power current rating of 61.47mAac. The maximum primary current is actually the output current rating divided by the transformer efficiency while delivering full power. Maybe ~90%.

If you don't have a T100mA fuse, then maybe an F200mA, or F160mA, fuse will give sufficient pass capability during start up to prevent nuisance blowing.
You should fit a fuse that prevents any cable (or equipment) downstream catching fire during a fault incident. A 1A fuse usually does that.

My soft start transformer of 230Vac, 6VA rating uses an F500mA fuse.

Fit a very high fuse rating into the Neutral side of the dual fuse IEC. You don't want the neutral fuse to blow leaving the Live fuse still powering all the live bits to 220Vac after you think the unit is apparently OFF !
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Last edited by AndrewT; 1st April 2016 at 01:19 PM.
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Old 1st April 2016, 01:41 PM   #5
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The Schaffner has capacitors that sit across the mains. They have a high load if you are unlucky to power on as the mains is at full voltage peak, probably take up 50mA momentarily.
These are excellent filters.
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Old 4th April 2016, 09:16 AM   #6
JAY X is offline JAY X  Spain
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Hi All!

Four eyes see better than two....Friday afternoon i went to an audio shop which also repairs, and after blowing a pair of 500ma fuses, we checked the psu and found that the primary wires were bad wired. After cutting the wrong traces, and placing two 63maT fuses...¡voilà!.. psu alive!.

It is all about checking, checking and checking...

JAY X
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Old 4th April 2016, 11:53 AM   #7
DF96 is offline DF96  England
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If you used a lamp limiter you would have saved money on fuses, and found the fault much more quickly.
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Old 4th April 2016, 05:32 PM   #8
JAY X is offline JAY X  Spain
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Hi!

I did not know about the lamp limiter. I have seen today some vídeos on youtube. What wattage should be the lamp?
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Old 4th April 2016, 06:25 PM   #9
Mooly is offline Mooly  United Kingdom
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60 or 100 watt usually. For very low power transformers an even smaller bulb can be used such as a fridge bulb.
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Old 4th April 2016, 07:18 PM   #10
DF96 is offline DF96  England
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For a 10VA transformer I would use a 40W or 25W lamp.
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