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Inrush current of toroidal transformers
Inrush current of toroidal transformers
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Old 26th March 2008, 01:23 AM   #1
hollowman is offline hollowman  United States
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Default Inrush current of toroidal transformers

I've just added an Avel-Lindberg Y236653 250VA 30+30 toroidal transformer to an existing stand-alone power supply.

Before the addition of the Avel Linberg Y23, the stand-alone PS already incorporated a Triad bobbin-type transformer (for balanced power use) and two tiny Amveco 0.5A toroidals. This transf. arrangement works fine with a 250V/1.5A "AGC"-type slow-blow (?) fuse on the 115V "Mains" AC line.

The addition of the Avel Linberg Y23, however, not only blew the 1.5A fuse, but a subsequently-tried 7.0A (AGC slo-blo) as well.

It's possible I'm using the wrong type of slow-blow fuse.

Any quick suggestions here?

Thx,
-hm
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Old 26th March 2008, 01:41 AM   #2
MikeW is offline MikeW  United States
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Check your connections. You may have a short. It should not blow a 7 amp fuse. A CL-60 thermistor will help with the inrush.
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Old 26th March 2008, 03:14 AM   #3
hollowman is offline hollowman  United States
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Forgot to note: the fuses used thus far have been type "313". Anyway ...
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Originally posted by MikeW
Check your connections. You may have a short. It should not blow a 7 amp fuse. A CL-60 thermistor will help with the inrush.
Thx for the CL-60 thermistor tip.

No shorts -- that's the first thing I ck'd. The manuf notes that slow-blow fuse's rated current is actually a sustained current; the fuse can allow several times the rated current (inrush) for a fraction of a second (or so). I wonder if the inrush current of the Y23 toroid is surpassing the fuse's max/inrush?
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Old 26th March 2008, 04:29 AM   #4
tinitus is offline tinitus  Europe
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Inrush current of toroidal transformers
A 250VA toroid should not blow fuse...some say up to 500VA is safe without softstart
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Old 26th March 2008, 05:35 AM   #5
hollowman is offline hollowman  United States
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Default CL-60 thermistor

Quote:
Originally posted by tinitus
A 250VA toroid should not blow fuse...some say up to 500VA is safe without softstart
I saw that too in the datasheet. Another possible culprit may be the parallel wiring of the other transformers -- a large Triad bobbin-type + two small (0.5A) toroids -- with the 250VA. Although this seems unlikely as the former operated just fine with a 250/1.5A type 313 fuse.

A question re earlier-suggested CL-60 thermistor...
Where is the best circuit position to add this? The datasheet seems to indicate after the rectifier. But don't we want to soften the blow on the tranf. primary side? At least that's where my potential problem lies.
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Old 26th March 2008, 10:59 AM   #6
MikeW is offline MikeW  United States
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The Line or Neutral or both on the Primary side. Look at some of the First Watt power supplies.

http://www.firstwatt.com/downloads/f4_om.pdf

page 15
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