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selecting Fuse value — Parallax Forums

selecting Fuse value

ArchiverArchiver Posts: 46,084
edited 2000-11-02 01:02 in General Discussion
Hi, I need to knwo how can i select the value of my fuse instaled on the primary side of 220 VAC transformer to compabtible with the UL standard. I have a board with transformer of 12 VA and 220 VAC input and it uses a fuse 3.15 A how the designer of this board select this fuse.

Waiting for reply
With my best regards
Yasser El-haddad

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2000-10-27 23:50
    Original Message
    From: yaserh <yaserh@g...>
    To: <basicstamps@egroups.com>
    Sent: Friday, October 27, 2000 5:03 AM
    Subject: [noparse][[/noparse]basicstamps] selecting Fuse value


    Hi, I need to knwo how can i select the value of my fuse instaled on the
    primary side of 220 VAC transformer to compabtible with the UL standard. I
    have a board with transformer of 12 VA and 220 VAC input and it uses a fuse
    3.15 A how the designer of this board select this fuse.

    **********
    A 12 watt load on 220 VAC would only draw 12/220 =55.5 ma. With a power
    factor as low as 50%, the current would double to 109 ma. A cheap
    transformer could have an inrush current of 20x, or 2.2 amps. It seems that
    3.15 amps is high. Often, a slow blow fuse is used to accomodate inrush
    current. So maybe a 1 amp slow blow would suffice.

    Ray McArthur
  • ArchiverArchiver Posts: 46,084
    edited 2000-11-02 01:02
    The NEC states for primary fusing of 600 volt class transformers :
    If current is greater than 9 amps, your fuse can be 1.25 times that current
    rating (or next higher standard fuse rating).
    If current is 2 - 9 amps, your fuse can be up to 1.67 times that current
    rating.
    If current is less than 2 amps, your fuse can be up to 3.00 times that
    current rating.

    Also for your future reference...
    The NEC states for secondary fusing of 600 volt class transformers :
    If current is greater than or equal to 9 amps, your fuse can be 1.25 times
    that current rating (or next higher standard fuse rating).
    If current is less than 9 amps, your fuse can be up to 1.67 times that
    current rating.


    So following the primary transformer rule, your primary current is 12 VA /
    220 V = 0.055 amps.
    0.055 amps is less than 2 amps, so 0.055 A x 3.00 = 0.16 amps.

    Therefore your fuse should be 0.16 amps OR LESS.

    I'd use a Littelfuse 3AG type (250 volt fuse) slow-blow fuse, 313 Series,
    0.150 amp. [noparse][[/noparse]Digikey F306-ND]
    (Always use slow-blow type on transformers, the inrush will blow regular
    fuses)

    Jim Howard

    >
    > From: yaserh
    > Reply To: basicstamps@egroups.com
    > Sent: Friday, October 27, 2000 1:03 am
    > To: basicstamps@egroups.com
    > Subject: [noparse][[/noparse]basicstamps] selecting Fuse value
    >
    > Hi, I need to knwo how can i select the value of my fuse instaled on the
    > primary side of 220 VAC transformer to compabtible with the UL standard. I
    > have a board with transformer of 12 VA and 220 VAC input and it uses a
    > fuse 3.15 A how the designer of this board select this fuse.
    >
    > Waiting for reply
    > With my best regards
    > Yasser El-haddad
    >
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