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
Waiting for reply
With my best regards
Yasser El-haddad
Comments
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
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
>