CALCULATE FUSE SIZE

There is a power transfer between the primary and secondary of a transformer. Even though there is a voltage difference between the primary and secondary of the transformer, the power transfer is equal except for transformer loss.

We can calculate the power in one winding by finding the power in the other winding using voltamps. To find the current in the primary, we find the voltamps ( power ) in the secondary. Then take the secondary voltamp value and divide it by the primary voltage ( 120 ). Note, you can use any primary voltage depending on the voltage in your area.

EXAMPLE 1
Secondary = 600VCT @ 80MA ( .080 amps )
600 X .080 = 48 ( voltamps )

add 10% to allow for transformer loss and the primary current is about 1/2 amp

Don't forget to include all the transformer secondary windings. If there is a filament winding then figure the voltamps for that and add to the high voltage voltamp figure. Then divide the total secondary voltamps by the primary voltage ( 120 ).

EXAMPLE 2
Secondary 1 = 600VCT @ 80MA ( .080 amps )
600 X .080 = 48 ( voltamps )
Secondary 2 = 6.3VCT @ 3 AMPS
6.3 X 3 = 18.9 ( voltamps )

add 10% to allow for transformer loss and the primary current is about 6/10 amp

The fuse value needs to be larger than the actual current or it will blow when powered up. Add 20% to the actual primary current for the fuse value. Power supplies usually have a current surge when they power up so use a slow-blow fuse. For example 2, we would use a 8/10 AMP slow-blow fuse or a 1 AMP slow-blow fuse if the turn-on surge is higher than expected.

FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE
FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE
FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE
FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE - FUSE