# How large my primary and secondary fuses should be for this transformer?

#### szabikka

Joined Sep 3, 2014
90
Hello everyone!

I need your help with the following problem. I have to choose primary and secondary fuses for a PCB transformer with the following values: It has one secondary winding with 12V output voltage and it has a power rating of 6 VA. This is all I know about it. I'm planning to use it with 230 V mains (it's rated for that) and want to power a load that draws no more than 300 mA peak current. What kind of fuse values should I use and what is the reason behind those values (safety standards, etc)?

#### k7elp60

Joined Nov 4, 2008
561
Hello everyone!

I need your help with the following problem. I have to choose primary and secondary fuses for a PCB transformer with the following values: It has one secondary winding with 12V output voltage and it has a power rating of 6 VA. This is all I know about it. I'm planning to use it with 230 V mains (it's rated for that) and want to power a load that draws no more than 300 mA peak current. What kind of fuse values should I use and what is the reason behind those values (safety standards, etc)?

Doing the calculations on the maximum current of the secondary, 6VA/12V = 0.5 amps. For the primary 6VA/230v = 0.026 amps or 26mA. Since your peak current on the secondary is 300mA or 0.3A I would recommend only only one fuse, in the primary.. That would be a Littlefuse part number of 312.031. This fuse has a voltage rating of 250V and 0.03125 amps. Fuses normally have a voltage and current rating. The voltage rating ensures the blown fuse will cease the current flow when the current is exceeded. The lowest voltage fuse that I could find is 32 volts. Therefore I cannot recommend a fuse for the secondary.

#### Sensacell

Joined Jun 19, 2012
2,564
I would not recommend a fuse rated so close to the load current.

Fuses are crude devices, cutting it so close will result in nuisance fuse blows.

High voltage fuses can be used on low voltage circuits, but not the other way around.
The fuse knows nothing about the voltage- until it ruptures, then in matters.

Think about the voltage drop across an intact fuse, it's minimal, even when used in a high voltage circuit.

#### Dodgydave

Joined Jun 22, 2012
8,826
I would use a 1Amp fast blow on the secondary side.

#### szabikka

Joined Sep 3, 2014
90
I would use a 1Amp fast blow on the secondary side.
Thank you for your answers! Dodgydave, I have read that slow blow fuses are better in case of transformers, because on start-up there are large currents for very short times, until the magnetic force is built up around the windings. Is this true or I should go with the fast-blow fuse as you recommended?

#### szabikka

Joined Sep 3, 2014
90
I would not recommend a fuse rated so close to the load current.

Fuses are crude devices, cutting it so close will result in nuisance fuse blows.

High voltage fuses can be used on low voltage circuits, but not the other way around.
The fuse knows nothing about the voltage- until it ruptures, then in matters.

Think about the voltage drop across an intact fuse, it's minimal, even when used in a high voltage circuit.
Sensacell, how much headroom is recommended in my example for the fuses?

#### ericgibbs

Joined Jan 29, 2010
9,560
hi sz,
This PDF has some useful information regarding fuse selection.
A simple way to consider the purpose of fuse protecting a project is:
A transformer secondary fuse is to protect the transformer from a continuous over current load, due to a failure in the project being powered.
The primary fuse is to protect the mains supply from a continuous over current due to failure in the transformer.

E

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#### Sensacell

Joined Jun 19, 2012
2,564
Sensacell, how much headroom is recommended in my example for the fuses?
Read the guidelines attached to Post #7, this will illuminate the complexity of the answer.
Throwing out a rule of thumb will give you a false sense of knowledge of a surprisingly complex subject.

#### k7elp60

Joined Nov 4, 2008
561
opinions vary