Transformer ratio question

[bwd111]

Trying to find the turn ratio of the primary coil to the secondary coil in a step down transformer in order to decrease aprimary voltage of 120v to a secondary voltage of 24 volts. I was thinking 100 turns to 24 turns?

 

[tvtech]

Hi and welcome bwd1111

Primary and Secondary wires on transformers are different thicknesses. Primary normally the thinner wire with many more turns than the Secondary thicker wire on a step down Transformer.

I am so old I cannot remember the ratios though...

I am sure someone here will help.

Regards,
tvtech

 

[Nigel Goodwin]

Trying to find the turn ratio of the primary coil to the secondary coil in a step down transformer in order to decrease aprimary voltage of 120v to a secondary voltage of 24 volts. I was thinking 100 turns to 24 turns?

Where did you get the 100 from? - hopefully it was a typo?.

 

[crutschow]

The number of turns required for the primary depends upon the primary frequency and voltage, the core size and the flux density the core can tolerate before saturating.

Once you have determined the number of primary turns, then you can readily determine the number of secondary turns based upon the primary to secondary voltage ratio.

 

[rumpfy]

You ask about 'Turns Ratio'.
The 'Turns Ratio is the same as the 'Voltage Ratio'. So, if you want to transform 120 Volt to 24 Volt, then you will need a turns ratio of 120/24= 5.0
However, as crutshow says, the REQUIRED number of turns on the primary side cannot be guessed because it depends on the factors crutshow mentions. The power required to be transformed affects the core size because the wire gauge needs to be thicker for a higher power transformer. The design of these things is somewhat cut and try, because of the restricted number of core sizes available.
When the number of turns is decided for the primary, then only one fifth of that number of turns will be needed on the 24 volt side. As tvtech says, the secondary wire diameter will be thicker than the primary wire by the ratio square root 5

 

[crutschow]

In practice the secondary generally has a few more windings then the theory predicts due to the primary and secondary winding resistance and other loses.

 

[Nigel Goodwin]

In practice the secondary generally has a few more windings then the theory predicts due to the primary and secondary winding resistance and other loses.

Unlikely to be a concern here, as it's a VERY early class question - the sort of thing you do at school in Physics about 12/13 years old, they certainly won't be looking for practial applications instead of theoretical ones.

 

[Miles Prower]

Trying to find the turn ratio of the primary coil to the secondary coil in a step down transformer in order to decrease aprimary voltage of 120v to a secondary voltage of 24 volts. I was thinking 100 turns to 24 turns?

How many turns depends on the operating frequency and the volts per turns ratio. 100 turns to 24 turns would require a helluva big core to sustain a 1.2V/N ratio at 50Hz or 60Hz (which may become necessary, depending on how much current the load needs). (This also becomes a good deal more practical for higher operating frequencies.) The formula you want is:

Vn= 4fSB

Where:

f -- frequency in Hz
S -- Core cross sectional area in meters^2
B -- Core flux density in Webers/meter^2

Therefore, if assuming:

B= 1.2Wb/M^2 (typical value for silicon steel, and with a margin of error here to ensure core saturation doesn't happen)
S= 2.0 sq. in. (Convert: 2.0 sq. in. = 1.29E-3 M^2)

Vn= 4 * 60 * 1.2 * 1.29E-3= 0.37V/N

Npri= 120/0.37= 324.32 (325 design nominal)
Nsec= 24/0.37= 64.86 (65 design nominal)

There could be empirical adjustments depending on requirements such as voltage regulation, core losses (better quality lams and care in stacking reduce core losses) copper losses (goes to VR).