Transformer turn ratio

[miketan3904]

Hi guys! I am currently working on a project that needs to use a transformer. The transformer that I will be using is rated for 230V-115V-0 at its input and 6V-0-6V at its output. I am going to apply 240V at the primary side and I will be using one the output of the transformer (I will be taking the output from the wire labelled as 6V and 0V). Here's something that I am not sure about it and I would appreciate if anyone can clear my doubts.
1. Is the turn ratio for each of the output equals to 230/6?
2. Can I calculate the input voltage applied to the primary winding by using the secondary voltage and the turn ratio? (I do know that there is some error in the transformer and I won't be getting the exact input voltage)

 

[rjenkinsgb]

A power transformer secondary voltage will often only be accurate under full load conditions.

Without a load, the voltage will be higher; usually, the lower the transformer power rating, the higher the unloaded voltage will be, proportionally.

That's the "regulation" rating of the transformer.

Having said that, as long as the load is fixed, there should be a consistent ratio between primary and secondary voltage.
eg. A small transformer may give 8V from a "6V" secondary with no load, but increase the primary voltage by 5% and the secondary should also increase by around 5%

A large power transformer may well change by less than 10% from no load to full load.

 

[Grossel]

A large power transformer may well change by less than 10% from no load to full load.

Those are probably equipped with a step regulator that make sure the secondary voltage are within certain limits.

 

[rjenkinsgb]

Those are probably equipped with a step regulator that make sure the secondary voltage are within certain limits.

No, that's not needed with good conventional transformers; it's more likely to be used where the incoming supply can vary with load.

eg. See the data for these - the "No load voltage" factor; it's 7% for a 200VA down to 2% for 1000VA and up.

https://docs.rs-online.com/8619/0900766b80695dc3.pdf

[Edit - stray word removed]

 

[Diver300]

Very small transformer may start to saturate the core when the voltage is near the rated voltage, so the output voltage will not increase at the same percentage rate as the input voltage.

If you can find a transformer with a higher input voltage, like 415 V, then the output at 230 V will be less, but it will be more linearly related to the supply voltage.

I've used one of these:- https://www.screwfix.com/p/british-general-8-24v-8va-bell-transformer-module/8707p to step down and isolate mains voltages to make reading easier and safer.

 

[MaxHeadRoom78]

If you want to find the turns ratio, just add a temporary small overwind of say 10 turns and measure the voltage.
For a toroidal, it is around 2t/v for a IE style, around 4 to 5t/v.
Max.

 

[dr pepper]

To answer your questions loosly I would say you are correct.
However divers comments are correct in my opinion, esp with small cheap transformers.