transformer higher than rated output

[qtommer]

hello good people

I recently got a transformer rated at primary 240VAC-115-0 stepped down to secondary 9VDC-0-9VDC. I assume the ratings are in R.M.S.

I hooked the inputs using the 240VAC-0 line and scoped the output VAC lines. From the scope I expected a 24.45VACp-p voltage (converted from 18V rms amplitude) but I got a peak to peak amplitude of 31volts instead.

Do transformers actually tend to exhibit higher rating than usual or could it be due to tolerance issues? Anyone faced this before??

thanks!

 

[crutschow]

18Vrms would give a peak voltage of 1.4 x 18 = 25.2V

The high no-load voltage is normal. The transformer's rated voltages are for full load. Due to winding resistances the no load voltage is thus higher. Cheaper transformers tend to have higher winding resistance and thus a greater variation between no-load and full-load voltages.

 

[awt]

i agree wt crutschow. Another thing is the input voltage of 240V; in the actual sense, it could be more than that.

 

[crutschow]

i agree wt crutschow. Another thing is the input voltage of 240V; in the actual sense, it could be more than that.

Good point. The input transformer voltage obviously affects the output voltage.

 

[Diver300]

You don't say what the VA rating of the transformer is. The smaller the transformer, the worse the regulation. Transformers are rated to give the rated voltage at maximum load. At low loads the voltage will be higher.

https://www.farnell.com/datasheets/494477.pdf shows that those 6 VA transformers have a regulation of about 24%, which would account for what you are seeing.

 

[qtommer]

The high no-load voltage is normal. The transformer's rated voltages are for full load. Due to winding resistances the no load voltage is thus higher. Cheaper transformers tend to have higher winding resistance and thus a greater variation between no-load and full-load voltages.

Ahh i see! i did a little research and found out the phenomena of no load versus full load. However these sites/+ 1 book gave me regulation formulas but were unable to physically describe why the voltage with a full load drops. Why does this happen? any analogy?

i agree wt crutschow. Another thing is the input voltage of 240V; in the actual sense, it could be more than that.

ahh i see..but if i measured at the mains, i got exactly 240Vrms. How does the input voltage vary? will it change when loaded as well like crutschow said?

https://www.electro-tech-online.com/custompdfs/2011/04/494477.pdf shows that those 6 VA transformers have a regulation of about 24%, which would account for what you are seeing.

transformers available in my little town (somewhere in Asia) are mostly rated in r.m.s ratings..no VA rating is available however, thanks alot for the tip on the regulation ratings

 

[Diver300]

The physical size of the transformer will give an indication as to the rating. The resistance of the windings and the stray inductance will both contribute to the regulation.
Small transformer cores have less magnetic flux, so they need more turns to get the same voltage. So small transformers have more turns in a smaller space, so much more winding resistance. That is one reason why small transformers have worse regulation.