# Are there standardized values for the turn ratio of transformers?

I am simulating a dc battery charger. The circuit contains a step down transformer

I will need to lower the tension from 311 V (Vp) (or 220 RMS) to 15 V (Vp). One possible option for the turn ratio would be nprimary=311, nsecondary=15. But I highly doubt I could find practical transformers with this turn ratio. So is there any table/reference where I can find standard (or at least common) values of transformers turn ratios?

• When you look in a list / catalog of transformers, they are typically sorted by ratio, not by number of windings. Nov 11, 2020 at 18:17
• ... and they will be rated in primary and secondary RMS voltage - not peak voltage. Standard secondary voltages will include 6, 9, 12, 15, 18 and 24 V rms with 115 / 230 nominal primaries. Nov 11, 2020 at 18:22
• The number of turns does more than apply a simple ratio - it also indicates how much power the transformer can move from primary to secondary. Consider your 311:15 transformer idea.... If you used 21 turns on the primary and only 1 on the secondary, you'd get (about) the same ratio. But you already know that ain't gonna work! Transformer design is a complicated subject, you're best off just buying one off-the-shelf that delivers the voltage you need and (at least) the current you need. BTW, you do NOT need to hit 15V exactly in your circuit - You have a regulator. 16V would be ok too. Nov 11, 2020 at 22:54