I'm using the following transformer with the output in a parallel configuration such that the input is 230V and the output is 12V. considering the tranformer is rated for 100VA this means the maximum output current is 100/12 = 8.3A.

Would this be 8.3A DC or 8.3A rms?

  • 5
    \$\begingroup\$ RMS - transformers don't work with DC. \$\endgroup\$ Apr 11, 2023 at 22:46
  • \$\begingroup\$ Just to add a little confusion to the mix -- some may be rated in terms of rectified DC output, Hammond classic tube/valve transformers for example (I think?). But VA can't be [ab]used that way; VA / VAC(RMS) = IAC RMS is the most reasonable interpretation here. \$\endgroup\$ Apr 12, 2023 at 0:05
  • \$\begingroup\$ Note that with rectification there isn't just "voltage drop" in diodes: current will need to be "derated" depending on load and filtering (choke input somewhat unpopular for lack of DC chokes & choke components). \$\endgroup\$
    – greybeard
    Apr 12, 2023 at 6:50
  • \$\begingroup\$ Nothing in the product details that I can see suggests you can parallel the secondary windings KMN. It's usually a non-no anyway. \$\endgroup\$
    – Andy aka
    Apr 12, 2023 at 9:44
  • \$\begingroup\$ @Andyaka Why cannot the output windings be paralleled if each secondary winding produces the same voltage? \$\endgroup\$
    – KMN
    Apr 12, 2023 at 11:13

1 Answer 1


Assuming you connect the two 12V secondary windings in parallel, then the rated transformer output will be 12V AC RMS @ 8.3A RMS.

If you connect in series, you will have 24V AC RMS @ 4.1A RMS.

However, the relationship between the AC & DC voltages and currents will depend on how you setup the rectification and smoothing, and the nature of the load.

This leads to the "Transformer Utilization Factor" which is defined as the ratio of DC power, to AC VA load on the transformer. You'd hope this would be 1, but in most simple rectifier circuit it is much lower.

The absolute maximum values (assuming perfect diodes) for the common setups are as follows

  • Half-wave rectifier ~0.3
  • Centre-tapped rectifier ~0.7
  • Bridge rectifier ~0.8

(Detail of how these values are calculated can be found at the above link.)

Real world losses (such as diode drop) will lower this further.

In your case, a 100VA 12V transformer with a bridge rectifier should therefore not be expected to deliver more than 80W @ 12V DC, which is ~6.6A, and allowing for diode losses will probably be nearer 75W.


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