PSU transformer rating

I have a handful of PSU toroidal transformers that are rated for 12v at 2A. My tube heater circuit draws 1.89A on the DC side.

What do you think? Too close to max rating for comfort?

I would overate by 50%.

If the 1,89A is rectified current with a diode bridge, you need minimum 1,89A x 1,6 = 3,024A.

with a calculator in hand, take your pick

and never trust manufacturer transformer ratings and leave some safety overhead - I don't know 20% or something.

BluegrassDan said:

I have a handful of PSU toroidal transformers that are rated for 12v at 2A. My tube heater circuit draws 1.89A on the DC side.

What do you think? Too close to max rating for comfort?

Click to expand...

Definitely.  You have to allow for the losses in rectification and smoothing. If you rectify 12VAC you get about 16V raw dc, probably dropping to 15V on load. At 1.89 this is over 28W, which is way over the rating of the transformer. Any series resistance you use to drop the volts to 12V just dissipates this extra energy.

Sowter has some design guide for different types of rectifier:

http://www.sowter.co.uk/rectifier-transformer-calculation.php

Mains transformers tend to be rated very close to saturation so it is very easy to overload them especially with the peak currents you get with semiconductor based rectifiers with capacitor input filters. This causes all sorts of problems including over heating and flattening of waveforms. I now habitually rate mains transformers at twice the dc power.

Cheers

Ian