Power Supply Transformer Loading & Measuring-Quick Quest

[beatpoet]

I got a job with an electronics supplier, so now I get all parts at factory cost :grin:

I have been eyeing up these 25.2V center tapped transformers for use in racked +/-15V power supplies.

Unloaded however, the transformer puts out @14V per leg. I was expecting more like 12.1 x 1.414 (@17+V) unloaded.

Am I wrong that these are out of spec, or do meters just read in RMS? When I add the rectifier/capacitor, will I get the full peak which would be 14V x 1.414???

I'd hate to start building and figure out that there's not enough voltage for my regulators, even a +/-12V :!:

Thanks for any help in advance :guinness:

 

[JohnRoberts]

There are several considerations.

The transformer winding will typically be rated for it's loaded voltage. So with no load it will be higher.

DC unreg consideration is that bottom of ripple saw tooth is still above the dropout voltage of the voltage regulator.

Then you need to factor in you expectation of mains low line voltage. This is more of an issue in some regions than others.

Finally after you have insured that you have adequate voltage, you need to look at high line with minimum load conditions to spec voltage of PS caps and check that you don't exceed regulator max input voltage or power dissipation capability.

So there are several moving parts to consider. From memory I think I used a 28V center tapped winding to generate adequate unregulated for standard 78/79 3 term regulators at modest current. There may be differences between regulation (voltage drop due to winding impedance) of different transformer approaches (torroid, EI, split bobbin, etc) so it's always good to breadboard one up, and test with a variac for hi/low line.

JR