# Help with transformer load and limits

#### tbfunk

Joined Dec 9, 2010
30
So I have a transformer that claims to be rated for up to 208v input and at that voltage will output 12v-0v-12v or 24v. Thinking I knew what I was doing I hooked it up directly to 110v household mains, thinking I can use this lowered voltage to get more amps out the back. When I put power to the transformer it showed 8.5v out or 17v together and after rectification I have a nice 23.7vdc. I can seems to get the math right and I guess I am struggling with it. The biggest load I have tried for far measured at 6.6amps dc and the transformer didnt even warm up. My math for far is: 3.75va is rated for 208v and 110v is roughly half that so double to get roughly 7.5va. Then my output ac voltage is 17v so 7.5/17=441mA roughly. This seems really incorrect and I keep reading through the transformer design chapter of Vol II AC on this site and still dont get how to use VA properly.

My question is how big of a load can I put on this transformer before blowing it up if the transformer is rated at 3.75va.

#### crutschow

Joined Mar 14, 2008
27,234
A transformer's output voltage is rated at full load.
It will be higher at no load, which is what you are seeing.

If the transformer rating is 3.75VA then its rated RMS output current is 3.75 / 24V = 156mA.
This current rating does not change when you lower the input to 110V since that rating is based upon wire size.

For the rectified output you need to derate the current about 40% due to the higher RMS value of the high peak current through the rectifiers, giving a maximum DC output current of 94mA.