Cornet - Hammond 270BX transformer out of spec.?

TChung · « **on:** 21 Jan 2004, 06:19 am »

I took AC voltage measurements on the Hammond 270BX transformer with 2 different multimeters and these are the readings:

Spec. = 5V, reading = 6.22V
Spec. = 6.3V, reading = 7.85V
Spec. = 225 - 0 - 225, reading = 333 - 0 - 333
The transformer was not connected to PCB.
I also measured the voltage of the power line and it was 125V.

I wonder if this transformer is defective because the discrepencies are so very big. Will I damage the circuit?

Thanks,
Tom

GRD · « **Reply #1 on:** 21 Jan 2004, 06:22 pm »

Those are about the same readings I got when I measured the output voltages my 270BX transformer before installing in the Cornet. Example 317v - 0 - 317v with 120V line power. Your readings look OK.

Brian Clark · « **Reply #2 on:** 21 Jan 2004, 06:50 pm »

Your readings are "off load" Tom. Once current is drawn through the windings the copper losses will reduce the voltages to the appropriate values.

In circuit the voltages should less than 10% off.

Brian.

TChung · « **Reply #3 on:** 21 Jan 2004, 11:38 pm »

Thanks to both for your responses to a newbie's question. I am feeling better with continuing now.

Tom

hagtech · « **Reply #4 on:** 22 Jan 2004, 05:05 am »

Yes, those are unloaded measurements. Everything changes when pushed to the specified current. There is a lot of series resistance in the secondary windings. You can actually measure resistance, then calculate voltage drop for a given load. It comes out pretty close to specs.

Also note the Hammond 270BX is rated for 117Vac primary (I think). Putting 120Vac or higher into has a proportional change in outputs.

Then there are core losses, etc. Copper has a very large positive temperature coefficient, too. Fortunately the Cornet circuits are quite tolerant of variations. You can really tweak on the sonics by changing rectifiers.

jh