Transformer Question

[law_freak]

I'm working on a radio from a 1956 Oldsmobile. This question relates to the power supply.

Why are the primary and secondary of the power transformer not isolated? Why is 12VDC applied to both the center tap of primary and the center tap of the secondary?

I understand how a vibrator power supply works, but would have figured that only the primary winding would have 12VDC applied to its center tap.

Schematic attached. The unlabeled line from the connector is 12VDC from the battery.

Radio model #983334 by Delco

[IanB]

Both sides of the transformer share a common ground reference. It is common in motor vehicle applications to reference everything to the vehicle chassis, which is presumably what is happening here.

Note that it is not possible to apply 12 V DC to a single terminal like the center tap of a transformer. All voltages are potential differences, and therefore to assign a potential difference of 12 V DC you must specify the two points in the circuit between which this potential difference exists.

[chris_leyson]

Why are the primary and secondary of the power transformer not isolated? Why is 12VDC applied to both the center tap of primary and the center tap of the secondary?

You could just as well have grounded the secondary center tap but my guess would be that it might have something to do with common mode noise on the +12V.

[floobydust]

It's to get an extra 12V on the secondary side. The 280V is referenced to +12V so a bit less output voltage (268V) needed from the vibrator/transformer.