This confirms my original posting that the magnatising current is greatly
increased when the core shunts are removed, as previously stated use two
with primaries in series, then lifting the earth connections on the seconary
windings connect in series and make up a full wave bridge, not forgeting to
put a resistor across each diode to equalise the voltages.
----- Original Message -----
From: "Brenda Hayes" <[email protected]>
To: <[email protected]>
Sent: Wednesday, July 15, 2009 11:35 AM
Subject: LF: Microwave oven transformers
I have recently done some tests on a pair of identical transformers
removed from two identical ovens.
Perhaps unfortunately, I removed the magnetic shunts before doing any
no-load testing. After half an hour, with 235V applied to the primary
windings and no secondary load, the iron was far too hot for comfort..
At this stage, I measured the iron losses with an accurate wattmeter that
certainly takes account of the crummy power factor..
At 50 watts apiece, for no-load losses, I had visions of cooling fans etc.
My solution was to wire a double-wound 230/32V [150VA] transformer in
step-down "auto" style to feed the ex-microwave transformer primaries.
This step-down transformer showed negligible iron loss of 4W or so. With
a new primary voltage of 200, the standing iron loss dropped to around 30W
per ex-microwave transformer. Naturally, the secondary voltage has also
dropped. I now have about
1600 - 0 - 1600V from the two transformers when the primaries are
While the earthed ends of each HT winding could be lifted and FW bridge
rectification employed, I was reluctant to follow this path due to
There is another trick that I found useful for safely matching
non-identical transformers, provided they are much the same size:
Apply 230V to the HV secondary windings and compare output voltages -
usually 23/26 volts or so.
Watch where you put your fingers.