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Dave · March 10th 07, 05:33 PM posted to rec.radio.amateur.boatanchors

wrote:

On Mar 9, 5:00 pm, wrote:

What is generally indicated when a power transformer is very hot, and
the 7w power transistor in its vicinity is also very hot? The radio
works but will often "go quiet" after 10 minutes or so of operation.
Any suggestions would be greatly appreciated.
Pete

I meant resistor, not transistor

There are two likely situations as to why a transformer runs 'hot'. The remedies
have been briefly discussed. Let me add some background.

First, the design of the transformer uses a magnetic material that is
non-linear. As the magnetizing force is increased to higher levels the B-H curve
goes sub linear and the cores starts to saturate [the self inductance decreases]
and more primary current is drawn into the transformer [more heat].

The transformer equation for 60 Hz sine waves is E = 4.44*f*N*B*A. Where f
equals line frequency, N equals the number of turns on the primary, B equals
flux density, and A equals cross sectional area of the core primary. Since f, N
and A are fixed, an increase of voltage requires an increase in flux density
[b]. B equals the permeability [u] time the magnetizing force [H]. The value u
is very non linear and decreases in transformer material as H is increased to
higher values above the 'linear' region.

Conclusion, transformer get hot. Solution: reduce the primary voltage as
suggested in other responses.

Second, older designed electrolytic capacitors used a 'wet' dielectric. Simply
because capacitors get old [don't we all] the degree of 'wetness' dries out and
the dielectric dissipation factor increases. The capacitors, in common language,
become lossy. The isolation resistance can change from millions of ohms to a
value of a few tens of ohms.

Conclusion, I^2*R losses increase in the lower [lossier] resistance. Some
capacitors can be reformed. If there is large design margin, spec value divided
by stress value [eg; 450 VDC rating/200 VDC operating condition], then it is
possible to 'reform' the capacitor by gradually increasing the operating voltage.

Preferably, the older electrolytic capacitors should be replaced.

Either or both situations should be checked to assure that the 'hot' transformer
is not over stressed.

Note: transformers typically run at 95 percent efficient. So, some heating is
normal.

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