Re: overloading wirewound resistors


John Larkin wrote:

The original design used a switcher, but it had a tendency to blow up.
The virtue of using a real resistor is that we can bring the thing up
and do a lot of self-tests with no direct silicon path from V+ to
ground or V-. When there's this much power involved, you don't want to
just pray and hit the circuit breaker. The boost supply is used at
powerup for the preliminary fet tests, as a safe, current-limited test
supply voltage. Later on it's used to precharge the filter caps before
we kick in the big power supply, and it's finally used as the boost
voltage for kicking L di/dt into gradient coils.

I've always wondered what it's like to power up the prototype of, say,
a megawatt AM transmitter.


The early ones were tube designs, and used a pair of custom output
tubes. Modern solid state Am broadcast transmitters are modular designs
where they use whatever number of identical RF output stages they need
to achieve the rated power. They are built & tested at the module
level. Harris Broadcast builds high power solid state transmitters.
They have pulled all the white papers off their website that described
the designs. Now its another crappy website with loud music. :(

http://www.broadcast.harris.com/

Their first generation design used banks of matched TO-3 transistors
in each tray. The transmitter ran directly off either three phase 208,
or the standard 240 volt single phase service. No power transformer,
just a big modulated switch mode power supply to generate the AM
signal. I've seen piles of trays damaged by direct lightning hits here
in Central Florida from a 5 KW transmitter for a station near Leesburg.

The transmitter detects failed modules and switches them out of the
system, then notifies the engineer of the problem by email.

<http://www.google.com/search?hl=en&rls=com.microsoft%3Aen-us%3AIE-SearchBox&rlz=1I7GGLD&q=harris+broadcast+am+transmitter&btnG=Search>
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