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HV PS Questions: Overload Protection Circuit

PostPosted: Sun Jan 21, 2007 7:01 pm
by k3tue
Looking at the schematic for the Power Supply for 360 Watt Class E Transmitter and Class H Modulator (http://www.classeradio.com/psm_modulator_power_supply.gif), specifically looking at the Overload Detector, I think I understand that the Overload Detector circuit just opens the keying line voltage when enough voltage drop is seen across the 0.1 ohm resistor. I did some calculations ans am curious about something.

Using 30A as a reasonable overload condition (125V at 20A + 62V at 8A)
P = I^2 x R
P = 30^2 x 0.1
P = 900 x 0.1
P = 90W

Why then is the 0.1 ohm resistor only rated at 25W?

Even at 28A (the rated peak) you will need to dissipate 78.4W through this power resistor.

What am I missing?

PostPosted: Sun Jan 21, 2007 8:01 pm
by kf1z
Well, I believe at carrier, the resistor is taking what, about 7 ish watts...?

The resistor is only taking more than that on an average basis....
(modulation peaks)


And I believe that the way the class - H modulator works ( that power supply is designed for) the peak current is 20 amps total (maybe wrong...)

In which case the total wattage the resistor would ever see would be 40w.

But, that's only peaks, and I'm sure those 25 watt resistors would handle it... as long as it's on a heat-sink......


Maybe someone elsse will comment too, and explain better,,,,
But I know they are in use!


I wonder if you've considered using the overload circuit that was designed for the PWM modulator?
The overload/efficiency meter.....

That overload circuit is VERY fast, and likely more reliable than the other one. (yes, more complicated, and more expensive, but worth it)



Bruce G

PostPosted: Sun Jan 21, 2007 9:09 pm
by k3tue
Bruce G, kf1z wrote:I wonder if you've considered using the overload circuit that was designed for the PWM modulator?
The overload/efficiency meter.....

That overload circuit is VERY fast, and likely more reliable than the other one. (yes, more complicated, and more expensive, but worth it)


It looks interesting, but cost and complication will lead me to let that one go for now in favor of simplicity. But thanks for pointing it out.

PostPosted: Tue Jan 30, 2007 11:48 pm
by steve_qix
Hi !

I would use the new, fast circuit. There is a PC board for it - working and ready to go. The fast circuit will actually protect your MOSFETs from meltdown, and also provides a very good measure of sequencing.

The new circuit also implements the efficiency meter, which is absolutely wonderful in letting you tune up your transmitter, no matter what state of un-tuned-ness it is in, WITHOUT an oscilloscope.

The new circuit interfaces with either new class H modulator OR the new PWM modulator. I've been using this circuit for several years now, and it's very nice.

There is one "expensive" component on the board - the current sensor - an LTS 25-NP, which I think costs about $15.00 . Well worth the price !

There is also a new class H modulator schematic.

The new documentation can be found at:



Or on the class E web site

Regards,

Steve