I have been told that toroid rings used in output matching of a somewhat powerful transmitter can be damaged by exposing them to too much wattage for too long a time.
Case in point, the ATU for a 20-Watt carrier current transmitter has a warning not to leave the coupler in the Test Mode for more than 5-minutes, and my understanding is that the toroid can over-heat and become damaged.
IF a toroid does become damaged by overheating, how can the damage be detected?
Is the damage permanent, or can the toroid be saved by cooling it back down?
There is an informative paper available on the Micrometals Inc. website titled "Iron Powder Core Selection For RF Power Applications". It says the following: "It is important to limit the operating temperature of inductors and transformers using iron powder cores. Long term operation of iron powder cores above 80 °C can cause a permanent reduction in both Q and inductance". That's 176 degrees F.
Note that it says "long term". Short term temperature can go higher without causing damage, but there is no data on what the temperature limit is for a short term. The document does say that intermittent temperature peaks can be averaged over time and the average shouldn't exceed 80 degrees C.
I've heard the same thing about ferrite beads. Too much heat ruins their electrical properties.
One indicator of abuse is discoloration of the paint on the toroid. This was observed with the output filter toroids in my Ramsey AM-25. Initially, this was operated as detailed in the instructions and doing so resulted in discoloration of the PC board under the power transistor and the output inductors. The toroid color darkened quite a bit but it is unknown if the properties changed. It would seem that if the paint darkened then the toroids were operating at a temperature higher than 80 C as mentioned above.
Subsequently, the toroids were replaced with a more suitable type and the power input was lowered to 100 mW and there has been no problem since.
Neil
Is there a temperature sensing device that can be placed with the toroid to allow monitoring its temperature?
Or, if there is possibility of too much heat, a small fan might be a good safety measure.
In about 1989 I had a Part 15 experiment
running on the 3rd floor of a apartment house in
West Hartford, CT.
The transmitter was a Panaxis AM-100. A great
transmitter, but as many of you know, it would
only output on 600, 750, 1000, 1200, and 1500 kHz.
It had a 6 MHz osc. that got divided, and the above
channels were the only options. 750 kHz was dead
during the day and not bad at night, either. So
I went with 750.
I had ABSOLUTELY NO IDEA how to get this
transmitter to put out a signal that would travel
a decent distance.
I grounded the transmitter to an outlet
strip up on a table where the "operating
position" was. (It was more like a big pile
of junk.) I took a "ferrite bar" out of a radio
and hooked that up to the AM-100 output somehow.
I was not a tuned system. I was sort of using the
"ferrite bar" as an RF transformer, I guess. I believe
that was the logic. So theoretically 100 mW was going
into that "transformer?" I didn't know anything about
damage or saturation then. I just let it run.
It got out to my neighboring streets, but I
think it was mostly being reradiated by power
lines. Also, I grounded the thing to an outlet
strip as mentioned, and kept the ground lead
and transmit antenna under 3 meters. The
intent was to be legal. But I suppose the
ground connection from the outlet strip all
the way down to the cellar was radiating, too.
Still, it was an honest attempt to go by the rules.
I just wonder what was really going on inside that
little "ferrite loop transformer."
This particular set-up on 750 kHz
had no name. I wish I had called
it "something" so it could be put on
the list of all of the Part 15 stations
I've tried to make.
Bruce, The Dog Radio Group
Dear Bruce:
There is a secret used by governments and religions to fix history.
It is called "revisionist history".
Instead of reporting what truly happened, as you have regarding the Panaxis 100 experiment which lacked a name...
With revisionist history you tell the story the way it should have been. You give it a name. Let's call it the "Panaxis Ferrite Bar Experiment".
From now on the revisionist version is the story to tell, forget all about the truth.
Here is the link referred by PhilB. It contains a lot of information.
Carl, I believe the reason for not running in "test" mode for an extended period of time is to prevent overheating the dummy load resistor(s) which dissapate the transmitter power while testing.
MRAM is right about the dummy load resistors, I was thinking wrong when I imagined that the torids were put under stress during the ATU test mode of a carrier current system.
I'll bet adding a cooling fan across the resistors would extend the time of safety for operating in the test mode.