Ham Radio Forum

Hi. I have construct this linear 500w rms (2kw pep) (50ohm) with 12 mosfet irfp360, http://tzitzikas.webs.com/linear500w.jpg

schematic.
The manufacturer and other radio amateurs from a greek forum have construct it successfylly.
The manufacturer tells that the linear requires only 2 watts r.f driving power for full 500w output r.f
power. The supply is 70-110vdc and for maximum power with 110vdc tha current is 13A (without am modulation).
Without driving power, we must adjust the current at 200mA (when the linear is cold).
My problem is this: When i tried to adjust the current at 200mA without driving power, when the current close to
160mA (by tuning the variable resistance 5Kohm) suddenly the current takes the value 3A and some smoke comes out
from the input transformer.

some photos from my linear and pll here:
http://www.tzitzikas.webs.com/linear10.JPG
http://www.tzitzikas.webs.com/fet.JPG
http://www.tzitzikas.webs.com/linear11.JPG
http://www.tzitzikas.webs.com/linear12.JPG

With 2w input driving power from my pll and supply at 110vdc, and 150mA current without input r.f power, the output power was

only 60w and the current was only 3,5A without modulation.
Witch do you think is the problem? Thank you very much and sorry for my English.

Unless you have made a circuit wiring mistake or have used a wrong resistor value, I think you probably have a defective IRFP360 mosfet.

sparky wrote:Unless you have made a circuit wiring mistake or have used a wrong resistor value, I think you probably have a defective IRFP360 mosfet.

the mosfet are new.

Of course, one of the mosfets could be bad even if they are new. I agree that the most likely problem is a bad mosfet.

i sort all connections and i think the problem with oscillation stop. I adjust the current at 200mA without driving power, and after with 3w input driving power with 106vdc power supply the output was 200w. The current was 6A. I can't understand what is the problem and my linear gives only 200w. The manufacture claims that with 110v and 2w driving power the linear gives 500w output R.F power. Do you have to propose any change to give the linear more output power? Maybe in R.F transformers? (to change the number of turns?). all mosfet gives 500ohm between drain-source. Thank you very much

new photos

http://tzitzikas.webs.com/fet20.JPG
http://tzitzikas.webs.com/linear20.JPG
http://tzitzikas.webs.com/linear21.JPG
http://tzitzikas.webs.com/linear22.JPG
http://tzitzikas.webs.com/fet21.JPG

Hi. the mosfet that i am using (IRFP360) are no name. Do you believe that the linear gives less power than the prototype because the mosfet are no name instead of the prototype which is using International Rectifier mosfet?

There is know way to know where the mosfets you are using were manufactured, where the manufacturer isn't identified. However, I think it is more likely that one of the mosfets is simply defective than that the manufacturer is producing substandard devices.

Kendall