Hi all,
here is my follow up on the FET replacement in my 2215A, and the adding of an optional fan.
Using a P9NK70ZFP (the working one I started with) gave a FET housing temperature of 68 degree Celsius;
Now I received the 6R280E6, that brought the temperature (only) down to 63 degree Celsius?
Based on the 10x lower ON-resistance, dissipation should have been way lower, but it clearly did not work this way...
I still didn't like this, so I looked a bit further.
I had a look at the different PS schematics of a 2232 and a 2235 and learned that in the 2232 they lowered R908 from 3k to 2k2, but the serial resistance R909 was still 39 Ohm;
In the 2235 however the kept R908 at 3k, but the completely removed R909 ?!?!
A quick calculation with an input capacitance of 3nF and a series resistor of 39 Ohm learned, that the rise (and fall) -times would be around 200ns, so even with the better FET that could explain the dissipation.
So at first I lowered R909 from 39 Ohm to 18 Ohm, (a second 39 Ohm in parallel), that lowered the FET's temperature to 58 degree Celsius.
Since I don't like having no serial resistance to the gate of the FET (it might start to oscillate?), I replaced the original R909 of 39 Ohm with a carbon resistor of 4,7 Ohm, and kept R908 at 3k.
I have the scope running for a while now, and the FET temperature has settled at around 54 degree Celsius, this is still without fan.
Since I have the place for both the components and the fan, I will put one in a soon as it arrives.
I will use the half-bridge rectifier circuit using the one diode, capacitor and resistor, this will give a 9V DC for the fan.
If the electronics inside the fan allow for this low voltage I will keep it at that, if not, I will add the second diode to raise the voltage to 12V
To be continued when the FAN arrives,
Un saludo,
Leo
satbeginner