Re: VLF: tonite on the 36km band...

[DK7FC <selberdenken@posteo.de>]

Hello Claudio,

Thanks for the suggestion.
I know these thermostats. There is one in the power supply that i used for VLF some months ago. The thermostat failed, the fan stopped, the supply became so hot that the solder on the PCB melt until the current was interrupted :-) Now that thermostat has been removed and the fan runs all the time (just a small silent one) :-)
There will never be a full protection. Something can fail.

But there is another circuit that automatically lowers the DC bias gate voltage of the linear mode PA. Using normal diodes which are thermally coupled to the heat sink, close to the FET. If the FET becomes to hot, then the forward voltage of the diode will drop, which will reduce the drain current. This is not yet realised, but considered. These FETs are biased to about 3.75 V and they have a 0.2 Ohm source resistor. I could use 0.1 Ohm maybe and 3 diodes plus a resistor to produce the gate voltage...

73 to Italy,
Stefan



Am 06.05.2017 20:41, schrieb Claudio Pozzi:

On Thursday 04 May 2017 22:25:43 you wrote:

> Hello Markus,

>

> Very good! With that SNR you could decode the 100 character message.

>

> The 3 char transmission for tonite failed. The fan has blown itselfe

> from the table, so the PA overheated again. One FET is dead and two 0.2

> Ohm resistors are dead as well. Time to build up the new design but i

> have to wait for some components...

> Now repairing the old design and continue to transmit...

>

> 73, Stefan

 

Hi Stefan. I know this fail!

 

I suggest a thermostat on the heat sink that remove the MOSFET DC supply.

 

http://uk.rs-online.com/web/p/thermostats/2282636/

 

73 de Claudio ik2pii