The Cranky Sysadmin A world of technology, fun, and ignorant rants.

December 22, 2011

When in Doubt, Put in a Load Resistor

Filed under: Electronics,ham radio — Cranky Sysadmin @ 8:23 pm

I am experimenting with various oscillators and having a lot of fun. It’s the first time that I am having a lot of success putting together analog circuits that work. I am also having my brain-farts. I am working on a Vackar VFO. It turns out to be a stable VFO with a nice clean sine wave, but that has nothing to do with my brain-fart.

I was working with the actual oscillator section (the far left portion in this diagram) thinking to myself, “Wow, that jfet doesn’t have a load resistor”. Well, after putting it together and replacing the zener diode with a 6V voltage regulator, I powered up the oscillator. It worked great! Perfect sine-wave although it still drifts. I was messing with the trimmer on the gate of the JFET, when my power supply made a loud hum for a few seconds and all oscillation stopped. The JFET blew.

It took me two more JFET’s to realise that the circuit did indeed include a load resistor. It was the 270 ohm resistor in series with the power supply. I didn’t see this because I was so focused on the oscillator circuit. In my head, I had replaced the zener diode with a voltage regulator already and I ignored everything to the right of the choke (RFC). The lesson learned is of course, that if you think you need a current limiting resistor, put one in. Even if you don’t think you need one, put one in. Now I have to go buy more JFET’s. In the meantime, here is a picture of my first attempt at “ugly construction”.

"Ugly Construction" of a Vackar VFO with no current limiting resistor!

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