Adding an indicator LED on RF generator, where should I?
In my RF generator I want to add an indicator ON/OFF LED
Obviously I could add it after the regulator, but I do not feel comfortable to have such a device connected directly across the resonators, as this might induce non linearities and produce harmonics. But maybe a big resistor (say 10k?) in series with the LED can eliminate this?
Then I could add it in the shielded PSU but the LED is on the front panel and a long cable would have to be used to connect it to the shielded PSU (which is at the back of the enclosure) and I am afraid that this might induce noise to the rest of the oscillator from the PSU and the benefit from shielding the PSU lost.
One place I could add it is at the base of the 2n2222 regulator with a resistor to the ground (or even without a resistor, since there is already a 1k there?). Will it be ok to add it there and will it be of any benefit? Won't this, together with the regulator, act as a constant current generator then, such as the BC547/LED combination in the RF amplifier?
It wouldn't be across the tuned circuit, it would be across the supply and in parallel with 150uF and 100nF capacitors so there is no chance of changing the RF characteristics, besides, it would have a series resistor giving even more isolation.
Why are you considering adding it to the base of the regulator transistor, all that could do is upset the regulation, you want only the Zener to be responsible for setting the voltage.
I think you misunderstand how the constant current generator works, the LED is only used like a crude low voltage Zener, the current through the 10K series resistor helps it to maintain about 1.6V across it. The transistor drops about 0.6V across it's base-emitter junction so there will be 1.0V on the emitter. The 33 Ohm emitter resistor therefore has to pass a constant (I=V/R) 1/33A or 30mA. It isn't a particularly precise way to do it because LED forward voltages vary, even within a batch and they are also prone to change characteristics when light falls on them.
Brian.