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In Reply to: RE: Q:DN2540 CCS protection diodes posted by deathtube 667 on January 05, 2017 at 18:06:34
Why are you putting them back to back? You only need to limit the gate to 12 volts above the source.
Follow Ups:
Why doesn't he set R1 with a pot as opposed to a fixed value because current can change with different tubes of the same type?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
The current through the circuit is set by the CCS, not the tube.I build and set the current of my CCS on the bench with a low voltage bench supply and a 100 ohm resistor in place of the tube before I install them.
Once installed, even though there is now a tube and a lot more voltage, the current is always the same as it was when I set it.
That proves that the CCS is working.
The only thing that will change when you sub in a different tube (of the same type) with different transconductance (do to age or condition) is the voltage at the plate.
If you want a certain voltage at the plate you don't change the current through the circuit, you change the bias condition of the tube by changing the value of it's cathode resistor. When you do, the current through the circuit stays the same.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/06/17
You can, and I do, set the current with a 9V battery source on the bench top, before installation, and it remains constant in the actual circuit.
It is useful to battery bias even the depletion mode MOSFET's. 4x of the 3.3V button cells will do. The voltage across the cathode resistor is then larger and a single 1% resistor can be used instead of a pot dialed in to set it as precisely. This does require two 9V batteries to check the current set level though...:)
cheers,
Douglas
also, at high currents it would also be of use to use one of the 10M400( or the higher voltage units ) bits since they have a larger bias voltage, and the lower FET does better with more voltage across it. Watch the gate stopper value on them, as with more voltage across the DN3545 or DN2540 the capacitance is lower and you'll probably find the value needs to be higher.
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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