solid state switch to full charge and discharge capacitor

[John Parker]

I need to full charge and discharge capacitor with some solid state device like transistor (FET) or avalanche diode/transistor. The problem is that this device have to turn on at high voltage and turn off on low voltage.
Any idea?

 

[cowboybob]

I need to full charge and discharge capacitor with some solid state device like transistor (FET) or avalanche diode/transistor. The problem is that this device have to turn on at high voltage and turn off on low voltage.
Any idea?

Please define the parameters for us. For instance, to what voltages levels are you charging/discharging the cap? What voltage levels will be the "turn on/turn off" signals?

 

[John Parker]

Please define the parameters for us. For instance, to what voltages levels are you charging/discharging the cap? What voltage levels will be the "turn on/turn off" signals?

I want to build a solid state marx generator powered by a 9V battery. Spark gap need higher initial voltage and therefore I need an other switch such as relay or avalanche diode or MOSFET. The last seems the best to me. I want to get the whole 9V on every capacitor and finally it will be an 5 stage stuff with 9V*5stage=45V output. It will be plenty enough as first experiment.

 

[dr pepper]

Drive a fet from a 555 or a tl494, you can adjust the frequency to the size of the capacitors.
To get the most from the ladder and to double the input voltage you might be better with a H bridge, you can use 4 seperate fets or a bridge driver ic, L298 springs to mind buts thats a dual H and a bit overkill.

 

[John Parker]

Drive a fet from a 555 or a tl494, you can adjust the frequency to the size of the capacitors.
To get the most from the ladder and to double the input voltage you might be better with a H bridge, you can use 4 seperate fets or a bridge driver ic, L298 springs to mind buts thats a dual H and a bit overkill.

Thanks but IC make my circuit too difficult. A MOSFET can fully open above 4V and much faster than any IC. I think I need a voltage divider to put 4V up to the top of capacitors but I don't know where I can connect the gate because I lose current at the voltage divider what is undesirable. What if I put two resistor instead of one between one side of capacitors and connect gate between these? The total value of these will be equal to the other side but I can adjust the ballance. What do you think?

 

[dr pepper]

You could do it with discretes but thats too much work for me.
If your going that route look up blocking oscillators, nice and simple and often used for generating ht, they were common as ccfl drivers in scanners before they got led's.