Greetings from my first post here! (not counting that test one, sorry about that).
What I have are 24 channels to control. Each channel drives an almost purely capacitive load. The capacitance of the load for each channel is low (in the fempto range). This is really a DC circuit, and I really only need for each channel to charge its capacitor and hold the voltage across it. The voltage for each channel would preferably be in the range of 0-400V. I would like to be able to control each channel from a computer linked to a cheap microcontroller such as the PIC18F45K20. Cheap cheap cheap is the overall name of the game here.
What I have tried so far is to setup an NPN transistor in the common emitter configuration, with 400V going through a resistor and into the collector of the transistor. Emitter hooked to ground. Capacitive load hooked between collector and emitter.I then had a separate power supply of 10V that drained into a potentiometer which then sourced a resistor going into the base of the transistor. By using the potentiometer to vary the voltage going to the base resistor (which in turn varied the base current), I was able to vary the voltage that was going to the capactive load. I had a circuit like this for each of the 24 channels. I also included an op amp in the original design for each channel as a voltage follower on the potentiometer. I have attached a hand drawn schematic.
After realizing that hand adjusting the potentiometers for each channel wasn't a feasible option in the long run (and probably not as safe as I'd like it), I decided to try out computer control (e.g., a GUI with sliders for each channel). What I have come up with is to replace the hand adjustable pots with digital pots that I can control via I2C with a microcontroller. I found some dig pots that each have 2 channels on a chip, and can be configured for 8 different I2C addresses (i.e., 8 of that same model chip on one I2C bus). So I was thinking 2 I2C buses with 6 chips a piece (total of 24 channels), then use 2 digital pins from the MCU to write to each I2C bus (4 digital pins total). Communication from PC to MCU done via serial commands over a USB to TTL connector. I've also attached a simple schematic of that.
The idea with using I2C communication is that digital outputs are cheap and common whereas it seems analog outputs will run a higher cost. If I had a cheap MCU with 24 analog output channels then I could just directly vary the voltage to each transistor base, but it seems like it will cost quite a bit more than the PIC's $2 price, but I could be mistaken of course.
I am just wondering overall if there is a better way to do this. The common emitter design was just the first thing that seemed cheap and like it might work so I went with it. The charge time for each channel doesn't have to fast as you could probably guess by the manual control. A few seconds would even be acceptable, so low current is not huge issue. Making the computer programs isn't an issue either (that's probably what I'm best at).
Please give me some feedback and suggestions on this. Thank you very much for your time.
What I have are 24 channels to control. Each channel drives an almost purely capacitive load. The capacitance of the load for each channel is low (in the fempto range). This is really a DC circuit, and I really only need for each channel to charge its capacitor and hold the voltage across it. The voltage for each channel would preferably be in the range of 0-400V. I would like to be able to control each channel from a computer linked to a cheap microcontroller such as the PIC18F45K20. Cheap cheap cheap is the overall name of the game here.
What I have tried so far is to setup an NPN transistor in the common emitter configuration, with 400V going through a resistor and into the collector of the transistor. Emitter hooked to ground. Capacitive load hooked between collector and emitter.I then had a separate power supply of 10V that drained into a potentiometer which then sourced a resistor going into the base of the transistor. By using the potentiometer to vary the voltage going to the base resistor (which in turn varied the base current), I was able to vary the voltage that was going to the capactive load. I had a circuit like this for each of the 24 channels. I also included an op amp in the original design for each channel as a voltage follower on the potentiometer. I have attached a hand drawn schematic.
After realizing that hand adjusting the potentiometers for each channel wasn't a feasible option in the long run (and probably not as safe as I'd like it), I decided to try out computer control (e.g., a GUI with sliders for each channel). What I have come up with is to replace the hand adjustable pots with digital pots that I can control via I2C with a microcontroller. I found some dig pots that each have 2 channels on a chip, and can be configured for 8 different I2C addresses (i.e., 8 of that same model chip on one I2C bus). So I was thinking 2 I2C buses with 6 chips a piece (total of 24 channels), then use 2 digital pins from the MCU to write to each I2C bus (4 digital pins total). Communication from PC to MCU done via serial commands over a USB to TTL connector. I've also attached a simple schematic of that.
The idea with using I2C communication is that digital outputs are cheap and common whereas it seems analog outputs will run a higher cost. If I had a cheap MCU with 24 analog output channels then I could just directly vary the voltage to each transistor base, but it seems like it will cost quite a bit more than the PIC's $2 price, but I could be mistaken of course.
I am just wondering overall if there is a better way to do this. The common emitter design was just the first thing that seemed cheap and like it might work so I went with it. The charge time for each channel doesn't have to fast as you could probably guess by the manual control. A few seconds would even be acceptable, so low current is not huge issue. Making the computer programs isn't an issue either (that's probably what I'm best at).
Please give me some feedback and suggestions on this. Thank you very much for your time.
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