jakehop Posted August 26, 2010 Report Share Posted August 26, 2010 Hi guys,I'm new here at the forum, and I've been searching for a solution to my problem without any luck! I need to design a circuit, where I can control a negative voltage with a PIC (in this case, the AT-platform). I've got the PIC's PWM-function, as well as digital potentiometers to control the circuit. However, the PWM is limited from 0-5V+ DC, and the digital potentiometers have certain limits to the voltage at their taps.The negative voltage should be -25 to -70 Volts DC. -50 Volts will also be OK if -70 is totally out of the question. So far, I've got a opamp working as an inverting amplifier, but even the best rail-to-rail model can only give me -22 volts from the +5V sources that I have. I thought about having the opamp controlling an elevated LM337 to get the -25V to -50V, but I can't get it figured out in my head, as I constantly bump into the limits of the PIC or the digital potentiometers.Also, linearity is important for me. The PWM allows me a resolution of 8 bits - 256 steps. If this proves a problem, I'll use a DAC to get 1024 steps.Does anyone have a clue as to how I can solve this?Kind regards, Jake Quote Link to comment Share on other sites More sharing options...
Silent Jack Posted August 27, 2010 Report Share Posted August 27, 2010 There are numerous different models of the PIC MCU and many of them are pretty affordable. Have you tried using the pic to just perform a switching function, and then using an inductor, caps and a 555 to produce the levels you are trying to obtain? The PIC should be able to monitor and control process by tracking a sample voltage that is stepped down to levels it can handle. Quote Link to comment Share on other sites More sharing options...
jakehop Posted August 27, 2010 Author Report Share Posted August 27, 2010 Hi Jack, thanks for your reply!I am having a hard time of conceptualizing your idea. However, the circuit that I am looking to integrate this solution into already has a negative voltage source, that I would just need to attenuate. It's like -80-90V, that I'm looking to get down to -50-25V. I would like to get some linearity. My digital potentiometers can't handle this voltage, and I was hoping to get some kind of solution that made use of the 8bit D/A's in my pic, so I could get a resolution of, say, 0,1V in that 25V of potential.Is this impossible? I'm currently working with a optoisolator, however it's not nearly as linear as I could hope for, even though the manufacturer claims exactly that! Quote Link to comment Share on other sites More sharing options...
Silent Jack Posted August 27, 2010 Report Share Posted August 27, 2010 You could try dividing the voltage and using a capacitor to smooth things our for clearer readings. Quote Link to comment Share on other sites More sharing options...
Hero999 Posted August 30, 2010 Report Share Posted August 30, 2010 You need a negative supply, even the best rail-to-rail op-amps will only give you 0V, if the negative supply voltage is 0V and there's no load current.The easiest way is to use a buck-boost converter.http://en.wikipedia.org/wiki/Buck Quote Link to comment Share on other sites More sharing options...
jakehop Posted September 7, 2010 Author Report Share Posted September 7, 2010 You need a negative supply, even the best rail-to-rail op-amps will only give you 0V, if the negative supply voltage is 0V and there's no load current.The easiest way is to use a buck-boost converter.http://en.wikipedia.org/wiki/Buck Quote Link to comment Share on other sites More sharing options...
Hero999 Posted September 7, 2010 Report Share Posted September 7, 2010 When I was talking about a buck-boost, I meant using the PIC as a controller not buying a pre-made controller-IC but if you already have a negative supply then you don't need a buck-boost converter.I think you're already going down the right path with op-amps but you're correct in that finding one which will work up to 50V will be a problem and 70V is probably certainly out of the question so a buck-boost might be your only option after all, in which case you should read the Wikipedia article I linked to earlier. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted September 14, 2010 Report Share Posted September 14, 2010 The easiest way to generate -50V is from a battery. The PIC output can switch a MOSFET that is in series with the battery and the load. The positive terminal of the battery is connected to ground and the negative terminal is connected to the load. Quote Link to comment Share on other sites More sharing options...
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