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mkr
Joined: 08 Aug 2006
Posts: 49
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| Linear driver for PID control |
 Posted: Mon Apr 30, 2007 5:10 pm |
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Hello Friends,
I am trying to source out for a driver transistor/MOSFET or device to control a PID valve. The valve spec. are 0-5v and current 0-320mA. The voltage and current characteristics are linear. I want to control the valve using the 12 bit ADC module in the MCU, which should not be a problem. 0 is 0v and 4095 is 5v to the corresponding ADC output.
Typically the MCU pin current is 20mA, which means I need a 2 stage driver. MCU will drive the 1st stage driver and then this drive will drive another drive. Is it right or can we do this in a more effeicent way. Can i drive the driver using the MCU ADC output itself.
I know its not the right forum, but I have come across experts in this forum who could answer both hardware and sw issues.
Your suggestion are most welcome.
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Thanks
mkr |
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JAYMILLER
Guest
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| valve control |
| Posted: Tue May 01, 2007 5:38 am |
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I've used the LM12CLK power opamp for similar projects for years.I know,a bit pricey, overkill for 99% of most needs, but it's 'bulletproof', can't be destroyed. Easy to use,setup as a voltage to curent converter. The LM675 is a cheaper,less powerful op amp which you could also try, if the money's tight.Nice thing about opamps, easy to understand,reliable.
HTH
Jay |
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cbarberis
Joined: 01 Oct 2003
Posts: 172
Location: Punta Gorda, Florida USA
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| Posted: Tue May 01, 2007 6:07 am |
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| Quote: | | The valve spec. are 0-5v and current 0-320mA. The voltage and current characteristics are linear. I want to control the valve using the 12 bit ADC module in the MCU, which should not be a problem. 0 is 0v and 4095 is 5v to the corresponding ADC output. |
If I understand you correctly the above valve opens or closes in a linear fashion based on the current/voltage magnitude?
And you want to use your level read by the adc (12bit) to directly control this valve?
If the above questions are true, it would seem to me that what you want is a voltage output generated by the adc counts, this could be done either by an external dac and current drive amplifier or just taking your adc reading and using your PIC pwm module generating a 12 bit equivalent pwm output, which you would then filter to convert to dc and drive your current amplifier that controls your valve. I have never used a PID valve, I would imagine that this device has the mechanical characteristics to provide some form of direct pid action? |
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mkr
Joined: 08 Aug 2006
Posts: 49
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| Posted: Tue May 01, 2007 6:41 am |
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You are right cbarberis. Its typically a voltage follower (unity amplifier). Its not easy to find one with the spec. I have. Yes, the ADC output will drive an op amplifier. Eg. Vin = 1.23v then Vout = 1.23v with higher current drive. I am looking at few LM series opamp from national.
Let me know if you have used a similar opamp.
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Thanks
mkr |
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cbarberis
Joined: 01 Oct 2003
Posts: 172
Location: Punta Gorda, Florida USA
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| Posted: Tue May 01, 2007 7:10 am |
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| Sounds like the old LM759 would be a good fit here, however, I am not sure that part is still available. Another method is to use a simple op-amp and use discrete power mosfets or bipolar transistors on the output and then close the feedback loop back to your op-amp. |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Sanity check |
| Posted: Tue May 01, 2007 7:19 am |
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| mkr wrote: | | Yes, the ADC output will drive an op amplifier. Eg. Vin = 1.23v then Vout = 1.23v with higher current drive. |
I hope you do realize that the ADC takes an analog INPUT and gives a digital OUTPUT. If you want an analog output you need to either use a DAC or a PWM output.
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The search for better is endless. Instead simply find very good and get the job done. |
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