Post by thread:Circuit for 0-1 amp variable current

Hi I have a proportional valve that requires variable current to operate. At approximately 800 mA the valve is fully open. It is proportional from approx 0 amps to 800 mA. I want to drive the valve with a pic or stamp. Can anyone recommend a circuit that a pic or stamp can drive which can output that range of variable current? Maybe PWM at a high frequency? Any help would be great. Thanks Jon

Jon Petty wrote: > Hi > > I have a proportional valve that requires variable current to operate. At > approximately 800 mA the valve is fully open. It is proportional from approx > 0 amps to 800 mA. I want to drive the valve with a pic or stamp. O.K. Easilly done, use a resistor ladder on an 8 bit port to be a crude D-A. Use that signal to drive a Transistor or FET. What is the control protocol? (How does the PIC know what value to send?) {Quote hidden}> Can anyone recommend a circuit that a pic or stamp can drive which can output > that range of variable current? > > Maybe PWM at a high frequency? > > Any help would be great. > > Thanks > > Jon -- * | __O Thomas C. Sefranek spam_OUTtcsTakeThisOuTcmcorp.com |_-\<,_ Amateur Radio Operator: WA1RHP (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz hamradio.cmcorp.com/inventory/Inventory.html http://www.harvardrepeater.org

Jon Petty wrote: > I have a proportional valve that requires variable current to operate. > At approximately 800 mA the valve is fully open. It is proportional > from approx 0 amps to 800 mA. I want to drive the valve with a pic or > stamp. > Can anyone recommend a circuit that a pic or stamp can drive which can > output that range of variable current? Maybe PWM at a high frequency? This was discussed a month or two back. PWM from a PIC is the perfect solution. The inductance of the solenoid smoothes the current for you. You need a commutating diode of suitable performance, voltage and current similar to the solenoid and reasonable response speed. -- Cheers, Paul B.

part 0 1203 bytes By using a constant current control we automatically compensate for heating effects at the coil. You MUST heat sink the FET! While this exact circuit has not been tested, I have built similar circuits that have worked fine. You may have to tweak the value of the 2k resistor to calibrate the circuit so that 100% PWM = 1 amp 50% PWM = .5 amp, etc. The capacitor value may have to be adjusted, as it is dependent upon the PWM frequency. Fr. Tom McGahee {Original Message removed}

Hi, Nice circuit btw: some PowerFETs need gatevoltages near ot higher then a standard Opamp can have as an max. outputvoltage with a Vcc of 5V, so I would suggest a VCC for the Opamp higher then 5V, 8V should be ok, 12V are ok. >Jon, >Attached is a PDF file called 1amp.pdf that will take a PWM signal >and convert it to a constant current from 0 to 1 amp. Michael Wieser .....m.k.wKILLspam@spam@nextra.at Service and Audiodesign

At 08:52 PM 3/30/00 +0200, you wrote: >Hi, > >Nice circuit > >btw: some PowerFETs need gatevoltages near ot higher then a standard Opamp >can have as an max. outputvoltage with a Vcc of 5V, so I would suggest a >VCC for the Opamp higher then 5V, 8V should be ok, 12V are ok. Thomas, looks good, but I'd really suggest compensating the amplifier, as it looks possibly unstable to me the way it's drawn, considering the very high gate-source capacitance of a power FET and the low-valued load resistor. 1K series to inverting input and 10n from output to inverting input should do it. Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

At 02:16 PM 3/30/00 -0500, you wrote: Oops, I meant to say "low valued sense resistor" not "low valued load resistor" (there is no load resistor), and of course I am talking about the *op-amp* output wrt the capacitor. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam.....interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

The pic will have a lookup table based on several sensor inputs to determine valve position Thanks Jon In a message dated 3/30/00 5:31:44 AM US Mountain Standard Time, EraseMEtcsspam_OUTTakeThisOuTCMCORP.COM writes: << O.K. Easilly done, use a resistor ladder on an 8 bit port to be a crude D-A. Use that signal to drive a Transistor or FET. What is the control protocol? (How does the PIC know what value to send?) >>

Thanks Tom I'll look over the schematic. I appreciate the help. Jon In a message dated 3/30/00 9:12:51 AM US Mountain Standard Time, tom_mcgaheespam_OUTSIGMAIS.COM writes: << Jon, Attached is a PDF file called 1amp.pdf that will take a PWM signal and convert it to a constant current from 0 to 1 amp. By using a constant current control we automatically compensate for heating effects at the coil. You MUST heat sink the FET! While this exact circuit has not been tested, I have built similar circuits that have worked fine. You may have to tweak the value of the 2k resistor to calibrate the circuit so that 100% PWM = 1 amp 50% PWM = .5 amp, etc. The capacitor value may have to be adjusted, as it is dependent upon the PWM frequency. Fr. Tom McGahee >>

Michael wrote > btw: some PowerFETs need gatevoltages near ot higher then a standard Opamp > can have as an max. outputvoltage with a Vcc of 5V, so I would suggest a > VCC for the Opamp higher then 5V, 8V should be ok, 12V are ok. > There are logic level power FETs that overcome the gate voltage problem. BTW The FET could be driven efficiently with direct PWM and the PIC monitors the current with a peak current detector as the feed back to the PIC A/D, in turn altering the PWM to maintain constant current. I throw this side issue in as this is what I am building at this very moment with a motor control scheme. The advantage of the PIC in the loop is the ability of move from constant current, to other control curves freely under software control. I am just using the peak detector from Microchip AN531 _____________________________ Lance Allen Technical Officer Uni of Auckland Psych Dept New Zealand http://www.psych.auckland.ac.nz _____________________________

Thomas McGahee wrote: > Attached is a PDF file called 1amp.pdf that will take a PWM signal > and convert it to a constant current from 0 to 1 amp. Cute but inefficient. Unless you are concerned about EMI from the wiring, simply PWM-ing the coil direct with a logic FET and commutating diode would be far easier! > By using a constant current control we automatically compensate for > heating effects at the coil. What's the magnitude of this resistance change for copper wire? Is it significant to the application? (e.g., if the application is closed- loop anyway, it will be compensated, or if variations are only made slowly, it will stabilise and become part of the calibration curve.) > You MUST heat sink the FET! Using switching regulation avoids this! -- Cheers, Paul B.