2 state driver board circuit

Discussion in 'The Projects Forum' started by Klix, Jan 9, 2015.

  1. Klix

    Thread Starter New Member

    Jan 9, 2015
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    Hi folks,

    I'm looking for some help to built a simple circuit which will output 2 separate analog voltages based on input PWM states (High/Low). As an example, I would like to output 20VDC when the PWM signal is high and 5VDC when the PWM signal is low. The PWM input will come from Arduino and there will be an external power supply to provide 24 VDC to the driver circuit.
    It would be nice to have 2 pots to dial in the 2 different voltages. The PWM time base will need to be very slow (2 - 4 hz).

    This driver is for a proportional solenoid valve (Asco 8202 series).
    Some specs for the solenoid valve:
    24VDC Valve
    Flow is controlled by applying straight voltage from 0VDC to 24 VDC
    Operating Current: 100 - 500 mA
    Coil Resistance: 25 Ohm

    Any help with the components required and circuit diagram would be greatly appreciated!
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I wouldn't describe the input as PWM; it sounds like it is just a standard port pin on a 5V microcontroller with two states, high and low.
     
  3. Klix

    Thread Starter New Member

    Jan 9, 2015
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    This is correct however for my application, the duty cycle will be changing. The Arduino will be acting as a PID with temperature inputs coming from thermocouples. The PWM duty cycle will be changing to provide longer high pulses when more heat is needed, etc... These details may be irrelevant to this circuit though...
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is crude hack at it. Adjustable only by changing the power resistors, but it might get you started.

    121.gif
    The NFET would be almost any "logic-gate" power switching one.
     
  5. Klix

    Thread Starter New Member

    Jan 9, 2015
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    Great! That looks like a simple solution. I'm just learning about this type of stuff right now... Question: what would be the difference if I was to use something like a TIP120 NPN transistor opposed to the suggested NFET?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The Collector-to-Emitter saturation voltage Vcesat for the TIP120 would be almost 2V, while the Drain-to-Source Vds of the NFet would be a few mV. That would change the required resistance to get 5V and 20V.
     
  7. Klix

    Thread Starter New Member

    Jan 9, 2015
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    Hi guys,

    So I found a schematic from another fellow that is doing the same type of setup as me, however his proportional valve is driven from a 12VDC supply. I have been analyzing this schematic to determine if this will work with a 24VDC supply and from what I can tell, it will work. That said, I only have very basic knowledge of electronics and I may be missing some details. I looked up the datasheets for main parts and I believe that everything would still fall in spec. Could you please have a look at this and let me know if this will work as-is or if there any parts that would need to be replaced?

    Would any of the resistors need to be changed given that the my valve has the following specs:
    Voltage 0 to 24VDC
    Operating Current: 100 - 500 mA
    Coil Resistance: 25 Ohm

    Remember I am looking for and output of 20 VDC when input signal is high and 5 VDC when input signal is low.

    2StateValveDriver.JPG
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The valve current is either I= E/R = 20/25 = 0.8A (1)
    or, 5/25= 0.2A. (2)

    If you start with a 24Vdc supply, the power dissipation in the regulator is P=IE = 0.8*(24-20) =3.2W (1)
    or, 0.2*(24-5) = 3.8W (2)

    So U1 would have to be on a big heatsink. I would use the TO-220 version.

    The proceedure for calculating the required values for R1, R1, R3, and R4 is on the data sheet of the LM1086.
     
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