Bladder fill/empty Circuit?

Discussion in 'The Projects Forum' started by birdirt, Sep 20, 2012.

  1. birdirt

    Thread Starter New Member

    Sep 14, 2012
    Hello all!

    I have designed the following circuit, but I am hoping some one can check it for me please.

    The designed function is as follows:

    Once the circuit is switched on, a time delay begins at TD1, and once it expires Pump1 turns on, and the Bladder inflates to a preset maximum pressure. At this point, the pressure switch sends a current to the NPN transistor, which in turn energizes the relay, diverting power to TD2 and Pump1 turns off. After the preset delay, Pump2 turns on, deflating the bladder to a preset minimum pressure. At which point the relay is switched back to TD1 and the cycle starts again.

    The pump speeds are controllable via PWM1 & PWM2 Modules.
    The system has a battery backup and regulated 12VDC supply to the circuit. I have an UPS module and a 12V regular Module as shown from the 240AC power supply.


    I have first calculated the resistor values over the SMC pressure controller and the transistor switch base current etc, as follows:

    Parts Specifications:
    SMC pressure controller Specs:
    Current Consumption Ismc = 40mA
    Find Resistance for SMC pressure controller - Rsmc:
    Rsmc = 12V/0.04A
    Rsme = 300ohms

    Relay1 12V DPDT
    30mA Coil

    All Diodes are to be IN14001 or IN14004

    Transistor BC547A
    he = 110
    Vbe = 0.7

    First find Ib for the Transistor.
    Ib = (Load Current / Hfe) x 2 to ensure saturation.
    Ib = 0.030A / 110 x 2
    Ib = 0.27mA x 2
    Ib = 0.54mA

    R2 = Vin - Vbe / Ib
    R2 = (12 - 0.7) / 0.00054
    R2 = 20925 Ohm or 22K

    Assuming the Resistance of the SMC controller and R2 are in paralell.
    R1 = 1 / (1/Rsmc + 1/R2)
    R1 = 1 / (1/300 + 1/2092)
    R1 = 295 Ohm

    TD1 = 1.1W when Relay is on.
    I = W/V
    I = 1.1 / 12
    I = 0.091A or 91mA

    R4 = V / I
    R4 = 12 / 0.091
    R4 = 131 Ohms

    R4 = R5
    R5 = 131Ohms

    R6 = R7
    I = W (pump) / V
    I = 2.5 / 12
    I = 0.208A or 208mA
    R6 = 12 / 0.208
    R6 -= 57.6 Ohm

    R6 = R7
    R7 = 57.6Ohm


    Some questions I have are:
    1. Will this circuit work as planned?
    2. Do I need a resistor for the path accross RC1 coil?
    3. Are the formulas and calculations correct?
    4. Are the Diode and Transistor suitable?

    Any advice would be appreciated, I am running out of time and just need a hand, any help would be appreciated please.

    Thanks very much,
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Looks ok to me, question, when the pressure sensor has no pressure applied, is the output positive (high) or 0 volts (low) also are you using model version 65, as model 25 uses npn output?

    why do you need R4 to R7?
  3. birdirt

    Thread Starter New Member

    Sep 14, 2012
    Thanks for replying!

    The open collector output is from an internal PNP transistor. The model is the 65, or as shown in this link is option 'B'. (it actually has two outputs but I will use just one: )

    From what I understand, when there is no pressure, the output would be positive (high). (I think), and when the pressure reaches the preset point it will ground the base of the internal PNP, allowing current to flow through the PNP open collector.

    With regard to the Resistors R4 to R7, don't I need these resistors to limit the current through the Time Delay Relays, and so the pumps don't exceed their maximum power?

    Or will they only draw what they need?

    Thanks very much, I am new to all this, two weeks ago I started learning the basics, ir volts, current etc.

  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    The pumps will draw only what they need, omit the resistors.
    Last edited: Sep 20, 2012
  5. birdirt

    Thread Starter New Member

    Sep 14, 2012
    Thanks so much.

    I really appreciate your assistance.