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DAC and 4-20ma output

Discussion in 'Electronic Design' started by sandeepa, Feb 5, 2008.

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  1. sandeepa

    sandeepa Guest

    What is the resolution of the DAC generally used in temperature
    controllers for a 4-20ma output ?
    I have made a temperature controller using PIC16f877a taht gives gives
    either relay outputs or pulses.Now i want to give 4-20ma output as
    well.So i need a DAC to convert the o/p from the PIC.
    I was looking at AD420 which has a resolution of 16bits.However, in
    such apps where the proportional band is around 20degrees, i think i
    dont need so much resolution.A 8bit DAC iwth even a 5V reference will
    give me 19.5mV, which is more than i need.Is that correct ?
    I am confused about this whole DAC thing.Please help.

  2. Frank Buss

    Frank Buss Guest

    This depends on the requirements. But if you don't have a good sensor and
    circuit for measuring the temperature, e.g. a PT1000 with 3-wires and
    ratiometric measurement, you don't need good output resolution anyway,
    because your input resolution is already worse.

    Searching the internet looks like there are products which outputs 4-20mA
    for 0°C to 50°C, with 0.5°C resolution, which would work with a 8 bit DAC.
    It depends on the accuracy and speed you need, but you could use the PWM of
    the PIC16f877a, for example like this:

    V_PWM is the low-pass filtered output of the PWM signal of the PIC (a
    resistor in series and a capacitor to ground). The lower the frequency of
    the low-pass filter, the cleaner the output signal, but slower. But
    temperature doesn't change very fast, so you could use 1 Hz update rate.
  3. Frank Buss

    Frank Buss Guest

    A safety note: This is only a concept drawing. The 4-20mA standard
    ISA-50.1-1982 (R1992) allows up to 32.7V for the current loop. If there is
    a short circuit or the PIC PWM output has a bug, your resistor will burn.
    Add a 50mA fuse, use a 2 watt resistor for R_sense and use a big
    transistor, like the BCP56, which can dissipate some heat.
  4. Jamie

    Jamie Guest

    No you don't need a DAC,.. You already have what you need.
    PWM the output into a unity gain (buffer) amp to control
    a current mode transmitter circuit..
    when not using that option, just don't PWM the output and
    use it as a standard IO line.

    "I'd rather have a bottle in front of me than a frontal lobotomy"

    "Daily Thought:
    if I only had one ""
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