DAC and 4-20ma output

[sandeepa]

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.

Thanks.

 

[Frank Buss]

What is the resolution of the DAC generally used in temperature
controllers for a 4-20ma output ?

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.

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.

It depends on the accuracy and speed you need, but you could use the PWM of
the PIC16f877a, for example like this:

http://www.frank-buss.de/tmp/4-20.4.png

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.

 

[Frank Buss]

http://www.frank-buss.de/tmp/4-20.4.png

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.

 

[Jamie]

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.

Thanks.

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.


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