DAC and 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.

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.

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

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.

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

ems.de

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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