Forums

DAC and 4-20ma output

Started by sandeepa February 5, 2008
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.
sandeepa wrote:

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

> 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. -- Frank Buss, fb@frank-buss.de http://www.frank-buss.de, http://www.it4-systems.de
On 2=D4=C25=C8=D5, =CF=C2=CE=E74=CA=B128=B7=D6, Frank Buss <f...@frank-buss.=
de> wrote:
> Frank Buss wrote: > >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. > > -- > Frank Buss, f...@frank-buss.dehttp://www.frank-buss.de,http://www.it4-syst=
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sandeepa wrote:
> 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. -- "I'd rather have a bottle in front of me than a frontal lobotomy" "Daily Thought: if I only had one " http://webpages.charter.net/jamie_5"