Hi all and nice to meet you!
I have to design a 4-20mA current loop output for a digital manometer from 1-5V obtained from the PWM of the uC. I have to make it from OPA and discrete because of pricing issue. AD694, XT110 or similars are too expensive.
I performed a search on the forum but none of the circuits proposed in old threads suits ny needs. I simulated all of them in LTSpice and I never obtained a compelling result.
Here are my goals:
1) 3-wire system so: a common ground, Iout and 24V supply
2) supply of the transmitter is 24Vdc ora 24Vac, bear in mind the supply for the current loop could have a large amount of ripple, something about 4-5V at 50Hz (the transmitter has an half wave rectifier)
3) OPA supply will be a regulated supply single or dual (obtained from loop supply)
4) Rloop about 100 to 500 ohms
5) Transmission line could be long, it will be placed in an industrial environment to control inverters connected to electric motors.
I am trying to reverse-design an existing product that is designed to work with both 24Vdc and 24Vac and I see that it uses a BCP54, so an NPN BJT to control the loop, and a LM358 double OPA to control the BJT.
The collector of BJT is directly tied to the + terminal of the only filter capacitor after the rectifier diode, so if the device is powered with 24Vac the BJT is fed with about 32Vdc and a large amount of ripple. Then there is an LM317 that fed 16Vdc without ripple to the + of the OPA, after the LM317 +5V and -5V are generated for the uC and the sensor. The - of the OPA is fed with -5V.
I put in attachment a rough idea of what I have in mind.
Can you help me with a schematic that can help me to achieve a result?
Any help will be very appreciated.
Many thanks
I have to design a 4-20mA current loop output for a digital manometer from 1-5V obtained from the PWM of the uC. I have to make it from OPA and discrete because of pricing issue. AD694, XT110 or similars are too expensive.
I performed a search on the forum but none of the circuits proposed in old threads suits ny needs. I simulated all of them in LTSpice and I never obtained a compelling result.
Here are my goals:
1) 3-wire system so: a common ground, Iout and 24V supply
2) supply of the transmitter is 24Vdc ora 24Vac, bear in mind the supply for the current loop could have a large amount of ripple, something about 4-5V at 50Hz (the transmitter has an half wave rectifier)
3) OPA supply will be a regulated supply single or dual (obtained from loop supply)
4) Rloop about 100 to 500 ohms
5) Transmission line could be long, it will be placed in an industrial environment to control inverters connected to electric motors.
I am trying to reverse-design an existing product that is designed to work with both 24Vdc and 24Vac and I see that it uses a BCP54, so an NPN BJT to control the loop, and a LM358 double OPA to control the BJT.
The collector of BJT is directly tied to the + terminal of the only filter capacitor after the rectifier diode, so if the device is powered with 24Vac the BJT is fed with about 32Vdc and a large amount of ripple. Then there is an LM317 that fed 16Vdc without ripple to the + of the OPA, after the LM317 +5V and -5V are generated for the uC and the sensor. The - of the OPA is fed with -5V.
I put in attachment a rough idea of what I have in mind.
Can you help me with a schematic that can help me to achieve a result?
Any help will be very appreciated.
Many thanks