Good morning Peter,
This is a very good question. As you have seen from the manual there are a number of different options for connecting 4 - 20 mA sensors to a DT80. There are also a number of traps for the beginner but they do have advantages in different situations.
While the DT80 has an internal shunt resistor there are some cases when an external shunt must be used in preference. The question is when to use an external shunt and when to use the internal shunt.
As a rule of thumb:
- If you have a remote site or power is a problem or if you are sampling slowly then use the internal shunt.
- If you have a good reliable external power supply or are sampling quickly then you would be better using an external shunt.
- If you have other devices connected to the current loop (HMI's, PLC's, Process Controllers) then you MUST use an external shunt.
Configurations C3 and C4 in DT80 User manual use the internal shunt resistor. The internal shunt resistor in the DT80 range is switched in and out of circuit when required.
This turns the the current loop on, then it is read and turned off again. This is really great at saving power on remote sites but when other devices like PLC's etc are reading the same loop it will disconnect the sensor from them as well so they can't read it either.
Also if the sensor requires a warm up period then you have to add a measurement delay to allow the sensor to stabilize before a reading is taken.
Configurations C1 and C2 use an external shunt resistor. If power is not a problem or you are sampling quickly or have other devices reading the same current loop then use an external shunt resistor as the loop will always be powered on. No warm up times needed and always in circuit,
Answering your questions directly:
C4 will greatly simplify the wiring and powering of the sensors but uses the internal shunt (See item A above)
Yes will supply the power at Ext to the channel and Ext # to the channel # terminals. But the I (I for Indigo) channel type will return the reading in mA. If you want the output in % of loop then use the L (L for Lima) channel type.
Yes this will work for any loop that can be powered by 7 to 30 VDC.
If you want the loop to be read by a PLC then use an external shunt resistor for (See item C above)
While there are currently two Ext * and Ext # they are physical connected internally in DT80 so you can only use one power supply.
You can not connect two power supplied and select between them.
Yes you can use the same supply for multiple sensors.
If you connect to the Ext* Ext # then you can use a small power supply.
If you use configs C1 or C2 then be aware that the sensors are in parallel so might need to up the power rating of the supply. (depends on the number of sensors you drive,
Roger
Good morning Peter,
This is a very good question. As you have seen from the manual there are a number of different options for connecting 4 - 20 mA sensors to a DT80. There are also a number of traps for the beginner but they do have advantages in different situations.
While the DT80 has an internal shunt resistor there are some cases when an external shunt must be used in preference. The question is when to use an external shunt and when to use the internal shunt.
As a rule of thumb:
- If you have a remote site or power is a problem or if you are sampling slowly then use the internal shunt.
- If you have a good reliable external power supply or are sampling quickly then you would be better using an external shunt.
- If you have other devices connected to the current loop (HMI's, PLC's, Process Controllers) then you MUST use an external shunt.
Configurations C3 and C4 in DT80 User manual use the internal shunt resistor. The internal shunt resistor in the DT80 range is switched in and out of circuit when required.
This turns the the current loop on, then it is read and turned off again. This is really great at saving power on remote sites but when other devices like PLC's etc are reading the same loop it will disconnect the sensor from them as well so they can't read it either.
Also if the sensor requires a warm up period then you have to add a measurement delay to allow the sensor to stabilize before a reading is taken.
Configurations C1 and C2 use an external shunt resistor. If power is not a problem or you are sampling quickly or have other devices reading the same current loop then use an external shunt resistor as the loop will always be powered on. No warm up times needed and always in circuit,
Answering your questions directly:
1. C4 will greatly simplify the wiring and powering of the sensors but uses the internal shunt (See item A above)
2. Yes will supply the power at Ext* to the channel * and Ext # to the channel # terminals. But the I (I for Indigo) channel type will return the reading in mA. If you want the output in % of loop then use the L (L for Lima) channel type.
3. Yes this will work for any loop that can be powered by 7 to 30 VDC.
4. If you want the loop to be read by a PLC then use an external shunt resistor for (See item C above)
5. While there are currently two Ext * and Ext # they are physical connected internally in DT80 so you can only use one power supply.
You can not connect two power supplied and select between them.
6. Yes you can use the same supply for multiple sensors.
If you connect to the Ext* Ext # then you can use a small power supply.
If you use configs C1 or C2 then be aware that the sensors are in parallel so might need to up the power rating of the supply. (depends on the number of sensors you drive,
Roger
