i got my PLC analog input required two wire 4-20mA with max 12VDC in the + and -, please somebody help, my instrument send 4-20mA but 24VDC in the + and -, how to reduce this voltage ? if i connect,my analog input burnt.
Most instruments can run the 4-20mA loop with 9 to 36 volts. Try using a 12VPS on your instrument. If not, you may need to replace the instrument with one that will work on a 12V powered loop.
The voltage across your instrument will only rise to a level that can produce 20mA, therefore it should all work OK. If you are really worried then find the impeadence of the instrument and put a resistor of the same value in series, but you really do not need to.
Typically, a 4-20 mA loop will be powered from a 24 V DC source. The circuit is a series one, with the typical arrangement being +ve 24 V terminal -- transmitter +ve terminal -- transmitter -ve terminal -- analogue input +ve terminal -- analogue input -ve terminal -- power supply -ve terminal. In additon, the analogue may have a resistor (either externally or internally) to convert the current signal to a voltage. Typically, this is 250 ohms, giving an input to your PLC of 1-5 V. The transmitter will normally require a minimum of 10 - 14 V across it to work, but will soak up whatever the difference is between the supply voltage and the input to the PLC.
What kind of PLC type and input card type are you using ?
I ask because what you write seems very unlikely. The +- 12V that the analog input can take is probably the voltage that will be generated over the measuring resistor when max current is applied to the input.
There is a resistor in the input circuitry for the analog input, it could be 150 ohms (only an example). When 20mA is sent to the input 3V will be generated over the resistor, and this goes directly to the AD converter in the input card.
If you have a two-wire 4-20mA transmitter that is fed with 24VDC, then you will NOT have 24 VDC on the input, only the voltage that is generated over the resistor.
24V can only get to the analog input by some wiring error !
