why we call 24VDC powered transmitters (Two wire) as analog transmitters? although the voltage is DC?

please provide a reference article for calculating loop resistance or some can suggest how to calculate it for Siemens pressure transmitter 2-wire. i'm using shielded cable with a resistance of 43ohm/km.

Hope your post will be helpful.

 

analog and digital differentiation is due to signal nature, not power supply. in your context 24vdc is power supply. if you have analog ac modulated 24 dc power, then it would be analog tx.

in other cases you have digital on 24 dc modulated, that's digital transmitter, or you can have power distributed separately.

regards,
anri

 

Vikki,

The manual gives the formula
Minimum supply voltage = 10.5V + (0.02 x network resistance)

So if you go 24 - 10.2 = 13.8 Volts

13.8 V / 0.02 A = 690 Ohms

This is the total loop resistance (including your input module (typical 250 Ohms)with a 24 VDC supply

They also give a complicated formula for very long cables taking capacitance into effect but I wouldnt worry about that unless you are getting up over 500 meters

Regards
Roy

 

Thanks roy,

but still I have some confusion whether this 500 meters is for a loop or just the distance between the field instrument and the cabinet?

Also in your post "So if you go 24 - 10.2 = 13.8 Volts" where this 10.2 comes from?

what I understand network resistance is only the cable resistance + 250 ohms (power supply). am I right?

Best regards,
Vicky

 

Vicky,

500 meters from the transmitter to cabinet. That's not a hard number, I just meant if its less don't even worry about the capacitance calculation. I have seen transmitters at the end of 10+ km of overhead (telephone) wire. I don't think capacitance is an issue for the analog (4-20) portion of the signal but it may effect the HART signal.

The transmitter needs at least 10.2 Volts to generate 20 mA so if you had a 24V supply it would work but you would only be able to supply a low resistance load 24 - 10.2 = 13.8 Volts to drive your load. Subtract 5 V for your input leaves 8.8 V for your cable. Other brands of transmitters have a different minimum Voltage. Look at most transmitter manuals and they show that in a graphical form.

Note: In most cases transmitters will allow a much higher supply so if your cable is really long you can also increase the supply voltage to overcome cable resistance.

If you have a variable voltage power supply I suggest you try it.

Power supply - multimeter - transmitter - power supply.

Set the power supply at ~9 Volts. You will find that the tansmitter works fine at the low end of its range but it wont be able to send 20 mA.

Regards,
Roy