3bC D MM 3595527 0008247 9 mm ~ TECHNICAL | 50-7a INFORMATION JUNE 1977 B-7/-03 LOAD RESISTANCE OF 2-WIRE TRANSMITTERS (with 4-20 ma dc output) Quite often we receive inquiries into the load resistance capabilities of our various 2-wire transmitters and especially into means for "extending" these load capabili- The information supplied in the fol- lowing paragraphs should clarify many of your questions. ties. A typical loop consists of the three basic 1. They are: dc power supply, current regulating transmitter, and the load consisting of the input resis- tance for board instruments and the trans-~ elements shown in Fig. mission wire resistance. by Ohm's law: Vioad = = V supply xmtr R _ Vsupply load T YVsupply = Vemtr +21 Vioad T*Rigad "Rioad Vamtx In Fig. 1: (1) (2) or (3) The maximum allowable load resistance is determined for the condition when I is maxi- mum (20 ma or .020 amps) and when Vymty has been dropped to its minimum value required to operate the transmitter. Rioad max -020 Therefore: _ Vsupply - Yxmtr min (4) Knowing the minimum transmitter voltage drop and the value of the power supply voltage, the maximum allowable load resistance can easily be calculated from Equation (4). The maximum allowable supply voltage (Table 1) should never be exceeded to prevent per- manent damage to the transmitter. Values of maximum load resistance with maximum sup- ply voltage are also listed in Table l. Tl pe 2-WIRE + XMTR 2 - ] Vv. _it pc as xmtr Vv === POWER = supply 7P> SUPPLY 1 In the early years of electronic de current transmitters it was common to arrange the receiver instruments in series. Receiver input signals were designated as 4-20 mA dc, and they had various values of input (load) resistance. Supply voltage for transmitters was usually 45 V de so that a number of re- ceivers could be placed in the series circuit even though each contributed as much as 250 ohms to the load of the circuit. Today, nearly all F&P receivers are designed for 1-5 V input, and they have internal amplification which uses practi- cally no current from the transmission loop. Receivers may therefore be con- nected in parallel across a single 250 ohm resistor which provides 1-5 V. The only load seen by the transmitter is the 250 ohm plus other items which might be series connected plus line losses. Consequently, a 24 V supply is now usually sufficient. Frank Dutch Product Mgr. Minimum Rioad, max Maximum Rioad, max Type Allowable with: Allowable using max. Remarks Transmitter Drop Across Vsupply Allowabie Transmitter 24 vde | 45 vde Vsupply (Vamtr, min) supply | supply 2-wire Force-Balance or Motion-Input 16 V dc 400 1450 50 V dc 1700 -- Transmitters with ohms ohms ohms 4-20 mA Output . 50ER3000 Rsistance-to- 17 Vi de 350 1400 -- Current Transmitter ohms ohms 50 V de ohne 50EM/ET3000 - 17 Vi de 350 -- 30 V de Millivolt or Thermo- ohms ee orwinels couple-to-Current Table 1 Transmitter 25 V de ~~ 1000 50 V de 1250 on 45V ohms ohms Terminals 50DP2000, S0AP2000 P2 -~ SOLP2000 12 V de 600 1650 50 V dec 1900 co Delta-P Cell ohms | ohms ohms ( } Transmitters / LO, ~ BaTS B08