4–20mA Current Transmitter

Building a 4–20mA cur­rent trans­mit­ter and receiv­er for some upcom­ing automa­tion projects.

After a bit of online search­ing for a pre­ci­sion 4–20 mA cir­cuit tester, I decid­ed to build my own ver­sion.
I based my trans­mit­ter off of an YouTube video by youtu­u­ba. It was a long but very detailed video on how he built a sim­ple 4–20 mA trans­mit­ter. The design is based on the Burr-Brown (Texas Instruments) XTR105 mono­lith­ic 4–20mA, 2‑wire cur­rent trans­mit­ter IC.
The cir­cuit in the video was a very basic design with an empha­sis on the small­est foot­print pos­si­ble.
I was not con­strained by size and decid­ed to add reverse volt­age oper­a­tion and over-volt­age surge pro­tec­tion, along with an exter­nal tran­sis­tor to improve accu­ra­cy. I also used pre­ci­sion 250 Ω 0.1% ±25ppm/°C resis­tors for the set­point resis­tors R1 and R2.

After the first pro­to­type board was com­plet­ed and test­ed, I decid­ed to slot mount the PCB in a Hammond 1455B802 alu­minum enclo­sure. The revi­sion 1.2 board size was changed with only minor adjust­ments to some traces to fit the case.

With the reverse volt­age pro­tec­tion diode bridge, it does­n’t mat­ter which ter­mi­nal is con­nect­ed to the + or — loop con­nec­tions.
The trans­mit­ter is loop pow­ered, and requires an exter­nal 12 to 25 volts DC sup­ply for pow­er.
I used some BAT46 Schottky 150mA diodes for the bridge cir­cuit with a for­ward volt­age of around 520 mV @ 20 mA of for­ward cur­rent. Total loop volt­age drop thru the bridge at 20 mA is only around 1.04 volts.
After the diode bridge cir­cuit is a Vishay 5KP26A-E3/54 uni-direc­tion­al 26 volt tran­sient volt­age sup­pres­sor (TVS) diode. I end­ed up using a 5,000 watt TVS diode in a P600 pack­age, because that was the only thing in stock. A 500 watt TVS diode would be more than ade­quate for this cir­cuit. I also picked a Reverse Standoff volt­age of 26 volts, as I only planned on using a loop volt­age of 24 volts. The XTR105 is rat­ed for loop volt­ages of up to 36 volts which can be accom­mo­dat­ed by increas­ing the Reverse Standoff volt­age of the TVS diode to the next val­ue over the required max cir­cuit voltage.

I used an exter­nal 2N4922G Bipolar NPN tran­sis­tor to iso­late the major­i­ty of the pow­er dis­si­pa­tion from the pre­ci­sion input and ref­er­ence cir­cuit­ry of the XTR105 IC. This is rec­om­mend­ed in the data sheet for improved accu­ra­cy and extend­ed tem­per­a­ture range.

The tran­sis­tor is mount­ed to the alu­minum case with a nylon 4–40 screw and nut with a ther­mal pad pro­vid­ing elec­tri­cal iso­la­tion between the two. I also used a 1/8″ ther­mal GAP pad between the XTR105 IC and the top of the case to pro­vide addi­tion­al ther­mal stability.

The gain adjust­ment poten­tiome­ter is adjust­ed for a loop cur­rent of 20.000 mA with the switch set to 20 and a loop volt­age of 24 volts. I use a 250 ohm resis­tor in series with the loop while adjust­ing the gain.

Once the 20 mA set­ting is cal­i­brat­ed, check the 12 and 4 mA switch set­tings to ver­i­fy that the read­ings are with­in 10 µA of the set­ting. When con­nect­ed to an oper­a­tional loop the front LED will illu­mi­nate indi­cat­ing the cir­cuit is operational.

The front pan­el switch is a C&K 7103SYZQE sin­gle pole dou­ble throw switch with three posi­tions (On-Off-On). Front pan­el con­nec­tions are Pomona dou­ble bind­ing posts that will accept either stan­dard banana plugs or wire con­nec­tions. Both posts are black in col­or as the diode bridge cir­cuit will allow any polar­i­ty connection.

The next project in progress is a 4–20 mA receiv­er with 0 to 5 volt out­put, based on the Burr-Brown (Texas Instruments) RCV420 cur­rent-loop receiv­er IC.

Component infor­ma­tion is embed­ded in the EagleCAD board file com­po­nent attribute set­ting.
EagleCAD V7.7 board & schemat­ic ZIP file for 4–20mATransmitter

OSH Park print­ed cir­cuit boards for the 4–20mA Current Transmitter