BA307E, BA327E, BA308E & BA328E intrinsically safe ... - Ex-Baltic

259.4 Display backlightThe BA307E, BA327E, BA308E and BA328E looppowered indicators can be supplied with a factoryfitted backlight that may be loop or separatelypowered.When loop powered the backlight produces greenbackground illumination enabling the display to beread at night or in poor lighting conditions. Noadditional power supply, intrinsic safety interface orfield wiring are required, but the indicator voltagedrop is increased. When separately powered thebacklight is brighter, but an additional intrinsicsafety interface and field wiring are required.described in sections 3 and 4 of this manualremain valid with the loop powered backlight.9.4.2 Separately powering the backlightThe optional backlight may also be powered from aseparate safe area power supply via an intrinsicallysafe interface as shown in Fig 18.Fig 16 Terminals for optional backlight9.4.1 Loop powering the backlightThe backlight is loop powered by connecting it inseries with the indicator 4/20mA input as shown inFig 17, which increases the maximum indicatorvoltage drop from 1.2 to 5V.Fig 18 Separately powered backlightWhen separately powered the backlight draws aconstant current when supply is equal to or greaterthe minimum specified voltage. Below this supplyvoltage the backlight continues to function but withreduced brilliance.Current Minimum voltageBA307E & BA327E 22.5mA 9.0VBA308E & BA328E 34.7mA 11.0VAny certified Zener barrier or galvanic isolator maybe used, providing the output parameters do notexceed:Uo = 30V dcIo = 200mAPo = 0.84WFig 17 Loop powered backlightThe input intrinsic safety parameters of thecombined indicator and backlight are the same asfor the indicator alone. The EC-Type ExaminationCertificate states that for intrinsic safetyconsiderations, under fault conditions the outputvoltage, current and power of the combinedindicator and backlight terminals 1 & 13 will notexceed those specified by clause 5.7 of EN 60079-11 for simple apparatus, which simplifies systemdesign and documentation.Providing the increased voltage drop can betolerated, the intrinsic safety and system designThe following equivalent internal capacitance andinductance between terminals 12 & 14 should besubtracted from Co and Lo of the intrinsically safeinterface powering the backlight to provide themaximum permissible cable parameters.Ci = 13nFLi = 8µHTwo separately powered BA307E or BA327Eindicator backlights may be connected in parallel toa single channel 28V, 93mA Zener barrier orgalvanic isolator with no noticeable reduction inbrilliance on a 24V dc supply.