DESIGNER’S HANDBOOK 4189350049C EN
Page 62 of 206
5.6.7.2.1
Alternative 1
–
How to solve the common mode problem above
If the dimmer configuration is slightly changed, as shown in the figure below, where the dimmer potentiometer uses
Vref instead, and it is separated from the current loop, then the system will work fine.
This solution will be possible using the DEIF standard rudder library where dimmer and measuring circuit is
configured to be connected to the same AX1 module and it will work for all XDi sizes (96, 144 and 192).
5.6.7.2.2
Alternative 2 - how to solve the common mode problem
To fix the problem in the above example it is important rot educe the common mode voltage to below 15V, so you
can either reduce the supply voltage for the current loop to below 15V (instead of 24V).
With the RTA 602 sensor the minimum voltage across the sensor at 20mA must be at least 7V and across a single
AX1 current input the drop will be max. 1.3V (20mA x 65
Ω) so the supply must never be below 8.3V at 20mA.
If a 10V to 15V power supply is available for the current loop it will work fine.
It would also be possible to insert a Zener diode in series with the wire going from +24V to HI1+, as shown below,
If the power supply is a stable 24V (not a battery) this will work fine with a 10 to 15 volt Zener diode. The power
rating for the Zener diode should not be below 0.5W (15V and 20mA is 0.3W).
