Mettler Toledo IND310drive Manual De Instalación página 23

The maximum current that an MTX POWERCELL consumes is 75 mA. So if the
IND310drive is configured with one POWERCELL card (31L30x), then
800mA/75mA = 10.7. Rounded down, this value means that the IND310drive
supports only 10 MTX POWERCELLS without using an external power supply.
This is the typical configuration for replacing a METTLER TOLEDO Cougar terminal.
If the POWERCELL card is mixed with an analog channel or two POWERCELL
cards are used simultaneously (31L31x or 31L33x), then 500mA/75mA = 6.7.
The value rounded down to six means the IND310drive supports only 6 MTX
POWERCELLS without using an external power supply in those configurations.
The MTX POWERCELLS consume more current than the CMOS POWERCELLS. On
the average the CMOS cells use 50–55mA and the MTX cells use 60–65mA.
METTLER TOLEDO chooses to use the maximum current of 75mA per cell for
calculations to ensure that the system will always have sufficient power supply
current.
NMOS POWERCELL
The NMOS cells always use a pit power supply because they require different
voltages than provided by the IND310drive and formerly by the METTLER TOLEDO
Cougar terminal. The IND310drive only supplies voltage to a switch in the pit
power supply to turn it on and off. However, the terminal voltage must be
considered. The METTLER TOLEDO Cougar supplies between 21V-24V, and the
IND310drive POWERCELL interface only supplies 12V.
The older pit power supply (09170168000) can operate with voltages from 5V to
30V and is completely compatible with the IND310drive. The new pit power supply
(90898000A) cannot be used with the IND310drive because it requires an 18V to
24V input. The alternative (90898000B) accepts 9V–12V inputs and is
compatible with the IND310drive.
Analog
Analog load cells look like resistors to the IND310drive. The specification states
that the IND310drive supports eight 350-ohm load cells. The eight load cells
connect in parallel so the equivalent resistance seen by the IND310drive is 350/8
= 43.75 ohms. Note that the calculation, equivalent resistance = (ohms)/(number
of load cells) , only works if all load cells have the same resistance. It does not
work if the scale has a mixture of resistances, such as mixing 1,000-ohm cells
and 350-ohm cells.
The equivalent resistance of any scale must be greater than 43.75 ohms for the
IND310drive to support it. As an example, consider the twelve 1,000-ohm load
cells. The equivalent resistance is 1,000/12= 83.3 ohms, which is greater than
43.75 ohms; so the IND310drive will easily support twelve of the 1,000-ohm load
cells.
To determine if the IND310drive can support twenty 750-ohm load cells use the
calculation 750/20= 37.5 ohms. Since 37.5 ohms is less than 43.75 ohms, the
IND310drive will not support twenty 750-ohm load cells. Table 2-1 uses a similar
IND310drive Installation Manual
2-5