Cooper Bussmann DCM 433 manual Getting Started with the DCM, Amplifier, Tare Weight Compensation

Getting Started with the DCM 433

1.Excitation Supply. If remote sensing is not used, connect +Sense to +Excitation, terminal 3 to 4, and connect -Sense to -Excitation, terminal 1 to 2.

2.Apply power to the DCM 433 and adjust the Excitation supply. Turn SWB-1 “ON” for a fixed 10 Volts. This will set the supply to 10.2 ±2.5% and provide the best temperature stability. For voltages between 5 and 10 Volts, place SWB-1 in the “OFF” position and set the Excitation Supply with potentiometer E. SWB-2 may be in either position.

3.Turn the power to the Model DCM 433 off and connect the load cell to the DCM 433 Excitation terminals and the Amplifier Inputs.

4.Turn on power to the DCM 433.

5.Turn SWA-3 “ON”. (To set output ZERO current with pot D.)

6.Select the expected full-scale signal range according to the table with SWA-4 and SWA-5. If the output from the load cell is not known, set both SW-4 and SW-5 “ON” for the 40-50mV range.

7.Set SWA-2 “OFF” for 4mA or SWA-2 “ON” for 0mA output ZERO.

8.Adjust D potentiometer for desired ZERO current.

9.Turn SWA-3 “OFF”. Expect output current to change.

10.Apply no load or dead weight to load cell.

11.Adjust TARE potentiometers A and B for the same ZERO current set in Step 8. SWA-1 “OFF” provides a bridge output balance of -3mV to +15mV, and “ON” between +15mV and +25mV.

12.Apply full-scale load and adjust SPAN (GAIN) potentiometer C for the desired full scale output current. Set Range switches SWA-4 and SWA-5 as required.

13.Remove full-scale load and check ZERO output current. Adjust FINE TARE potentiometer A if required.

14.Recheck full scale as in Step 12.

15.End.

Note: If the amplifier is used without using the DCM 433 Excitation Supply, the external power supply low side must be connected to the -EXCITATION terminal 2 on the DCM 433, or one of the inputs must be tied to terminal 2, -EXCITATION. This provides a DC return path for the finite amplifier input current.

Amplifier

The amplifier is a true differential input, low drift Instrumentation Amplifier with less than 100pA input current. It has a common mode range of 5 Volts with respect to the – EXCITATION supply terminal and a minimum of 100dB rejection of the common mode voltage. The input amplifier and excitation supply are DC isolated from the AC line and the current output.

The isolated output current source has a compliance of 20 Volts to allow the DCM 433 to operate with zero to 1000 Ohms of loop resistance.

Tare Weight Compensation

The Model DCM 433 has two different zero controls. One is called the OUTPUT zero and can be set to 0mA or 4mA with SWA-2. The other zero control is called TARE. SWA-1 allows the selection of one of two TARE ranges, - 3mV to 15mV or +15mV to +25mV. Potentiometers are available for COARSE and FINE TARE adjustments.

Connecting to a Sensor

Any amplifier has a finite input current, which must have DC return path to the amplifier power supplies. This path is automatically provided when the DCM 433 Bridge Excitation Supply is used to excite the sensor. If an external supply is used, one side of the external supply must be connected to the Model DCM 433 common, Terminal 2. Be sure that the common mode voltage limits are observed. This would generally limit the external power supply to 10 Volts assuming that half the voltage would be common mode, as is the case when exciting a full bridge.

When the full-scale output of a sensor is measured in millivolts, say 10 millivolts, care must be exercised in wiring systems. At 10 millivolts full scale, each microvolt (10-6volts) contributes 0.01% of full-scale output. Wire connections can generate microvolts of potential due to contact potentials. These will also be thermoelectric potentials and thus vary with temperature differences. All wires used in connecting up the DCM 433 should be of the same material. If any intervening connections are made such as a terminal block, the terminal block connecting points should have good thermal contact so they will always be at the same temperature and thus cancel each other.

CF 26 4 4/2001