The load cell readout is a simple readout for common miniature load cells that work as DC biased resistive bridge circuits.
The sensitivity dial (2) is used to calibrate the unit to the connected load cell.
In most case the zero dial (3) can be set by applying zero force (=not touching) on the load cell and then turning the dial for a 000 display (1).
A load cell is connected with a 5-pin DIN connector (5) with screw lock.
12 Volts power is supplied to the unit with a standard regulated AC mains adaptor (6).
With the decimal point dial (7), the decimal point in the LCD screen (1) is set to the correct value for the load cell. The range of the screen (1) is 1999. This dial does not affect the input sensitivity, it simply switches one of the four decimal points in the screen on.

If the load cell readout does not work while it is connected correctly, the protection circuitry may have been activated. This problem is corrected by disconnecting the supply power (6) for at least 15 seconds.

During the Dental Materials Hands-on Workshop the values for the sensitivity dial are on top of the device. Moreover, for this workshop the rest of this document is not of interest.

The output of the pre-amp is available from the BNC connector (8). The output voltage is 100x the voltage given off by the load cell.

Because there is considerable variation in sensitivity among different cells, the load cell readout has a wide adjustment range.

Calibration

In most cases the calibration has already been carried out. In such cases, these values can be used to set the sensitivity and offset dials.
In the case of an unknown load cell, calibration generally takes the following three steps:

In many applications is it not necessary to log the value of the zero shift dial, because it is very simple to apply zero force.

Technical specifications

Amplifier

differential impedance ≈ 5kW
Gain: 100x, ±1%, up to like 10Hz (-3dB, to reject 50Hz electromagnetic mains interference).

Input noise, 24hrs reproducibility after 5min warm-up, and thermal drift ≈ 3µV_p-p or 1µV_eff not per degree, but over the entire temperature range (0 to +60 Celsius), with shorted inputs.
If you don't believe it, read this: it uses one half of a high quality NE5533 dual opamp with 2x 4.99k input and 2x 499k feedback resistors for a fine low noise differential amp, but with average thermal stability. The trick is that it basically uses the second opamp of the NE5533 as a thermometer of the first one. The thermometer opamp drives a TO220 power transistor, which is folded over the dual opamp with a small heat sink. This guaranties a constant chip temperature, which we have set at +60 Celsius. 

Noise of the NE5533 is about equal to the thermal noise of a 5k resistor, which is like nothing, especially at (only) 10Hz bandwidth, so if there is no drift up to this temperature, then why still 1µV_eff? There are two reasons. An NE5533 produces like 0.5µV_eff of low frequency noise. Below a few Hertz bipolar integrated circuits produce a constant noise voltage because of so called 1/f noise, no matter how much you slow it down. It got doubled because of the heater trick; underneath 0.5Hz the 1/f noise of the second opamp gets added.

Power supply

Operating supply voltage +/- 12 Volts DC from a standard regulated AC adaptor. Supply current is about 100 mA. The warm-up supply current is initially 400 mA at switch-on, decreasing during 2 minutes.

Protected up to +/- 60 Volts DC on the supply input. The protection circuit is reset by disconnecting the supply connector for about fifteen seconds.

The crowbar type protection circuit is triggered when the supply voltage exceeds 12.5 Volts or when static electricity causes more than 10 mA ground fault current from the load cell connector to the supply connector.