Three-Wire Circuit

The preferred circuit for use with a single strain gage in a quarter-bridge configuration is the three-wire circuit shown in Fig. 516.3. In the two-wire circuit, both leadwires are in series with the strain gage in one arm of the Wheatstone bridge. In the three-wire circuit, the first leadwire remains in series with the strain gage, but the second leadwire is now in series with dummy resistor R 4 between the negative input and output corners of the bridge. Referring to Fig. 516.3, if these two leadwires are the same type and length and exposed to the same temperature, their resistances will be equal. The two respective bridge arms will therefore be equal in resistance, the bridge is again resistively symmetrical about a horizontal line through the bridge output corners, and the bridge remains balanced regardless of leadwire temperature changes, so long as the two leadwires are at the same respective temperature. And because only one leadwire is in series with the strain gage, leadwire desensitization is reduced about 50% compared to the two-wire configuration. The third wire in Fig. 516.3 is a voltage-sensing wire only and it is not in series with any of the bridge arms, therefore it does not affect bridge balance or temperature stability.

Fig. 516.3 - Three-wire quarter-bridge circuit.

While the three-wire circuit offers several advantages over the two-wire circuit, in some special applications involving, for example, slip rings or feed-through connectors, not enough connections may be available for a continuous three-wire system from the gage site to the instrument terminals. In these cases, use of a two-wire lead system between the strain gage and the connector, and a three-wire circuit between the connector and the measuring instrument is recommended to minimize the total length of the two-wire system.

The foregoing discussion applies primarily to measurement of static strains with a measuring instrument that provides decoupling between the bridge circuit and the amplifier input terminals. For measurement of purely dynamic strains when only the peak-to-peak amplitude of a time-varying strain signal is of interest, the two-wire system may sometimes be used effectively by selecting a signal-conditioning amplifier that provides for ac-coupling of the input signal, to "block" the effects of temperature-induced changes in leadwire resistance on the strain signal.

In summary, benefits of the three-wire circuit include intrinsic bridge balance, automatic compensation for the effects of leadwire temperature changes on bridge balance, and increased measurement sensitivity compared to the two-wire configuration. The three-wire hookup is the recommended configuration for quarter-bridge strain gage circuits for static strain measurement. The two-wire circuit can sometimes be used effectively for special situations such as dynamic-only measurements with ac-coupled instrumentation, or in static strain applications where the length of the two-wire system can be kept very short.

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