Bridge circuits and multimeters

** Bridge circuits work by finding the ratio of two resistors - one is a known reference and the other is the one on test.

Digital multimeters work much the same way - they also find the ratio between a resistor built into the meter and the external one on test, then display the result as a reading in ohms. They do this very accurately, very quickly and very reliably.

Analogue multimeters are not near so accurate as bridges or DMMs because the meter movement is subject to many sources of error and misreading due to parallax etc. There is really no advantage in using them for testing resistors.

** No surprise.

DMMs long ago won the game of resistance measurement.

...... Phil

In the 'old days', ie before digital, it was easy to make a meter which indicated a small current/voltage, without necessarily having very good calibration. Just sensing a 'no current' condition was good enough. Then you could put this in a bridge, and measure the null. The unknown resistance was then measurable in term of known resistors to their accuracy, without worrying about the accuracy of the meter.

Now we have digital meters, where the inherent accuracy can be made as high as necessary ( at a cost ), so its easier to measure resistance with these. The limitation with digital is usually at least as bad near zero as anywhere else, so there is no advantage in using one in a bridge type circuit just as a null indicator.

But bridge circuits still have a place, particularly where you need to resolve reactive components into resistive and inductive/capacitive parts.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
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Back in the dark ages there was a thing called a "differential meter".

It used a bridge and had a series of 10-position knobs across the front. You tweaked the knobs for a null on the analog meter movement then you read the value off the knobs to many decimal places.

--as Phil said, minimal need for needle interpolation.

When a readout could be had economically in the blink of an eye on a Nixie/LED/LCD, these units became obsolete.

A multimeter is quicker, easier, usually more accurate, usually less expensive, and universally available these days. You'd be hard pressed to find an example of where a bridge circuit can beat a good DMM, this is why the bridge has practically gone the way of the Dodo. I guess you could say one advantage of the bridge is that's "it's more fun" to twiddle with the knobs, if that's what you like to do.

Dave.

You might begin by comparing specifications for resistance bridges and DMMs. But don't sell the bridges short. A General Radio 1666 DC Resistance Bridge, for example, offers more than an order of magnitude greater accuracy than most any DMM. In addition, it has a measurement range from 1 uohm to 1 Tohm!

You might also consider how 4-terminal resistance measurements are made.

Look at the specs here:

Chuck

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Do not forget the cute little meters that used a motor to do that tweaking...

Bridge circuits have one big advantage, however, namely temperature compensation. This is especially important with strain gauges, where the full-scale change in resistance is very small, due to the elastic limit of the sensor wire. (You want the indicated strain to return to 0 when the load is removed, so you can't go stretching the wires.) Bridge circuits are also useful when made of capacitors, e.g. capacitive gauges, and for phase shifting, e.g. Wien bridges.

Classical Wheatstone bridges are also still useful for very high impedance voltage measurements--DVMs operating above the 200 mV range use voltage dividers on their inputs, typically presenting a 20M ohm resistance. If you can use a bridge to get the voltage down to less than 200 mV, the DVM impedance goes way, way up, and the measurement gets correspondingly better. You don't need to null the bridge exactly before doing the measurement, so this is a practical technique even today.

Cheers,

Phil Hobbs

Frat boy: "Wanna play TV?" Sorority gal: "Huh? How do you play?" Frat boy: "I'll twiddle with your knobs and you can watch my antenna go up."

Sorry. Rich