Wheatstone Bridge Circuit and Theory of Operation

Wheatstone Bridge Circuit and Theory of Operation1 of 7 http://www.electronics-tutorials.ws/blog/wheatstone-bridge.html Home (http://www.electronics-tutorials.ws) » Miscellaneous Circuits (http://www.electronics-tutorials.ws/category/blog) » Wheatstone Bridge Search ► Bridge Rectifier ► Wheatstone Bridge ► Bridge Types Wheatstone Bridge Wheatstone Bridge The Wheatstone Bridge was originally developed by Charles Wheatstone to measure unknown resistance values and as a means of calibrating measuring instruments, voltmeters, ammeters, etc, by the use of a long resistive slide wire. Although today digital multimeters provide the simplest way to measure a resistance, The Wheatstone Bridge can still be used to measure very low values of resistances down in the milli-Ohms range. The Wheatstone bridge (or resistance bridge) circuit can be used in a number of applications and today, with modern Operational Amplifiers (http://www.amazon.com/gp/aws/cart/add.html?ASIN.1=0750659149&Quantity.1=1&AWSAccessKeyId=AKIAIOB4VMPIMBIMN7NA& AssociateTag=basicelecttut-20) we can use the Wheatstone Bridge Circuit to interface various transducers and sensors to these amplifier circuits. The Wheatstone Bridge circuit is nothing more than two simple series-parallel arrangements of resistances connected between a voltage supply terminal and ground producing zero voltage difference between the two parallel branches when balanced. A Wheatstone bridge circuit has two input terminals and two output terminals consisting of four resistors configured in a diamond-like arrangement as shown. This is typical of how the Wheatstone bridge is drawn. The Wheatstone Bridge When balanced, the Wheatstone bridge can be analysed simply as two series strings in parallel. In our tutorial about Resistors in Series (http://www.electronics-tutorials.ws/resistor/res_3.html), we saw that each resistor within the series chain produces an IR drop, or voltage drop across itself as a consequence of the current flowing through it as defined by Ohms Law. Consider the series circuit below. As the two resistors are in series, the same current ( i ) flows through both of them. Therefore the current flowing through these two resistors in series is given as: V/RT. I = V ÷ R = 12V ÷ (10Ω + 20Ω) = 0.4A The voltage at point C, which is also the voltage drop across the lower resistor, R2 is calculated as: VR2 = I × R2 = 0.4A × 20Ω = 8 volts Then we can see that the source voltage VS is divided among the two series resistors in direct proportion to their resistances as VR1 = 4V and VR2 = 8V. This is the principle of voltage division, producing what is commonly called a potential divider circuit or voltage divider network. Now if we add another series resistor circuit using the same resistor values in parallel with the first we would have the following circuit. As the second series circuit has the same resistive values of the first, the voltage at point D, which is also the voltage drop across resistor, R4 will be the same at 8 volts, with respect to zero (battery negative), as the voltage is common and the two resistive networks are the same. But something else equally as important is that the voltage difference between point C and point D will be zero volts as both points are at the same value of 8 volts as: C = D = 8 volts, then the difference is: 0 volts When this happens, both sides of the parallel network are said to be balanced because the voltage at point C is the same value as the voltage at point D. Now let’s consider what would happen when we reverse the position of the two resistors, R3 and R4 in the second parallel 11/19/2014 10:40 AM Wheatstone Bridge Circuit and Theory of Operation 2 of 7 http://www.4A × 10Ω = 4 volts Now with VR4 having 4 volts dropped across it. ammeter or classically a galvanometer between points C and D. which is also the voltage drop across resistor. Then we can see that the resistance ratio of these two parallel arms. When this happens the parallel network is said to be unbalanced as the voltage at point C is at a different value to the voltage at point D.4 = 4 volts The result of swapping the two resistors is that both sides or “arms” of the parallel network are different as they produce different voltage drops.ws/blog/wheatstone-bridge. Calculate the output voltage across points C and D and the value of resistor R4 required to balance the bridge circuit. Then the difference is: 8 . So we can see that a Wheatstone bridge circuit can be used to compare an unknown resistance RX with others of a known value. would result in the two arms being balanced and the value of RX. for example. R4 will be: VR4 = 0.electronics-tutorials. have fixed values. If we connected a voltmeter. R1 and R2. (substituting R4) known as shown. will result in a zero voltage output. 11/19/2014 10:40 AM . Wheatstone Bridge Circuit By replacing R4 above with a resistance of known or unknown value in the sensing arm of the Wheatstone bridge corresponding to RX and adjusting the opposing resistor. results in a voltage difference between 0 volts (balanced) and the maximum supply voltage (unbalanced). R3 until the meters read zero. and R3 could be variable.html branch. the same current flows through the series combination and the voltage at point D. R3 to “balance” the bridge network. and then varied resistor. and this is the basic principal of the Wheatstone Bridge Circuit. R1 and R2 are known or preset values. Wheatstone Bridge Example No1 The following unbalanced Wheatstone Bridge is constructed. the voltage difference between points C and D will be 4 volts as: C = 8 volts and D = 4 volts. Then we can see that balance occurs when: The Wheatstone Bridge equation required to give the value of the unknown resistance. With resistors. RX at balance is given as: Where resistors. R3 and R4 reversed. ACB and ADB. however. is a passive resistive sensor which converts changes in visible light levels into a change in resistance and hence a voltage. or whether a light source is ON or OFF.ws/blog/wheatstone-bridge. The light dependent resistor typically has a resistance of several Megaohms (MΩ’s) in dark or dim light. the voltage across the output terminals is 0 volts. or LDR. When the bridge is balanced. pressure or strain. etc. Light dependent resistors can be used for monitoring and measuring the level of light intensity. also known as a cadmium-sulphide (Cds) photocell. ACB For the second series arm. Wheatstone Bridge Light Detector Balanced bridge circuits find many useful electronics applications such as being used to measure changes in light intensity. ADB The voltage across points C-D is given as: The value of resistor.html For the first series arm. piezoresistive sensors (strain gauges) and temperature sensors (thermistor’s).Wheatstone Bridge Circuit and Theory of Operation 3 of 7 http://www. The types of resistive sensors that can be used within a wheatstone bridge circuit include: photoresistive sensors (LDR’s). Wheatstone Bridge Light Detector 11/19/2014 10:40 AM . By carefully attaching the LDR to the Wheatstone bridge circuit above. R4 required to balance the bridge is given as: We have seen above that the Wheatstone Bridge has two input terminals (A-B) and two output terminals (C-D). There are many wheatstone bridge applications for sensing a whole range of mechanical and electrical quantities.electronics-tutorials. One of the resistors within the bridge network is replaced by a light dependent resistor. down to about 30Ω in bright sunlight. When the bridge is unbalanced. the output voltage may be either positive or negative depending upon the direction of unbalance. positional sensors (potentiometers). but one very simple wheatstone bridge application is in the measurement of light by using a photoresistive device. An LDR. about 900Ω at a light intensity of 100 Lux (typical of a well lit room). we can monitor and measure any changes in the light levels as shown. electronics-tutorials.html) Christmas Lights Sequencer (http://www.html) | I-V Characteristic Curves (http://www.1=1& AWSAccessKeyId=AKIAIOB4VMPIMBIMN7NA& AssociateTag=basicelecttut-20) List Price: $40.amazon.electronics-tutorials.html) Tags: Attenuator (http://www.1=1& AWSAccessKeyId=AKIAIOB4VMPIMBIMN7NA& AssociateTag=basicelecttut-20) Practical Electronics for Inventors (http://www.electronics-tutorials. as well as swapping the positions of the LDR and VR1. 100% Free DataSheet (PDF) Over 21000000 DataSheet. so by replacing the above photo-resistive LDR light sensor for a thermistor.electronics-tutorials.com /gp/aws /cart/add. RX due.ws/blog/relay-switch-circuit.ws/blog/optocoupler.html) Wheatstone Bridge (http://www. The relay turns “ON” when the voltage at point C is less than the voltage at point D.ws/blog/convert-atx-psu-to-bench-supply. we can use them in a variety of other Wheatstone bridge applications.html/comment-page-1#comments) 11/19/2014 10:40 AM .ws/tag/miscellaneous) Resistance (http://www.electronics-tutorials.ws/blog/voltage-multiplier-circuit. The LDR can be any cadmium-sulphide device that has a high impedance at low light levels and a low impedance at high light levels. strain gauge.60 (http://www. But the bridge circuit is also suitable for measuring the resistance change of other changing quantities.html) Price Disclaimer Relay Switch Circuit (http://www.electronics-tutorials. “half-bridge” or “quarter-bridge circuit arrangements providing thermal compensation or automatic balancing of the Wheatstone bridge.electronics-tutorials.electronics-tutorials.ws/blog/wheatstone-bridge. The op-amp is connected as a voltage comparator with the reference voltage VD applied to the inverting pin and the variable resistor.ws/blog/wheatstone-bridge.1=1& AWSAccessKeyId=AKIAIOB4VMPIMBIMN7NA& AssociateTag=basicelecttut-20) Guide to Passive Devices (http://www.electronics-tutorials.1=0071771336& Quantity. It's Free No Login.ws/tag/passives) Miscellaneous (http://www. Multi Fast Search System. applied to the non-inverting input.electronics-tutorials.ws/blog/variable-voltage-power-supply. « Voltage Multiplier (http://www.ws/tag/attenuator) Passives (http://www.ws/blog/wheatstone-bridge.electronics-tutorials.amazon.ws/blog/pulse-width-modulation.html) Unregulated Power Supply (http://www. Note that the circuit can be used to act as a “light-activated” switching circuit or a “dark-activated” switching circuit simply by transposing the LDR and R3 positions within the design.electronics-tutorials.html?ASIN.html) I-V Characteristic Curves (http://www.electronics-tutorials.html?ASIN. Adjusting VR1 sets the voltage at point C to balance the bridge circuit at the required light level or intensity.html The LDR photocell is connected into the Wheatstone Bridge circuit as shown to produce a light sensitive switch that activates when the light level being sensed goes above or below the pre-set value of VR1.amazon.ws/tag/resistance) 26 Responses to “Wheatstone Bridge” Older comments (http://www.ws/blog/passive-devices.electronics-tutorials.ws/opamp/opamp_1.electronics-tutorials.electronics-tutorials.com /gp/aws /cart/add.html) (http://www. Also more than one resistive sensor can be used within the four arms (or branches) of the bridge formed by the resistors R1 to R4 to produce “full-bridge”.html) Variable Voltage Power Supply (http://www.amazon. for example.electronics-tutorials.html) Pulse Width Modulation (http://www.html?ASIN.com /gp/aws /cart/add.html) » Other Good Tutorials in this Category 7-segment Display (http://www.ws/blog/voltage-multiplier-circuit.Wheatstone Bridge Circuit and Theory of Operation 4 of 7 http://www.1=1& AWSAccessKeyId=AKIAIOB4VMPIMBIMN7NA& AssociateTag=basicelecttut-20) Convert ATX PSU to Bench Supply (http://www.ws/blog/unregulated-power-supply.ws/blog/i-v-characteristic-curves. the Wheatstone bridge circuit can be used to measure and amplify small changes in resistance.1=0071771336& Quantity.1=0071771336& Quantity.html) (http://www.electronics-tutorials.ws/blog/christmas-lights-sequencer-circuit.00 (http://www.electronics-tutorials. and other such transducers.html).html?ASIN.ws/blog/7-segment-display-tutorial. When used with Operational Amplifiers (http://www.com /gp/aws /cart/add.electronics-tutorials.html?ASIN.html) Optocoupler Tutorial (http://www. pressure sensor.amazon.ws/blog/i-v-characteristic-curves.1=0071771336& Quantity. VR1 sets the trip point voltage VC. The Wheatstone Bridge has many uses in electronic circuits other than comparing an unknown resistance with a known resistance.html) Voltage Multiplier (http://www.1=0071771336& Quantity. to changes in light intensity as we have seen above.com /gp/aws /cart/add.electronics-tutorials.1=1& AWSAccessKeyId=AKIAIOB4VMPIMBIMN7NA& AssociateTag=basicelecttut-20) Current Price: $18. html?replytocom=4771#respond) November 5th.electronics-tutorials.google.electronics-tutorials. for example.d2s&tt=0&captcha_provider=nucaptcha) 34 11/19/2014 10:40 AM .electronics-tutorials. Reply (/blog/wheatstone-bridge.ws/blog/wheatstone-bridge. a light sensor.electronics-tutorials.html& title=Wheatstone%20Bridge%20Circuit%20and%20Theory%20of%20Operation&ate=AT-ra-4dabe1c24a1ae8c8/-/-/546c2c719599 uid=546c2c71d59bd9d2&ct=1&pre=http%3A%2F%2Fwww. is one where one or more of the resistive elements changes value with respect to some external influence.d2s&tt=0&captcha_provider=nucaptcha) (http://www.html/comment-page-1#comments) Leave a Reply Your email address will not be published.ws/blog/wheatstone-bridge. 2014 (http://www. 2014 (http://www.ws/blog/wheatstone-bridge.electronics-tutorials.php?v=300&winname=addthis&pub=ra-4dabe1c24a1ae8c8&source=tbx32-300&lng= US&s=google_plusone_share&url=http%3A%2F%2Fwww.html/comment-page-2#comment-4774) Older comments (http://www.html& title=Wheatstone%20Bridge%20Circuit%20and%20Theory%20of%20Operation&ate=AT-ra-4dabe1c24a1ae8c8/-/-/546c2c719599 uid=546c2c7121349be7&ct=1&pre=http%3A%2F%2Fwww.ws%252Fblog%252Fwheatstonebridge.electronics-tutorials.com/bookmark. an LDR.80120444%2Cd. Whereas a balanced bridge is one where the resistive elements are adjusted or fixed to give zero differential output voltage.ws%2Fblog%2Fwheatstone-bridge. a temperature sensor.com.addthis.ws) A dynamic bridge as I understand it.google.ws/blog/wheatstone-bridge.html Serge could you please explain the difference between the balanced and dynamic bridge please? Reply (/blog/wheatstone-bridge.pk%2Furl%3Fsa%3Dt%26rct%3Dj%26q %3D%26esrc%3Ds%26source%3Dweb%26cd%3D5%26cad%3Drja%26uact%3D8%26ved%3D0CDcQFjAE%26url%3Dhttp%253A% tutorials.Wheatstone Bridge Circuit and Theory of Operation 5 of 7 http://www.ws%2Fblog%2Fwheatstone-bridge.html/comment-page-2#comment-4771) Wayne Storr (http://www.electronics-tutorials.80120444%2Cd. etc.html?replytocom=4774#respond) November 5th.com.html%26ei%3DVCxsVNu7F4njaLqHgPgH%26usg%3DAFQjCNGIZqRKdCxq1Ef7WCK8HBbRHQqqVw%26sig2%3DYjm75Nt A%26bvm%3Dbv.pk%2Furl%3Fsa%3Dt%26rct%3Dj%26q %3D%26esrc%3Ds%26source%3Dweb%26cd%3D5%26cad%3Drja%26uact%3D8%26ved%3D0CDcQFjAE%26url%3Dhttp%253A% tutorials.php?v=300&winname=addthis&pub=ra-4dabe1c24a1ae8c8&source=tbx32-300&lng= url=http%3A%2F%2Fwww.html%26ei%3DVCxsVNu7F4njaLqHgPgH%26usg%3DAFQjCNGIZqRKdCxq1Ef7WCK8HBbRHQqqVw%26sig2%3DYjm75Nt A%26bvm%3Dbv. 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