I am working on a power meter project where I want to measure an AC current using a shunt resistor. After research, I found that the use of this resistance is rarely. Is it possible to know the reason for this and what are the other possible solutions?
This was my first project idea, where I used a current transformer. now I thought of the shunt resistor but I read about the problem of isolation. is there any solution to replace the current transformer with the shunt resistor without getting the problem of isolation
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\$\begingroup\$ You want a very small resistance so you aren't significantly affecting the equipment under test. If you used a resistor that was "infinite", no current would flow. So obviously that's the wrong choice. If you use a resistor of "zero" value, you would get no voltage to measure across that resistor (E=IR.. If R=0, then E=0...) So the right value is somewhere in between. It's up to you to pick a resistor that is small enough not to affect the circuit under test, but large enough to develop enough voltage for you to measure. \$\endgroup\$– Kyle BCommented Aug 18, 2022 at 18:03
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\$\begingroup\$ Are you asking why the shunt resistor value is very low, or why shunt resistors are rarely used for AC current measurement? Current transformers are often used for AC current measurement, but I don't know how often they are used compared to shunt resistors. \$\endgroup\$– Peter BennettCommented Aug 18, 2022 at 18:13
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\$\begingroup\$ no i am asking why the shunt resistor are rarely used for AC current measurement. \$\endgroup\$– ShamilCommented Aug 18, 2022 at 18:16
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The resistance can be very small, because we have simple means to amplify those small signals accurately - the modern op-amp is a marvel of engineering in that regard.
The reasons are related. We desire:
Low burden voltage - that's the voltage drop across the shunt. An ideal current measurement device has zero resistance. Practical ones try to approach that ideal, so the shunt has low resistance so it doesn't affect the circuit it's placed into very much. We care that the circuit doesn't suffer from the voltage drop.
Low dissipated power - shunts have non-zero temperature coefficient, so as they heat up from burden power (burden voltage x current), their calibration conductance changes (Amperes per Volt). Low burden voltage aids in lowering the dissipated power. As some power dissipation is inevitable, the higher power shunts use various techniques for temperature compensation, e.g. special alloys, temperature sensors, etc.
what are the other possible solutions?
Current transformers work for AC current sensing and offer inherent isolation, especially useful when the voltages and/or currents involved are very high,
Magnetic field sensors, typically Hall sensors, but also fluxgate sensors for higher accuracy applications, measure the magnetic field produced by the current flowing through the conductor. Those can be used in both AC and DC applications.
What ends up being used depends on the needs of the application: safety requirements, thermal budget, market acceptance, sensitivity, linearity, noise level.
answered Aug 18, 2022 at 18:04
Kuba hasn't forgotten MonicaKuba hasn't forgotten Monica
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\$\begingroup\$ thanks for the reply, but in the majority of projects and articles I found that the use of the shunt resistor for AC current measurement is not common unlike current transformers, so I want to know the reasons. \$\endgroup\$– ShamilCommented Aug 18, 2022 at 18:11
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\$\begingroup\$ Isolation is probably the biggest factor. Meters and measuring circuitry are very likely directly accessible to the user whereas whatever is being powered by the AC supply might have any dangerous voltages tucked away someplace safe. If you bring mains voltage onto your board, you have to deal with creepage and a whole host of other safety issues that aren't as much of a headache if you have kVs of isolation between your otherwise-low-voltage system and the wall. \$\endgroup\$– virCommented Aug 18, 2022 at 19:18
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\$\begingroup\$ Shunt resistors are made to be temperature independent. \$\endgroup\$ Commented Aug 18, 2022 at 21:22
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\$\begingroup\$ @Shamil It's "not common" maybe in industrial uses when the current is measured on a panel or at an electrical substation, since there the isolation is usually a requirement. On the level of a PCB, currents are routinely measured using current shunts, and there's billions of those things - most switching power supplies have a shunt that measures AC current. Sometimes the shunt is cleverly integrated into a semiconductor component, e.g. it may be the channel resistance of a mosfet. \$\endgroup\$ Commented Aug 18, 2022 at 21:30
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\$\begingroup\$ @MissMulan Anything can be a shunt resistor, and sometimes they have to be an inherent part of another component. For example, I've seen low-burden bench current meters that use the fuse itself as a shunt, and have a rather sophisticated observers of fuse temperature and thus its shunt resistance. Channel resistance in mosfets is also often used as a current shunt, and there too the tempco is much higher than in a shunt that's an expensive chunk of metal. Various techniques can compensate those tempcos. Passive compensation requires matching of time constants as well. \$\endgroup\$ Commented Aug 18, 2022 at 21:33