How to Choose Shunt Resistors for Power Electronics and Automotive Applications

When you choose shunt resistors for power electronics or cars, consider accuracy and their performance. The key factor is to achieve a measurable voltage drop without wasting too much power. A quality shunt resistor allows you to monitor current safely and reliably. Choose shunt resistors wisely to avoid costly mistakes.

Key Takeaways

• Know what your project needs. Think about temperature, size, and how much current you need to measure when picking shunt resistors for cars.

• Find a good balance between resistance and power. Pick a shunt resistor that gives a clear voltage drop for good measurements. Try to use less power so your system works well.

• Use Ohm’s Law to pick the right shunt resistor. Figure out the resistance you need by looking at the voltage drop and current you want. This helps you measure current the right way.

• Handle heat the right way. Make your PCB well and use ways to keep things cool. This stops shunt resistors from getting too hot in strong power systems.

• Look at the datasheet. Always check the power rating, resistance, and tolerance of the shunt resistor. Make sure it fits what your system needs.

Application Needs

Automotive Requirements

When picking a shunt resistor for cars, you need to know what your system needs. Each car system is different. You should think about things like temperature, size, and how well it works. The table below lists the main things to look for in shunt resistors for cars:

Requirement	Description
Application Temperature	You need to know the temperature range for your system. This helps you pick the right shunt resistor.
Installation Size	The resistor must fit in the space you have in your car system.
Current Measurement	The amount of current you want to measure will change what resistor value you need.
Power Loss and Heating	Less power loss and less heat help your system work better and last longer.
Temperature Coefficient	A low temperature coefficient keeps your measurements right when the temperature changes.
Pulse Load Considerations	The resistor must handle quick changes in current to work well.

Tip: Always check the temperature and vibration in your system. If the temperature changes, the resistance value of a shunt resistor can change too. This can make your measurements wrong. Vibration can break the resistor or change its value over time. Good cooling and strong mounting make your system more reliable.

Current Range

You need to know the current range before you pick a shunt resistor. In battery management systems, the current can be very high. The table below shows the usual current ranges you might see:

Current Range	Description
200A to 500A	This is the normal range for battery management systems in electric cars.
Up to 2,000A	Some systems need shunt resistors for even bigger currents.

Battery management systems use shunt resistors to check and control current flow. These systems need good current monitoring to keep batteries safe and working well. If you pick the wrong resistor, you might get bad accuracy or even break your system. Always match the resistor to your system’s current needs and where it will be used. This helps you make good current monitoring systems for cars.

Choose Shunt Resistors

Resistance & Power

When picking shunt resistors, focus on resistance value and power rating. These two things decide how well the shunt resistor works. The resistance value sets the voltage drop across the shunt. If you pick a higher resistance, the voltage drop gets bigger. This makes it easier to measure current. But higher resistance also means more power loss. This can heat up the shunt resistor and make your system less efficient.

If you use lower resistance, you lose less power. Your system stays cooler and works better. But the voltage drop gets smaller, so measuring current is harder. You need to balance a good voltage drop with low power loss. Here is what happens when you change resistance:

• Higher resistance gives a bigger voltage drop for easier measurement.

• More resistance also means more power loss, which can hurt your circuit.

• Lower resistance keeps power loss low, but makes voltage drop harder to measure.

You must also check the power rating. The power rating shows how much power the shunt resistor can handle. If you go over the power rating, the shunt can get too hot or break. Always pick shunt resistors with a power rating higher than the most power you expect.

Note: Power loss in a shunt resistor uses this formula:
P_loss = I^2 * R
Lower resistance values mean less power loss and better system efficiency.

Ohm’s Law & Shunt Resistor Value

You can use Ohm’s Law to pick the right shunt resistor value. Ohm’s Law says V = I * R. V is the voltage drop, I is the current, and R is the resistance. When you know the current you want to measure and the voltage drop you can read, you can solve for R.

For example, if you want to measure 100A and your system can read a voltage drop of 50mV, use the formula:

R = V / I = 0.05V / 100A = 0.0005Ω or 0.5mΩ

This value is common in high current automotive systems. Many shunt resistors for these systems have resistance values as low as 0.5 mΩ. You often find these resistors in a 2512 case size, and they can handle up to 3 W of power. One example is the ULRG3-2512-0M50-FLFSLT, which is a 0.5-mΩ shunt resistor with a 1% tolerance.

When you choose shunt resistors, always check the power rating. Use the formula for power loss to make sure your shunt resistor can handle the heat. If your current is 100A and your resistance is 0.5mΩ, the power loss is:

P_loss = (100A)^2 * 0.0005Ω = 5W

You need a shunt resistor with a power rating above 5W for this case. If you use a resistor with a lower power rating, it may get too hot or break.

Low Resistance Shunt Resistors

For high current jobs, pick low resistance shunt resistors. These resistors help keep power loss low and make your system work better. You can see the trade-offs in the table below:

Advantages of Shunt Resistors	Disadvantages of Shunt Resistors
Small and inexpensive	Can lose a lot of power
Can be made on PCB	Changes the measured circuit because of voltage drop
Many low resistance options	May not work for low voltage jobs

Low resistance shunt resistors are very helpful in cars and power electronics. They keep your system cool and efficient. But you must make sure the voltage drop is still big enough for your measurement circuit to read. If the voltage drop is too small, your readings may not be right.

When you pick, always balance these points:

• The shunt resistor must have low enough resistance to keep power loss down.

• The voltage drop across the shunt must be high enough for your measurement system.

• The power rating must be higher than the most power you expect.

Tip: Always check the datasheet for the shunt resistor. Make sure the power rating, resistance value, and case size fit your needs. Good choices help you avoid overheating and measurement mistakes.

You can pick shunt resistors that match your current range, voltage drop needs, and power rating. Careful picking will help your system work safely and efficiently.

Accuracy & Impact

Voltage Drop

It is important to know how voltage drop works. The voltage drop across a shunt resistor is the main way to measure current. This voltage drop changes when the current changes. If the voltage drop is not steady or correct, your current reading will be wrong. In cars, even a small change in voltage drop can make a big mistake in current readings. Things like temperature, shunt resistor quality, and your system’s design all matter. You need to control these things for good measurement. If you do not, your current measurement will not be accurate.

Tip: Always check the voltage drop when the temperature changes. This helps you keep your current readings right in real life.

Measurement Error

Many things can make your current measurement wrong. You should watch out for tolerance, thermal EMF, and other errors. Tolerance shows how much the real resistance can be different from what is printed on the shunt resistor. If the tolerance is high, your measurement will not be as good. For very exact jobs, always pick a shunt resistor with low tolerance.

Thermal EMF is another thing that can cause mistakes. It happens when two different metals touch inside the resistor. This can make a small voltage that adds to the voltage drop, mostly when the current is low. In battery systems that are not working, this error can be important. Here are some main points:

• Thermal EMF makes an error voltage at low currents, especially when batteries are not being used.

• This error can hurt measurement if you do not fix it.

• The thermocouple effect makes thermal EMF where different metals meet in resistors.

• In very exact jobs or with low ohm resistors, this effect can be a big deal.

You should also think about the total measurement error, which includes tolerance, thermal EMF, and other things. Always look at the datasheet for these numbers. By knowing about these errors, you can make your current measurement better and get results you can trust.

Practical Factors

Heat Management

When you use a shunt resistor in high-power systems, heat is a big issue. You must control heat to keep your system safe and make it last longer. There are some ways to help with heat:

• Make PCB traces wide so they carry more current and spread heat.

• Put thermal vias under the shunt to move heat away.

• Pick a resistor with the right power rating for your needs.

• Use heat sinks if your system gets very hot.

• Keep copper on both sides of the shunt even to stop voltage issues.

Good layout and placing the shunt in the right spot help with heat. Always check your design to make sure heat can leave the resistor.

PCB Design

How you design your PCB changes how well you can measure current. You need to put the shunt in the right place and make the layout good for power.

If you use the right layout, the shunt’s temperature coefficient of resistance stays steady. The solder joint thickness can change resistance and TCR. More solder means higher resistance and TCR, which can hurt your measurements.

You should use a Kelvin connection for the voltage sense lines. This helps your shunt resistor work its best. Try to keep traces and pads as even as you can.

Form Factor

The material and how the shunt resistor is made change how it works over time. Pick the right type for your job. The table below shows some common materials and what they do:

Material/Method	Characteristics
Constantan	Stays stable with heat, resists rust
Nichrome	Stays stable with heat, resists rust
Wire Wound	Good for jobs with lots of power
Thin Film	Best for jobs that need exact and quiet work
Metal Foil	Most accurate and stable

You also need to think about price, rules, and getting parts when you pick a shunt resistor. Car makers must follow strict safety and performance rules. Work with suppliers you trust and plan ahead to avoid problems. Use more than one supplier and track your supply chain with data. This helps you keep your system working well.

You can find the right shunt resistor by using these steps:

1. Figure out the working current and add a power margin of 20% to 30%.

2. Make sure your measurements are accurate and check your system’s tolerance.

3. Think about the temperature coefficient and the place where you will use it.

4. Pick the best packaging and PCB layout for your shunt resistor.

5. Look at cost, supply chain, and if you need any certifications.

Do not use a resistor with a value that is too high. This can make a big voltage drop and hurt your device. Always match the shunt resistor to your system. Check the datasheet before you decide which one to use.

 

 

 

 

 

Written by Jack Elliott from AIChipLink.

 

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