Questions About Choosing a Shunt Resistor

In summary, this person is trying to do a power analysis on their target device, which runs on 3.3 volts. They used a 10 Ohm resistor as a shunt resistor to try to lower the 5V source to 3.3 volts, but are having trouble because the oscilloscope lacks a proper power consumption line. They need to know more about the tricks they will need to use to make the analysis work.
  • #1
Yoyo G
I'm trying to do a power analysis on my target, which runs on 3.3 volts. Without the resistor, the average current while running is roughly 140mA.
I used a 10Ohm resistor as a shunt resistor with a 5V power source. The voltage difference between VCC PIN and GND was measured using a shunt resistor linked between 5V and the target VCC pin. (The decoupling capacitors have been removed.) On the other hand, the oscilloscope lacked a proper power consumption line.
Is there a problem with my setup? Is there any guidance on how to choose a shunt resistor?
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  • #2
Uh what? You are using a shunt resistor to try to lower your 5V source to 3.3V? Are you serious?
  • #3
What is the target ?
Where is the usual 3.3 V regulator ?
Can you measure the input current to the 3.3 V regulator ?
Keep the bypass capacitors, do not disconnect them.
  • #4
What? This doesn't make much sense to me. What are you trying to do? Can you post a schematic or block diagram?
Yoyo G said:
the oscilloscope lacked a proper power consumption line.
What is that? I've used a lot of oscilloscopes and measure current many times, but I don't know what you are referring to here. Are you wanting to use the scope to multiply current and voltage to display power? Why would you need that for a fixed PS voltage?

I think you're not communicating your question well enough for us to help. Tell us more.
  • #5
Feels like quite a mess. Based on the description, you wish to see some current-over-time diagram, or possibly power-over-time diagram? At what time scale?

The most fundamental problem with your setup is, that while serial resistors would split power and (for resistors! In theory!) it would enable measuring the power on the other resistor, when it's 3.3V and VCC (means an integrated circuit) it is no longer working since an IC requires constant voltage supply to function properly.

So you need to pick a shunt which is small enough to not disturb the VCC during measurement. 10Ohm would sure not do. You need further tricks to make this work, but we need to know more to know what tricks will you need.

1. What is a shunt resistor and what is its purpose?

A shunt resistor is a type of electrical resistor that is used to measure the current flowing through a circuit. It is connected in parallel with the load and creates a known voltage drop that can be measured to determine the current. Its purpose is to provide a low resistance path for the current to pass through, allowing for accurate current measurement without affecting the circuit's overall performance.

2. How do I choose the right shunt resistor for my application?

Choosing the right shunt resistor involves considering the maximum current that will be flowing through the circuit, the desired accuracy of the measurement, and the power rating of the resistor. It is important to select a shunt resistor with a power rating that is higher than the maximum power expected in the circuit to avoid overheating. Additionally, the resistance value of the shunt resistor should be low enough to minimize voltage drop but high enough to provide an accurate measurement.

3. Can I use any type of resistor as a shunt resistor?

No, not all resistors are suitable for use as shunt resistors. Shunt resistors are designed specifically for current measurement and have low resistance values and high power ratings. Other types of resistors may not be able to handle the high currents and could affect the accuracy of the measurement.

4. What is the difference between a fixed and adjustable shunt resistor?

A fixed shunt resistor has a predetermined resistance value and cannot be changed, while an adjustable shunt resistor allows for the resistance value to be changed. Adjustable shunt resistors are useful when the current being measured may vary, as the resistance can be adjusted to provide a more accurate measurement.

5. Are there any safety considerations when using a shunt resistor?

Yes, it is important to ensure that the shunt resistor is properly rated for the maximum current and power expected in the circuit. If the resistor is not rated correctly, it could overheat and potentially cause damage to the circuit or pose a safety hazard. It is also important to follow proper handling and installation procedures to avoid any electrical hazards.

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