Questions About Choosing a Shunt Resistor

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Discussion Overview

The discussion revolves around the selection and use of a shunt resistor for measuring power consumption in a circuit operating at 3.3 volts. Participants are exploring the setup and methodology for accurate current measurement, as well as the implications of using a shunt resistor in this context.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes their setup using a 10Ohm shunt resistor with a 5V power source and questions the validity of their approach.
  • Another participant expresses confusion regarding the use of a shunt resistor to reduce voltage from 5V to 3.3V.
  • A participant inquires about the target device and the presence of a 3.3V regulator, suggesting that the bypass capacitors should not be removed.
  • Concerns are raised about the clarity of the original poster's question, with requests for a schematic or further explanation of the oscilloscope's role in measuring power consumption.
  • One participant points out that using a shunt resistor in series may not work effectively for an integrated circuit that requires a constant voltage supply, suggesting that a smaller shunt resistor may be necessary to avoid disturbance during measurement.

Areas of Agreement / Disagreement

Participants express disagreement regarding the appropriateness of the setup and the methodology for measuring current. There is no consensus on the best approach or the validity of the original poster's configuration.

Contextual Notes

Participants highlight potential limitations in the setup, including the removal of bypass capacitors and the choice of shunt resistor value, which may affect measurement accuracy. The discussion also reflects uncertainty about the intended measurement technique and the role of the oscilloscope.

Yoyo G
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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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Uh what? You are using a shunt resistor to try to lower your 5V source to 3.3V? Are you serious?
 
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.
 
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.
 
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.
 

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