Questions About Choosing a Shunt Resistor

  • Thread starter Yoyo G
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  • #1
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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?
 

Answers and Replies

  • #2
berkeman
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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?
 
  • #3
Baluncore
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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
DaveE
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What? This doesn't make much sense to me. What are you trying to do? Can you post a schematic or block diagram?
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
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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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