Manually choose shunt resistor

Is there a way to manually set the shunt resistor? Here is why I ask… My device is normally operating in the 6 mA range when awake. I have a mode where 50 mS long pulses will draw as much as 1 amp of current. The behavior that I am seeing is that my device resets when this even occurs. If I remove the JouleScope I don’t have reset problems. So I suspect my voltage is drooping and the reset occurs. Maybe the dynamic switch to a smaller resistor is not fast enough for my system?

I will do further trouble shooting on my own system, but bottom line is: how do I manually set the shunt resistor value? Thanks.

Hi @chet and welcome to the forum! Sorry to hear that things are not going as smoothly as they should.

Yes, you can manually set the shunt resistor value, but you will lose some dynamic range. In the “Control” widget in they UI (which is shown by default when you select ViewOscilloscope), select Current Range 2A:

Now, the more puzzling question is why is your target device reseting? Your device will normally be operating in Joulescope’s 18 mA range using a 1.11 Ω shunt resistor. Joulescope will autorange in 1.2 µs, max. Even if your edge rate is slow and Joulescope briefly autoranges to 180 mA (0.11 Ω) before autoranging to 2 A (0.01 Ω), the voltage drop will be 0.11 Ω * 1 A = 110 mV. Even that amount should not cause your target device to brown out.

I have some questions:

  1. What is the approximate capacitance of your target device? For a 1 A jump, I normally recommend at least 10 µF, which is a reasonable amount normally needed for your target device to work properly even without Joulescope.

  2. How long is your cabling and what gauge wire are you using? At 1 A, 22 gauge hook-up wire’s 53 mΩ/meter resistance can really add up. Wikipedia has a good table showing wire resistance per length.

  3. What is the maximum voltage drop that your target can handle before reseting (likely due to brown-out reset)?

Thanks for the excellent feedback. I need to get a separate scope probe on my device and see what is happening. Good to know about the manual setting. Sounds like if my device is well behaved I should not need the manual setting. I will do some further testing and report back.

BTW, I am loving the JouleScope! Its another set of eyes I have never had for my work. My project involves a small solar panel, a lipo battery, and lots of coding to manage the power. The JouleScope is going to be my best friend as I go through this process.

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