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Looking at this question in a design exercise. I'm trying to work out where the impedance to the 9V power supply comes from.

It's given the typical complex impedance of a PCB track at 1kHz is 0.1 + j0.286ohms

So we would get a total impedance of 0.3ohms.Is this the impedance to the 9V supply?

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  • \$\begingroup\$ Notice that they're asking for the impedance to the 9 V supply, not the impedance of the 9 V supply. \$\endgroup\$
    – The Photon
    Aug 11, 2021 at 15:23
  • \$\begingroup\$ But what would be the cause of impedance to the 9V supply? If it's the voltage at the top of R1, then no resistive components between R1 and the supply? \$\endgroup\$
    – Dr. Shakamoto
    Aug 11, 2021 at 15:40
  • \$\begingroup\$ Presumably the track between the supply and the load would be the source of impedance "to" the supply. \$\endgroup\$
    – The Photon
    Aug 11, 2021 at 15:46
  • \$\begingroup\$ Okay thanks, that's what I'd thought. Then is it a case of multiplying the current (2.9A) by the resistance of the track. And then doing 9V - (that voltage). To get the voltage above R1? \$\endgroup\$
    – Dr. Shakamoto
    Aug 11, 2021 at 15:58

1 Answer 1

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It's given the typical complex impedance of a PCB track at 1kHz is 0.1 + j0.286ohms

Think about what you are saying here. PCBs are not usually big. For instance, a PCB trace of 1 mm width on 1 oz copper would need to be 21 cm long to have a resistance of 0.1 Ω.

Then, given that we are talking about a PCB trace length of 21 cm (about 8 inches) it is totally unfeasible for it to have a reactive impedance of 0.286 Ω at 1 kHz: -

Inductance = \$0.286 / (2\pi 1000\$) = 46 μH and that ain't happening in your circuit, not by a mile.

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  • \$\begingroup\$ ah, you make a good point. I think the correct inductance should be 0.2μH. What kind of typical complex impedance do you think you'd expect on a PCB track at 1kHz? \$\endgroup\$
    – Dr. Shakamoto
    Aug 11, 2021 at 11:48
  • \$\begingroup\$ Using 0.2 μH implies an impedance of 1.26 milli ohm. \$\endgroup\$
    – Andy aka
    Aug 11, 2021 at 12:15
  • \$\begingroup\$ and that's as overall impedance. is it possible to think about that in terms of a complex impedance at a frequency of 1kHz? \$\endgroup\$
    – Dr. Shakamoto
    Aug 11, 2021 at 12:37
  • \$\begingroup\$ 1.26 milli ohm is the impedance of 0.2 μH at 1 kHz. \$\endgroup\$
    – Andy aka
    Aug 11, 2021 at 12:50
  • \$\begingroup\$ so could that be written in the form 0.1 + j1.26 mΩ? \$\endgroup\$
    – Dr. Shakamoto
    Aug 11, 2021 at 12:51

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