# Impedance to supply voltage  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?

• Notice that they're asking for the impedance to the 9 V supply, not the impedance of the 9 V supply. Aug 11, 2021 at 15:23
• 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? Aug 11, 2021 at 15:40
• Presumably the track between the supply and the load would be the source of impedance "to" the supply. Aug 11, 2021 at 15:46
• 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? Aug 11, 2021 at 15:58

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.

• 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? Aug 11, 2021 at 11:48
• Using 0.2 μH implies an impedance of 1.26 milli ohm. Aug 11, 2021 at 12:15
• and that's as overall impedance. is it possible to think about that in terms of a complex impedance at a frequency of 1kHz? Aug 11, 2021 at 12:37
• 1.26 milli ohm is the impedance of 0.2 μH at 1 kHz. Aug 11, 2021 at 12:50
• so could that be written in the form 0.1 + j1.26 mΩ? Aug 11, 2021 at 12:51