Why is impedance mentioned in DC circuits? - Page 1
Forum section: electronics / beginners

author: MrBond
author_rank: Contributor
timestamp: January 10, 2022, 01:21:21 pm
content: Hi, whenever i watch Dave and many other guys on youtube the word impedance is thrown around every time even if the circuit being discussed is purely DC, no AC to speak of. But yet again and again impedance is mentioned. Now if you know the formula for calculating impedance you need a frequency, which DC does not have. It's just confusing.

author: Grandchuck
author_rank: Super Contributor
timestamp: January 10, 2022, 01:39:54 pm
content: With no reactance, Z = R

author: Terry Bites
author_rank: Super Contributor
timestamp: January 10, 2022, 02:33:53 pm
content: Impedance can be resistive or reactive. Its a catch all term for any component or network that impedes the flow and/ or direction of a current. For example a dc current will increase in an indcutor until at some point in time it reaches a maximum value. "Time" being the operative word. DC circuits are not necessarily scalar but may also contain vector quantities.The DC operating point in any circuit cannot be achieved intantaneously. V=IR after a bit!

author: M0HZH
author_rank: Regular Contributor
timestamp: January 10, 2022, 03:28:37 pm
content: Even in a DC circuit, the reactive part of the impedance matters when you're looking at transients or ripple/noise.

author: MrBond
author_rank: Contributor
timestamp: January 10, 2022, 03:47:28 pm
content: Quote from: Grandchuck on January 10, 2022, 01:39:54 pmWell ok, but why use the term in the wrong way thus making it more confusing for people that just began learning about this stuff, such as myself?

author: MrBond
author_rank: Contributor
timestamp: January 10, 2022, 03:50:22 pm
content: Quote from: M0HZH on January 10, 2022, 03:28:37 pmSo let me get this straight, you're supposed to treat ripple as a frequency that introduces the idea of impedance into the circuit.

author: voltsandjolts
author_rank: Unknown
timestamp: January 10, 2022, 03:55:57 pm
content: Impedance (resistance and reactance) are always there, to some extent.Perhaps the reactance is so small you can't measure it with the equipment you have, but its there nonetheless.In every wire, pcb trace, connector and component....

author: Grandchuck
author_rank: Super Contributor
timestamp: January 10, 2022, 04:08:19 pm
content: Quote from: MrBond on January 10, 2022, 03:47:28 pmNot the wrong way. I can see how it is confusing. It is just that impedance, Z, is always appropriate to use when discussing the opposition to the flow of current. All practical circuits have impedance (except superconducting ones ... not often relevant). Some circuits have reactance that can be ignored (often called purely resistive ones). In those, resistance, R, is appropriately used.

author: M0HZH
author_rank: Regular Contributor
timestamp: January 11, 2022, 10:45:19 am
content: Quote from: MrBond on January 10, 2022, 03:50:22 pmYes, ripple is in fact an AC signal superimposed on your DC signal; when you are looking to remove ripple you're supposed to analyze the circuit in terms of impedance. The same goes for transients like switching the circuit on or off, adding/removing a load or any significant change in voltage or current. That's why you have filtering capacitors for example, they present a high resistance but a low impedance for noise/ripple/transients etc.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 11, 2022, 12:28:37 pm
content: If a circuit that has a diode in series with a resistor, would we call that impedance or just resistance? The current flow into the circuit is certainly not proportional to the voltage.

author: Bud
author_rank: Super Contributor
timestamp: January 11, 2022, 02:12:25 pm
content: It is called dynamic resistance or non-linear resistance.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 11, 2022, 02:22:19 pm
content: A network

author: Benta
author_rank: Super Contributor
timestamp: January 11, 2022, 10:19:19 pm
content: Quote from: MrBond on January 10, 2022, 03:47:28 pmNo one is using this in the wrong way.It's a semantics question:Impedance and Admittance are the top-level terms, so if you use those, you'll never be wrong. The attached table should help (Impedance = 1 / Admittance and vice versa):

author: radiolistener
author_rank: Super Contributor
timestamp: January 12, 2022, 01:06:32 am
content: Quote from: MrBond on January 10, 2022, 01:21:21 pmThere is no confusing. If you use DC, you can put 0 Hz in the formula, because DC is actually AC with a frequency 0 Hz.Impedance is important, because it allows to understand how much current can flow through circuit. If impedance has reactance, you can understand circuit behavior for a transient process.

author: brabus
author_rank: Frequent Contributor
timestamp: January 12, 2022, 03:59:37 pm
content: Some say there is no such thing as DC.What you see as a DC voltage or current is the result of a stable dynamic system, obtained through a proper balance of impedance modules, transfer blocks and feedback phenomena, even in the simplest circuits with no apparent reactive component.Furthermore, one can say "high impedance" or "low impedance" when the reactive component is simply unknown. Take for example an opamp: the input stage is often JFET, thus presenting very high resistance and very low capacitance. Short said: a very high impedance.

author: Cerebus
author_rank: Super Contributor
timestamp: January 12, 2022, 04:09:43 pm
content: Quote from: MrBond on January 10, 2022, 03:47:28 pmIt's not "the wrong way", 'impedance' is just a broader term that includes 'resistance' and 'reactance' in the same way that 'number' is a broader term for 'integer', 'fraction' or 'real'.

author: KT88
author_rank: Frequent Contributor
timestamp: January 13, 2022, 10:45:46 am
content: Resistance alone doesn't exist in the real world. It can be used to simplify the circuit analysis if one is interested in the steady state behaviour of a DC signal - but this is always a simplification of actual real-world circuits.Mentioning impedance most of the time reminds us not to forget about this fact.