Impedance of a power source: Whats the difference in high and low? - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: March 15, 2017, 01:45:40 am
content: This has always been a mystery to me. I understand the impedance of a load. Makes sense you can measure it in ohms of resistance or in the case of a coil inductance. But it starts to get confusing when people say the AC outlet neutral and the ground is a high impedance or low impedance. For example speakers are usually 8 or 4 ohms inductance and you have to match them to your amp. Whats in the amp makes it work better with one or the other? I realize you want to match a higher current amp with lower impedance but how does this all tie together. How can the outlet of AC be one or the other?

author: retrolefty
author_rank: Super Contributor
timestamp: March 15, 2017, 02:03:44 am
content: An old maxim (from my ham radio days) is that maximum efficient POWER transfer only occurs when the source impedance matches the load impedance. I don't know if that adds any understanding to what you are asking, but there it is.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: March 15, 2017, 02:09:22 am
content: Quote from: retrolefty on March 15, 2017, 02:03:44 amThats what I don't get. How do you measure the source impedance? Put different loads on it, then what do you look for? How hot it gets?

author: GoneTomorrow
author_rank: Regular Contributor
timestamp: March 15, 2017, 02:26:29 am
content: Just talking crap here, but wouldn't source impedance be like, the ESR of a battery? I.e. what causes its voltage to drop as the current it's sourcing increases. So a 4 ohm amp with 1A flowing on a speaker would be dropping 4V, and the speaker would also be dropping 4V if it were matched impedance.

author: Brumby
author_rank: Unknown
timestamp: March 15, 2017, 02:37:01 am
content: Quote from: GoneTomorrow on March 15, 2017, 02:26:29 amGive the man a cigar!!That is exactly the example I was going to start with.Quote from: GoneTomorrow on March 15, 2017, 02:26:29 amThat's not quite how it works.You are (basically) correct about 4V - 1A for a 4 ohm speaker - but the amplifier (power supply) will have a significantly lower impedance than the speaker.The Maximum Power Transfer point is not found in a lot of real world situations. Take a power generation plant, for example. If it were to deliver 100MW to the grid at MPT, it would have to dissipate 100MW on site. Not a great idea.

author: Brumby
author_rank: Unknown
timestamp: March 15, 2017, 02:49:18 am
content: If you are wondering about "resistance" versus "impedance", then don't fret - it's similar territory ... sort of.In itssimplestform, "resistance" is a DC measurement and "impedance" is an AC measurement - but they both refer to how they affect the passage of electric current. **As such, a low impedance power supply will provide a nominal voltage, with the ability to supply high levels of current, whereas a high impedance power supply will not be able to produce a lot of current at all.Exactly what "high" and "low" mean here is very dependent on the environment. In a power station supplying mains power to homes and industry, a generator with a "high" impedance is no good for that job, but it would almost look like an ideal power supply on someone's test bench. (Mental image here ....)** Beginners Section type answer

author: hurricanehenry
author_rank: Regular Contributor
timestamp: March 15, 2017, 02:53:33 am
content: The key is that you said source IMPEDANCE and not source RESISTANCE.You probably already understand this.As mentioned by others, you want to match the source and the load impedances.The AC source impedance can be derived through a mix of being calculated and also measured. The components will have resistance and reactance. So you calculate it based on frequency. Then you match it with the load, which operates at that frequency as well.

author: Audioguru
author_rank: Super Contributor
timestamp: March 15, 2017, 04:04:43 am
content: A speaker resonates like a bongo drum, bong! How is the resonance controlled and damped? By the extremely low output impedance of a modern amplifier, the specification is its "damping factor".Many good audio amplifiers have a damping factor of 400 so the output impedance is 8 ohms/400= 0.02 ohms created by low impedance emitter-followers at the amplifier output and lots of negative feedback.Then when the speaker tries to resonate it becomes a generator but the low impedance of the amplifier shorts its generation causing it not to move differently from the audio signal that is feeding it.Try to turn a generator or push a speaker cone that is shorted. It is very difficult but they move or resonate easily when not shorted.Many years ago, vacuum tubes were used in audio amplifiers and they had a high impedance. The output transformer was made to match the speaker impedance and the speakers were made to poorly damp themselves.

author: Brumby
author_rank: Unknown
timestamp: March 15, 2017, 04:19:04 am
content: Quote from: hurricanehenry on March 15, 2017, 02:53:33 amBe careful here."Matching" source and load impedances in a lot of situations does not mean making them the same - unless you are specifically seeking to implement MPT (with clear reason) - but in a lot of situations this is not a good idea.As mentioned above, mains power generation and amplifier output stages are but two examples. In these situations "matching" essentially means ensuring the load is driven to function as intended and the source is operating within its capabilities.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2017, 07:29:11 am
content: If the circuit is linear and obeys superposition, then:Treat it precisely like any other two-terminal element (aka one port). Apply a voltage and observe the current flow, or vice versa. Or apply a small change in voltage, and observe the change in current.If superposition works, then it doesn't matter where the voltage/current comes from. It can be the source itself!Read up on Thevenin's theorem (and the Norton equivalent) for more detail.Tim