How to make variable impedance for high power AC source - Page 1
Forum section: electronics / beginners

author: ConcreteAnswers
author_rank: Newbie
timestamp: July 28, 2025, 04:34:43 am
content: HelloI have an AC voltage source capable of 0-100V and would like to be able to simulate the effects of different loads on it. I would like the frequency to go up to 100kHz if not feasible 20kHz would be okay.I would like to simulate a load as small as up to 50 ohms, and with a reactive component of +-50 ohms. (I am not sure on the numbers for reactive completely, could possibly be less). (So max current draw of 2A). This should be controllable.I am struggling to think of a solution that uses either MOSFETs or BJTs in their linear regions, these voltages are well beyond the abilities of op amps. I’m not too worried on cost (unless ridiculously expensive) but would prefer answers that don’t have very high complexity analog circuitry, and many control stages.If there are any changes that would make the circuit significantly easier (maybe half the voltage, or half the max current draw) I’m open to that as well.Thank you for advice!

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2025, 07:16:28 am
content: I envision something like a class-B or class-AB power amplifier driven by a multiplier to produce a voltage variable resistive load, which is then driven by a variable frequency sine source. This covers the resistive part and is similar to an active power factor correction circuit. The feedback to the multiplier is then phase shifted by something like a voltage or frequency variable all-pass filter to simulate the reactance. A gyrator using operational transconductance amplifiers might work for this and it could also be done with a phase locked loop and additional circuitry.Digital signal processing could do it of course, but either solution is not simple.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 28, 2025, 07:44:53 am
content: Quote from: ConcreteAnswers on July 28, 2025, 04:34:43 amCould you also provide the lowest frequency? It's difficult to achieve a wide bandwidth in phase-shifting circuits.

author: ConcreteAnswers
author_rank: Newbie
timestamp: July 28, 2025, 08:55:49 am
content: Absolute lowest I expect would be 500Hz. If 500Hz - 100kHz is too large a range I could try with 500- 20kHz instead

author: ConcreteAnswers
author_rank: Newbie
timestamp: July 28, 2025, 08:59:06 am
content: Quote from: David Hess on July 28, 2025, 07:16:28 amSorry I haven’t used most of these components so not quite sure what you mean. Particularly using a class A or A/B power amplifier being driven by a multiplier. Thank you though! It is seeming like an excessively tough problem…

author: Whales
author_rank: Super Contributor
timestamp: July 28, 2025, 11:19:01 pm
content: Question: What do you mean by "simulate"? Are you wanting to simulate this whole thing in software on a computer; or do you want to buy/build actual equipment and experimentally test it?Software simulation of this is reasonably accessible. Most electronics sims (eg falstad, ltspice, QUCS) will do what you want within only a few minutes of usage (or hours if you're not familiar with them).For hardware: you can buy off the shelf audio amplifiers that will meet something similar to your AC source needs.

author: ConcreteAnswers
author_rank: Newbie
timestamp: July 29, 2025, 01:30:07 pm
content: Quote from: Whales on July 28, 2025, 11:19:01 pmNot simulate in software. I take a high powered AC source and have a circuit that imitates a load on it.E.g if I want to test a load of 50 + 50jor 60 - 100j I can do it under high voltage (100VAC) and a range of frequencies and it should behave as if the source is under that load.

author: MrAl
author_rank: Super Contributor
timestamp: July 29, 2025, 05:16:26 pm
content: Quote from: ConcreteAnswers on July 28, 2025, 04:34:43 amDo you have to change the impedance in a completely varable linear fashion, or can you step it up one step at a time.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2025, 05:16:55 pm
content: If you want a bode plot representing the output response including phase of the AC source, I may know a better way.This requires a fast resistive load which is switched on and off, which is easy enough to build or perhaps buy. It has to be fast to support your frequency requirements. The load is applied, asynchronously to the AC frequency, and the output voltage variation is captured by a DSO. The waveform is then differentiated, and then run through an FFT that reports phase and magnitude. The output of the FFT should probably be averaged as well, but maybe it would be better to accumulate the response. The FFT is aligned with the trigger to correctly calculate phase. The result is a magnitude and phase plot representing the response of the AC source, similar to what a VNA would produce.This procedure is one of the three ways to do vector network analysis with a DSO. I do not see any reason that it will not work with an AC instead of DC source.This requires more than an entry level DSO since none of them support phase and magnitude FFTs, and usually do not support differentiation, although oddly enough there are plenty of old DSOs which will work. None of the Chinese DSOs will do it so far as I know.

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2025, 05:35:25 pm
content: Kepco makes high-power op-amp power devices up to multiple kW, but they are pricey.Conservative ratings and reliable.