Do I need an AC power supply or is a Variac suitable? - Page 1
Forum section: electronics / testgear

author: F4
author_rank: Contributor
timestamp: November 30, 2024, 12:24:23 pm
content: I recently came across a need to inject 15v AC into a circuit that had its own rectifier et all on the board.Is a dedicated AC power supply and a variac the same exact thing? Looking around at dedicated AC power supplies, they are few and far between compared to DC power supplies so it made me start to wonder.The main difference I see is that an AC power supply is not limited by the AC voltage from the wall, but what are the other reasons to have a dedicated AC power supply over using a variac?

author: tggzzz
author_rank: Super Contributor
timestamp: November 30, 2024, 12:38:36 pm
content: What power do you need?A variac is merely an auto-transformer where the wiper picks off a variable number of turns. It is not isolated from the mains input.

author: F4
author_rank: Contributor
timestamp: November 30, 2024, 12:56:53 pm
content: Thank you.I have an isolation transformer to put in front of the variac if needed.In terms of power, 1000 watts would cover 95% of needs.

author: glitchcatcher
author_rank: Contributor
timestamp: November 30, 2024, 01:12:49 pm
content: A variac does not have variable frequency, setting the voltage accurately and stably might be challenging and advanced features like programming, arbitrary waveforms and much more are missing.But if you just need to supply some AC voltage with mains frequency here and there, it should suffice.You can look into AC power supplies like Chroma 61504/61604 but yeah, they are hugely expensive.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 30, 2024, 03:45:15 pm
content: For power line AC work, a Variac plus an isolation transformer are your lowest cost option.Lowest cost doesn’t means cheap, the sheer amount of iron and copper makes this things expensive, but not as expensive as a true AC source.Additionally, it will be far more rugged. A simple fuse can protect against shorts or overloads.

author: TimFox
author_rank: Super Contributor
timestamp: November 30, 2024, 04:41:50 pm
content: The simplest and probably cheapest solution and is to use a mains to low-voltage transformer that supplies approximately 15 V rms at the current required by the load.If you need to vary the voltage, then a Variac before the primary of that transformer can be used.(With a Variac, you might want to use a low-voltage transformer with more than 15 V rms nominal output, since the Variac can reduce the voltage.)

author: _Wim_
author_rank: Super Contributor
timestamp: December 01, 2024, 06:18:23 am
content: Quote from: F4 on November 30, 2024, 12:24:23 pmWhy not connect a DC power supply to the + and - terminals of the rectifier? This is typically what I do and avoids the need to inject AC

author: F4
author_rank: Contributor
timestamp: December 02, 2024, 02:04:07 pm
content: Quote from: _Wim_ on December 01, 2024, 06:18:23 amI actually thought this too, might give that a try, thank you.

author: _Wim_
author_rank: Super Contributor
timestamp: December 02, 2024, 08:35:54 pm
content: Quote from: F4 on December 02, 2024, 02:04:07 pmYou probably know this, but if the DUT has a bipolar supply, you need to connect 2 DC power supplies with their common connected to the center tap of the transformer (=gnd of the DUT), and the positive and negative DC voltage to the + and - of the bridge rectifier.