Dummy load for AC using a DC one - Page 1
Forum section: electronics / beginners

author: WimDH
author_rank: Newbie
timestamp: December 23, 2021, 02:13:05 pm
content: Hey guys,Following the forum for a while, but never subscribed. Now I've found a good reason to do so: I have a questionI'm doing a project with current transformers, and I need to create an AC load to generate an input form my current transformers. My initial idea was to connect some resistors to a 6V transformer to generate the current which I will feed to my CT (Current Transformer).Now I'm thinking of building a dummy doad that can do (besides CC and CP) CR, and connect it to a rectifier and use it as load for the transformer, instead of my resistors.Like this, I can control the current flow way better than with resistors, and I don't need to buy power resistors, which will most likely eat dust once the project is finished.I can't find anything on the Internet, so I'm thinking: what am I missing here, why did no-one come up with this before?Any ideas, thoughts, what I should (not) do?Thanks a lot!

author: Ian.M
author_rank: Super Contributor
timestamp: December 23, 2021, 02:28:44 pm
content: Consider the current waveform. An ideal DC constant current load wrapped in a bridge rectifier will result in a near squarewave AC current, with risetime/falltime determined by the Vf of the diode in the bridge, and the supply waveform slope near its zero crossings.However the classic MOSFET + OPAMP constant current load has poor transient response due the MOSFET gate capacitance, and when the voltage across it is below the minimum required to meet the target current the OPAMP output will rail turning the MOSFET hard on. Its therefore likely to let through an uncontrolled current spike at the beginning of each half cycle, shortly after the zero crossing as the OPAMP slews from railed to controlling Vgs in the MOSFET's linear region.Neither case would be desirable for current transformer calibration

author: ogden
author_rank: Super Contributor
timestamp: December 23, 2021, 02:37:57 pm
content: DC load could be used in constant resistance mode, yet there will be diode bridge forward voltage artefacts as already explained. I would use three (relay-) switchable resistive, somewhat thermally-stable loads for 3-point calibration together with decent AC current measurement instrument.

author: mawyatt
author_rank: Super Contributor
timestamp: December 23, 2021, 02:48:22 pm
content: Use in CR mode should work OK if the Current Transformer DUT can support the extra couple diode drops due to the bridge rectifier without significant error.Caution with CT is advised as the voltage can rise to harmful levels if the current path is broken or reaches a high impedance level on the secondary, so keeping the effective resistance in CR mode low is wise.Best,

author: WimDH
author_rank: Newbie
timestamp: December 23, 2021, 02:59:46 pm
content: The dummy load will be on the primary side, generating s smaller signal on the secondary of the CT, which will be fed to a microcontroller.I'm talking about a 1-5A load on the primary side, and doing CR on my load. If correctly done, the secondary waveform should be a sinewave (more or less).Maybe I can put a resistive load in parallel with the dummy load to deal with the rectifier diodes drop voltage.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 25, 2021, 09:50:11 pm
content: Possibly TS wants an AC current source but not AC load?I used Howland current pump design to make my 3-phase 10 A AC current source. It is convenient if output has to have common ground. If not - any current source design should do (+ oscillator or other signal source).