AC Dummy load transformer tester

[nosaj]

So I was listing to the ARRL Electic tech podcast an this ham was talking about an AC Dummy load transformer tester. i got the pages and thought I'd share and see what you guys thought.
The schematic has an error which i've included the correction.

nosaj
QST.pdf
QST.pdf

[nosaj]

Basically on the podcast he was using it to identify specs of unknown transformers
nosaj

[vintagekiki]

The schematic in QST is correct.

The diode is an internal part of the N-Channel Mosfet transistor IRF542
https://datasheetspdf.com/pdf/1322671/STMicroelectronics/IRF542/1

[Helmholtz]

The original QST circuit is wrong, as the two body diodes would short one of the halfwaves of the transformer under test.

[vintagekiki]

Dummy load for transformers and power supplies

https://www.eevblog.com/forum/projects/ac-electronic-load-circuit/
AC Electronic Load Circuit

test_circuit.pdf

https://www.deeptronic.com/wp-content/uploads/2013/10/Power-Suplay-Tester.gif
Power Suplay Tester

http://danyk.cz/zatez_en.html
Electronic resistor - variable dummy load for power supplies

[Helmholtz]

I don't like the corrected version either, because it won't work symmetrically:
While the bias of the upper Mosfet is essentialy constant during its conduction phase, the bias of the lower Mosfet varies with the momentary AC voltage.
This makes the active load non-linear and introduces DC current. Consequently the results will be flawed.

I think the gate voltage supply should not be grounded but connected between gates and sources (i.e. floating).

[vintagekiki]

Mosfet transistors IRF542 should represent some kind of "strength (power) electronic rheostat" in the Dummy load function.
simplified schematic
ac-load.GIF

[vintagekiki]

#6

1)
QST_julio .pdf
page 52-53

[nosaj]

If you go to this link they have the amended link for the corrected schematic I linked to in the first post as well as the podcast where they discuss it. show 39
nosaj

[vintagekiki]

The diode is an internal part of the N-Channel Mosfet transistor.
In that case on corrected schematic should be displayed a complementary P-Channel Mosfet transistor.
1)
https://www.eevblog.com/forum/testgear/adjustable-ac-load/
Adjustable AC load

[Helmholtz]

I already said what I don't like with the (corrected) circuit.

Generally the loading graph (V over I) of a transformer is a straight line, so the uneven curve shown in the article can't be real.
I suppose it reflects mains voltage variations during measurement.

The voltage of a transformer drops with increasing current according to V = Vo - Ri*I, where Vo is the open circuit voltage and Ri is the internal resistance of the transformer.
As this is a linear function, the graph is a straight line and Ri determines the negative slope.
(The core has no influence.)

To construct a straight line it takes only 2 points. As the open circuit voltage at I = 0 is typically known, one needs only one more V-I pair to draw the graph.
So a single load resistor suffices.

[vintagekiki]

Please share you suggest in schematic form, will be helpful to interested DIY builders.

[Helmholtz]

In the "corrected" version from post #1, the Mosfet source terminals are interconnected (in the middle).

My proposal is to lift the gate supply zero from ground ( no ground connection required) and connect it to the common source point instead.
This makes sure that both Mosfets always have equal and constant Vgs bias.

But still the Mosfets would have to be selected for balanced operation. It is essential that the load resistance is the same for positive and negative currents.

[vintagekiki]

Schematic please if possible.
From the comments, I can't imagine how the AC Dummy load practically (schematic) looks.
I have been successfully using a power supply load similar to www.deeptronic.com for ten years.
It’s not AC Dummy but it does the job.
1)
https://www.deeptronic.com/electronic-circuit-design/variable-dummy-load-for-power-supply-testing/
Variable Dummy Load for Power Supply Testing

[J M Fahey]

What´s wrong with plain old wirewound resistors?
Robust, symmetrical, linear, cheap, and .... ummmmm ....... "resistive"