4.6 Worked Example
A laminate core is to be tested in accordance with
Under a constant ac current of up to 3A, 1V at 50Hz in
the primary test winding, the secondary voltage is measured and limits are
applied in terms of the rectified mean voltage (rms scaled).
To produce 1V at 3A,
R = 1V / 3A
R = 0.333 Ohm. Choose 0.5 Ohm standard value.
V = 3A x 0.5 Ohm
V = 1.5V
W = I x I x R
W = 4.50 Choose 5W standard part.
Resistor is 0.5 Ohm, 5W. Voltage produced will be 1.5V.
4.6.2 Step-down Transformer
Look for a stock transformer with, say, a 5A secondary
current rating. A stock transformer will usually have a 110 and/or 230V
VA rating is that of the part plus the current sense resistor.
VA = 3A x (1+1.5)V = 7.5VA
Choose a 7.5VA (or greater) transformer with a 5A low-voltage secondary.
Toroidal transformers are ideal, since they will have low losses.
Primary: 230V Secondary: 2 x 6V @ 2.5A.
The secondaries are connected in parallel to provide a
6V, 5A output.
The step-down transformer will supply the part under test
and the current-sense resistor in series. The step-down transformer's
secondary voltage, Vs, being:
Vs = 1.5V +1V = 2.5V
The AT3600 must supply 2.5 x 230:6 to the primary of the transformer.
2.5 x 230 / 6 = 95.8V
The AT3600 can supply 270V, so 95.8V is well within its capability.
It is convenient to use the MAGX test at first, since
this will confirm that the desired current is being generated.
In the previous connection diagram, the return or 'low'
end of the sense resistor has been connected to node 5, and the higher
voltage end of the resistor connected to node 3.
For optimum accuracy, it is important that the physical 'low' connection
is also designated as the 'low' terminal in the program.
4.6.5 AC INTERFACE Set Up
Next, you must set up the External Source in the Editor.
The source type is 'AT Output Transformer', and you are required to enter
the nominal turns ratio of the transformer.
Remember that the AT3600 will trim the output voltage by comparing the
desired voltage (from the program) with that measured across the
programmed sense terminals.
The ratio is used to establish a target for voltage ramp-up, when the
tester starts the test. Using the correct ratio will optimize test speed.
In this case, the ratio is modified because, when the
desired 1.5V is achieved across the resistor, the step-down transformer
must also supply the 1V across the part under test. For 1.5V programmed,
the step-down transformer delivers 2.5V.
Ratio = 230:6 x 2.5:1.5
That is, for every 1V programmed, the AT3600 must supply
63.9V to the primary of the step-down transformer.
4.6.6 MAGX Test
In the MAGX test, simply enter the required voltage, V,
across the resistor to program the constant current, I. In this case,
With an AT3600 connected to the PC Editor and the circuit
assembled as shown previously, you may now press the 'Measure' button to
run a MAGX measurement.
If the current measured is as desired, then all is well.
If not, simply adjust the program voltage to suit. If the tester should
fail to return a result, check the wiring and settings carefully.
4.6.7 VOCX Test
Now secondary connections may be made for VOCX tests.
Note that VOCX has the option to measure rms voltage or "mean sense rms
For the mean sense option, the tester measures the
rectified mean voltage and multiplies that by 1.11.
This option is offered as a means of comparing the measurements with those
made by mean sensing analog meters and calculated parameters that require