V
OLTECH
AC
I
NTERFACE
U
SER
M
ANUAL
98-072
ISS
7
PAGE
10
1.3.2 AC Source and STEP-UP UP Transformer combined
Note: When using an Amplifier AND an additional external step-up transformer connected to the output of an
Amplifier to increase the output voltage, a non-polarised capacitor must be wired in series with the output of
the Amplifier and the drive of the step-up transformer due to offsets in the Amplifier which cannot be trimmed.
Selecting a capacitor:
C
= (Im * N
2
)
/
( 2 *
π
* Fm * Vm * 10%)
C
VoltageRating
= ( 2 * Vm * 10%) / N
Where:
C
is the Capacitor value (Farads)
C
VoltageRating
is the voltage rating of the capacitor (V)
Fm
is the test frequency (Hz)
Vm
is the measured voltage of the component under test (V)
Im
is the measured current of the component under test (A)
N
is the ratio of the step-up output transformer
Worked Example:
Using a 1:3 transformer connected to the output of an Amplifier and a Test parameter of
Vm = 440 V
Im = 80 mA
F = 50 Hz.
Find the series Capacitor needed.
C
= (0.08 * 3 * 3) / ( 2 * 3.14 * 50 * 440 * 0.1)
=
52.11 uF
C
VoltageRating
= ( 2 * 440 * 0.1) / 3
=
29.3 Volts
Therefore, a non-polarised capacitor with a value of around 50
uF and ≥
30 V must be wired in series with the
output of the Amplifier and the drive of the step-up transformer
