Constant Current Sources
A True Constant Current Source: The WT2 'Low Power' Test Port
The WT2's propriatary 'low power' test port circuit delivers a true constant current to
the test terminals. Though the voltage is limited to the USB supply rail (+/-2V
from center bias of 2V), the output appears to be a true constant current with an output
impedance in the megohm range. In this case Vout is directly proportional to the
An interesting advantage of the constant current source high impedance is the ability
to measure high impedance devices. An example would be the ability to accurately
measure capacitors as small as 100 pF. A special capacitor
measurement mode setup is used to provide a DC return path and null out any
residual parasitic capacitance. This FAQ
discussing measurement of inductors, resistors and capacitors may also be of
Approximated (Faux) Constant Current
The circuit shown here has been used in the the speaker industry for many years to
simulate a constant current. By applying a voltage divider equation, it can
then be seen that as long as the device impedance is relatively small, the bulk of
the applied voltage appears across the series resistor, and the current is more or
less constant. This assumption however begins to quickly fall apart when the ratio
is less then 10:1.
Impedance peaks of several hundred ohms are quite common in modern loudspeakers making
high value series resistors desirable. Additionally the driving amplifier needs to
have sufficient drive voltage to achieve a usefull signal level. Even the 1k
series resistor that is shown is a little too small (but it does make the math
easier). Using a simple test jig of say a 100 ohm series resistor hung on the
output of a PC sound card might be convenient, but it provides neither constant
current or constant voltage.
Constant Voltage (not very common)
Completing the measurement circuit topologies is constant voltage. In this case a low value current sensing
resistor is placed inline with the circuit. The voltages across the
load and sensing resistor are then differentially amplified and measured.
The math is similar to that of the faux constant current source, except that the
ratio is now inverted. And once again, it would be desirable to have a 10:1
ratio. The advantage is that achieving high drive level is relatively easy but at
the expense of not being constant current. Another significant
disadvantage is that a shorted test load will produce a very high current.
Hi-ZP Port - Achiving Constant Current Using Software
One of the selectable WTPro methods uses a software feedback loop to produce the same
effect as a true constant current source. Initially Z is measured, and
then V1 is adjusted knowing I=V/Z, the result being a constant current. This
itterative process takes several loops to narrow down V1, and the resulting I, to an
A much faster WTPro method when measuring driver TS parameters is to first find the Fs
impedance peak. The 'Zo points' that define the Q are a function of Re and Zmax,
allowing Zo to be precalculated, and from that V1 can be pre-adjusted.
Again, the result is that of a constant current but without needing a software
feedback loop. In other words, this produces the same results but is much
The remaining WTPro options include simple voltage (no feedback at all), or constant
voltage using feedback.
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