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Measure current vs voltage or voltage vs current over limited range with good accuracy

This was built on a phenolic board which was mounted on a plastic box.
The box serves two purposes: It holds the circuit off the workbench, and
it makes a nice, stable mount for the potentiometer.

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I realized that soon I am going to have to measure the voltage as a function of current for a lot of little devices. Having done quite a few using clip leads and resistors, and wanting to make things a little easier on myself, I put together the curve tracer described on this page.

Current as a function of voltage for a infrared LED. Horizontal axis is 200 mv/ division
and the vertical axis is 1 ma per division. The knee is seen to b
e pretty sharp at 1 volt.
In the case of this measurement, the oscilloscope was adjusted to place the origin in the
lower left hand corner of the screen.

Using this fixture, I can easily measure the IV curves of components in two quadrants. If I want to see the curve in the other two quadrants, I can reverse the device's connections and adjust the centering on the scope accordingly. This can also be used with voltmeters, especially when a high degree of accuracy is requited.

Because this was designed to characterize a narrow range of components, the range of currents only extends up to approximately 50 milliamps, mostly limited by the drive capabilities of Q1, but also limited by RLimit so as to not damage devices under test, and a maximum combined voltage device under test voltage and RLimit voltage of 2.5 volts. The circuit can be scaled for other ranges.

The common connection to the circuit (similar to "ground") is the feedback
input to the TL431, therefore the power supply must be fully floating free of ground.

The use is very simple: I set my digital oscilloscope to XY mode and infinite persistence. The X and Y inputs are connected to the -voltage and current test points on the test fixture. The voltage between the voltage test point and common is the voltage across the device. It is negative. The voltage from the current test pot and common is the voltage drop across a 100 ohm resistor in series with the device under test; the voltage across the 100 ohm resistor is not included in the voltage on the voltage test point.

To see current as a function of voltage, as shown in the photograph earlier on this page, connect the voltage test point to the oscilloscope's X input and the current test point to the oscilloscope's Y input. If your scope does not have variable or infinite persistence available, then this is going to be more useful in conjunction with voltmeters.

The circuit's operation is very simple. U1 provides a total of 9 volts. U2 generates a 2.5 volt reference voltage with respect to the "ground" connection of U1. The cathode of U2 is at the same potential as the feedback input of U3, which is used as the common voltage for this circuit. One end of the device under test is held at the common voltage (U1 "ground + 2.5V) by current through R5 and Q1, as controlled by U3. This feedback to U3 allows measurement of the voltage across the 100 ohm 1% resistor without adding the burden of that voltage drop to the voltage measured across the device. Pot R2 along with current limiting resistor R3 cause the current through the device under test to vary. The voltage across the device and the current through the device are made available for display.

The current is manually adjustable to allow for the testing of devices that are affected by self heating, and therefore need to be allowed to settle before a measurement is made, or in some cases, can have their curves traced if the know is adjusted very slowly.

Powering the Circuit
Please note, since the circuit common is created with U3 (and virtually by U2), it is important that the power supply be floating if the circuit common is grounded, such as when using with a grounded oscilloscope. If the test points are connected to floating digital voltmeters, then it doesn't matter whether the power supply is grounded.

The 78M09 regulator is not really needed; it use reduces the noise in the circuit a little bit by attenuating the ripple coming in from the "wall wart", and besides, it made the design a little easier by freeing me up from cosidering the variations in line voltage. Basically input voltage that is high enough for the regulator to be in regulation but not so high that it damages the regulator is fine. I think that a 9 volt transistor radio battery in place of the regulator would work fine, though it creates the worry of the battery running down.

The map above makes it easy to remember which terminal is which.

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Contents ©2011 Richard Cappels All Rights Reserved. Find updates at www.projects.cappels.org

First posted in November, 2011. Minor corrections 8 November 2011.

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