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V/I Curve Tracer

 

Brian Rippie

Introduction

 

This article shows how I made a Voltage/Current Curve Tracer. A curve tracer is a device that will show you a devices voltage vs. current characteristic. With this you can see how the device under test will perform with respect to voltage and/or current. As a robotics hobbyist, I have a small budget to work with and cannot afford to pay for a professional tracer. I was lucky to find an old Tektronics type 564B storage oscilloscope for only $30 in my local area. This is perfect for trouble shooting circuits on the hobby level. With the V/I Curve Tracer, I can test components that I plan to, or have used for my designs. I found a schematic on-line to make a simple tracer to use with my scope. I will try to show you how I built mine with minimal out of pocket expense, which for you, depends on how resourceful you are.

 

 

 

 

Construction

 

I used whatever hardware I had sitting around that I had scavenged from things I (or friends) were throwing out. I assembled the circuit as shown on the schematic below. S3 allows selecting between two voltage levels. One level using the full voltage from the secondary winding and the other from the center tap for half the secondary voltage. The Tracer would be more consistent if R1 were one ohm but I did not have that value sitting around. I could have used five 5 ohm resistors in parallel to reach one ohm, I but thought this unnecessary as I wish only to verify curve shape and not try to derive dynamic values from the curve.      

 

 

Figure 1

 

 

Identifier

Description

T1

Transformer, secondary can have one or more different voltage taps, rescued from the trash.

S1

A two pole rotary switch from any supplier.

S2,3

I used two scavenged slider switches from an old R/C car. Be sure S3 has the same amount of contacts as to mate with your transformer.

R1-9

These resistors may be acquired from any supplier.

R10

I scavenged this one from an old mass spectrum analyzer! (It was trashed and beyond recovery) or you can get one from any supplier.

D1

I ripped it out of an old alarm clock a friend was throwing out.

PC Board 

From any supplier.

Box

Project box to contain the parts.

Table 1

 

 

Operation

 

By selecting different resistance pairs with S1 you can control the V/I pattern displayed on the scope. R6-R9 will vary the amount of current (or Vertical portion) that passes through the device under test. R5 and R10 are used for the horizontal portion (or voltage) of the curve. Adjusting R10 will give you greater control over the voltage. D1 is used to trim the negative going half of the test signal. You can use this if you are testing a small signal diode that you are afraid of damaging with too strong of a negative swing.

 

 

Picture 1

 

Picture 1 shows the rat’s nest I made of the V/I curve tracer. I have labeled the most prominent components visible in the image.  

 

Picture2

 

In picture 2 is the face of the tracer. In the upper left is S1 and in the upper right is R10. You will see three bolts sticking out of the box. The two top are for signals going to the oscilloscope and the one on the right is for GND. The switch labeled “VH VL” on the left controls which secondary voltage from the transformer is applied to the rest of the circuit. Just below the “- +” label is the 1x2 header (not visible in picture) I used to connect the components which I intend to test. Last, on the lower right is the switch used to insert or remove the diode from the circuit, effectively removing the negative voltage swing.     

 

In picture 3 I show how the CRT of the scope will be divided into 4 quadrants. I also show vectors indicating the direction of current and voltage in accordance with their sign. Please refer to this drawing as I go through and test a resistor, diode and capacitor. 

 

Picture 3

 

 

Resistance

 

Keeping with Ohms Law, resistance equals voltage divided by current (R=V/I), we see a sloped line that is representative of resistance. As voltage changes from negative to positive, the current follows. This obeys Ohms Law in that the resistance is constant, and in order for the equation to be balanced, then the current must follow the voltage. (Pay no attention to the man behind in scope!)

 

Picture4

Diode

 

A diode will only allow current to flow in one direction. Looking at picture 5 you see that as voltage goes negative (to the left) the current is zero (not going up or down). When the voltage precedes positive (to the right) the current flows positive (going up) with low resistance, this is keeping with the function of a diode. I say low resistance because the slope of the curve in the 2nd quadrant is very steep.

 

Picture 5

 

 

Capacitor 

 

In a capacitor, current leads voltage by 90 degrees (Picture 6). When the sinusoidal wave of 60Hz is applied to the cap, you can see that when voltage is zero the current has a value and when current is zero the voltage has a value. Since the voltage and current are not in phase with one another, an elliptical shape appears.

 

Picture 6

 

 

Conclusion

 

What many people may think of when reading this article is an Octopus. An octopus which is another curve tracing circuit is a close relative to the V/I curve tracer but is a simpler animal. It does not allow for different currents and adjustment to the voltage but it is certainly in the same family.

 

The V/I Curve Tracer is a useful tool for trouble shooting circuits down to the component level. On the other hand, if you are new to electronics and wish to have a better understanding of what is happening within components and have access to an oscilloscope I suggest making a curve tracer. I have shown the curves to friends with electrical or electronics background and it has always started conversation and a desire to test just about every thing under the sun. However, when I show it to non-technical people all I seem to get is blank looks and all-around lack of enthusiasm. That's until I whip out some good old fashion Lissajous! (People seam to love those seminally spinning images.)   

 

References

 

This circuit is built from designs found on-line at….

 

http://www.ionpool.net/arcade/tech/octopus.pdf

http://www.techlib.com/electronics/curvetrace.html

http://www.armory.com/~rstevew/Public/TestEquip/CurveTracer.pdf

 

Sample Curves

 

A collection of example curves are found here….

 

http://www.fullnet.com/u/tomg/gooteect.htm

 

Safety Safety Safety

 

I do not accept any responsibility for any damage to any equipment. I am not promoting that people disassemble existing devices to pirate parts for this design or any other design. Anytime someone works with electronics all safety precautions should be observed and all safety equipment should be used. No device should ever be powered while work is performed.