Preface: This is a DIY design that you can create yourself, but I preferred to go ahead and get this one pre-built. Trying to run down enclosures for DIY builds can be a real pain.
If you’d like to build one yourself (which I’d highly recommend), below you can find a great article on this design and how to improve on it. This is a great DIY circuit, it’s not hard to make and it’s very useful in the right hands.
I have been talking back and forth with AF5FV on Ebay and I’ve learned a few small things worth mentioning.
This device can be used for in-circuit testing (for most circuits). This means, it can be used without needing to remove components for individual testing. This is really where the unit shines and I sort of knew this before, but didn’t mention it directly.
There are a few small things to mention with that. First of all, realize this unit applies a 15V or 30V test signal. Don’t try and use it in a circuit designed to run at 5V. While it may not do anything to harm it, as most components can take 15V, I can’t guarantee that. So, do so at your own risk.
Another note on in-circuit testing, you must keep in mind what other components are wired along the same node as the component being tested. If you have a diode and the curve is an ellipse, you most likely have a capacitor in parallel with the diode. This is just a short example of the sort of scenarios you will encounter for in-circuit. It’s still very useful, the majority of the time. You are basically getting a visual represenation of the hardware in your circuit, it’s really neat.
Note on internals:
If you look inside of the AF5FV Octopus, you’ll find a transformer, in this case a dual-tap 15V/30V, a ceramic rotary switch, variable and fixed resistors, a few small caps and then jacks/wiring.
I have been taking Semiconductors-I for my Senior/Junior year of Computer Engineering. I decided it was well worth the $50-$70 cost of entry to be able to visually show Current vs. Voltage (I/V) graphs on my oscilloscope. It’s called the “AF5FV Octopus” and it does an excellent job! It’s powered by a standard 3-pin plug (like a PSU). Inside of the unit you’ll find all analog parts, there is no sampling done.
This is in contrast to most curve tracers for under $75. Nearly all of those contain digital circuitry and thus they are using sampling. The main benefit to those types of curve tracers is the fact they can make the classic stacked I_C vs. V_CE graphs that you see with all BJT (Bipolar Junction Transistor) datasheets You can also do it with this one, but you’d need to combine different scope images at various Volt/Amperage settings.
These graphs can be created for almost any type of electrical component (active or passive). I have to say, I am glad I did get one. It’s extremely interesting to be able to see it play out before your eyes. It also allows you to design circuits involving amplification with optimal characteristics (for your design parameters). Some of the other common uses for tracers include large signal modeling and bias network design.
I can even use this sort of I/V graph for my vintage capacitor store. The I/V graph shown below for the capacitor (the elliptical shape) is a visual representation of ESR (Equivalent Series Resistance). You can find a bad capacitor based on if the ellipse is tilted, away from 90 degrees.
Note: An analog/CRT (Cathode Ray Tube) oscilloscope is recommended for use with curve tracers. This is due to the fact that digital scopes have to use ADCs (Analog-to-Digital Converters) in the signal path. Analog scopes never have to convert anything, this is why they are in fact called “Analog” oscilloscopes. If that doesn’t make sense to you, just remember if there are no ICs (Integrated Circuits) present, it’s analog.
The reason you don’t want ADCs in the signal path is because of the fact, no simulated/sampled signal will ever have 100% of the original content. This is why anyone working with audio signals or communication systems is going to prefer either an analog scope or a hybrid analog scope. A hybrid scope combines the clarity of an analog scope, with the storage capability of a digital scope. These were most popular in the 70’s and 80’s, before digital scopes surpassed analog (in most uses).
Audio rant: This is even true for DSD (Direct Stream-Digital), but it’s a TINY amount that is missing with DSD. Those who are big into DSD, like myself, will tell you what is missing falls onto the HW capturing the original sound waves. Anything with transducers (basically, all microphones), that is the primary flaw in audio right now. We are still relying on the same technology for audio capture on the front-end, as we were 50 years ago. It’s the back-end that has improved so much,
The reason CRT/Analog oscilloscopes usually weigh in at 10-20+ pounds, is because all the components are discrete (no ICs, or very limited in scope if they exist). We know that the voltages to push a CRT is much higher than any digital scope. Knowing what we said earlier about a component’s physical dimension and the current carrying capability, it should be clear that the components involved with the CRT are going to be large/heavy. Also, a CRT in general, is much heavier than a comparable digital screen.
In the below picture you can see the CRT of my Hitachi V-1065 scope, even more interesting is that delay line! The coil of cable placed just above the narrow end of the CRT is simply for inserting delays in the time constant. Most analog scopes have something similar to this, but this coil is especially hefty.
This is a perfect example of what makes an analog scope, analog. In a digital scope, you won’t find any delay line such as this. Most digital scopes employ a form of sampling in order to get the same form of control as a delay line gives an analog scope.
The analog delay likely weighs ~1-2 pounds, while the digital delay won’t be more than a few grams. The main point of a delay line is to allow you to see the input signal, before the scope triggered on the waveform. You are stepping backward in time.
Do you own a curve tracer? Tells us about your favorite uses for the device!
You can see the unit I am using right here:
(I have been in contact with the designer and he says he is currently building more, since he has sold out of units.)
He also has this simpler $50 unit, but it lacks the ability to test for both 15V and 30V (if that is important to you).
A very interesting DIY Curve Tracer article, with excellent info:
Excellent Curve Tracer Reference:
Another way to show ESR (visually), using an oscilloscope and a function generator:
More Curve Tracer Experiments:
One thought on “New Hardware Day: “AF5FV Octopus” I/V Curve Tracer”
[…] you check out the ads, be sure to notice the Curve tracer being used that you saw featured in a previous article. This is a great use for the device, as long as the capacitance is above like 0.05uF or so. At that […]