Computing stuff tied to the physical world

Component Tester – part 2

In Hardware on Feb 1, 2012 at 00:01

After yesterday’s introduction, we’re ready for some more insight…

To summarize: a straight line going through (0,0) represents a purely resistive effect. The slope of the line is related to actual resistance. With resistors, once you know the voltage, you know the current (and vice versa).

Here’s a diode, i.e. a component with very specific properties (this shows why it’s called a semiconductor!):

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With negative voltages, it just blocks (horizontal line, infinite resistance). With positive voltages it’s essentially a short circuit (vertical line, almost zero resistance). Note the “knee”: a diode starts conductiong at about 0.7V.

Here’s a blue LED:

DSC 2905

Very much like a diode (the “D” in LED stands for diode, after all). Except that the knee is higher, at around 3V.

Here are three zener diodes of 3.3V, 5.1V, and 9.1V, respectively:

DSC 2913 DSC 2914 DSC 2912

Note first of all that these diodes were connected in reverse compared to the diode and LED shown earlier, so the graphs are rotated by 180° compared to those. A zener is a regular diode, in that it conducts normally at around 0.7V. The difference is that when it’s blocking, it will at some point “avalanche” and start conducting anyway. This very specific voltage is what makes zeners special. But note how that avalanche knee is round and inaccurate for low voltage types. Zeners for less than 6V or so are not very precise for regulating the voltage – but 9.1V is fine.

Neat, huh? Each type of component has its distinctive analog signature when viewed on a CT!

So far, you’d be forgiven to conclude that a Component Tester is simply a hardware function plotter. With the horizontal axis being the voltage applied, and the vertical axis being the current flowing through the component.

Ah, but wait… here’s a 1 µF capacitor, showing that capacitors are fundamentally different beasts:

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This is where things start to go crazy. No current at maximum and minimum voltage? Lots of current at zero volts? Positive and negative current at that zero-volt position? What’s going on here?

The thing to keep in mind is that this is not simply a function of voltage vs current. We’re applying a sine wave – a voltage which very uniformly and smoothly varies between -10V and +10V. Think of a swinging pendulum, oscillating over and over again in a constant pattern.

Note also that the component is being driven through a 1 kΩ resistor, limiting the maximum current through it. So we’re looking at the capacitor while it’s in fact part of a circuit – i.e. a 1 kΩ resistor in series with our 1 µF cap.

Let’s start at the right. The capacitor is fully charged to +10V, and our voltage is starting to decrease. When the voltage is +9V, the cap is still +10V, so it starts sending out charge in the form of current to try and regain the balance. So a positive current flows out when the voltage is at +9V. If that voltage stayed at +9V, it would soon stop, since the charge drops, and the capacitor reaches +9V equilibrium again. But as this happens, the voltage keeps on dropping. In fact, it drops faster and faster, so more and more current leaks out while catching up.

At 0V, the rate of descent (dare I say slope or derivative?) is maximal, as you can see when you look at a sine wave. So at that point, the capacitor is leaking charge as fast as it can – at the rate of 4 mA in this case.

The voltage doesn’t stop dropping, though. I keeps on dropping to -10V, although it’s slowing down again. So the current still flows out of the cap, but slower and slower. At -10V, the voltage is no longer dropping at all, and the charge will have caught up – no more current, i.e. 0 mA.

Now the roller coaster ride repeats the other way around. The capacitor has -10V charge (lack of charge, if you wish to look at it that way), and voltage is about to start rising again. This time, charge has to be fed into the cap to try and equalize voltages, and so the current is now negative.

And sure enough, the lower negative side of the circle goes through the same changes. Until we reach +10V again.

So what you’re looking at is not a function, but the path of a point in space, racing around a circular path (ok… oval, since you insist). That point in space leaves a trail on the screen, and that’s the resulting image.

Phew! Still there?

The reason this happens, is due to the fact that a capacitor has state (or memory, if you like). It will respond to an external voltage differently, depending on the amount of charge it currently holds. Applying +5V to an empty cap will generate a different current than applying +5V to a capacitor which is currently charged up to +10V, or whatever. Current will start to flow to balance things out, but this requires time.

Very loosely speaking, you could say that capacitors “live in the time domain”. Unlike resistors – which just resist the same way under any circumstance.

Here’s the trace of an inductor (the secondary coil of a small transformer in this case):

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Hey, it looks like inductors also have state! And yes indeed, they do. Capacitors and inductors are very similar, electrically. They both “live in the time domain”, although through very different mechanisms.

The state of a capacitor is its current charge level, i.e. the “amount of electricity” inside it at any particular time.

The state of an inductor is the magnetic field level it has created. When you send an electric current through a coil, that coil becomes an electro-magnet, and starts generating a magnetic field around it. When the current stops, the magnetic field wants to keep going. But it can’t and it starts fading – while it does, an electric current is generated in the opposite same direction. This effect (plus a little resistance) is what causes the tilted shape shown above.

As you can see, it has the same weird effect: no current at maximum or minimum voltage, and either positive or negative current at zero volts.

The point of these little demos was to show how current and voltage stop being linearly inter-related with caps and inductors. Because they mess with time. The charge which came in today could come out tomorrow, for example.

With constant voltages, capacitors and inductors are boring. But when their time effects are pitted against voltages which change over time, then nifty things can happen. It’s probably fair to say that the discovery of DC (direct current) brought electricity to the world, whereas AC (alternating current) brought electronics to the world.

For measuring DC, you can get by with a voltmeter. For AC, you need a voltmeter-over-time, a.k.a. an oscilloscope.

I hope this gives you a feel for what’s going on in electronic circuits. The behaviors shown here are universal, i.e. caps will behave like this every time, no matter what else sits around them, and getting an intuition about how these components react to voltages is a fantastic way to figure out all sorts of more complex circuits.

There’s tons more to explore about signals and circuits: filters, phase effects, crazy stuff called “complex numbers” (values with a “real” and an “imaginary” part, go figure!), switching perspectives from the “time domain” to the “frequency domain”, and Fourier transforms. None of this matters, if all you want is to turn on a lamp or work with digital signals. But if you’ve ever wondered how electronic stuff really works: trust me… it’s fascinating.

Is anyone interested in any of this? I’d love to write a series about it one day, where intuition comes first, insight a close second, and where all the mathematics involved will become totally obvious (seriously!).

PS. Here’s my intuitive summary of what R’s, C’s, and L’s do (and what makes each of them unique):

  • Resistors turn electrical energy into heat (no way back with a resistor)
  • Capacitors turn voltage differences into electric charge (and back)
  • Inductors turn electrical current flow into magnetism (and back)
  1. One mistake: Inductors don’t generate a current in the “opposite” direction when you turn off the power. It’s the same direction. Just like a capacitor resists voltage changes, a coil wants the current to stay the same.

    This is why a protective diode across a relay coil works. When you turn a relay off, the current tries to keep going.

    • Ah, ok – that voltage vs current aspect clarifies it niccely. Fixed, thx.

    • The collapsing field of a relay will generate a reverse voltage, this is why the protection diode is placed in reverse across the coil from the usual current flow.

      In effect, yes it is trying to keep the current flowing in the same direction, but doing so it is driving the now disconnected input -ve with respect to the output it is pushing the current to.

      I think!

      Blimee it’s been a while since I’ve had to do the theory! Next you’ll be making me break out the left and right hand rules!

  2. Interestingly, this can visualise the equivalence/reciprocity of C v. L

    If the UIT is an air cored inductor, the kinks due to non-linearities of the core material on the ellipse will go away. Still tilted since there is still the resistance of the wire; imagine an R in series with a pure L.

    So mapping this over to the C case, a series R becomes a parallel R. Add this real component across the C under test and sure enough – the ellipse tilts. The rest is all scaling to get the same size and tilt angle.

  3. Thx – I’ve added a little overview at the end to summarize R’s, C’s, and L’s – as I view them, intuitively.

  4. Neat! Almost makes me wanna buy a scope myself…

    I wonder what the plot for a memristor would look like?

  5. Excellent article! Many thanks for your efforts.

  6. Great article! You bet we’re interested in a series on how electronics really works. Your work is truly appreciated.

  7. Chalk one up for me as member of the interested ones. Definitely! You have brought imaginary numbers and all that vague sinus, cosines (heck, maybe even tangents) secondary-school talk to life with your scope images. The coil pic might have had a magnified version upon click; it seems more interesting than the others. Memristors: Osma beat me to it. So new, even my spelling correction doesn’t know about it ;-) Graphs of numbers of hares and foxes are still missing. What’s their circuitry? Please, please, do continue! (wonder if this comment does pass ? 8-)

  8. +1 for the series!

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