Hameg scopes have often included a “Component Tester” (CT) and mine’s no exception. It’s a really nifty way to identify a component and understand its basic characteristics. It requires a sine wave signal and an oscilloscope:

Don’t fret too long about the above circuit (copied from this PDF, which I found via Google). It’s just to show that setting up something like this is very easy – but you do need an oscilloscope with X-Y capability.

The basic idea is to apply a sine wave of say 10 VAC @ 50 Hz to the part you want to identify, and to then display voltage over versus current through that component.

My scope has the equivalent of the above simple CT circuit built in:

Two pins, on which a 50 Hz voltage is applied which varies between +10V and -10V in the form of a pure sine wave. For some reason, the scope won’t let me take screen dumps to USB in this mode, so I’ll use camera shots.

Here’s what you see with nothing connected (note the full scale: ±10 V on X and ±10 mA on Y):

The horizontal axis shows the applied voltage, and as you can see, no current is flowing. Because air insulates!

Let’s short the two pins with a copper wire (I’ve reduced the image scale to reduce this weblog post’s length):

Of course: no matter what voltage we try to put between the pins, the wire will force it to 0V, and will simply pass -10 .. +10 mA of current. As you can see in the schematic, there’s a 1 kΩ resistor in series to limit the current.

These two images of open vs shorted set the stage. Now let me insert a couple of different components, so you can see how they behave when subjected to this 10 Vpp sine wave. First, let’s insert a 1 kΩ resistor:

Make sense? Now have a look at a 10 kΩ resistor:

So what we have so far, is resistance varying from zero ohm (shorted) to infinite ohm (open), with two values in between. It all ends up as a straight line, with the slope varying from horizontal to vertical. That’s it: resistance!

If you want an explanation: this is Ohm’s law, visualized. Voltage and current are proportional, i.e. V = I x R.

So much for the basic stuff. Tomorrow, I’ll show you a couple of considerably more interesting components.

Yesterday, I tried to get to grips with how a capacitive power supply works. Real samples are a bit messy, though.

So let me try something else this time, and simplify a bit:

The setup is very similar, but I’m leaving out the zener diode and the rest, and more importantly, I’m going to feed a real 50 Hz sine wave signal into this circuit, using my little sine wave generator.

Here again, because scopes need to measure with a common ground, I’m placing that common ground in between the resistor and the cap, and I’m using the scope’s internal “invert” feature to treat this as if the channel 1 probe were connected the other way around. Here’s what we get with this new setup:

The vertical scale of the resistor was adjusted to display the same amplitude for the resistor as for the capacitor.

• the yellow line is the voltage over the capacitor
• the blue line is the voltage over the resistor
• the red line is the sum of the yellow and blue lines

The scale of the red line is not quite accurate, but its shape is. So the red line is in essence the input signal.

So what’s going on here?

As you can see, all these signals are 50 Hz sine waves. That’s quite remarkable already. Obviously the red line is a 50 Hz sine wave, since that’s what we’ve been feeding in. But so is the voltage over the capacitor, the voltage over the resistor, and hence also the current through this circuit!

What you see, is a set of sine waves which differ only in phase and in amplitude:

• the voltage over the capacitor (yellow) lags the input signal (red): it’s forever trying to catch up
• the current through the capacitor, i.e. the voltage over the resistor (blue), is leading in phase

And something else, as we saw yesterday: the current through the capacitor is related directly to the slope of the capacitor’s voltage change (i.e. its derivative). When the yellow line is steepest, the blue line is at its highest.

Let’s throw one more calculation into the mix: power. Power is input voltage (red) times input current (blue):

Looks very sine wave’ish again! There is some amplitude variation, which can probably be attributed to signal asymmetry or amplitude differences between the different sine waves. The phase of this wave is different from the ones already shown, but note that it’s also twice the frequency, i.e. 100 Hz.

Let’s step back for a moment. With a purely resistive circuit (as used in a resistive transformer-less supply), the current and voltage would be in lock step, i.e. sine waves with exactly the same phase, and the power consumption would be maximal (high current at times of high voltage).

With this capacitive setup, currents get “moved around”. That means power consumption will be less than when voltage and current match up (since this gives the largest possible results).

I’ll include one more screenshot, this time using the same vertical scale for all signals (except power):

This puts things more in perspective: the voltage over the capacitor (yellow) is slightly lagging the input signal (red) now. And the input current (blue) is out of phase w.r.t. the input signal (red). So the power consumption (red x blue) is substantially lower than with a resistive circuit: when the current is maximal, the input voltage is only a fraction of its maximum range. IOW, we’re drawing current when it “costs” little.

This is why a capacitively coupled supply is cheaper: the electricity company charges us for real power (i.e. V x I). We’re being charged for what happens inside a pure resistor.

But we’re doing a lot more: we’re taking charge out of the AC mains line on one half of the cycle and pushing it back on the other half. It might seem as if that doesn’t use up energy, but it does: current through a wire causes resistive losses (in the form of heat), and “returning” that energy one half cycle later causes those losses again! So the electricity company sees its electricity turn into waste heat, and they can’t charge us for it.

Here’s a thought experiment: suppose you had a huge capacitor, and hooked it up directly to AC mains, next to the electricity meter. According to what we’ve seen, it’ll track the 230V cycles, with current exactly 90% out of phase with AC mains voltage. Huge currents would flow at the time of zero volts. You’d be charged relatively little for the large out of phase current that flows (not quite zero because a practical capacitor has some internal losses that look like a resistive component from the outside). But your garden would be nicely heated by that current in the resistance of wires between the electricity company and your meter…

You can read more about “real”, “reactive”, and “apparent” power on Wikipedia.

(with a tip of the hat to Martyn for helping me understand this stuff a little better)

(Thanks for your overwhelming understanding w.r.t. not making the Low-power supply available as kit!)

The concept of the transformer-less capacitive power supply still puzzles me – intuitively I still don’t get it:

(the above image was copied from this excellent site with a web-calculcator for it all)

So what exactly is going on here? Tomorrow, I’ll simplify that circuit in an attempt to really get it, but for now let me show you what I see on the oscilloscope. What I did was feed a ≈ 100 Vpp signal into the above circuit, which is essentially the same as in the Low-power Supply.

To interpret the graph, you need some info:

• the scope ground was connected between the capacitor and the resistor
• the yellow trace is the voltage over the resistor
• the blue trace is the voltage over the capacitor
• the yellow trace is inverted, i.e. negative voltages at the top, positive at the bottom
• zero is the middle of the screen for both signals

Here’s the scope capture:

Ignore the fact that these waves are hideously complex, ignore the red lines for now, and also note that watching voltage over a resistor is the same as watching current through that resistor. Voltage and current are always proportional in a resistor, that’s what defines a resistor (Ohm’s law: voltage = current x resistance).

So what are we seeing here?

Well… when the yellow line is high, the blue line rises sharply. When the yellow line is zero, the blue line is flat.

That makes a lot of sense: the resistor is charging the capacitor. And similarly for negative values, it’s discharging the cap (and then charging it negatively).

So ignoring the zener and the rest of the righthand side of the circuit, this is really all that’s going on: when the input voltage is highly positive, current flows in one direction, charging the cap and dropping to zero, and when the input voltage is highly negative, the whole process unfolds in the opposite direction.

One more piece of the puzzle: current is “charge per time unit” (in units: Amperes is Coulombs per second).

In other words, the capacitor accumulates the charge pushed into it, in either polarity. And while it does so, the resistor “takes the heat”, so to speak: it limits the current by creating a voltage drop over itself.

Please let this sink in, dear reader. It’s essential to get a solid intuitive grasp on what’s going on.

Note also that it really makes no difference at all how complex the input signal is. The resistor and capacitor work in tandem, sharing the task of dealing with that signal. In other words: the capacitor is always trying to catch up!

This is where it gets interesting.

You may or may not have given up in high school when it came to advanced maths / calculus – derivatives and integrals, in particular. If so, then get ready to finally get to grips with these incredible concepts.

Let me explain the red lines in the above image – they are generated by the built-in math functions of this scope. One red line is the integral of the value measured across the resistor, and the other red line is the derivative of the voltage across the capacitor. But here’s the big surprise:

• the integral of the voltage over the resistor is the same as the voltage over the capacitor!
• the derivative of the voltage over the capacitor is the same as the voltage over the resistor!

What’s the point? Well, this means that I didn’t have to measure both signals to see what’s going on. I could have omitted the blue trace, because it can be calculated from the yellow trace (and vice versa). Even though these traces have completely different shapes, they are in fact totally inter-locked and inter-related.

As I said before, the voltage over a resistor is proportional to the current through it. So the derivative of the voltage over the cap is the same as the current through it (the current flowing through the resistor and the capacitor is always the same, since they are connected in series).

The derivative is the rate of change, i.e. the slope of the graph. Integrating the current (i.e. the derivative) is like adding “all the little currents together over time. A capacitor is no more and no less than a “current integrator”.

And that’s exactly the same as saying that a capacitor accumulates charge. It’s like a tiny rechargeable battery, it takes the current pushed through it and it stores that current (as charge). As the charge accumulates, the voltage rises. Loosely speaking, this is the same as saying that it pushes back harder and harder against the incoming current. At some point it pushes back so hard, that no more current comes in. At that same point, there will be zero volts over the resistor, and the voltage over the cap stays constant. Check the graph to see where that happens.

One last observation is that the blue line is a lot smoother than the yellow line. That’s not surprising: when you integrate (accumulate) a jittery signal, things tend to smooth out. That’s why capacitors are also a fundamental component in filters, i.e. circuits which let some frequencies through more and others less. That schematic we’ve been looking at here is also known as an RC circuit – if you ignore the zener and the rest. One way to look at an RC filter is to see the capacitor as the sluggish part, and the resistor as taking up the slack. So with any input signal, the voltage over the cap is related to the low frequencies, while the resistor follows more the high frequencies.

Did this explain how a capacitive power supply works? Probably not. But first we need to get to grips with what a capacitor does, and hopefully this little experiment helped you get some intuition for what’s going on.

I’ll try to take this further tomorrow, by simplifying things a bit. Stay tuned!

To avoid switching noise near the work bench, I wanted to drive my LED strips from a plain ol’ transformer -> bridge -> capacitor supply. It would have to supply about 2A @ 12V for my LEDs. Here’s the setup:

Ignore the diodes at the top, these were 1N4004 diodes which weren’t quite up to the task. The ones at the bottom right are Schottky diodes for a lower voltage drop, rated at 5A. The capacitor is 2200 µF, but ripple isn’t an issue.

Alas, the heat from this thing is excessive. I even went for an artsy swiss-cheese design:

Trouble is the transformer (and surprisingly not the diodes!) – I measured them both, with everything closed:

Diodes heated up to 75°C – but the transformer went all the way up to well over 90°C within the hour! AC mains consumption is 50W, so this silly thing is eating up half the power while trying to behave like a stove…

The transformer is a cheap 2x6V @ 2.5A unit I had lying around (draws 3.7W, even without any load). I’ll keep this as a high-ripple 12..20 VDC power brick for testing – but it definitely can’t be used continuously @ 2 Amps!

PS. If you’d like to meet up – I’ll be presenting JeeStuff at HackersNL coming Thursday evening in Utrecht.

To follow up on yesterday’s post, here’s a similar TCXO oscillator which is considerably more accurate:

It’s a 10.000 MHz unit which has been calibrated and claims a 0.1 ppm accuracy. That’s 10 MHz ± 1 Hz, i.e. 7 digits. Here’s the output signal, which appears to be AC coupled:

Not bad for a low-cost unit. I got it from the RFcandy shop. Current consumption is 0.94 mA @ 5V.

Once you start looking into this stuff, all sorts of things pop up on the web. There’s a group of amateurs who call themselves Time-Nuts (terrific name!) and who aim to keep track of time as accurately as they can – with pretty amazing results. In a comment yesterday, John Beale pointed me to a web page with info about the Rubidium clocks now available cheaply on eBay.

I found this page most entertaining, summarizing the history of accurate time-keeping in a few slides.

The RTC Plug uses a DS1340 chip to keep track of time. It has its own on-board coin cell and 32 KHz crystal, so that it can keep running when power to the JeeNode is switched off.

The 32,768 Hz crystal is rated at 20 ppm, with 3 ppm aging in the first year. A year has about 3600 * 24 * 365.25 = 31557600 seconds. With 20 ppm, the RTC Plug can be some 630 seconds off at the end of the year, i.e. close to one minute per month – worst case, that is. It’s fine for lots of uses, but evidently not all…

Here’s a new plug prototype, designed by Lennart Herlaar:

It’s based on the DS3231 chip, which includes a “TXCO”, i.e. a Temperature Compensated Xtal Oscillator. There are many ways to improve the stability of a crystal oscillator, but this one is pretty nifty because it’s low-power (unlike an oven-based technique, which puts the crystal in an controlled temperature environment). The idea is that the chip periodically measures the ambient temperature, and then adjusts the capacitive loading of the crystal according to a calibrated function of temperature. IOW, it knows how to compensate its on-board crystal.

The DS3231 is specified at 2 ppm for temperatures 0 .. 40°C, i.e. ten times more accurate than the above crystal. So this clock will stay on time with an error of at most one minute per year, i.e. about a second per week.

For something, eh… slightly better – you could consider a Rubidium Frequency Standard with 50 ppt accuracy, i.e. well under a millisecond per year error. But don’t expect to run it off a coin cell – it draws about 11 Watt :)

If there’s enough interest, this plug could be added to the shop…

There’s one dirty little secret about most digital storage oscilloscopes (DSOs) which puts them way behind the capabilities of their analog brethren: screen update rate.

For a scope to present a trace on screen, it has to do a lot of signal processing. After all, 1 GSa/s means its acquiring 1 billion data values per second. And although we’re not able to really see that with our eyes, we do see things that stay on the screen for a few milliseconds.

One trick is to turn on persistence. On analog scopes, that’s a pretty nifty feature whereby the image generated from the beam hitting the screen is made to stay visible for a while. Either via a phosphor coating which keeps on glowing for a while, or by constantly re-sending the same beam pattern to keep the visual display going.

Digital scopes can simulate this. All they need to do is leave the pixels on their LCD display as is, right?

Unfortunately, it’s a lot more complicated than that. If you just refresh the screen a few times a second, then you’re still going to miss a huge amount of detail. Let’s take a periodic 1 MHz signal: to display everything measured, the display needs to be updated 1,000,000 times per second, which means each “sweep” would only remain visible for 1 µs. That won’t do – our slow eyes wouldn’t see a darn thing, especially glitches which occur very infrequently. So what a DSO does, is simulate persistence by merging new sweeps into what’s being shown, and then take old sweeps out of the picture a bit later (try implementing that – efficiently – !).

To test this stuff, I used the following sketch (which is based on this pin flipping trick):

It generates pulses at ≈ 1 MHz, but every once in 10,000 pulses, it messes up the timing. Note that interrupts are turned off, so this thing is as stable as the clock it’s running on (a JeeNode with a 16 MHz resonator in this case).

One glitch every 10,000 pulses at ≈ 1 MHz means there are 100 glitches per second. That should be constantly visible on the screen, right? Well… not so on my scope. I enabled 10s persistence, i.e. sweeps will fade after 10s:

That’s two glitches (the actual display is a bit erratic, but the position of the glitch pulses is absolutely repeatable).

IOW, this scope is showing only one out of every 500 pulses on the screen, on average!

This is why DSO manufacturers report a “waveform/sec” value in their specs. In this case, we are getting 2,000 waveforms per second, even though a scope could theoretically sweep 500,000 times per second on this.

An analog scope would have no problem whatsoever with this. To show the same two pulses in each sweep, it could trigger a new sweep 500,000 times per second, or maybe even “only” 100,000 but that would still show 10 glitches per second, i.e. a nearly constant visual display of the glitches. Fifty times more often than my DSO, anyway.

That 2,000 wf/s figure is an optimistic one, BTW. With more channels enabled, or things like signal stats calculated or the math function applied, this value can drop substantially. Meaning you’ll see even less glitches.

What this tells me, is that the Hameg needs about 500 µs of processing time to get 1 channel of acquired data onto the screen in its most basic form.

Don’t get me wrong: technically that’s an astonishing achievement. It’s not just copying a few bytes and setting a few pixels. It’s (always!) performing the emulated-persistence trick – because with persistence turned off, I still see a glitch every few seconds, which means that the LCD display is showing the glitch for a substantial fraction of a second – much longer than the 1 µs (or even 500 µs) sweep rate would suggest. Besides, you can see that fading is also implemented, so it’s not just drawing pixels but actually fooling around with color intensities.

There are some scopes which get much higher waverform/sec rates, such as the Agilent 2000X (50,000 wf/s) and the Agilent 3000X (1,000,000 wf/s) series. But they are twice the price, and even have lower specs in other areas.

Does it matter? Not for me. Being aware of this is good, though – far more important than fixing it. Note also that IF – that’s a very big if – you know what you’re looking for, then there are usually other ways to find these things. In this case, triggering on a negative pulse width under 900 ns captures all those glitchy pulses, for example.

As so much in life, it’s all about trade-offs. Analog / digital, brands, and of course… your needs and budget.

Recently, the radioBlip sketch reached yet another milestone: it’s been running for 500 days on one LiPo charge!

Seems like a good time to start a new duration test, this time with a JeeNode Micro:

Since I don’t want to have to wait yet another 500 days, I decided to up the ante:

The JeeNode Micro includes holes for a CR2032 3V battery holder, so why not try that for a change, eh?

The sketch was modified slightly, to set the proper clock divider on startup:

I’ve set it up as node 18, and it’s been blipping happily:

Let’s see how many days that little JNµ can dance the Sputnik shuffle…

V-scoring is a low-cost way to cut up printed circuit boards. I use it in combination with routing to get al ’em plugs made out of larger panels. It consists of slightly cutting into the PCB material from both sides – when done properly, the resulting boards can be snapped off quite easily by hand.

Recently someone gave me a really neat breakout panel for SOIC and TSSOP IC’s. These are useful to try out SMD chips on a breadboard. Except that the board I got was still panelized…

Can’t look a gift horse in the mouth, and since I don’t have a mini bandsaw or such, I had to find another way.

As it turns out, it’s quite easy to do the v-scoring by hand:

The basic idea is to lightly scratch all the lines about three times, and once there’s a decent groove, just take the hobby knife and cut two more times, pressing down fairly hard. Rinse and repeat for all the cuts on both sides, and things will come apart by pressing down firmly. Here’s what I ended up with after about 15 minutes:

The edges can be a bit rough, as usual with v-scoring, but those rough bits can easily be scratched off with the knife.

And this is what it’s all about:

Tiny chips, ready for breadboard use!

Have you noticed that solder fumes always tend to rise up into your nose? As the type of solder I’m using is particularly irritating, I decided to do something about it, ending up with a very low-tech solution…

Normal fume extractors (Dutch link) tend to suck the air in, filtering it through a carbon filter to actually remove the fumes from the air, but I find them a bit unwieldy. Yet another power cable to clutter my workbench.

Here’s my solution:

A small 12V brushless PC fan, which happens to still start up and rotate reliably at 3.6V, as provided by three Eneloop AA batteries. Stuck to a battery pack (with on/off switch) using hot glue, which also doubles as weighted base for it all. The fan is slanted slightly downwards, to better cover the area on the table.

The result is whisper quiet and generates a micro breeze which gently diverts the solder fumes away from my nose. This particular fan draws only 30 mA @ 3.6V, so it’ll last some 70 hours on a single battery pack. And since these are rechargeable batteries of which I keep a constant supply, that’s plenty for me.

Long live re-use!

To try and improve noise levels during measurements (and as general ESD precaution), I went “green”:

That’s a green ESD mat, covering almost the entire workbench. It’s hooked up to the radiator for grounding. Note that the mat only provides a “dissipative” connection to ground, the surface still has several MΩ’s of resistance. It’s just to get rid of static electricity and to offer a clean protective working surface. Got the mat from Farnell, BTW.

Here’s what I see when pushing a scope probe onto the mat:

A clear 50 Hz pattern of a couple of volts (the amplitude increases when the probe is pushed more firmly onto the mat). This is with a standard 10 MΩ high impedance probe. The big puzzle is: where does this come from?

My explanation for now, is that the scope ground is “floating” a bit, due to the different devices hooked up to the house wiring. Note that the mat is tied to the radiator, not to the ground of AC mains. Since there isn’t any current flowing through the heating system (I hope!), I’m inclined to trust it more as being the “real” ground potential.

It’s no doubt completely harmless. Measuring AC current between these two ground levels with my most sensitive multimeter (a Voltcraft VC940), I see a very occasional blip of up to 0.20 µA flowing. Well under a microwatt.

To give an idea how crazy things can get, here’s the mat with nothing connected – turning it into one big antenna:

Who knows, maybe one day we could harvest even this sort of energy, eh?

A long time ago, I got this DDS-60 kit, which is a small circuit based on an AD9851 DDS chip:

It has everything on board to generate a sine wave from 0..30 MHz, and my intention was to hook it up to a JeeNode (as part of a long term plan of mine to set up a more extensive wirelessly-controlled electronics lab):

Never got it to work at the time, but now with the new scope, there really is no excuse anymore. First check, as indicated in their build instructions, is to verify that the crystal oscillator is feeding a 30 MHz clock into the chip:

Looking good. Very impressive rise and fall times, BTW.

When driven at 30 MHz, the AD9851 output frequency is settable in steps of 0.006984919 Hz. In other words, a multiplier of 1000 will generate a sine wave of ≈ 7 Hz.

Here’s the output when programmed to generate 10 MHz (multiplier 1431655765, i.e. 9999999.9977 Hz):

Whoa… it’s 10 MHz, but a far cry from a sine wave. Ah, but that’s not really surprising: this thing uses DDS to synthesize a sine wave, as recently described on this weblog. With 30 MHz sample rate, i.e. 3 samples per wave, it’s not really possible to create a decent 10 MHz sine wave (not even a symmetrical shape in fact, as you can see).

But the AD9851 has a trick up its sleeve: it includes a “6x” multiplier option, which causes it to internally generate a reference frequency which is 6x the incoming clock, i.e. 180 MHz in this case.

Using that, and adjusting the frequency setting to work at 180 MSa/sec, we get a much better approximation:

Still not perfect, but by analyzing the FFT of this signal, we can find out what’s going on:

This output signal is made up mostly of a 10 MHz sine, with another peak at 90 MHz.

Unfortunately, the output circuit on this board isn’t working yet (this is what probably threw me off when trying this circuit before), so I can’t test the effect of the 60 MHz low-pass filter yet. It won’t filter out the 30 MHz residue visible in that last picture, but should definitely reduce the frequency components over 60 MHz.

Ok, all the important bits seem to work – I “only” need to troubleshoot that analog back end a bit more.

Update – I found the problem: the SMD trimpot was fractured, i.e. no contact. I’ve replaced it with a fixed 220 Ω resistor for now – this brings the output to ≈ 680 mVpp (or 350 mVpp into 50 Ω) – the sine wave output is now considerably cleaner, but several of the frequency peaks ≥ 90 MHz are still present:

I suspect that the 30 MHz clock is “feeding through” somewhere, perhaps better decoupling would avoid that.

The quickest way to describe what Fritzing is about, is to quote the first paragraph on their site:

Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to work creatively with interactive electronics. We are creating a software and website in the spirit of Processing and Arduino, developing a tool that allows users to document their prototypes, share them with others, teach electronics in a classroom, and to create a pcb layout for professional manufacturing.

It has been around for a while, evolving and improving at a steady pace. Here’s a screen shot of the main window:

The neat bit is that support has now been added for the JeeNode, as well as support for the main two prototyping / extension mechanisms, the JeePlug and the Carrier Card. Just search for the “JeeLabs” keyword:

So now you can use Fritzing to design and document projects, and create printed circuit boards for them!