Archive for January 30, 2015

Multimeter accuracy and calibration confusion

As a bit of a follow up to my miliiohmeter project, I’m taking a step back to assess the standard by which I measure things. Having been schooled with a science background (Chemistry, Biology, Physics, Mathematics), the importance of good data, good results, good science is deeply ingrained in me. I believe this important in every walk of life, as an assist for critical thinking and to debunk the media’s annoying tendancy to throw meaningless statistics and skewed numbers at us to convince of whatever they want to convince us of. The tin-foil hat will however remain off tonight. I’ve had three beers after all.

Like most hobbysits, I accept my multimeter as not only the gold standard for everything I do with electronics, but it is also my eyes into what those pesky electrons are actually doing in there. Without it, the study of electronics would be horribly boring. We’d see lumps of circuitry that either did what it was supposed to, or failed in a puff of acrid smelling smoke – the reek of overload.

Recap of the Problem at Hand

In my milliohmeter project, I had reached an impasse. I created the thing to enable my multimeter to measure low resistences (>10Ω) down into the 100s of micro-ohms range since pretty much all DMMs without a dedicated function for this fail badly below 10Ω and especially below 1Ω. To make it work, I need a constant current source. I chose 1A as this made everything line up nicely. Ohm and his law states that 1V = 1A x 1Ω. My DMM, with this box in-between, would clearly read mV as mΩ. I put it together in my usual way of cobbling schematics, lots of fussing and reading.

It worked, after a fashion. It gave me a reading reasonably close to anything I measured with it. Perhaps a bit higher than it’s stated value and tolerance would suggest but close and certainly far better than my multimeter could do. It seems it’s about 2.2% out of where it should be. This could be a number of things or a combination of things. The set resistor that enables the LM317 to act as a constant 1A current source is actually a bunch of parallel resistors to dial into that sweet spot. CircuitLab told me this would be 1.155Ω, the datasheet for the LM317 told me it would be 1.25Ω. The actual measured value I got was approximately 1.245V drop across the parallel arrangement which is close to where it should be, or where I think it should be. I used standard 5% carbon film resistors to make this parallel arrangement with the addition of the critical 100Ω trimpot to calibrate away any oddities in that 5% tolerance of the resistors.

This is dandy, just build it up, trim it up and the things works right? Well sort of.

The datasheet says it should be 1.25Ω, which for 1A means a voltage drop of 1.25V, 50mV off isn’t bad, it’s a 5% error but how can I be sure that will net me 1A out of the thing? The LM317 has it’s limits too based on a variety of factors and that will need to be trimmed out in addition to the 5% resistor tolerance. Then there’s the other things, losses in the protoboard I’m using, loses in the leads, stray capacitance, quantum fluctuations – it never ends.

The only thing I needed to be sure of is that the thing is outputting 1A as close to exactly bang on as possible so that I could get an accurate reading. I needed to calibrate to that.

Unfortunately, as previously chronicled, my multimeter’s current ranges are quite limited to 10A, 200mA, 20mA, 2mA. For 1A I am forced to use the 10A range which gives me an output of 01.00A. Spot on yes, but lacking in that last digit to make sure it’s within the tolerance I need. I need to read 1.000A at least. 1.0000A would be even better! Given that my readings on a 1Ω 1% resistor was 1.022Ω that makes it 2.2% off-tolerance, and I’m pretty sure that’s not the resistor.

Electronics’ Dirty Secret

One of the first things I noticed about electronics when I began playing with it as a child is that the numbers never quite add up in reality. Every time I look at my mutlimeter when I take a measurement, I always shrug and say “close enough” and this can’t be helped. It’s frustrating when one’s math on paper makes nice round exact numbers yet the reality shows us we are just a little bit off. Part of this is due to the fact that we live in the real world and all the things we normally take for granted as not existing – like the resistance of conductors and PCB traces, as well as the noise they induce being antennas, tend to add up and creep into our measurements. Add to that the (in)tolerance of parts and the meter itself you have a mess.

As always, we, the scientists and experimenters, try to minimize this “noise” by buying bigger and better test equipment calibrated by some boffins in lab coats. This is all fine, if you have money. I don’t.

All I have is my Mastech MC8222H Chinese made $70 meter and that is the most accurate instrument I own. To me, this is my de facto gold standard as I simply have nothing better to compare it to.

The Mastech MC8222H is not a bad meter especially for it’s price. It has many annoyances I am not fond of and fluctuates like hell, but it works and has all the features I need for general electronics work on my humble hobbyist bench.

It is s 2000-count 4-digit display which would make this measurement easy if not for two things: it is lacking a 2A range on the current measurement. I can measure 200mA just fine, I can measure 10A just fine, but not in-between and keep that third decimal point. That’s not even the whole story. This is merely talking of it’s display resolution which says nothing of it’s accuracy.

A cursory look at it’s badly translated manual booklet tells me something else I’m not terribly fond of. Though the DCV function has a standard 0.5% accuracy, the DCA on the 10A range has an appalling ±2%+5 accuracy in the best possible case. For those that don’t know, the +5 figure means ±5 digits, meaning the least significant digit could be off by as much as 5 in addition to the ±2% accuracy window.

Acceptance

So here I am, with a bunch of adding intolerances. The resistors to set the constant current of the LM317 can be out by ±5%, the LM317 can be out itself by a bit, the meter I’m trying to calibrate it to can be off by ±2% and then some and that’s before we even take into account all the micro anomalies in terms of materials and construction. What is one to do?

The answer for right this moment is: nothing. I cannot calibrate this thing any better unless I have one good known bit of it I can say is calibrated to within half a bee’s dick of it’s life of where it should be. To me that’s >= 0.1%.

Options

One option that is apparent is to get a 1Ω precision resistor meant for calibration. I did a quick poke about and was unable to find one but I’m sure they exist. With that, I could dial the current source down until it reads 1Ω and then know i’ll be getting the best possible measurements from it. Not counting the error my multimeter will inject of course just being it. At least it would eliminate a couple of error sources straight away.

The other option of course is buy a multimeter worth owning. A brand name, one that is respected. A company that actually calibrates their meters before shipping them out and are known for reliability. The obvious boon here apart from being calibrated are that there will be a much better accuracy on the unit in general. We’re talking at least 10x better on the DCV and at least 2x better on the DCA. If i make it a 10,000 count one, I will get my much coveted missing digit also. The less obvious boons will be a meter I can rely on for twenty years that won’t drift much and has features like auto-ranging that will annoy me far less than the Mastech. I’ll keep both of course, always need two meters at least.

The obvious brand contenders are: Fluke, Keysight (Agilent), BK Precision and a couple of others I will consider after like Extech. I haven’t a budget yet, but when I do I imagine it to be about $300.

With this, i can dial in that current in to my satisfaction. Ironically, I bet some of these more expensive meters come with a low-Ω function which completely negates the purpose of this project but hey – that’s why we do these things, to learn. As you can tell, I’ve learned a lot. Like don’t buy cheap meters.

Milliohmeter basic construction finished, and testing success.

With only some minor comedic incidents

Last post on this topic, I explained the why I needed one and how I was going to build it. Go there for schematics and function details. Finally having gone electronics shopping last week, I was able to get the one critical part I needed for this project: a 100Ω trimpot. One measly part which took me a month to get off my ass and get. Such is life :) . I also managed to grab some beefier crocodile clips to make my test leads that much more durable and awesome looking, not least because I could actually fit the rubber boot over the two wires to each clip!

Soldering was trivial and I managed not to burn myself or set anything on fire, or wreck the board. I did cut myself though when I jabbed my hand with a pair of needle nose pliers though the damage to my hand is so slight I’m only mentioning it as I take that as a sign I’m actually doing something useful if I am bleeding.

mohmeter_total

With the board now fully assembled I thought “what they heck” and decided to power it on to see if it blows up or something. To my delight, it did not blow up and worked as expected! Hooray! That never happens.

Calibration goof up

This is just funny. Remember in the last post how I thought I was being smart by setting the output current to 1A instead of 100mA so that mV = mΩ? Well that part works a treat. My multimeter does read directly in milliohms no problem. The difficulty is in calibrating the damn thing. Like most DMMs, mine comes with 2mA, 20mA, 200mA, and 10A current ranges. In order to calibrate this thing so it’s actually useful, I need to have the LM317 outputting a constant current of as close to 1A spot-on as possible to get an accurate reading. Since the output needs to be 1A, I have no choice but to use the 10A range which will only give me a reading of 1.00A, which it did. This is great at first glance, spot on 1A right? Well no. What about the third digit? Assuming it rounds it, that means that it could potentially be ±5mA and I wouldn’t even know it! Had I stuck to one original design, I could have had it set to 100mA which i could dial in to within a bee’s dick of 100.0mA (±50µA making it all the more accurate. For lack of one extra digit on my multimeter, I cannot calibrate it any better than I have it now. Unless I devise some clever plan to give it a known resistance… chicken and egg scenario, that’s what I built the damn thing for! Oh electronics will get you :) . Anyone have a multimeter than can measure 1.000mA that I could borrow for 2 minutes?

mohmeter_test

These sorts of issues will plague me until I get a better multimeter, no doubt about it. When I build the power supply those extra digits will come in handy for calibration.

I measured a bunch of low-ohm resistors I had laying about and got reasonable results according to their stated values which is nice. I know it’s still a bit off, has to be, but it will do for now. A 1Ω 1% resistor I measured read 1.025Ω making it 2% off it’s stated value, so unless this resistor is out of spec, the thing needs some tweaking. I won’t know until I calibrate it properly.

Next steps

The next part is almost trivial, almost. I just need to safely whack it in a case and call it a day. I did notice the thing heats up quite a bit and I measured 83°C on the case of the LM317 (well within it’s limit) so that’s not a big deal. I may drive it harder, by using a 10Ω load and have it up the voltage to push an amp through it and see what the temperature shoots to. The thing isn’t really required to be on for hours or anything, just enough time to test a resistance quick, so I may just try to get away with some vent holes or (depending on the temperature at 10Ω) I may add a little fan to cool the bugger.

Then it’s case drilling and mounting everything, whack in a power LED. Oh and add an appropriate power supply for it, which I still do not have. Still, hard part over.

Kanger IPOW2

So recently I’ve had a problem in vapeland. I had everything so I thought, four batteries, two chargers, a bunch of clearmoizers and plenty of liquid as well as the fixings to make more. I noticed something disturbing happening. My batteries were not lasting as long as they should. I’m not talking about thirty minutes or an hour less, I’m talking about 20% or less of their former capacity. Something was amiss. At first, I assumed it was the charger. Such things are made in China and made to be cheap so it’s more or less expected they will fail at some point. A simple failure of it’s voltage reference would cause it to say it’s done charging when it isn’t. Which was exactly what I thought was happening.

Not so in this case. The purchase of a new charger did nothing to revitalize these poor batteries which were in fact just dying as all LiON batteries with too many charging cycles wearing out the battery. So the damage is two of my batteries out of four with another one being about half working. This is kind of scary since I depend on these things.

Having to spend an entire day out of doors an away from my home made AC box mod was rather frightening, I didn’t want to run out of juice. I knew of a brick and mortar on Yonge St. named 180° Smoke so I went there looking to replace with the same old de-branded 1300mAh spinner type battery that has served me so well.

Evaluating my options, it seemed to me that the usual spinner type batteries weren’t so competitive and awesome any more. Sure they can be bought for cheap online, but they aren’t great, and actually kind of a pain in the butt in many ways. They have no display or feedback for the charge state of the battery, they are variable voltage so need frequent tweaks to get a good vape with differing atomizer resistances and battery charge levels as well as the saturation of the clearo itself. Moreover, they have no readout at all aside from the flashy button LED which tells you very little if anything. So I saw the Kanger IPOW2 and thought “ooh that’s shiny”.
IPOW2

The IPOW2

For $30-40 here’s what you get – a variable power (wattage) integrated battery with a nice display and smooth interface. What more could you ask for? The design is nice and sleek and all flush mount, the display is a nice bright (and informative!) OLED display which shows the battery charge, output power, and atomizer resistance all at once. It is a bit longer and thicker than the spinners which is no big deal really. It’s not oversized. It comes in 1000mAh, 1300mAh and 1600mAh models to suit everyone’s size vs. battery capacity taste. The thread at the top is 510 but helpfully comes with an eGo adaptor in the box. Here’s one of my favourite things about it – it charges by a micro USB cable (also included). No more stupid screw in chargers!

Theory of Function

Before, variable power (as opposed to voltage) was only available to the owners of APVs (advanced personal vaporizing devices) for quite a bit more than your standard eGo twist clone. What this means in brief is that you get a consistent output from your device no matter what the atomizer resistance or battery charge level. From first vape to last it will be consistent. With the twist/spinners, one would have to keep adjusting the knob on the bottom to continually tweak to get the desired vape out of the thing which is fiddly and annoying honestly. With this, you can set it and forget it and just enjoy your vape.

The reason variable power (wattage) is so important and why it works is quite simple and just a bit of basic electronics. The atomizer coil is just a resistor by another name. Resistors are just heaters by another name :) . If you apply a voltage across a resistor, it will pull current through it and dissipate heat according to it’s resistance value. Basic Ohm’s law: voltage = current x resistance. So if your coil is 2.2Ω and your battery is set to 3.7V it will draw 1.68A of current (re-arrange the formula: current = voltage/resistance, 3.7/2.2=1.68). Easy. So you’d think you just set it at 3.7 and always get 1.68A through your coil to get a consistent vape, no problem. Catch for young players: what if the voltage changes, or the resistance?

Fact is, the little knob on the bottom of your twist will say it’s set to 3.7V, but remember it’s not made to be a laboratory grade voltage regulator, so it could be off by who knows how much. Not only that, but it could drift off depending on the load you put on it or any number of other factors. It can also drift and even fall off as your battery drains. All voltage regulators have what’s called a “dropout” voltage, meaning it needs a bit of overhead to regulate the voltage to the desired level consistently. If your battery voltage is lower than your dropout voltage plus your setting, you will not get your desired voltage. Likewise, over their lifespan, atomizer coils can increase their resistance causing the battery to work harder for the same output. This, I believe, is through heat expansion of the coil and it being gunked up with e-juice. Eventually, the coil just becomes unusable for a variety of reasons, but one thing variable voltage battery owners find is they have to increase the voltage to get the same vape as the atomizer ages. The point is, it’s the power output that is important. Regulating that is key to a consistent vaping experience.

So what is the variable wattage about and how does it solve these issues? Well, let’s look at what wattage actually is. A Watt is a derived unit of power. It could mean electrical energy, or thermal energy, but always 1W = 1 joule per second. It is a direct measurement of the amount of energy over time that something is outputting. This is why amplifiers, heaters, microwaves, everything uses it as their rating by which you buy them. According to the formulae, 1W = (current)^2 x Resistance, or it is current x voltage. By making power what we want to regulate, we let voltage fly free and allow it to be adjusted as need be to maintain a steady output.

This is essentially what I (and probably you) were doing by fiddling with that bottom knob on the spinners. We knew, by feeling, where our perfect vape was, and would adjust this knob up or down as needed. We were varying the voltage to match the changing resistance of our coil or the battery charge level to get the same power output. The variable wattage takes care of this for us. It’s voltage regulator will sense changes in output resistance and compensate for the battery charge level and focus on keeping that steady power output regardless of these two factors. If the atomizer resistance increases, the device ups it’s voltage to compensate.

The Review

Finally, after my tedious intro rand and technobabble, I’ll finally talk about the device itself.

I’ve been using it about 30 hours and I have to say I’m suitably impressed. It is certainly more fun to use than the boring old spinners and yes I’m quite dazzled by all sorts of electronic technology so having this as my PV appeals to me. It’s thickness is greater, as I mentioned, but I actually find this more comfortable to hold in my hand than the thinner spinners. The controls and display are all nicely flush mounted which to me not only looks good but is less to break when I drop it or knock it against something. Conversely, since the button is flush I have had occasional trouble finding it when not looking at the device which is slightly annoying. Nevertheless, the fire button has a nice tactile click and is not hard to actuate. I imagine it no more or less durable than the microswitches found in other batteries so I would still suggest against mashing it. Oddly, this button is in the shape of a shield, which brings to mind anti-virus software logos. I do not know why Kanger chose this but I don’t mind it either. A clear plastic light pipe surrounds the button to indicate when it is being operated and flashes like you would expect it to when turning it on and off, when it reaches it’s max firing time, and when the battery has run flat. I do note shadows in the oddly shaped lightpipe so it’s not quite as attractive as it could be be being nice and bright and lit up. Unlike some batteries, the LED powers off instantly. Guess they didn’t want to put a cap there to make it fade. Which is neither here nor there.

The OLED display is nice and bright and blue and perfectly flush mounted on the metal tube. One thing I absolutely love is that it shows you all information at once. In one glance you get your power setting, your atomizer resistance, and battery charge level. I’ve looked about and I cannot find a device for a comparable price that does that. Even some higher end APVs only show one of the three at a time and one has to hunt through menus in an awkward operating system to see everything one wants to know. Who wants that? It brings up another point – the user interface. It is dead simple.

All that is involved in using the thing is pressing a button. If one wants to adjust the power level, simply adjust the knob on the bottom. That’s it. Usual 5 click on-off. That’s it. No menus, no bullshit, just hassle free vaping.

IPOW2disp

The knob on the bottom is great too. On the twisters, I was pretty sure I was turning a linear potentiometer and could feel the wipers scraping inside. Those would definitely wear out or get gunky over time and I also had the feeling that really they needed a log instead of linear pot since the control didn’t feel linear at all. The knob on the IPOW2 is great, it just turns endlessly in either direction while the display updates your settings in 0.5W increments. So it’s an endless rotary encoder instead of a pot, grand.

This, the display and the fact it is variable wattage tells me clearly that somewhere in there is a little microcontroller. Your vaping kept consistent by digital micro processor control, what could be better?

Another big plus of this device is it’s integrated charging circuit. I was already frustrated with failing eGo chargers and hated having to buy a specific charger with a specific thread only to have them die on me. The IPOW2 uses micro USB. Done. The little display even shows charging progress.

So here’s the summary:

Pros
  • Consistent power output
  • Sleek design, comfortable to use in the hand, smooth flush mount controls and display
  • Bright OLED display with all information at once
  • Simple operation, button and encoder, no menus to fiddle with
  • Integrated Micro-USB charger
  • Temperature protected, double protected battery, multiple vent holes for heat and battery failure
  • 3-15W regulated output
  • Cheaper than APVs with the same feature set
Cons
  • Flush button hard to find by touch
  • Non-replacable integrated battery
  • Display at bottom, counter-intuitive
  • Battery display not proportional to how long it will last (they all are)

Overall, I’d declare it a winner. It’s a lovely offering a step up from the basic eGo batteries and cheaper than the APVs. I was doing my research into an APV for myself since the batteries seem to die faster than anything else, and I thought it prudent to start using replaceable battery models. This, however, proved to be both a nice stop-gap and convenient solution. For the average vaper, it is a no fuss device that just works. I think I will buy one or two more to replace my spinners entirely.

Zener Pre-regulator further testing

Finally I was able to do some electronics shopping and got the few (trivial) parts that I needed. They weren’t even fancy, just some jumbo crocodile clips and a few resistors and miscellaneous parts. Took me only a month!

Anyway, as I returned to my bench, I noted my pre-regulator already set up and remembered I needed to run some tests. Last time I had worried I had used a PNP instead of NPN darlington transistor by mistake and found that I was indeed using NPN and it was working just fine. I also noted some disturbing voltage readings when heavily loading it.

In the last post, the pre-regulator worked just fine with a bit of voltage variation over the range of loads I could be using. If using a high resistive load, the voltage could climb too high above 30V, dangerously increasing the voltage differential the poor LM317 has to deal with (which could cause an overload even with a pass transistor taking most of it), or too low when using a low resistance load (higher current) dipping below my absolute minimum required of 26V.

The maximum user selectable voltage output of my power supply will be 24V (actually 48V as it is ±24V and there is a negative mirror of the positive side, we’re just dealing with one side for now) so the LM317 needs at least 2V of dropout (or headroom) to output a good clean 24V which is how I arrive at 26V. Of course, electronics being electronics, I know better than to call it a day once I get 26.0V, a bit of padding needs adding.

I had previously measured 25.something volts when using a ~3A load (my home made resistor) which is below the minimum I need. Most frustrating. I postulated a bit of capacitance could beef it up so I gave it a go and tried a bunch of caps before settling on 100µF which gave me a nice 27.4V which is just plain ideal! Problem solved there. For now. I checked with higher resistance loads (up to 10kΩ) and found it gave me an output of 30.4V which is still acceptable to me.

Of course, this is only one piece of the puzzle. I’m going to have to check again to see how well it fares with the current limiter, then voltage regulator, then load after that. It is more than likely some tweaking will be necessary to keep it all nice and stable and happy.

Other measurements

Of course, just getting the right voltage and current out isn’t the whole story. My DMM updates kind of slowly, and at the 200V range I have to use, it gives me only one decimal point to work with below 1V so it’s hard to see quick or small variations in voltage over time. Naturally, I fired up my ancient oscilloscope to take a peek at what it’s actually outputting. To my understanding, no power supply is perfect, and some ripple is always present. What is acceptable ripple I have no idea but I always assume the less the better. Considering a 30V output, the unfiltered ripple would be 30V which is insane. Thanks to the huge filter caps I have on the thing I can reduce that a fair deal. This is the first time I tried to measure the ripply on my power supply.

powerripple

The result seems to be approximately 6mV in a curve that screams to me capacitor which is not surprising. A quick google search turns up a ripple value for your average computer PSU of 120mV on the 12V line which is twenty times what I measured from my home made power supply! Having measured my own hacked ATX PSU it was also noisy as hell, which I hear is endemic to switch-mode supplies. Sure I had my 6mV of ripple, but at least it was clean ripple! A nice clean waveform. It seems I’m on the right path. More measurements to come later as I add other modules. The later stages and in particular the voltage regulator *should* show even less ripple. I remember a section on the LM317 on ripple rejection…

I will also probably double check my results. I do remember I saw a Fundamentals Friday episode of Dave Jone’s excellent EEV blog on the very subject of measuring ripple and noise of a power supply, found here.

Another consideration was, with how hard I’m driving my transistors by pulling 3A through them, was heat. I have zero idea on how to calculate how big a heat sink I need for a particular application and naturally assume the bigger the better. I’m sure Dave Jones has something on that also come to think of it. Time to get back to class. In the meantime, because I hate leaving the bench when I can melt things straight away, I used my indefatigable logic to conclude that I will drive it as hard as I plan to run it and take a bunch of measurements. Then I will plan for at least 20% margin above that.

I used the thermocouple that came with my multimeter to measure the case temperature of the TIP142 transistor and ran it through a 3A load. The temperature climbed steadily after a few minutes to about 120°C which is bloody hot! I’m uncertain whether or not it would have climbed higher than that given time, but it was climbing very very slowly at that point. 120°C is exactly 80% of the TIP142′s max junction temperature of 150°C which sounds just fine but I’m not really satisfied. If I left the thing running for three hours would it approach or exceed 150°C? I have no idea. Better over-safe than melted and sorry.

I will definitely be using a larger heat sink for all six pass transistors used in my design, and of course there will also be forced-air cooling as well. Pretty much all semiconductors tend to start acting badly as they approach their maximum rated temperatures so it’s a good idea to keep them as cool as possible. Easily done.

If you are a budding hobbyist reading this I recommend building your own power supply, you’ll learn a hell of a lot in the process.

Update

Have a good look at the pic of my oscilloscope reading above. Note that it is set to 2ms/division. So each of those squares on the x-axis is 2ms. Note that there are four (close enough) divisions for each wave. 4 divisions at 2ms/div is 8ms. Now think of this. The AC cycle in our houses, which I have converted to DC in my power supply, is 60Hz or oscillating 60 times per second. Because I use a full-wave bridge rectifier, that frequency doubles because it inverts the negative voltage to positive voltage so it oscillates at 120Hz. So what is the wavelength? 1 second / 120 times a second is 0.0083333ms. So it will take 8 and 1/3 ms for each wave which is exactly what you see on my ‘scope :) .

Update part two

To give you a comparison, in the video linked above about power supply ripple and noise, Dave Jones compares the data sheets for two power supplies, one a high current switchmode, the other the Rigol DP832 commercial bench supply. The former had a 10mV rms ripple rating, and the latter had a 2mV peak-peak rating. If i am reading my oscilloscope correctly, I am showing 6mV peak to peak to I am actually very happy with that result. When I have more of it built, I will have to of course test it under a wide variety of loads and get a more accurate picture of the ripple coming out of it but it is encouraging.