Tag Archives: laptop

Fun with Failure

Have you ever come up with a neat idea?  An idea for some really useful sort of device?  And then you find out that it is, at least for now, a physical impossibility?

I had just such a neat idea this week.  At least, I thought it was a neat idea.  You see, for the past few weeks, I’ve been traveling around the country interviewing for various different jobs.  One of these jobs is with John Deere in Waterloo, Iowa.  Now for those of you who don’t know, it gets cold up in Iowa.  Their average January temperature is a mere 16°F.  I was imagining myself leaving work after a long day, going out into the cold, and then shivering in a very cold Jeep Liberty (my car) for the drive home.  So I’ve been thinking, wouldn’t it be nice if I could somehow heat my car while it’s outside?

A Solar-Heated Jeep?

Well, I don’t want to run a heater using my car battery, for obvious reasons.  However, I have this nifty 90W solar panel that I’m not really using at the moment (although it is acting as a glorified cellphone charger, and has been put to good use in the past).  Perhaps I could mount this to the top of my Jeep and use it to run a small heater.  My only question – would 90W be enough power to effect a substantial (20-30°F) temperature rise?

Initially, my gut feeling said no.  For one thing, from experience I know that my 90W photovoltaic (PV) panel will never produce 90W if mounted horizontally on the roof of my car.  Even on a perfectly clear and sunny winter day, it won’t be getting strong, direct sunlight.  So perhaps I could expect 60W at best, but on a cloudy day I might see less than half of that.  But more importantly, on a sunny day, wouldn’t I already be trapping more than 90W via my windows and the greenhouse effect?  Because if so, this effect has never produced a particularly warm interior.

Of course there’s only one way to answer these questions.  Science!  Particularly, the study of thermodynamics.  I’ve taken a couple of courses on the subject, but I still had to refer to my book for this formula, derived from the first law of thermodynamics:

An equation for the rate of temperature change for an object at a given power input/output.

This says that the rate of change of an object’s temperature (dT/dt) is equal to the power (P) absorbed or released by that object divided by its mass and its specific heat.  So for my first experiment, I decided to heat the interior of my Jeep by idling the engine and turning the heat to full-blast.  I then hopped out and remotely measured the interior temperature during cooling.  How did I accomplish this?  I temporarily re-purposed my Doom Box, which now contains both a LM34 temperature sensor and an XBee wireless transceiver.  Here’s a snapshot of the latest PCB revision, powered by an ATMega644P MCU:

Doom Box PCB (Rev3)

So here’s what the temperatures looked like during my first test.  The yellow vertical line below indicates when the engine was shutdown and cooling began:

Test #1 Graph

I should note that the Jeep’s temperature measurement was filtered by a 40 second (eight sample) moving average filter implemented within LabVIEW.  Also, the internal temperature sensor was placed on the floor in front of the back seats.  Exterior temperature measurements were taken manually using a digital thermometer and are represented by a linear trend-line shown in red above.

So during this test, my jeep was outdoors, parked in the shade, from 3-5PM.  You’ll notice that during this time the exterior temperature dropped by about ten degrees as the sun started to set.  However, the temperature inside the Jeep dropped slightly faster, as you would expect.  There was also very little wind during this test.

Now to apply the formula!  The drop in temperature between t = 4000s and t = 6000s is easily determined from the graph: 4.7°F or 2.6°C.  The trick is in determining values for mass and specific heat.  For this first test I decided to simply approximate the mass of air and metal within the car.  I calculated that the Jeep contained about 4kg of air at a cp of 1005J/kg-K as well as about 20kg of steel (e.g. in the seats and wheel) at a cp of 490J/kg-K (it’s pretty interesting to note that air has a higher specific heat than most metals).  I decided to ignore small pieces like plastic and seat stuffing, as well as everything under the hood (an assumption which proved later to be somewhat stupid).

So plugging all of that into the equation above yields a power loss of just about 18W.

Well now, eighteen watts is pretty surprising.  I wasn’t expecting losses to be that low for an interior-exterior temperature difference of 25°F.  This called for a second round of testing.  This time, I moved my Jeep into an insulated, closed garage.  Doing this allowed me to get rid of temperature variations due to solar radiation and wind.  It also prevented rapid changes in exterior temperature.  I then inserted a controllable heat source (an old laptop) into the car, located a few feet away from my interior temperature sensor.  The laptop was configured in such a way as to draw about 46W via a 120VAC inverter.  In theory, this level of power should have raised the Jeep’s internal temperature by about 12°F per hour.  Did it?  Well, I think the graph below speaks for itself:

Test #2 Graph

Yep, that’s definitely a failure.  It turns out that an input of 46W could only raise the car’s interior temperature to about five degrees (F) above ambient.  So the bottom line is my 90W solar panel isn’t going to have much of an effect.

So what went wrong with that first test?  Why did I calculate the need for only 18W?  Two reasons.  First, I didn’t account for all the residual heat in the engine compartment.  This helped keep the Jeep’s interior temperature higher than it would have been otherwise.  I likely also underestimated both the mass and heat capacity of the car’s interior.  In my defense, it’s not exactly an easy thing to determine.  In thermodynamics classes, you get problems about isolated, well-defined blocks of aluminum and ideal, constant heaters.  In reality, you get an ill-defined mixture of materials and variable heat sources.

Just out of curiosity, I used the first few minutes of my second test to make a better guess at the Jeep’s mass and specific heat.  In five minutes (300s), the temperature rose by about 0.7°F or 0.4°C.  Assuming this rise came only from heat input by the laptop, the term mcp = 34,500J/K.  That’s actually not too far from my initial guess for the first test, which worked out to 13,800J/K.  What this likely means then is that in my initial outside test, the Jeep was losing about 45W instead of just 18W.  But I’m also guessing that a lot of heat was still coming in from the engine compartment.

Anyway, I hope these results aren’t too confusing.  I’m not making any guarantees about the correctness or validity of my methods or assumptions.  What I’m really trying to say here is that you’ll need a lot more than 90W of PV panels to heat your car’s interior.  That, I can say with confidence.  I’d love to hear comments on this, particularly if you’re an expert in thermodynamics!  Where have I gone wrong here? 🙂

ThinkPad T43p, T61p Power Consumption Tests

One of my biggest hobby electronics projects thus far has been the development of a solar power system, used primarily for home automation (for details see Part I and Part II). In the past, I’ve used it to partially power my five-year-old ThinkPad T43p laptop, which also acts as the brains of the system. Unfortunately, given the limited number of photovoltaic (PV) panels I possess (not to mention the lack of a good mounting location for these panels), I could only run the laptop off-grid for about six hours on a sunny day.

My ThinkPad T43p - This is one of my stock images.  If you look closely, you'll see that the lappy is actually in standby, despite what you see on the screen.Now that my PV system is (mostly) inactive, and I’ve got my Kill-A-Watt meter back, I’ve taken some time to quantify just how far I can reduce the power consumption of my T43p by disabling components. While I was at it, I ran the same tests on my more modern ThinkPad T61p. And if you’re like me, I think you’ll find the results rather interesting. 🙂

However, before I begin, I must tell you that this morning I’m mourning the loss of my ThinkPad T60. What, another laptop? Yes, admittedly, I have something of an obsession with ThinkPads. I bought the T60 as a replacement/upgrade for my T43p. I got a great deal on it from a guy on EBay, too – just $230 for a mint condition, perfectly functional PC. Even the battery still held about 80% of its design capacity. Unfortunately, after owning and operating my T60 for about six straight months, it shut down and now won’t start.

Dead, Partially-Disassembled, ThinkPad T60
I’m not really sure what’s happened; I wasn’t around when it failed. I had it running as a server in a remote location for several weeks. Then one day I just couldn’t connect. It appears the motherboard has been damaged somehow, but certainly not from any physical impact. Perhaps an electrical storm? I’ve tried everything – pressing the power button 10+ times as some forums have suggested, removing components, etc. It just won’t turn on. When plugged into AC, the battery and plug lights come on, but nothing more.

Well, at least I can still use it for parts. I’ve already swapped one of its 2GB RAM sticks into my mom’s virtually identical T60. And the battery fits my T61p, so now I can carry that along as a spare. I may look into buying a new motherboard, but it’d probably cost me another $80-100. Oh well… so far this is the only blemish on my ThinkPad experience.

Testing Power Consumption

P3 Kill-A-Watt MeterIn order to perform these tests, I’ve used a P3 Kill-A-Watt power meter. This is a super handy gadget for anyone interested in determining how quickly their devices are using electrical energy. It’ll measure up to 15A at 125VAC. In addition to current, voltage, and real power, this device also measures frequency, VA, and power factor.

I should quickly mention that while I trust the Kill-A-Watt’s real power measurements, I’m not sure it accurately computes power factor and VA for devices with non-sinusoidal current draws. And most laptop power supplies draw a very distorted current waveform due to the nature of their input rectifiers and capacitance. Here’s one example I recorded a few months back using the supply for my T61p and a nice hall-effect current probe:

ThinkPad T61p Current and Voltage Waveforms

The Kill-A-Watt claims this signal represents a power factor of 0.59. Although I haven’t crunched the numbers, my guess is that this is a little on the low side. Of course, I’m no expert in power-factor correction, particularly with such odd waveforms…

Well let’s get on to the good stuff. During this testing, I always allowed my laptops to reach a steady-state condition before recording anything. Thus, I took no data during boot-up, program launches, etc. I also ran all tests using a fully-charged battery. The following numbers would of course be somewhat higher if the battery were charging. However, just how much higher they’d be may vary depending on the battery’s state of charge. So in an effort to eliminate some variability, the battery in each laptop was first fully charged. The data I collected was then condensed into one simple graph (click to enlarge):

T43p Power Consumption Graph (Watts)Now you can look at this however you like, but I prefer to read it from left to right. So beginning on the left, in blue, you’ll see that the T43p draws a minimum or “base” power of 16W. As we move right, the numbers you see represent how much additional power is required by each specific component. For example, next in the sequence you’ll see that 1W is consumed to trickle charge the laptop’s battery. I determined this by first running the laptop without any battery connected. I then installing a charged battery pack and recorded the difference in measured power.

Moving right along the above graph, you’ll see that the T43p’s LCD backlight consumes seven watts. Next, I used the laptop’s power management software to set the CPU to its maximum clock speed, which required an additional four watts (and at this point, no additional software was calling for CPU time). Inserting the optical drive drew another watt. Surprisingly, enabling bluetooth consumed six more watts, while 802.11 wireless only called for 2W. Loading the CPU by enabling World Community Grid called for 10W. Next, running the hard drive with HD Tune required three more watts. Finally, I stressed the graphics card using the OpenGL benchmark within CineBench R10. This drew an additional six watts, bringing the T43p’s maximum power consumption to 56W. Not bad.

Next up, I performed the same tests with my main workhorse, the ThinkPad T61p:

T61p Power Consumption Graph (Watts)Interestingly, the T61p consumed slightly less base power than the older T43p – just 15W! Its backlight did draw a bit more power at 9W, but this isn’t terribly surprising since its screen is bigger. What I found particularly impressive was the ability of the CPU to throttle back, even when set to its highest clock rate. The difference between an unloaded and loaded CPU at full clock was 22W. The more modern graphics card of the T61p consumed about twice as much power as that of the T43p – fully 13W, bringing the total to 67W. Still not too bad. I should also note that with the T61p there was no noticeable difference in power consumption (e.g. from trickle charging) with the battery removed.

So there you have it! There’s quite a bit of power to be saved by cutting back your CPU speed (if your laptop allows it) as well as disabling your wireless radios and optical drive. Since I’ll be using my T43p as a server, I’ll also be able to disable itsLCD screen, which will get me very close to a base power of 16W. Not quite as good as an some netbooks, or the SheevaPlug, but still not bad given the processing power it affords me.

Oh and if you’re curious, here are my CineBench R10 results for both machines:

  • T43p Rendering (Single CPU – Pentium M 760 at 2.0GHz): 1694 CB-CPU
  • T43p Shading (OpenGL – 128MB ATI Mobility Fire V3200): 1015 CB-GFX
  • T61p Rendering (Single CPU – Intel T9300 at 2.5 GHz): 3061 CB-CPU
  • T61p Rendering (Multiple CPU – Intel T9300 at 2.5 GHz): 5757 CB-CPU
  • T61p Shading (OpenGL – 256MB NVIDIA Quadro FX 570M): 4212 CB-GFX

Fun with FLIR

A little while back I posted about testing a Nichrome heater using a FLIR thermal camera. While I had the opportunity, I snapped a couple other pictures. First off, my laptop, a Lenovo Thinkpad T61p. As you may have guessed, the top image of the following composite was taken with the FLIR camera. The orange areas indicate high temperatures while the blue areas indicate lower temperatures.

Lenovo T61p Thermal Image

While it may look like flames are shooting out the left side of the laptop, that’s actually just the desk being heated by the CPU fan. And I’m sure you can guess where the processor is located – just under the caps lock. On the lower right part of the hand rest, you can also see the outline of my three fingers in blue. But those aren’t my fingers – what you’re seeing is merely a thermal “shadow” left by a brief touch that slightly cooled the plastic.

You’ll also notice that the screen is warmest right along the bottom edge. I’m guessing that’s where the back-light is located. And in case you’re wondering, the highest temperature in this image is approximately 95F.

Soldering Iron Thermal Image

This second and final image is, you guessed it, a soldering iron. And as you can see, the hottest part of this particular iron (indicated by white) isn’t the tip, but just above the tip. I also noticed that the iron’s temperature controller wasn’t all that accurate. The hottest point in this image is just over 600C, while the iron’s controller claimed 800C. Perhaps I hadn’t given it time to ready a steady-state temperature distribution?