Authors Posts by David Weight

David Weight


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We had the opportunity to visit the Motiv8 Forums and talked to several of the exhibitors there about the latest developments.
In this video we are talking to , a company who have developed new techniques to add electrically insulating, but thermally conductive surfaces onto aluminium to make a surface which performs much better than the existing polymer solutions.

We’ll be publishing interviews with some of the other exhibitors over the next few weeks, looking at test equipment, thermal solutions and new lighting designs!

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Here is a video in which we show you the build process of a new electronics bench, we also go through some of the decisions that were made.

We’d love to hear any comments.

Thanks for watching.

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Observant viewers may have noticed that there haven’t been any videos filmed in Dave’s workshop for a while. After a move a couple of years ago, the previous workshop was lost and I’ve been stuck without one since then.

However, the new house had a back garden. And who needs a large back garden when you don’t have a workshop!

From Dave's workshop build

The first step was to apply for planning permission-approval from the local government to build a structure. After submitting some basic drawings and fees, approval was granted. time to break ground!

From Dave's workshop build

Doesn’t look like much here, but lots of weekend later, basic foundations were dug. They consisted of a trench around the perimeter of the workshop, and a less deep area in the middle. For insulation, the middle section was then partly filled with expanded polystyrene. Where the ground was not level on the outside, wooden shuttering was added to contain the concrete pour.
The reason for having the foundations above ground was that I wanted this building to work as a garage later as well, so I wanted the right hand side to be roughly level with the road and driveway.

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On Workshops

As the mark 4 workshop nears its fitting out period, thoughts on laying out and designing workshops are currently occupying much of my waking thoughts.
This will be my fourth workshop and they’ve got a bit better every time. My first workshop consisted of a shed which was fitted out with a bench and a vise.

From Musings on workshop design
From Musings on workshop design

Apologies for the pictures, these were taken many years ago! As my skills and budget increased, electricity appeared and insulation was added. This was a welcome improvement after trying to work on electronics by tea light! However, at 2.4m’ by 1.8m, space rapidly became an issue. With a paid gap year, workshop mark 2 made it’s appearance!

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So, we have finally finished our entry, a few hours before the deadline!

The competition is about building an eco friendly or renewable based circuit that uses a ChipKIT board, and is run by designspark, Elektor and Circuit Cellar. We built a maximum power tracking circuit, which performs impedance matching to transfer power out of a source efficiently. Basically all energy sources have some resistance as part of their construction (like the internal resistance), and we make sure that our circuit presents a load to the battery which wastes the least power.

Our one is a bit different from the current designs on the market, or indeed, in the research, as we used a discontinuous mode boost converter to give really good control over the circuit, and to use only voltage sensors. These are much cheaper and simpler than the current sensors used in some of the competing designs, and brings the circuit cost right down, to around £50 for our prototype.

One application in particular that would benefit with the addition of this MPPT is the new TEG being developed by BMW in conjunction with NASA which improves fuel efficiency and reduces CO2 emissions by up to 5% and is aiming to eventually replace the need for an alternator as it aims to produce enough electric power to run all the electrical components of the car. The energy and environmental implications of this will be huge, when taking into consideration that if successful, it could be applied to all vehicles (cars, lorries, etc) around the world, which is a massive reduction in oil consumption, and big improvement in energy efficiency as it re-uses the heat escaping from the exhaust in order to provide power to the vehicle. Considering that around 60% of the energy generated by a typical internal combustion engine is lost, half by heat absorbed by the engine cooling system, and the other half lost via exhaust heat, thus by reusing this exhaust heat the efficiency of the engine can be significantly improved.

So, the design. We started off with constructing a generic boost converter in LTSpice, using ideal components like switches instead of FETs. This keeps it simple for the time being, and allows the filter values and switching times to be selected.

From ChipKIT Challenge

And we can see that it is in discontinuous mode by looking at the current through the inductor

From ChipKIT Challenge

And we can see that maximum power tracking is working as the input voltage is half the output voltage,

From ChipKIT Challenge

Now this circuit has to allow for both changing voltages and changing loads to be useful in the real world. To do this, we need to sense the input and output voltage, and change how the switch operates accordingly. We used a ChipKIT to measure these voltages after having scaled them down with potential dividers. This allows for closed loop control, using a really simple bit of code that put the inputs into a 2D matrix, and returned the pre-calculated duty cycle to be fed to the switch! Really quick as well, which means we can respond to fast changing loads as well.

With the circuit looking like it’s working, we then designed a PCB for the main Boost circuit using DesignSpark PCB, a pretty good bit of free software for doing PCB design and schematic entry. It was the first time we used this, but the PCB turned out pretty well. We also needed another PCB for the control and interface circuitry, but we decided to prototype this on breadboard before we committed to a PCB.

From ChipKIT Challenge
From ChipKIT Challenge

Actually measuring the performance of the circuit is always something of a challenge for these kind of circuits. Fortunately, we have the Hameg 4ch scope for most of the measurements, with a Rigol scope when we got short of channels. We also had a differential probe for measuring the floating voltages, and a couple of current probes. Add to that a high current power supply for the input to the circuit, a second power supply for the chipkit and a third power supply for the interface circuitry. And a few multimeters, and temperature probe for monitoring the heatsinks, and a laptop for debugging the code. The bench started to get pretty full!

From ChipKIT Challenge

We’ll get the full results up in our report, but the circuit works as efficiently as competing solutions, but without using expensive current sensors. It also works in the discontinuous mode, which offers very good control of the output and a very suitable source for voltage stabilisation, ready for laptops, lighting etc.

We have our write up with some more pictures and the PCB files, abstract etc. all downloadable at the DesignSpark website, here,

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We have been working on an entry for the DesignSpark Chipkit challenge; there are lots of details here,

Basically the entries are circuits that have to use the ChipKit that we have covered before, and have some form of eco-friendly/power efficiency application. We have been building a maximum power point tracker, used for lots of renewable applications

We filmed this just after we got the circuit working, so a bit of a rough one this time. We are doing the filming for a proper one to follow, and we’ll get a report uploaded in a week or so!

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Another quick post before whilst we get our ChipKit tutorial finished today; we have been designing a circuit which is powered from a thermal battery. These have a fairly slow voltage ramp rate when they are first powered on, and so circuits need to be tested to make sure they work with this slow increase in voltage.
Repeatedly doing this with batteries gets expensive rapidly and we had a capture on the oscilloscope which describes how the voltage increases. We can save this capture as a comma separated values (CSV) file. We can output this CSV file from an arbitary waveform generator, so the obvious next step was to amplify the signal (increase the current) so we could run the circuitry from a power supply.

The easiest way to do this is to use a common collector circuit. This is a single transistor amplifier that can provide more current than the input (current gain), and outputs the same voltage-exactly what we want!

So lets pick some basic components and simulate

We’ll use a triangle wave input (in green), the output is in blue

The output looks basically good, but there is a voltage drop as well as some distortion at the lower end. We can re-run the simulation with a lower voltage input to see this distortion

As you can see, it’s not behaving itself. Let’s plot Vin vs Vout to see what’s happening.

It isn’t working as a (linear) current amplifier below 0.4V. This is because the transistor requires a certain voltage between the base and emitter in order for it to start turning on. For most applications, we can use a common potential divider biasing circuit as shown below.

However, this won’t work in this case as this common biasing circuit is AC coupled, and this amplifier needs to work for signals down to DC. What we can do to solve the distortion and biasing is to use negative feedback. Basically we look at what the actual output is and compare this with what we would like the output to be. The fastest way to implement this is the old favourite, the operational amplifier.

So lets put an op-amp in the circuit. This will handle biasing the transistor, as well as compensating for any non-linearity in the circuit. So the circuit will now look like,

So, let’s simulate with the same triangle wave as we had before (using an ideal op-amp)

And the Vin vs Vout plot

As you can see, the output now follows the input precisely (they are both on there, just overlaid!), and if we try it again with a different load (80ohms), the circuit should still amplify nicely.

Looks like it’s working, lets build it! The op-amp selection isn’t critical for this particular application as it isn’t running at a particularly high speed. The op-amp used was a MC4558CN. This has an 5.5MHz gain bandwidth product so this shall be more than fast enough for the application. It will be running off a single supply, but we don’t need to have an output near the power supply rails (0V & 8V), so we don’t need to use a rail to rail output amplifier.

Selecting the output transistor is also not critical. We are looking for an NPN transistor with a reasonable current gain. Also, as we are using this as a linear amplifier (class B amplifier, or one half of a push-pull output stage), we are expecting the transistor to dissapate a fairly large amount of power. A safe choice, and more importantly, one that was in stock, was the classic 2N3055 transistor. This has a transistion frequency of 3MHz, so we know at this point that our current gain will drop to 1. However, we are operating far below this frequency. Importantly, as this is in a TO-3 case, we can dissipate up to 115W of power into this transistor, assuming it is properly heatsinked.

A quick build on veroboard later,

The performance is pretty good. I put in a sin wave from the function generator and the output tracked the input really well.

It was tried with a 500 ohm load and a 4 ohm power resistor-performance was identical. But what about frequency performance?

If we take the freqency up to about 220kHz, then we start to get some distortion at the output.

Looking at a lissajous curve, we get

Below this frequency it works pretty nicely. For the last test, we have a look at the response to a step change in load. In this case I went from a 1k load to a 22R load and scoped the result. We could improve this with some capacitive terms in the feedback, but not too bad for a 20minute lash up job!

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Hi Guys,

Just a quick post today (we are working hard on the next video, honest!). We’ve had loads of questions about the bad user interface on the Hameg that I referred to in the video review, so some more detail coming up.

First things first-the user interface shouldn’t be a deal breaker for anyone thinking of buying one.
When I first use a scope, basically every time I can figure out how to use 99% of the features without going down the wrong menu or having to just try everything, step by step. However, with the Hameg, I still find myself with the (supplied printed) manual on my bench, so I can look up how to set up the advanced options like serial decode and the maths menu.
This is probably a function of a couple of points. For a start, the Hameg has a lot of buttons compared to similar scopes. On the one hand, this means you have to go through less menus when you’re trying to use a feature. On the other hand, it can be awkward trying to remember where your option is!

This is probably worse on this scope than on some others just due to the sheer volume of options and controls that are included, and that is fantastic.

The one other small point I will make is that on some of the menu options they are changed either by pressing the button a few times, or by rotating the menu wheel. These two controls are distinguished by a small mark on one corner of the box surronding the option, and this is pretty rubbish.

Overall though, using the basic features is intuitive, and the learning curve for the more advanced features is a worthwhile tradeoff for the speed of use-so still highly recommended.

Hope this helps!