Building an ATX power supply

Using ATX PSU as basis for a lab power supply is an easy way to get started when building a lab power supply. It solves all the hard parts about wiring the transformer and the voltage regulation stuff, and basically leaves only the wiring. Especially when one needs some high-current PSU, this a nice way to get a simple solution. So I had this on my ‘projects to do’ list for a while (and even started to get some components). So when I stumbled over a nice project by an italian electronics hobbyist I was intrigued. Francesco Truzzi did a great job not only on creating a nice breakout PCB, but also on adding some additional features. So when he offered to send his surplus PCBs to readers of his blog I took my chance.

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Review - Tektronix TBS1052B-EDO oscilloscope

As part of the Forget me not challenge in the Element14 community, I got an oscilloscope from Farnell to be used as part of the design challenge. Due to a communication mishap it was advertised as being the 200 MHz version, but in the end all participants got a TBS1052B-EDU. Now that I had some time to use it and get some hands-on experience with it, its about time to write a review.

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wireless power - status update 2

So what happened in the last two weeks? As already mentioned, I finalized the PCB and send it away. I did go with Würth in the end. They were not the cheapest of the lot, but provided the most bang for the buck. I got 5 day manufacturing time (and since they deliver with courier, probably even from Germany, delivery also takes only one day). Also, they did not charge extra for silkscreen or solder resist (as may happen with cheaper manufacturers). And last but not least I got Ni/Au plating for still the same price, which is not only better for long-term usage, but also better than HASL for SMD soldering. So I send the board files off to WEdirekt (no need to convert them to Gerbers), and thanks to Simon Leuz I even got a 10 percent rebate

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Wireless Power - the full schematic explained

Since the contest is now nearing its completion, I gather its about time to actually show and explain the whole circuit. I already went through some of the parts of it, and how I decided how to build it. But I never showed how these parts work together. So lets have a look.

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wireless power - status update

So the last couple of weeks was silent with regards to my project - I was just too busy. Being on a business trip left not so much time for doing electronic projects (and I forgot to take the Eagle project with me). And the aftermath was then rather busy at work.

But I got new transmitter coils from Würth in the mean time, and did some experiments with them (which need a write-up).

Apart from that I have finalized my schematic, and added all the missing details I already explained the most important parts of it, but there are still some minor tweaks here and there. So there will be a separate post about it.

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Building a multi-voltage reference for my DMMs

A while ago I saw in the Dangerousprototypes forums a nice project: MrRef. Its a small voltage reference, intended to verify that a digital multimeter is still within its spec. Normally one would use either a real high-precision meter (like the Agilent 3458A) or a really good, calibrated voltage reference. But these are not exactly cheap, and probably overkill for a 50$ (or cheaper) multimeter. So matseng took a not so expensive voltage reference (a MAX6350, but it onto small breakout board, together with 4 batteries, and had something which is at least more accurate than the normal hobbyists multimeter.

Seeing this I did something similar - but I used a REF5020 from TI instead:

The REF5020 voltage reference

but it was not as accurate as I would have liked (the initial accuracy is 0.1%), and the 2.048 Vv output voltage meant that its just over the range for a 2000 counts multimeter (so it read 2.05).

So when I bought a new multimeter last fall (two UT61E actually), I looked into building a new, more accurate reference, with a wider range of voltages.

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wireless power - battery under-voltage lockout with multiple cells

When designing systems powered by rechargeable batteries, especially LiIon batteries, its important to ensure that the batteries are not discharged too much. For primary cells this is not a problem (except that the device won’t work anymore), but secondary cells can be permanently damaged by such a deep discharge. normally this just results in permanently reduced capacity (which is bad enough). But a LiIon battery that has been discharged too deep can actually catch fire (that happens when its charged again, since during discharge internal shorts can form inside the battery). So an under-voltage safety mechanism has to be used to ensure that never happens. Basically all single cell LiIon batteries already come with such a circuit (most times inside the case already). Many battery packs too come with it, but especially pack for RC models do not, since the high current there make that difficult. A typical cut-off voltage is about 2.5 V, sometimes as low as 2.3 V. That is a voltage where the capacity might be permanently reduced already. So I would like to have the cut-off voltage higher, at about 2.9-3.0 V. That means building my own under-voltage-lockout (or UVLO)

Such a safety mechanism gets a little bit more complicated in the case of a battery pack, since there the cut-off needs to happen when just one cell gets discharged below this voltage. So lets look how we might implement that.

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