An Eccentric Anomaly: Electronics

Ed Davies's Blog


Pages tagged: Electronics

Electronics, electrics and where the magic smoke should/does go

Mike Phillips PV Water Heater

Previously I wrote about the problems with connecting PV panels directly to an immersion heater in which I suggested using multiple immersion elements switched by relays to achieve an approximate match to the PV output as the sunlight varies.

A year ago I had some email correspondence with Mike Phillips (mike@mjrphillips.co.uk) about designing such a system for use in Spain. Pleasingly, he's just reported that he's only gone and done it and has had a system working for a while.

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Resistance is Stainless

A couple of weeks ago Paul at the End Of The Road was wiring up some little bus bars using aluminium and M5 stainless bolts. I commented:

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LiFePO₄ Efficiency

Having just carefully charged some LiFePO₄ cells and also just read a thread on the Green Building Forum about battery efficiency I decided to do a little science and measure the efficiency of these cells.

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Charging LiFePO₄ Cells with a Maplin N27GG Bench Powersupply

Introduction

As previously mentioned, I have 10 of these 20 Ah LiFePO₄ cells. They've been in storage for a year and I was getting a bit concerned that their self-discharge might be reaching the point where they'd damage themselves by going under their minimum voltage so I fetched them and gave some thought as to how to charge them.

When I was playing with them before I used the 12 volt gel cell charger which I previously used for glider batteries, basically as a constant current source. This is not very satisfactory as the charger gets quite hot in the near-shorted state charging a single 3.3 volt cell and you have to watch things very carefully to make sure you don't overcharge the cell.

That was for the initial conditioning charge (when you first get these cells you need to charge them to 4 volts first time, then charge to some value a bit less than this subsequently) and for additional playing. Most of the time I was using them on my little solar panels with a Morningstar TriStar MPPT controller.

Inspired by Outtasight I got a Maplin N27GG bench power supply from Maplin in Blackpool while down that way on a family visit. It's in some ways quite cute but in others quite horrible. The first one I had blew up, moderately spectacularly.

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Which is more scary: volts or amps?

It both amuses and scares me how people are frightened of high voltages but often seem amazingly relaxed about low-voltage high-current situations.

High voltages are relatively easy to deal with. Wrap them up so people can't contact them with bodily parts and put an over-current protection device (fuse or circuit breaker) in the way so they're not likely to start a fire. For extra points, put an RCD in the circuit to defeat those more intent than normal on a Darwin award and even an AFCI to stop relatively small sparks causing fires.

High current circuits, particularly DC ones, are intrinsically harder to make safe even at low voltages as any poor connection, with higher than nominal resistance, can dissipate significant amounts of power as heat and so, maybe, cause a fire. Over-current protection doesn't help as normal operating currents can cause such problems.

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PV to Immersion Heater Problems

In my previous Photovoltaics for Domestic Hot Water post I said I'd write more on the use, caveats and interpretation of that calculator later. A recent thread on the Navitron forum triggers me to write about some of this.

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Crossover Cables and Red Tape

The other day Akkana Peck wrote about using a crossover cable to her Raspberry Pi. All good stuff but there are a couple of points worth adding.

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To Power Point Track Or Not

As mentioned in a previous post I have a few small PV panels out in the garden feeding an old battery via a Morningstar TriStar MPPT 60 charge controller. With the experience of playing with this controller I'm having some second thoughts about how worthwhile MPPT still is as PV panels get cheaper.

Hearing of two of these failing in a short time also helps to undermine my resolve. [Update 2013-07-24: …and another one bites the dust.]

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Off-Grid Batteries

The other day somebody asked on the Navitron forum about batteries for off-grid use specifically mentioning the use of lithium-ion batteries. My reply encapsulates the beginnings of my thoughts on the subject well enough and, unlike most forum posts, stands alone well enough that I thought it was worth re-posting here with some light editing.

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