A Draught Proposal

Model Engineer|4635

A Draught Proposal
A Draught Proposal
Peter Kenington

A step up (or down) Having been caught out by the availability of both 12V and 24V supplies on the same steaming bay and the same connectors, depending upon who was last (or is currently) using the club blower supply, I was determined to use my electronics background to come up with a ‘universal’ solution to this problem. The system was to have the following specification:

1. An ability to cope with a 12V or 24V DC supply without having to know, in advance, which was available or in use.

2. A pre-settable output voltage, which did not change whichever input voltage was being used (eliminating the need to carry around a multimeter to measure either or both of the supply and output voltages).

3. A ‘nice to have’ would be the ability to vary the speed of the blower, to allow it to be adjusted as the fire progressed from the initial lighting of charcoal or wood, through to a fully ignited coal-bed.

Fortunately, modern switched-mode regulators have the ability to meet all three requirements and, even more usefully, are available as ready-built circuit modules for only a few pounds. These are typically in the form of DC-DC converter modules, which provide exactly the functionality we require.

The converter board I chose was based around the XL6009 buck-boost converter chip from XLSEMI (photos 9 and 10). This chip is widely used in DC-DC converters for a range of applications and it is important to choose a board which has both ‘buck’ and ‘boost’ capabilities, since this device can be used for either, individually, or both together and different commercial circuit cards covering all three options are widely available. So - what do these bizarre terms mean?

Firstly, it is worth spending a minute or two discussing DC-DC converters in general, since many model engineers probably won’t have encountered them, other than in the form of pre-packaged phone chargers for use in the car, and the like.

DC-DC converters are seemingly magical devices which can turn a wide range of DC input voltages into an equally wide range of DC output voltages with very high levels of efficiency (typically 90%+). As a result, they don’t tend to get hot (or even warm) in use, at least at the current levels we are concerned with in a steam-raising blower application. This, in turn, means that we can safely house them in a plastic box, without a heatsink – more on this later. Unlike traditional mains transformers – those heavy things full of copper and iron which used to form the basis of power supplies for electronic equipment prior to about the mid-1980s – they are small and very cheap to manufacture. They achieve this by replacing the traditional mains transformer, which operated with a low AC (alternating current) frequency (50Hz in the UK and 60Hz in the US), with a very high AC frequency (400kHz in the case of the XL6009). This, in turn, allows very much smaller transformers (and also capacitors) to be used and these can be made more efficient by using exotic ferrite materials in place of the iron laminations of a traditional transformer.

Ah, but I hear you say, we are starting here with a low DC voltage and not 240V AC mains. This is indeed the case, but the device ‘chops’ the incoming DC current (by effectively turning it on and off very quickly, 400,000 times per second) to create a high-frequency squarewave which is, by definition, now AC. This can then be supplied to the small transformer, discussed above, thereby isolating the output from the input (one of the main functions of a transformer) and allowing the voltages present on each to be different. The above is a somewhat simplistic explanation of what is, internally, quite a complex device, but hopefully it helps to lift a veil on the black-magic contained within these devices.

Returning now to our strange terms: ‘buck’ and ‘boost’ and accepting, as we now hopefully do, that the input and output voltages are isolated from one another, then it follows that these voltages can have any relationship to one another. In other words, the output voltage can be higher than the input voltage, or lower, or exactly the same. There are clearly limits on the absolute voltages involved, but these are impressively wide, as we shall see.

The terms ‘buck’ and ‘boost’ simply refer to which way around these voltage are permitted to be; in a ‘buck’ converter the output voltage must be lower than the input voltage (typically by around 1.5V or so). In a ‘boost’ converter (and I’m sure you’re way ahead of me by now…), the output voltage must be higher than the input voltage. Finally (and I’m in great danger of being pedantic here), a ‘buckboost’ converter combines both options, allowing the output voltage to range continuously from rather lower than the input voltage to rather higher, with the only limits typically being the absolute range of each. The version I used covers an input voltage range of 3.8V - 32V and an output voltage range of 1.25V - 35V.

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