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Introduction

The inspiration for this project came from a reader who was asking for a way of lighting his glow plug that did not involve the use of something that could double as a hand warm. As a few months before this I had been looking at an article published in RCM&E in Dec 1977, and titled 'GLO-CONTROL', I decided that it was time to revisit this subject and come up with a more modern design. The original 1977 design was based around a linear voltage regulator, this meant that if the unit was powered from 12 volts and was driving a 2 volt plug at 3.5 amps, then the power the plug was taking was 2 volts x 3.5 amps = 7 watts, and the power that the regulators was having to waste was 12 - 2 volts x 3.5 amps = 35 watts. If used to drive a 1.5 volt plug then things were even worst, with the regulator dissipating 36.75 watts, which gives an efficiency of only 20%. Hence the comment that you could use this type of plug driver as a hand warmer.

It was decided to use one of the modern switching regulator IC's (LM2576) that are available these days, these work like a speed controller by switching the output on and off very fast (52Khz) and smoothing the output to produce the desired DC voltage. The advantage of this is that the regulator dissipates very little power and hence makes the whole thing more efficient.

With the unit powering a 2 volt plug at 3.5 amps, the regulator is only consuming about 1.2 amps from the 12 volt supply, which is 14.4 watts. The regulator IC only dissipates a fixed 1.75 watts, and so with the rest of the looses in the circuit the whole device is working at an efficiency of about 60%, compared to the 20% of the earlier linear regulator.

Circuit Operation

The LM2576 (IC1) that is used in this design has the advantages of being very robust, in that it has it own current limit which will protect it if the output is short circuited, and it needs very few external components to make it function, also it is an adjustable version which we can set the operating output voltage of by selecting the values of two resistors R1 and R2. The diode D1 is used to protect IC1 from being connected across the 12 volt supply the wrong way round. C1 provides input decoupling, D2, L1 and C2 smooth the output of IC1 to provide the regulated DC output. D2 should be a fast recovery diode such as a Schottky or Ultra fast recovery silicon diode like a UF5401, these types of diode are needed to obtain the high levels of efficiency, and a normal silicon diode is not suitable.

Construction

The unit is built on veroboard as per the layout drawing, the only points to watch here are that you break the copper track at the point shown, and that you beef up some of the tracks with a bead of solder, as they are carrying more current then the track is rated for. Also IC1 should be mounted on some form of heatsink, the prototype had a piece of aluminium 2 x 58 x 21mm.

As there are so few components there is very little to be said about the construction, and the only components that you need to watch the orientation of are the diodes and capacitors, the positive end of the diode D1 is indicated by a band around the body, and the electrolytic capacitors have their negative lead marked on the body.

The diode D2 has leads slightly bigger than the 1mm holes in the veroboard, so the 2 holes for this component will need to be opened out to about 1.2mm. The inductor L1 should be stuck onto the veroboard with some double sided servo mounting tape or sellotape.

If you wish to use the unit with 2 volt glow plugs only, then you may omit R1a and the switch SW1, and only fit R1b and R2. If you wish to use it for 1.5 volts plugs only then omit switch SW1 but fit R1a in a different position directly across R1b, and keeping R2 in the same position as before.

The prototype was built into an ABS box measuring 112 x 62 x 32 mm, but could be fitted in any suitable case. You may wish to build it into your flight box, in which case it can be mounted on the back of the power panel of your flight box, with the panel acting as the heatsink for IC1. The 5 amp ammeter shown is not necessary and can be left out if you want to save on the cost, but it gives you a good indication that the glow plug is taking current and has not blown, and can also indicate if the engine is flooded.

And Finally

I do hope that any of you winter flyers don't suffer from cold hands by building this more up to date and GREEN glow plug controller, if so you could always resort to warming your hands on over the engine.

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