Glo-Control MkII

By Ken Hewitt

Originally published in the July 1995 issue of
Radio Control Models and Electronics


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 D1 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. 


Parts List

Cct Ref.        Description
C1              100uF 25v
C2              470uF 16v

D1              1N5401
D2              MBR1045

IC1             LM2576T-ADJ

R1a             470R 1%
R1b             680R 1%
R2              1k2    1%

L1              100uH 5.4A

M1              0-5 Amp

Veroboard        20 x 25 Holes

SW1             SPST Switch


Author

Ken Hewitt,
21 The Brambles
Welwyn
Herts, AL6 0PG
England

or
C/O RCM&E,Nexus House, Broundary Way, Hemel
Hempstead, Herts. HP2 7ST.  


