12V Glow Plug Converter


Most small
internal-combustion engines commonly used in the model-building world
use glow plugs for starting. Unfortunately, glow plugs have an operating
voltage of 1.5 V, while fuel pumps, starter motors, chargers and the
like generally run on 12 V. This means that a separate battery is always
needed to power the glow plug. The standard solution is to use an
additional 2-V lead storage battery, with a power diode in series to
reduce the voltage by approximately 0.5 V. However, this has the
annoying consequence that more than 30 percent of the energy is
dissipated in the diode. Naturally, this is far from being efficient.
The converter presented here allows glow plugs to be powered from the
12-V storage battery that is usually used for fuelling, charging,
starting and so on.

12V Glow Plug Converter

12V Glow Plug Converter

A car battery can also be used as a power source. Furthermore, this
circuit is considerably more efficient than the approach of using a 2-V
battery with a series power diode. The heart of the DC/DC converter is
IC1, a MAX 1627. The converter works according
to the well-known step-down principle, using a coil and an electrolytic
capacitor. Here the switching stage is not integrated into the IC, so
we are free to select a FET according to the desired current level. In this case, we have selected a 2SJ349 (T1), but any other type of logic-level FET with a low value of RDSon would also be satisfactory. Of course, the FET
must be able to handle the required high currents. Diode D1 is a fast
Schottky diode, which must be rated to handle the charging currents for
C2 and C3. This diode must also be a fairly hefty type. The internal
resistances of coil L1 and capacitors C2 and C3 must be as low as
possible. This ensures efficient conversion and prevents the components
from becoming too warm. The resistor network R2/R3 causes 87 percent of
the output voltage to be applied to the FB pin of IC1.

This means that an output voltage of 1.5 V will cause a voltage of
approximately 1.3 V to be present at the FB pin. The IC always tries to
drive the switching stage such that it ‘sees’ a voltage of 1.3 V on the
FB input. If desired, a different output voltage can be provided by
modifying the values of R2 and R3. When assembling the circuit, ensure
that C5 and C1 are placed as close as possible to IC1, and use
sufficiently heavy wiring between the 12-V input and the 1-5-V output,
since large currents flow in this part of the circuit. A glow plug can
easily draw around 5 A, and the charging current flowing through the
coil and into C2 and C3 is a lot higher than this!


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