I'd like to design my own glow driver and am looking for information on
glow plugs. My understanding is typical resistance is about .3 ohms,
and that when they flood they can draw upwards of 5 amps or higher to
clear them. Depending on plugs the resistance varies some. The Pro
South driver uses a pwm scheme to drive the plug from four cells. I
assume the resistance of the plug changes as it starts to flood. What
are the using to determine the change to the duty cycle of the pwm, the
resistance of the plug?
Interesting project. I guess you want to attack this because you want the
satisfaction of doing it yourself, or you see some shortcomings with the
existing off-the-shelf designs.
I played with the idea in the 60's before the commercial control units were
available. ICs were just starting to be applied to reduce the size of some
"hybrid circuits". The "power" transistors were expensive and stained my
project budget. Rather than prattle along about design details, I'll tell
you about my initial experimental set-up.
As a first step, I mounted the same brand and number plug that I intended to
use in an engine head that I had removed from the engine cylindar. The head
was set into a small jig so that it was inverted. The jig was mounted on a
base large enough to hold the bread-board circuit and the battery pack. The
battery pack was set so that I could select the number of cells to be used
to "step" the approximate input voltage between 1.5 (single carbon zinc
cell) and 6.0V (4 cells). Today, with advanced chemisty cells, the voltage
will be slightly different.
Because I was flying during the winter "up north" at the time, I had the
entire assembly mounted so that I could slip it into my freezer. That way I
could cold-soak the entire head, plug, and cells. This was done only
because of the design specification for starting a cold engine at "0" F. I
wound resistance wire around the head (salvaged from a hot-plate element) so
that I could raise the head temperature to different ambient temperatures
and to quickly jack it up to simulate (badly) the head getting hot with
I went with descrete electronic components because ICs at the time were not
available to the casual experimenter. I used a mechanical switch to sense
the throttle servo position. One circuit design required me to wind a
current "choke" to limit in-rush current. I told you this was back in the
"Stone age". However, the basic test jig would work just as well today.
I used analog voltage meters to monitor the voltage drop across the plug
right at the plug, the output voltage at the board, the input voltage to the
board (battery voltage) and current meters to measure the plug current and
the input current (draw from the cells) to the board. This permitted me to
have real-time indications of both I & E. The initial cold resistance of
the plug circuit (from the selected "ground" point on the engine through the
plug connector and element) was measured using an ohm meter.
The inverted position of the head permitted me to use an atomizer and small
glass dropper to "wet" the plug with fuel. As a side note - putting the
fuel in a tightly sealed jar in the freezer sure showed up any extra water.
That taught me not to open the fuel can in the house after I had it outside
during the winter. The old metal cans were great vater vapor condensation
The rig permitted me to develop the initial current curve for the plug and
the current required to keep the voltage drop across the plug at a safe
level for the element once it came up to temperature. I would have liked to
measure the element temperature, but IR temperature sensors were available
only in high-end labs back then and the cost of the instrument and the
liquid nitrogen were well out of my budget. I ended up constructing a "dark
box" around the assembly with a small magnifier mounted directly over the
head and glow plug so that I could see the element's color.
I conducted the experiments at initial head temperatures between 0 F. and
250 F. (high temps to simulates "hot" starts") with the glow element "dry"
and with various levels of fuel wetting from misting it lightly (my wife
probably still wonders what happened to her expensive cologne atomizer) to
being completely flooded. The circuit and battery environmental
temperatures used ranged from 0 F. to 100 F.
Once I had the glow circuit designed and fabricated, I repeated the
experiments to tune the circuit performance and determine the crude point at
which it could be turned "off" by the throttle servo arm. Final tuning was
conducted by mounting the assembled engine and circuit on an engine test
stand. My wife put her foot down about my running the engine in her kitchen
and freezer - so that part had to wait until winter was cold enough.
The end result was a circuit assembly and three cells that just barely fit
into the cabin of a big Robin-Hood like design. I originally had a circuit
design that was "on" all of the time and would monitor the glow plug current
at a very reduced voltage. If the current increased beyond a pre-set value,
it indicated that the plug had cooled (down-side of maneuver flooding at
chopped throttle). It would switch "Heat" regardless of the position of the
servo arm switch. I had some problems with the phyical size of the assembly
and eliminated this feature for the final flying version.
The one thing that I added that really increased the advantage of the
device was an external power jack and a "pre-set" switch that permitted me
to power the circuit from a "ground power unit" (Three huge 1.5V surplus
cells in series to form a 4.5 V battery). This added the advantage of
having a large Amp/hour source for pre-heating using a lower controlled
current to the glow plug. If I put an insulated oven mit over the engine
and let the plug run at a low current for 15 minutes for pre-heat, the
engine would start instantly on even the coldest days. Try flying at 18
degrees below zero in a breeze while wearing artic gloves and a parka closed
over your face so that only your eyes are exposed. I had to keep the fuel
in a small "COX" fuel can inside the parka to keep it warm. And, I had to
insulate the GPU battery so that it would not cool too quickly once it was
taken out of a warm car.
Just for reference and part of an "old fart story", the initial engine used
was a control line engine fitted with an exhaust restrictor type throttle.
It didn't work worth a hoot. I lived hundreds of miles from a decent hobby
shop. An airline pilot friend who was flying the California to Tokyo route
took pity on me and started to bring me back engines with a real RC carbs.
This was before any of the local hobby shops could get Enya and the exchange
rate was 360 Yen to the Dollar! He also brought me my first "Futaba
proportional" radio. I was hesitant to switch to "Japanees Junk" from my
Kraft units. If I had known then what I know now---- I still have the last
engine he brought me - a early 1960's Enya "60 RC"
This is also off-topic, but might be interesting. Circumstances put me in
Japan in 1969 and I visited a RC club flying from recovered land near
Yokohama. It was definately a "rich man's hobby" in Japan back then. This
club flew only WWII scale of both Japaneese and U.S. Pacific Theater
aircraft. There was a small salt-water "bay" on the land and they even had
scale ships cruising around. One of the battle ships was bigger than the
Chris-Craft "recovery" boat they used. That was the first time I had ever
seen a RC "sub". I was astounded at the quality of the models. Many of
these guys hired whole teams of craftsmen to build and paint their ships and
aircraft. Most of the guys were high-level management with the Japanese
industries. Again, if I had known then --- and had been older and wiser -
I sure would have bought some Pacific Rim stocks.
Anyway, have fun with your project.
I built a simple glow plug driver that I used for many years. It
consisted of an LM317 voltage regulator operating in a constant
current mode. If I remember correctly, I set it up to output 3 amps.
The power source was the same 12volt battery used for my electric
There are many different versions of the LM317 voltage regulator.
Many of them will only handle 1.5amps. You need to get one that is
packaged in a "TO-3" case that will handle 5 amps.