From: mnelson@cbnewsl.cb.att.com (michael.c.nelson)
Newsgroups: rec.models.rc
Subject: Re: Controlling Glow Plugs
Summary: resistors
Date: 14 Apr 92 14:54:07 GMT
Article-I.D.: cbnewsl.1992Apr14.145407.27766
Organization: AT&T Bell Laboratories

In article <kduXYwC00WBK09fWFs@andrew.cmu.edu>, Michael Oliver Muller <mm7q+@andrew.cmu.edu> writes:
> From previous discussion, a glow plug has a resistance of about 0.5
> ohms, after it gets hot. 
> Would the following circuit work?
> 
> 
>                _____________/\/\/\/\_____________....________
>               |                3.5 Ohms                      |  
>               |                                              \
>            ------  12V Battery                               /   Glow plug
>              --                                              \   0.5 Ohms
>               |                                              /
>               |                                              |
>               |__________________________________....._______|
> 
>         Ohm's law: V = RI
>         Assuming a 3 Amp current, and that the glow plug heats up 
>         instantaniously,
>                    12 = 3.5(3) + 0.5(3)
>                    12 = 10.5 + 1.5
>                    12 = 12
> 
> Why doesn't a simple circuit like this work? There is a three amp
> current through the glow plug, and a 1.5V drop across it. It makes
> sense, yet no one markets this type of circuit. How about replacing the
> 3.5 Ohm resistor with an adjustable resistor to allow for variations in
> different glow plugs? (Do adjustable resistors exist?)
> 
> Any comments?
>     Mike Muller
>     mm7q@andrew.cmu.edu
> 

This kind of circuit works fine.  We would get three 10 ohm 10
watt and a 1 ohm 10 watt resistors from Radio Shack (Tandy
stores in Europe).  They are ceramic resistors about 1cm 
square by 5cm long.  Using silicone glue we'd glue them in a 
block.  Solder the 10 ohm resistors in parallel and then the 
1 ohm in series.


             10 ohm resistors
               (3.3 ohms)
           _____/\/\/\/\_____     1 ohm
       o__|_____/\/\/\/\_____|___/\/\/\/\_______o___
          |_____/\/\/\/\_____|                      |
					            \
					            /  Plug
  12v   				            \
					            /
					            \
       o________________________________________o___|

We found that 3.3 ohms was too hot during the summer.  Adding 
the 1 ohm resistor still gave us good starts.  During the
winter we'd add a shunt wire around the 1 ohm resistor to get
those cold blooded things to start.  Four 1 ohm 10 watt
resistors in series might be good compromise for more moderate
climates.  Add enough lamp cord and aligator clips to complete
the circuit.  

This circuit has some good points.  1) As the plug cools, its 
resistance begins to drop.  This reduces the total circuit 
resistance so the current goes up.  This helps clear the plug 
faster.  2) It is cheap!!!!  A couple dollars at RS and you're in 
business.  3) It won't fry itself if the leads get shorted for
a few seconds.  Some of the power panels will toast themselves
if shorted. 

Now the problems.  1) Power hog!!!!  The resistors are wasting
30+ watts of power.  By contrast your plug is using ~4.5 watts.  
We used a car battery so it didn't matter.  2) It gets hot!!!.
Engineering guide lines say we should be using 15 - 20 watt
resistors.  If you use it for more than a minute or so you'll
fry your hand on the resistors.  

Many of the power panels use a FET or transistor to pulse the
power to the plug.  The plug sees the full 12v but only for a
short period of time.  The average power to the plug is
controled by the period of the square wave.  These circuits
are alot more efficient but as mentioned above, will burn up
if the leads are acidentally shorted (all 12v across a FET of
.01 ohms.  Ouch!).  

My favorite is the McDaniel Ni-Starter (or Ni-Lite?  I
forget).  It's basically a 1.2V, 1200mAh nicad with a socket
connector for the glow plug.  Several other companies make 
them.  The 1.5V plugs get plenty hot and there are no cords 
strung everywhere to catch in the prop.  I've used the same 
sized batteries to cure some idle problems on an OS .90 four 
stroke.  I had a micro switch on the throttle servo that 
kicked in at 1/3 throttle.  It was fun to watch people scratch
their heads when I started the airplane with out hooking up a
battery.  

Mike Nelson
AT&T Bell Labs
Naperville, IL