From: Iskandar Taib <ntaib@silver.ucs.indiana.edu>
Subject: Re: Controlling Glow Plugs
Organization: Indiana University
Distribution: na
Date: Tue, 14 Apr 1992 19:30:42 -0500

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?


Yes it does, and yes they did. The old DAE power panels worked like
this. The problem with a circuit of this type is that it is very 
inefficient. You are dissipating far more energy in the resistor than 
in the glowplug. So your heavy 12 volt battery needs charging often. 

There is another consideration, so I've heard. Apparently as the glow-
plug increases in temperature the resistance of the plug increases, 
while the resistance across the resistor does not. Since the resistor
basically controls the current (the glowplug's resistance is a small
fraction of the resistor's) the current doesn't decrease very much.
Yet, the voltage drop across the glowplug will increase a large per-
centage of its original value. So at least in some situations this 
can lead to plug burnout.

A better (if as inefficient) circuit was published in Model Builder
or Model Aviation recently. It basically uses a voltage divider ar-
ray to feed the base of a NPN power transistor set up in a collec-
tor follower configuration. This keeps voltage constant across the 
plug.

Does anyone have a copy of the 555-based glow driver article? I have 
played with 555's and can't see how they arrange for the feedback to 
occur. Can someone snail mail me a copy? (We'll set this up over E-
mail first). I'll bet the cheap IRF-511 MOSFETs they have at Radio 
Shack would work great for the final stage....



-- 
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Iskandar Taib                          | The only thing worse than Peach ala
Internet: NTAIB@SILVER.UCS.INDIANA.EDU |    Frog is Frog ala Peach
Bitnet:   NTAIB@IUBACS                 !
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