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'voltage sensing'
1999\07\28@165057 by Louis

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face
I'm not that good in electronics, so i just ask a simple question here.
I have made a device to operate glow-plugs of model-airplanes.
Basically, it is just PWM-generation.
Well, the problem is, that i cant make the PIC "see" if the glow-plug is
actually connected.
It seems to be the old "current-sensing" problem, but i dont want to put
any extra components into this.
This is the circuit so far:

---     12V
P I      I
I I      ---->1
C I      ---->2  to glow-plug
 I      I
 I      I
 I      I
 I     D
 I----G   N-Channel Enhancement-Mode FET, Logic Level
 I     S
 I      I
---      I
       GND


The resistance of the glow plug is around 2-3 ohms.
Any suggestions ?


-------------------------------------------
It's better to light a candle
than to complain about darkness.
                              Confucius

1999\07\28@170407 by Andy Kunz

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>I'm not that good in electronics, so i just ask a simple question here.
>I have made a device to operate glow-plugs of model-airplanes.
>Basically, it is just PWM-generation.
>Well, the problem is, that i cant make the PIC "see" if the glow-plug is
>actually connected.
>It seems to be the old "current-sensing" problem, but i dont want to put
>any extra components into this.

Louis,

If you get the right design, you can actually see how hot the plug is and
regulate the temperature.  I know of guys who have done this.  Much more
reliable than basing the output voltage on the throttle position (you know,
turn on glow plug at 20% throttle, etc.)

Andy

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1999\07\28@184518 by Barry King

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> It seems to be the old "current-sensing" problem
Yes, as discussed here recently...

> but i dont want to put any extra components into this.
Is component count really that much of an issue?  If it is, there are
MOSFETs with extra drain and source terminals which can be used to
sense the channel current.  But you still have to measure this
proportional sense current.  That takes a little analog circuitry.


> The resistance of the glow plug is around 2-3 ohms.

Cold or Hot?

And do you need to sense the plug resistance?  Are you trying to
regulate the glow-plug temp so the engine will idle?

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
http://www.nrgsystems.com

1999\07\28@215248 by Harold Hallikainen

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On Wed, 28 Jul 1999 22:39:43 +0200 Louis <LouisspamKILLspamSNAFU.DE> writes:
{Quote hidden}

       How about a 10 K resistor across the FET, then a 47K or so
resistor from the drain of the FET to a PIC input.  With the FET off, the
drain should go high.  If the glow plug is absent, the 10K will pull it
low.

Harold


Harold Hallikainen
.....haroldKILLspamspam.....hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1999\07\29@152349 by Louis

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>Hi,
>
>I think it is very simple: put a 12-to-5 voltage divisor between Drain and
>Gnd. If you drive the FET, and the voltage decreases, then ghe glow-plug
>is o.k.
>
>I hope this helps:
>
>Imre

Yes, you are right, it IS just that easy. The simplest solution is often
the best. I was thinking too complicated.

Many thanks !

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I have seen the truth and it makes no sense.

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