Re: Measuring glowplug under voltage
- From: bill.sloman@xxxxxxxx
- Date: 16 Jan 2007 14:52:12 -0800
Rodge wrote:
Hi,
I have a broken Webasto heater and i would like to understand a bit
more about their measuring circuit.
To ignite the heater they use a combined glowplug/flame sensor which is
connected to 12v DC. After x seconds the fuel injects and the heater
starts burning. The voltage to the glow plug is cut and the glow plug
acts now as a flame sensor by measuring the temperature.
If you measure the plug at 20°C it give abt 0.6 ohm. The working value
is between 0.6 and 2 ohm.
Now during the heating of the plug, the unit even measures his
resistance and my question is how they can do this? Put AC through and
make a devider with a capaciter which has the same impedance as the
plug at the AC freqency and measure the voltage over the capaciter?
Put a square DC to the plug and measure resistance every time the
voltage is 0, or are their better ways of doing this?
Search on "Wheatstone bridge". You'd measure the current through the
glow-plug by measuring the drop across a second - almost certainly
smaller - resistor, perhaps around the 0.1 ohm level or lower. You want
to use Kelvin four-terminal connections on both resistors, because lead
resistances can be embarassing large at these resistance levels.
Farnell stock a bunch of "low ohmic value" resistors for this sort of
work, some of them with temperature coefficients as low as 20ppm/C and
+/-1% tolerances.
Capacitors aren't that good, particularly stuff that could carry the
sorts of current invovled. AC excitation (or reversing DC) is a good
idea for this kind of bridge - the thermocuple voltages that you tend
to see around a red-hot glow plug can otherwise be something of a
problem.
--
Bill Sloman, Nijmegen
.
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