Measuring glowplug under voltage

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.

--=20 Bill Sloman, Nijmegen

** Groper on the loose ...

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?

** Resistance is just the ratio of applied voltage to current flow in a conductor.

If the applied voltage is known, then resistance is inversely proportional to current.

It is easy to monitor the current flow in a circuit.

....... Phil

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?

Kind regards, Rogier Maxim do a high side current sense chip....... with milliohm resistance