Download flame current measurement

Application note AN 3.
FLAME CURRENT MEASUREMENT
What is flame current
When two electrodes are placed side by side in the flame and
if they are connected to an ac voltage source, an ac current
will flow.
Conduction in the flame takes place because the flame itself
is ionised. The relatively large and heavy positive particles in
the flame, called ions, are attracted to the electrode that is
negative. The small and ligth negative particles in the flame,
called electrons, are attracted by the positive electrode.
If both electrodes are equal, current in both halves of the sine
is equal (see fig. 1.).
Flame electrode
Flame
Burner
5 ←A
−
0
+
5 ←A
Fig. 1. Principle of ionisation flame safeguard
This principle can be used for flame sensing, by making the
two electrodes unequal.
If one electrode is at least 4 times larger than the other, the
current in both halves of the sine will not be equal. This is
caused by the difference in mobility due to size and weigth of
the positive and negative particles.
In practice in most cases the main burner is used as the large
electrode.
If the large electrode is positive, the small electrons are easily
attracted by it. On the contrary, the heavy ions do not easily
come in contact with the small electrode. This limits the
current (see fig. 2.)
The other way around, ions are easily captured by the large
electrode and also the electrons by the small electrode, so
current is not limited (see fig. 3.)
−
Ions
Electrons
+
+
0
−
1 ←A
Fig. 2. Flame signal during negative half--wave
+
Ions
Electrons
−
5 ←A
+
0
−
Fig. 3. Flame signal during positive half--wave
In figure 4. the current signal during one sine wave is shown.
Because positive and negative half wave are unequal, a dc
current is the result.
This is the signal on which flame sensing is based and which
can be in the range of 0.5 ←A to several tens of ←A depending
on applied voltage, position in the flame and measuring
circuit.
5 ←A
1 ←A
Fig. 4. Flame signal during full wave
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How can it be measured
The flame current can be measured with a common
multimeter measuring mV across a 1 Kτ resistor in series
with the electrode, giving 1 mV per ←A. (see fig. 5.)
1 Kτ
Electrode cable
Fig. 5.
What protection is needed
Ignition controls can be differentiated in dual rod systems
and single rod systems.
Dual rod systems
Dual rods systems have separate electrodes for ignition
and flame sensing and a multimeter can be connected to
the flame sensing electrode without any problem.
Single rod systems
Single rod systems use one electrode for ignition and
sensing.
A multimeter connected to the electrode will get disturbed
and it’s very likely that it gets damaged by the high voltage
ignition peaks. To avoid this problem, a circuit was
designed to be able to measure flame current without risk
of damage. (see fig. 6.)
Important part of the protection is the so called spark gap
A circuit like this, in combination with a battery powered
digital panel meter, is in use now for many years in the
Application Center of Honeywell Combustion Control
Center in Emmen without being damaged in single rod
systems.
Spark electrode
Spark gap
230V
R1
43K
R2
2k2
R6
464K
R5
43K
R3
2k2
R4
10k
C1
820n
D1
C2
100V
D3
20n
DVM
100V
464K
R7
1 ←A = 10 mV
D2
D4
Igniter
Diode D1 t/m D4: type 1N4148
Spark gap: Siemens B1--A230
Fig. 6.
For suggestions or comments, please contact the Application Center or send an E--mail to:
[email protected]
Combustion Controls Center Europe
Honeywell BV
Application Center
Phileas Foggstraat 7, Emmen
P.O. Box 83
7800 AB Emmen
NL--The Netherlands
Tel: +31 (0)591 6959 11
Fax: +31 (0)591 6952 00
ENAR--9003 0001R0--NE
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