Ballasts for discharge lamps. Osram HQI-E 150 W/NDL CL
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3 Ballasts for discharge lamps
8
Since the discharge reacts to increasing lamp current with falling voltage (which would cause the current to rise indefinitely until the fuse blows or another part of the circuit fails), the lamp current must be limited by a ballast during operation. This usually consists of an inductive circuit (choke), although in rare cases up to
400 W capacitive circuits are also possible (although this usually results in a shorter service life). In the blended lamp (HWL), the resistance of the filament serves as a series resistor for the high-pressure mercury discharge lamp. In most cases, additionally to the current-limiting element, an ignition device is needed to start discharge (see chapter 4 “Ignition and starting discharge lamps”).
In modern luminaires, an electronic ballast fulfils the function of igniting the lamp, limiting the lamp current and controlling the lamp wattage.
3.1 Inductive ballasts (chokes)
The voltage across the electromagnetic ballast increases as the current increases, therefore a stable working point can be achieved in the series connection of the discharge lamp and the choke.
Charting the equations results in the curves shown in
Fig. 5. The difference between lamp wattage and the product of lamp voltage and lamp current is called lamp power factor. It reaches values between 0.7 and
0.95 depending on the operating mode. The yellow curve was generated by using a higher lamp power factor λ
L
[(to be more exact: 1.05*(1-n/3)].
Typical voltage and current waveforms as shown in
Fig. 6 show that while the current is (approximately) sinusoidal, voltage is not. After the current zero crossing, the voltage initially increases (so-called re-ignition peak) to then fall to a relatively constant value (saddle)
(see also chapter 6.2.2 and Fig. 30). Voltage remains approximately the same beyond the maximum of the current, and has the same zero crossing as the current.
So there are areas with high voltage which count towards the effective value of the voltage but don’t contribute to the wattage as the current at that point in time is nearly zero. This results in lamp power factors deviating from the value 1.
If the lamp voltage is equal to zero, the voltage drop across the choke is the entire supply voltage, and the choke short-circuit current is reached. This is the maximum current that can flow through the choke inasmuch the current has no DC component (see chapter
6.2.9 for effects of direct current components).
The following curves are typical for 150 W and apply in the same way to other wattages.
C
PFC
... PFC capacitor
La ... lamp
U
S
... supply voltage
Ch ... choke
U
U
S
C
PFC
Fig. 4: Discharge lamp with inductive ballast (ignition unit has been left out, the various possibilities are featured in chapter 4 “Ignition and start-up of discharge lamps”)
Describing the relationships of current and voltage requires a system of differential equations which cannot generally be solved. The following approximation formulas describe how the lamp current and lamp wattage depend on the relationship of lamp voltage to supply voltage [3]:
100
80
60
180
160
140
120 P
L
I
L
5 % higher lamp power factor
2,5
2
1,5
1
40
0,5
20
0
0 0,5
U /U
S
1
0
Fig. 5: Lamp current I
L
, lamp wattage P
L over the ratio of lamp voltage to supply voltage U
L
/U
S
; Z=99 Ω for a
150 W lamp
Voltage in V Current in A
P
L
=
U
Z
S
2
⋅
n
⎜
⎝
1
n
3 ⎠
⎟
⎞
[ (
1 −
n 2
) 1 / 2
− 0 , 225
n
]
I
L
≈
U
Z
S
[ (
1 −
n 2
) 1 / 2
− 0 , 225
n
]
(Gl. 4.1)
(Gl. 4.2) whereby: (1-n/3) ....approximation for the lamp
power factor λ
L
P
L
U
S
..... lamp wattage in W
..... supply voltage in V n ..... ratio of lamp voltage U
L
to supply voltage U
S
Z ..... choke impedance
Time in ms
Fig. 6: Graph showing lamp voltage and current of a
150 W lamp when operated at a choke (applies in the same way to other wattages)
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