Technical information

9
This lamp behavior results
f
rom the relativel
y
f
lat zero
crossin
g
f
or sinusoidal current. When the current ap
-
proac
h
es zero, t
h
e p
l
asma temperature
d
ecreases an
d
th
e
e
l
ec
tr
odes
a
l
so
coo
l
do
wn. Th
e
r
eco
m
b
in
a
ti
o
n
of
electrons with ions reduces conductivit
y
. A
f
ter the zero
crossing, the conductivity is too low to take up the
current t
h
at t
h
e c
h
o
k
e wants to
d
r
i
ve.
A
s a resu
l
t
,
t
h
e
volta
g
e throu
g
h the lamp increases a
g
ain si
g
ni
f
icantly
unt
il
t
h
e
l
amp
“
re
ig
n
i
tes
”
.
Th
e
high
er vo
l
ta
g
e resu
l
ts
i
n a
high
er
i
on
i
zat
i
on rate t
h
at
i
ncreases con
d
uct
i
v
i
ty
a
g
ain so that volta
g
e
f
alls.
By contrast, current and volta
g
e
f
or the rectan
g
ular
wave
f
orms o
f
an electronic ballast chan
g
e si
g
ni
f
i-
cantly
f
aster
f
rom positive to ne
g
ative hal
f
-wave, or
have a faster commutation times
(
see cha
p
ter 3.2
“
Electrical ballasts
(
E
CG)
”
)
, so that the
p
lasma has
li
tt
l
e c
h
ance to coo
l
d
own.
Th
e
i
nstantaneous vo
l
ta
g
e
required
f
rom the electronic ballast is there
f
ore si
g
-
ni
f
icantl
y
lower than
f
or the choke. This is one o
f
the
advanta
g
es o
f
electronic ballast, as one o
f
the
f
ailure
mechanisms o
f
metal halide lamps is to extin
g
uish
due to hi
g
h re-i
g
nition volta
g
e. The re-i
g
nition peak
o
f
a lamp normall
y
increases over the service li
f
e, and
w
h
en
i
t excee
d
s w
h
at t
h
e supp
l
y vo
l
ta
g
e momentar
il
y
can prov
id
e, t
h
ere
i
s no
“
re-
ig
n
i
t
i
on
”
an
d
t
h
e
l
amp
goes out
(
see also chapter 6.2.2 “Increase of the
re-i
g
nition peak”
).
Wh
en operat
i
n
g
on a convent
i
ona
l
c
h
o
k
e, t
h
e
l
amp
watta
g
e runs t
h
rou
gh
a max
i
mum
d
epen
di
n
g
on t
h
e
lamp volta
g
e
(
see Fi
g
. 5
)
. The maximum occurs for a
lamp volta
g
e o
f
sli
g
htly more than hal
f
the supply volt-
a
g
e.
N
ear t
h
e max
i
mum t
h
e
l
amp watta
g
e c
h
an
g
es
on
l
y s
ligh
t
l
y w
i
t
h
t
h
e
l
amp vo
l
ta
g
e.
D
ur
i
n
g
t
h
e
l
amp
service li
f
e, the lamp volta
g
e increases, as also shown
in chapter 6 “Lamp li
f
ecycle, a
g
in
g
and
f
ailure behav
-
ior”. In order
f
or the lamp watta
g
e to chan
g
e as little
as possible, the nominal value
f
or lamp voltage is gen
-
erall
y
chosen near the maximum, there
f
ore at about
hal
f
the supply volta
g
e.
The im
p
edance o
f
the choke is rated at a certain su
p
-
ply
f
requency and certain supply volta
g
e. Deviations
f
rom the nominal supply volta
g
e will result in a di
ff
er
-
ent ballast curve and a related di
ff
erent workin
g
point
f
or the lamp and there
f
ore di
ff
erent lamp watta
g
e. To
li
m
i
t t
h
e assoc
i
ate
d
g
reater sprea
d
i
n t
h
e
l
amp para
-
meters
,
a maximum deviation o
f
5
%
f
rom the nominal
values is permitted in the short term
f
or the suppl
y
volta
g
e, or maximum 3
%
in the lon
g
term. For devia
-
tions over a lon
g
er period o
f
time, suitable choke tap
must be selected.
S
imilarl
y
, the choke impedance
must not deviate
f
rom the nominal values b
y
more than
2%
(
see also cha
p
ter 3.1.3 “Influence of deviations in
supply volta
g
e”)
.
A
s
p
er the IE
C
61167 standard, ballast units for MH
lamps must by protected
f
rom overheatin
g
throu
g
h
recti
f
ication. This can be done e.
g
. with a thermal
f
use
(
tested accordin
g
to IE
C
598-1, Annex
C).
3
.1.1
A
m
e
r
ica
n
ci
r
cui
t
s
fo
r
ballas
t
s
I
n t
hi
s context
i
t
i
s
i
mportant to note t
h
at t
h
e supp
ly
volta
g
e in America has a different frequency
(
60 Hz
)
.
A
s the inductive resistance o
f
the choke de
p
ends
on the
f
requenc
y
, in this case it is important to use a
desi
g
nated ballast
f
or the correspondin
g
f
requency. In
addition, both lamps and ballasts in the U
S
A are stan
-
dardized b
y
AN
S
I, the American National
S
tandards
I
nst
i
tute.
T
o operate t
h
e s
y
stems correct
ly
,
l
amps must
b
e operate
d
w
i
t
h
correspon
di
ng
b
a
ll
asts.
R
at
i
ngs
d
es-
i
g
nations are required by AN
S
I to be marked clearly on
all products, allowin
g
users to clearly identi
f
y system
a
g
reemen
t.
3.1.1.1 Autoleak trans
f
ormer or hi
g
h reactance auto
-
tr
a
n
sfo
rm
er
I
f
the supply volta
g
e is smaller than around twice the
lamp volta
g
e, as is the case, for example, in the U
S
A
or Japan, then the supply volta
g
e must
f
irst be stepped
u
p. A
g
ood way o
f
doin
g
this is use an autoleak trans-
f
ormer. Part o
f
the secondary windin
g
s act as lamp
choke.
O
n the one hand
,
this saves on material
,
and on
the other hand, a hi
g
her volta
g
e
(
open-circuit volta
g
e
)
is available to start the lamp. These t
y
pes o
f
ballasts
are typically more economical than a constant wattage
style ballasts at the expense o
f
watta
g
e re
g
ulation.
3.1.1.2
C
onstant watta
g
e ballast
A
constant watta
g
e
b
a
ll
ast suc
h
as t
h
ose w
id
e
l
y ava
il-
able in the
US
A consists of an autoleak transformer in
series with a capacitor. The advanta
g
e o
f
this circuit is
the reduced impact o
f
f
luctuations in the suppl
y
volt-
a
g
e and the possibility o
f
operatin
g
the lamp at supply
volta
g
es
(
110
/
120 V in the U
S
A, 100 V in Japan
)
that
lie within the ran
g
e o
f
the lamp volta
g
e
.
Fi
g
. 7: Autoleak trans
f
orme
r
C
... capac
i
tor
L
a ...
l
am
p
U
S
... supp
l
y vo
l
ta
ge
Tr ...
au
t
o
l
ea
k tr
a
n
sfo
rm
er
L
a
Tr
U
S
C
C
PF
C
... PF
C
capacito
r
L
a ...
l
am
p
U
S
... supply volta
ge
Tr ...
au
t
o
l
ea
k tr
a
n
sfo
rm
e
r
L
a
T
r
U
S
C
P
F
C
•
M
ax
i
mum perm
i
tte
d
supp
ly
vo
l
ta
g
e
d
ev
i
at
i
ons:
5
%
in the short-term, 3
%
in the lon
g
-term, use
ot
h
er tap on t
h
e c
h
o
k
e
if
necessar
y.
•
Maximum deviation of choke im
p
edance 2%.
•
T
he choke must be protected a
g
ainst overheatin
g
accordin
g
to the standard
(
thermal fuse
)
.
Fig. 8: Constant wattage ballas
t