What makes a fluorescent ballast remote mountable?

[Harry Muscle]

I've noticed that some companies offer fluorescent ballasts are have to be
mounted near the bulbs, as well as ballasts that can be mounted away from
the bulbs (remote mount). What is different about the ballast that
determines where it needs to be mounted? Can a normal ballast be remotely
mounted (say 4 feet away from a 32W T8 bulb), and if so, what guidelines do
I need to follow?

Thanks,
Harry

P.S. In case it matters, the ballast I would like to mount remotely is made
by Fulham, model Pony NPY-120-140-T8.

 

[Victor Roberts]

I've noticed that some companies offer fluorescent ballasts are have to be
mounted near the bulbs, as well as ballasts that can be mounted away from
the bulbs (remote mount). What is different about the ballast that
determines where it needs to be mounted? Can a normal ballast be remotely
mounted (say 4 feet away from a 32W T8 bulb), and if so, what guidelines do
I need to follow?

Thanks,
Harry

P.S. In case it matters, the ballast I would like to mount remotely is made
by Fulham, model Pony NPY-120-140-T8.

Electronic ballasts typically operate at frequencies between 25 kHz
and 100 kHz. When you mount an electronic ballast remotely you
increase the capacitance between the ballast output leads, and also
the series inductance of the leads themselves. At these frequencies
the load on the ballast created by the additional capacitance can
reduce the starting (open circuit) voltage at the ballast output, and
when combined with the series inductance of the leads can create even
lower starting voltages at the lamp. This is an especially serious
problem for instant start ballasts such as the Fulham.

Most ballast manufacturers give maximum ballast-to-lamp distances on
their data sheets, or expect that the length of the wires they provide
will not be increased. Fulham makes the Longhorse series of ballasts
specifically for remote mounting. And, I see the Fulham Workhorse,
Longhorse, Highhorse and Wham on their Web site, but not a line called
"Pony".

 

[Victor Roberts]

Their Pony series is actually their "custom" series that they make for
specific applications. In my case it came with a flourescent aquarium hood.
The Pony is actually a Rapid Start, unlike most of their other ballasts. Do
I have a better chance of getting a Rapid Start ballast to work four feet
from the bulb?

I was not aware that Fulham made any rapid start ballasts. Yes, rapid
start ballasts should allow somewhat longer lead lengths as their open
circuit voltages do not need to be so high --- but some of the new
programmed rapid start ballasts have open circuit voltages that are as
high as an instant start ballast. You need to ask Fulham about the
maximum distance between lamp and ballast.

Note that ballasts will usually still work if you exceed the
manufacturer's recommended lead length. The problem is that lamp
starting is compromised and lamp life will be shortened. Only in very
extreme cases will the ballast not start the lamp.

 

[Victor Roberts]

Why would lamp life be shortened? Wouldn't the lower starting voltage
lengthen the life?

Instant start lamps are started with cold electrodes, which are then
heated to thermionic emission temperature by the discharge. When the
discharge is operated with cold electrodes there is a high voltage
(~150 volts) between the electrode and the discharge. This is known as
the glow mode and quickly damages the type of electrode used in
fluorescent lamps. When the electrode reaches thermionic emission
temperatures, the voltage drop between the electrode and the discharge
drops at the range of 10 to 15 volts.

Since the rate of electrode damage is very high when operated in the
glow mode compared to operation in the arc mode, ballasts are designed
to make the glow-to-arc transition time as short as possible. ANSI
C82.11 specifies that the glow-to-arc transition time shall be no
longer than 100 msec.

If the ballast voltage is too low during starting, or perhaps more
importantly, if ballast current is shunted away from the lamp by
lead-to-lead capacitance or lead-to-ground capacitance, the lamp
current will be lower than normal during the glow mode and the heating
power delivered to the electrode will be decreased, increasing the
time spent in the glow mode and shortening the life of the electrodes.

The issue with programmed rapid start ballasts is less clear since the
electrodes should have been preheated to operating temperature before
the discharge voltage was applied. However, even in this case, a "hot
spot" must be established on the now uniformly heated electrode, and
the time needed for this to happen will be longer if the ballast
current is too low. I suspect that lead length has less impact on the
life of lamps operated on rapid start ballasts, but cannot point to
any data supporting that conclusion.

 

[TKM]

I was not aware that Fulham made any rapid start ballasts. Yes, rapid
start ballasts should allow somewhat longer lead lengths as their open
circuit voltages do not need to be so high --- but some of the new
programmed rapid start ballasts have open circuit voltages that are as
high as an instant start ballast. You need to ask Fulham about the
maximum distance between lamp and ballast.

Note that ballasts will usually still work if you exceed the
manufacturer's recommended lead length. The problem is that lamp
starting is compromised and lamp life will be shortened. Only in very
extreme cases will the ballast not start the lamp.

Extending the lead length of the secondary of a high frequency ballast also
adds to the radiating "antenna". What about the potential for increased rf
interference?

Terry McGowan

 

[Victor Roberts]

[snip]

Extending the lead length of the secondary of a high frequency ballast also
adds to the radiating "antenna". What about the potential for increased rf
interference?

Excellent question. When I first heard about the lead length
limitation I assumed it was an RF interference issue. After all, the
ballasts are certified to meet FCC Regulations with respect to limits
of radiated and conducted noise, and if the lead lengths are increased
we have increased the size of the antenna, as you have said, and will
get more interference.

However, I was later convinced by staff from Osram Sylvania who were
reviewing a ballast test rack set up at the LRC, that the loss of
starting voltage and current were more important than EMI when long
leads were used.

I suspect the relative importance of these two issues would depend
upon the setup in each individual application.