Discussion:
Using hot cathode lamps in cold cathode mode
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Steve
2015-05-18 20:52:50 UTC
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Is it possible to run hot cathode fluorescent lamps in cold cathode mode? I
want to build a portable UV lamp that runs off 6V, and want to use 2 9 watt
preheat germicidal lamps in cold cathode mode (to simplify the ballast design).

Any suggestions?
Andrew Gabriel
2015-05-18 22:14:15 UTC
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Is it possible to run hot cathode fluorescent lamps in cold cathode mode?=
I
want to build a portable UV lamp that runs off 6V, and want to use 2 9 =
watt
preheat germicidal lamps in cold cathode mode (to simplify the ballast =
design).
Any suggestions?
There are two aspects
o Striking it in cold cathode mode
o Running it in cold cathode mode

Running in cold cathode mode means under-running the tube so that the
tube current is significantly lower than designed and hence the tube
voltage is higher than designed for and the electrodes don't heat up
to thermionic operating temperature. This results in higher energy
electrons and ions impacting on the electrodes, and will sputter off
the thermionic emission coating - this doesn't matter much as you
aren't using the coating when running in cold cathode mode, but it
will generate dark shadows inside the glass at the tube ends. The light
intensity will be much less, but the energy spectrum will move towards
the higher frequency (higher energy) UV bands.

If you intend to start the tube in cold cathode mode but then run it
at full power (which will change to thermonic emission mode within a
second or two), the cold cathode starting will wear the thermionic
emission coating which is required to run the tube, so you will get
fewer tube starts out of it. This won't matter if the tube operates
for many hours each switch-on, but would give you a shorter tube life
if the tube only operates for a short time with lots of switch-ons.

One other thing to watchout for is that if your control gear can
provide enough over voltage to keep the tube running at more than
full power after the emission coating is all sputtered off (which
is really a dead tube), the electrode ends can change to run in
cold cathode mode, which at full tube current will make the tube
ends very hot, and can cause the glass to melt or crack, and can
ignite any nearby inflamable materials. Electronic ballasts usually
try to detect when the thermionic emission coating finally wears off
and stop driving the tube, to prevent this happening. One end will
always wear out before the other, and this causes the tube to partially
rectify (different voltage drop in each direction), and this is used
to detect that the tube is end-of-life and stop driving it before it
overheats.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
Steve
2015-05-19 00:32:21 UTC
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Raw Message
Post by Andrew Gabriel
Is it possible to run hot cathode fluorescent lamps in cold cathode mode?=
I
want to build a portable UV lamp that runs off 6V, and want to use 2 9 =
watt
preheat germicidal lamps in cold cathode mode (to simplify the ballast =
design).
Any suggestions?
There are two aspects
o Striking it in cold cathode mode
o Running it in cold cathode mode
Running in cold cathode mode means under-running the tube so that the
tube current is significantly lower than designed and hence the tube
voltage is higher than designed for and the electrodes don't heat up
to thermionic operating temperature. This results in higher energy
electrons and ions impacting on the electrodes, and will sputter off
the thermionic emission coating - this doesn't matter much as you
aren't using the coating when running in cold cathode mode, but it
will generate dark shadows inside the glass at the tube ends. The light
intensity will be much less, but the energy spectrum will move towards
the higher frequency (higher energy) UV bands.
If you intend to start the tube in cold cathode mode but then run it
at full power (which will change to thermonic emission mode within a
second or two), the cold cathode starting will wear the thermionic
emission coating which is required to run the tube, so you will get
fewer tube starts out of it. This won't matter if the tube operates
for many hours each switch-on, but would give you a shorter tube life
if the tube only operates for a short time with lots of switch-ons.
One other thing to watchout for is that if your control gear can
provide enough over voltage to keep the tube running at more than
full power after the emission coating is all sputtered off (which
is really a dead tube), the electrode ends can change to run in
cold cathode mode, which at full tube current will make the tube
ends very hot, and can cause the glass to melt or crack, and can
ignite any nearby inflamable materials. Electronic ballasts usually
try to detect when the thermionic emission coating finally wears off
and stop driving the tube, to prevent this happening. One end will
always wear out before the other, and this causes the tube to partially
rectify (different voltage drop in each direction), and this is used
to detect that the tube is end-of-life and stop driving it before it
overheats.
Thanks for your advice.

Do you have any suggestions as to where I could find a decent schematic for a
6V ballast to run one of these tubes in hot cathode mode? There are a few
schematics out there, but all require an unspecified special transformer.

I would run the device for short periods so as not to drain the batteries. I
can run two ballasts to work both tubes.

Steve
Andrew Gabriel
2015-05-19 07:59:11 UTC
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Do you have any suggestions as to where I could find a decent schematic =
for a
6V ballast to run one of these tubes in hot cathode mode? There are a =
few
schematics out there, but all require an unspecified special transformer.
I would run the device for short periods so as not to drain the =
batteries. I
can run two ballasts to work both tubes.
I would look for a low voltage compact fluorescent lamp of similar
power rating, and extract the electronic ballast from that.
I haven't seen 6V compact fluorescents, but 12V ones have been
available, although I would expect them to give way to LEDs if
they haven't already.

Some notes on reusing compact fluorescent integral control gear
here: http://www.cucumber.demon.co.uk/lights/diy/
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
m***@att.net
2015-05-19 18:14:31 UTC
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Post by Steve
Do you have any suggestions as to where I could find a decent
schematic for a 6V ballast to run one of these tubes in hot cathode
mode? There are a few schematics out there, but all require an
unspecified special transformer.
There are some at http://www.repairfaq.org/sam/samschem.htm#schinv .
Many of them give the mechanical specifications for winding the
transformer; a few of them use "standard" 120 V power transformers.

Matt Roberds
Victor Roberts
2015-08-29 14:11:03 UTC
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Post by Steve
Is it possible to run hot cathode fluorescent lamps in cold cathode mode? I
want to build a portable UV lamp that runs off 6V, and want to use 2 9 watt
preheat germicidal lamps in cold cathode mode (to simplify the ballast design).
Any suggestions?
No. Cold cathode fluorescent lamps have large area electrodes so the
current density is very low. You can, however, run the lamp in
Instant Start mode.
--
Vic Roberts
http://www.RobertsResearchInc.com
http://www.cflfacts.com
sci.engr.lighting Rogues Gallery http://www.langmuir.org
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