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Ballasts

H

Hiking

Jan 1, 1970
0
Hello,
...

The current control function of the ballast can be quite simple.

First, the ballast could be a current source. In that case there is no
real "interaction" with the lamp, as the ballast will always provide a
fixed current. I know this may be hard to conceive, but we use voltage
sources all the time and these are designed to provide a very wide
range of currents while keeping the voltage constant, or just about
constant. Without understanding how a current source works internally,
you should still be able to conceive of such an object as an analog of
the common voltage source.

Second, the simplest ballast is just a series impedance. A resistor or
inductor if used at power line frequencies or resistor, inductor or
capacitor of used at frequencies above about 5 kHz.

I wrote back:

Something wrong with the 2nd sentence above. Ok, think I got it, what
you're saying is:

Second, the simplest ballast is just a series impedance. A resistor or
inductor if used at power line frequencies, inductor or capacitor if
used at frequencies above about 5 kHz.

Ok, this may make sense now... the ballast is inline in the circuit, and
thus if the current "attempts" to increase, then the back-EMF increases
as a result of the additional magnetic lines of force cutting the
winding's conductors, effectively neutralizing the "attempted increase"
in current (actually, there would indeed be a very slight increase in
current, which would immediately be countered by back-EMF, which lowers
current, which lowers back-EMF, and the cycle starts over, on and on,
many a times second). This about right?

If my above understanding is correct, then I do understand what you mean
by saying that the ballast is analogous to a current source.

And one could say that as the arc in the lamp causes heat to increase,
and resistance/impedance to drop as result of higher temperature, the
current would then start to increase, but as mentioned above, would be
immediately countered by the ballast. You say that there is no
interaction between the bulb and ballast, I suspect, because one
perpetually pushes against the other (as per previous sentence), this
can be seen this way from both sides (lamp/ballast), and since this
interaction between the bulb and the ballast is stable, and for all
intents and purposes never changes, there is no real purpose in talking
about this as an interaction.



I have not received a response, so would just like to make certain that
I have a correct understanding of how the ballast works in a lighting
circuit. Does the above make sense? Thanks.
 
A

Andrew Gabriel

Jan 1, 1970
0
Second, the simplest ballast is just a series impedance. A resistor or
inductor if used at power line frequencies, inductor or capacitor if
used at frequencies above about 5 kHz.

Ok, this may make sense now... the ballast is inline in the circuit, and
thus if the current "attempts" to increase, then the back-EMF increases
as a result of the additional magnetic lines of force cutting the
winding's conductors, effectively neutralizing the "attempted increase"
in current (actually, there would indeed be a very slight increase in
current, which would immediately be countered by back-EMF, which lowers
current, which lowers back-EMF, and the cycle starts over, on and on,
many a times second). This about right?

No. You are describing an inductor's DC characteristic of behaving
so as to keep a constant current through it by generating an EMF
to counteract attempts to change the current flow. This is not
the effect which is used to regulate the current in an AC circuit,
where the inductor has an impedance which behaves like a resistor
would in a DC circuit, except it doesn't waste the power a resistor
would. (The effect you describe is responsible for the inductor's
AC impedance, but that's at a different level.)
If my above understanding is correct, then I do understand what you mean
by saying that the ballast is analogous to a current source.

And one could say that as the arc in the lamp causes heat to increase,
and resistance/impedance to drop as result of higher temperature, the
current would then start to increase, but as mentioned above, would be
immediately countered by the ballast. You say that there is no
interaction between the bulb and ballast, I suspect, because one
perpetually pushes against the other (as per previous sentence), this
can be seen this way from both sides (lamp/ballast), and since this
interaction between the bulb and the ballast is stable, and for all
intents and purposes never changes, there is no real purpose in talking
about this as an interaction.

I think you have the wrong model in mind.
Just think of the ballast as a resistor which only works with AC.
 
V

Victor Roberts

Jan 1, 1970
0
Hello,


I wrote back:

When? I don't remember seeing a reply.
Something wrong with the 2nd sentence above. Ok, think I got it, what
you're saying is:

Seems to be incomplete ...
Second, the simplest ballast is just a series impedance. A resistor or
inductor if used at power line frequencies, inductor or capacitor if
used at frequencies above about 5 kHz.

Ok, this may make sense now... the ballast is inline in the circuit, and
thus if the current "attempts" to increase, then the back-EMF increases
as a result of the additional magnetic lines of force cutting the
winding's conductors, effectively neutralizing the "attempted increase"
in current (actually, there would indeed be a very slight increase in
current, which would immediately be countered by back-EMF, which lowers
current, which lowers back-EMF, and the cycle starts over, on and on,
many a times second). This about right?

Yes, this is how the inductor works for time periods short compared to
the driving frequency. It is easier to understand ballasts if you
start with a resistor. I thought I had posted some graphs that
explained this. Did you see them?
If my above understanding is correct, then I do understand what you mean
by saying that the ballast is analogous to a current source.

Well "ballast" has two meanings. The oldest is an impedance that is
connected in series with a voltage source to create a pseudo-current
source. The second definition of a ballast is that it is the
pseudo-current source itself.
And one could say that as the arc in the lamp causes heat to increase,
and resistance/impedance to drop as result of higher temperature, the
current would then start to increase, but as mentioned above, would be
immediately countered by the ballast.

Forget about "heat." The primary cause of increasing current in
discharge lamps is increasing electron density, and this occurs in
"cold" lamps such as fluorescent lamps.
You say that there is no
interaction between the bulb and ballast,

I should have said "ideal ballast" or "current source". There is some
interaction between a real ballast and the lamp since lower lamp
voltage will cause the ballast to allow higher current, and higher
lamp voltage will cause the ballast to provide lower current.
I suspect, because one
perpetually pushes against the other (as per previous sentence), this
can be seen this way from both sides (lamp/ballast), and since this
interaction between the bulb and the ballast is stable, and for all
intents and purposes never changes, there is no real purpose in talking
about this as an interaction.

See above. There is some interaction, but it is secondary.
I have not received a response, so would just like to make certain that
I have a correct understanding of how the ballast works in a lighting
circuit. Does the above make sense? Thanks.

Response to a message posted here, or sent to me via e-mail? I haven't
seen either, but I have not checked the e-mail account I use here in a
day or so.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.
 
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