Cannabis Fact Contributed by:
Basil Fawlty
Cannabis Tip Submitted: August 27th, 2010
Images archived: 2010
Have you ever looked at your lamp and thought, "Hmm, the
bulb says 150W but surely this ballast thingy must use
some power too" Well if you did, youre absolutely right.
Some people say that the rule of thumb is about 5-10% of
the lamp's wattage. While this may be true for some
ballasts, it certainly isn't the case for all ballasts,
particularly cheap HID (High Intensity Discharge) lamp
ballasts.
We essentially want to know the ratio of the lamp wattage
(W) to total input power (VA, in Watts):
This is called the
Power Factor.
A good ballast will have a power factor above 0.9 (i.e.
more than 90% of the power it draws is used to power the
bulb). Cheaper (lower quality) ballasts often have power
factors of 0.5 or less. That's right, only half of the
power gets to the bulb. So a 150W lamp would be drawing
300W of power.
Let's look at a real world example. Here's a 150W HPS
lamp made by Globe. Notice they claim that this 150W HPS
is equivalent to 9 incandescent bulbs for a saving of 89%
in energy costs. This lamp has been used for about 8
months (about 4-5000hrs).
OK, let's
take our trusty multimeter and measure the voltage on our
120V line.
So now we've measured V = 121.6V. We now set our
multimeter to measure AC current and plug the leads into
the 10A slot on it.
WARNING! Never put the leads in the 10A slot for any
purpose other than measuring current! Failure to heed
this warning will at very least result in a tripped
breaker and a blown fuse inside your multimeter, if
you're lucky.
We then wire
the leads in series with the black wires of the lamp.
Let's turn on the lamp and see what the startup current
looks like.
Holy cow! 4.4A! That means that it's using 121.6V x 4.4A
= 535W! Let\'s figure out the power factor at
startup.
Power Factor = W / VA = 150 / (121.6 x 4.4) = 0.28!
That means that only 28% of the power is being used by
the lamp. Let's not forget that it's normal for HID lamps
to draw more juice at startup. Therefore, the lamp is
probably drawing more than 150W and is skewing our power
factor calculation.
So let's let it run for a while....
After a good
fifteen minutes has gone by, the light is at maximum
intensity. Let's have a look at the current flowing
through the circuit now!
As we can see, it has indeed dropped. It's now drawing
3.06A or 121.6 x 3.06 = 372W! Let's figure out the power
factor.
Power Factor = W / VA = 150 / (121.6 x 3.06) = 0.4
It's elevated to mention that as an HPS bulb ages, it
draws more power. I performed the same measurement
when the bulb was almost new and it was drawing 2.6-2.7A
at operating temperature then, which would give something
closer to a more realistic power factor of 0.5.
Since the power factor of the ballast doesn't change much
over time, we can work backwards to determine how many
watts the lamp is drawing part way through its
life:
W = PF x V x A = 0.5 x 121.6 x 3.06 = 186W.
To answer our initial question, "How much power does
my lamp really use?"
Bulb Wattage - 150W
Power used at Startup - 535W
Power used at Operating - 372W
I think the most elevated thing to learn here is that it
costs almost as much to run a 150W bulb on a low power
factor ballast as it does to run a 400W bulb with a high
power factor ballast.
It should also be apparent that one should be careful
about the assumptions they make when making calculations
for the installation of electrical circuits. You may
think that you can safely put 7-8 of these 150W lamps on
a 15A circuit but guess again!
In the next installment, I'll show you how to add a power
factor correction capacitor to increase the power factor
of your el cheapo ballast.
