On 11/25/2015 7:01 AM, firstname.lastname@example.org wrote:
"The first support call came from a customer with thirteen 23-watt CFL
bulbs installed in a workspace, as replacements for 60 W incandescent
bulbs. He indicated that the lights held in the ON position after only
a few actuations of our control relay.
[note that 13*23W = 299W -- the equivalent of *5* 60W bulbs. Hardly
anything to "worry" about!]
"We could not believe that, so we replaced the incandescents in our
test rig with one CFL bulb. It was true! A relay that would run 100,000
cycles, switching asynchronously on numerous incandescent bulbs ran 10
to 100 cycles with a single CFL. Astonishing.
[asynchronously == no regard for zero-crossings of AC waveform. 1000X
*fewer* relay cycles before CFL "wore out" the relay]
"So the race ensued to determine the failure cause. Relays stick closed
due to high switching currents, but how can a CFL bulb that draws 23 watts
suck that much current, enough to weld the contacts on a 10 amp relay,
one rated for incandescent service? We set up a fixture with a current
monitor to see.
[10A relay; 299W is < 3A]
"We found that some CFL bulbs draw huge inrush currents, peaking up to 17
amps, for a duration of 300 us or more!"
[half a 60Hz cycle is ~8300 us]
Watch the videos referenced in
-- which essentially
restates the issue more graphically.
Interesting. I would be curious about the relay tho. A lot of
"incandescent only" relays look pretty wimpy because they assume that
to be purely resistive. That is one problem with "engineering".
Instead of "how strong can I make it" the tendency is "how small/cheap
can I get away with".
On 11/25/2015 9:21 AM, email@example.com wrote:
Incandescent lamps only represent a "sizeable" load on startup. Once
the filament gets warm, current approaches that of a purely resistive
load. And, as startup can (theoretically) occur at any point in the
(half) waveform, the chances of the cold filament encountering PEAK
line voltage is small. E.g., if the instantaneous line voltage is
20V, then even a 10X reduction in cold resistance makes the lamp
look like a *warm* lamp seeing 200V (instead of 170V that it
would normally see at peak).
CFL/LED ballasts, OTOH, are *always* reactive -- rarely have power factor
correction. So, that "surge" happens every (half) cycle. And, ALWAYS
at the peak of the cycle (when the bridge is conducting!).
*Opening* the relay at this time usually poses the biggest risk for
contact weld as current is already flowing and wants to continue to
flow across the ever widening gap between the contacts. So, you draw
an arc and start melting metal.
Engineering is the endless pursuit of the "least bad" solution -- acknowledging
that all solutions are "bad" in some way...
On 11/25/2015 10:25 AM, firstname.lastname@example.org wrote:
Lots of folks/institutions have already done this. Your friendly neighborhood
search engine will save you the headache.
Essentially, the ballast just looks like a cap sitting on the output of
a bridge. So, you get all the current flowing through the bridge near
the peak of the AC (voltage) waveform and none at other times (diodes
being reversed biased as the cap stores a higher voltage than available
on the line, at that time).
By contrast, current waveform for incandescent lamp will just look like
voltage waveform scaled by some factor representative of the *hot*
resistance of the filament.
The implications of CFL/LED and other highly reactive loads on power
distribution can be pronounced. *Every* CFL/LED is drawing those
peak loads at roughly the same (unfortunate!) time in the waveform.
So, the copper wire is just acting as a heatsink for most of the time;
not conducting any current!
The incandescent bulbs draw a large ammount of startup current because the
reistance is very low before the filiment comes up to the operating
temperature. Same as with most common conductors except carbon and most
If you are just putting the bulb in a typical fixture with a mechanical
switch, youo should have no/few problems regardless of (practical)
"size". Do note that different spectra result in different growth
patterns. E.g., flowering plants tend to like the "blue-er" wavelengths.
Note that this is not the same as the "color temperature" of the bulb
but, rather, specific wavelengths of light in that composite "white".
You have an understated way of describing a ~$70 purchase! :>
When the socket limits the bulb used to 60watts, that means the socket
itself and the fixture it is in, have been designed to handle a
maximum of 60w of thermal load. A CFL or LED that is rated at 13.5w
will be fine if it physically fits in the fixture.
Web based forums are like subscribing to 10 different newspapers
and having to visit 10 different news stands to pickup each one.
You could if you sequentially switched them. Turning them all on at
once would turn the switch into an arc welder if it didn't trip the
I have 7 9 watt LEDs on a normal switch and you cna hear it when you
turn them on..
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