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Old 12-30-2011, 10:49 AM   #1
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Use 2-wire occupancy switch with fluorescents?


Hi;
Considering the cost difference between the 2-wire standard switches, and the 3-wire that are required for fluorescent fixtures, I had this thought:

Use a 2-wire switch, and install a capacitor (properly rated) in the fluorescent fixture, directly across the line (hot to neutral).

The theory is that the 2-wire switch does not work with fluorescent because there is no current through the circuit until the lamps fire, so the neutral conductor is required to power the switch.

But with the addition of the capacitor, there would be current through the circuit to operate the switch.

I believe that I have done this in the past with home automation controls. I used the standard incandescent dimmer module, but did not use the dim feature; just turned on and off.
I'm not sure what value cap I used, but it did work.
another method was to parallel the fluorescent luminaire with a small incandescent one.

Don't know whether this would meet code though; But since the modification is to the fixture, and not the wiring of the structure, it may be more of an issue with the UL approval of the fixture than NEC.

Any thoughts?

FW

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Old 12-30-2011, 11:21 AM   #2
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Use 2-wire occupancy switch with fluorescents?


Huh?

If I understand this right.....you are trying to find a short cut to not having a neutral in your box for the occupancy sensor?

As for occupancy sensors....they don't care of the load is incandescent or fluorescent...they still need a neutral and hot for power.

Fluorescent lights needing a 3 wire switch? If your talking just on/off....they work the same as an incandescent......

Capacitor across the Hot and Neutral in the light? Good way to make your own fireworks. If you go small enough, the impeadance would be low enough to not blow....but still....sounds like a classic hack job.

Sorry...but I think your trying to do something that you have no knowlege of.

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Old 12-30-2011, 11:35 AM   #3
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Use 2-wire occupancy switch with fluorescents?


Quote:
Originally Posted by KE2KB View Post
The theory is...
Don't know whether this would meet code though <--which code?
what are you trying to accomplish?
and why?
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Old 12-30-2011, 11:49 AM   #4
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Use 2-wire occupancy switch with fluorescents?


If you are trying to control the lights based on occupancy in two different areas, or a room that exceeds the capabilities of a single sensor then bite the bullet and buy the right switches. If, OTOH, you want to be able to turn off lights from two locations, but only have the lights actually come on when someone is in the room then you could wire the occupancy sensor in the line after the three way switches or before.
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Old 12-30-2011, 06:20 PM   #5
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Use 2-wire occupancy switch with fluorescents?


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Originally Posted by KE2KB View Post
Hi;
Considering the cost difference between the 2-wire standard switches, and the 3-wire that are required for fluorescent fixtures, I had this thought:

Use a 2-wire switch, and install a capacitor (properly rated) in the fluorescent fixture, directly across the line (hot to neutral).

The theory is that the 2-wire switch does not work with fluorescent because there is no current through the circuit until the lamps fire, so the neutral conductor is required to power the switch.

But with the addition of the capacitor, there would be current through the circuit to operate the switch.

I believe that I have done this in the past with home automation controls. I used the standard incandescent dimmer module, but did not use the dim feature; just turned on and off.
I'm not sure what value cap I used, but it did work.
another method was to parallel the fluorescent luminaire with a small incandescent one.

Don't know whether this would meet code though; But since the modification is to the fixture, and not the wiring of the structure, it may be more of an issue with the UL approval of the fixture than NEC.

Any thoughts?

FW
Your letter tells me you have NO understanding of electricity !
sorry not tring to be mean !
Your theory about switchs and neutral lines is way off the mark !

The capacitor in floro fixtures plays NO part in the operation
of the light.
It is only for power factor correction !

Dimming standard floros, will only sometimes partuailly work,
they need a special type of ballast to dim properly.

Please explain more clearly what you are tring to do?
then we might be able to help further.

Last edited by dmxtothemax; 12-30-2011 at 06:22 PM.
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Old 12-30-2011, 08:10 PM   #6
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Use 2-wire occupancy switch with fluorescents?


A basic two wire occupancy sensor will not work with fluorescent ballasts unless they are the core-and-coil type and do not use a starter. The reason is because the sensor derives its power by the fact that it is in series with the load. It has high resistance, therefore low current, and the voltage is divided between the sensor and the load.

If the load is electronic or has a starter, the voltage across it under low current conditions is pretty much zero, and the sensor cannot 'steal' enough power to keep it functioning.

A capacitor placed across the load would provide enough reactance to power the sensor, but the value would have to be chosen carefully. A capacitor in parallel with an AC circuit doesn't necessarily cause bad things to happen.

When a capacitor is placed in parallel with an AC circuit, it'll cause power factor to be raised, and it'll distort the AC waveform. If the power factor goes above 1.0, the voltage will become unstable. Not a good idea. The distorted waveform can cause electronic devices to fail, but this is rarely a problem.

A better idea would be to place a resistive load in parallel with the fluorescent fixture. This would eliminate all the problems associated with the capacitor, and is in fact, exactly what the sensor is designed to work with.

Rob.
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Old 12-30-2011, 09:05 PM   #7
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Use 2-wire occupancy switch with fluorescents?


So does this mean the occupancy swith has no neutral ?
and derives its operating power purely by the bleed currant thru it ?

Sounds like these 2 wire occupany switchs are not the right
switch for the intended application !

Are there not switchs, that do work with floros ?
If so I would be using them, instead of messing
around with putting caps in circuit
which could be potentially dangerous !
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Old 12-30-2011, 10:15 PM   #8
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Use 2-wire occupancy switch with fluorescents?


Quote:
Originally Posted by dmxtothemax View Post
So does this mean the occupancy swith has no neutral ?
and derives its operating power purely by the bleed currant thru it ?

Sounds like these 2 wire occupany switchs are not the right
switch for the intended application !

Are there not switchs, that do work with floros ?
If so I would be using them, instead of messing
around with putting caps in circuit
which could be potentially dangerous !
The two wire type have no neutral. Power is derived through the load.

They work fine provided the load has low enough impedance.

I haven't done resi sensors in a few years, back then there were none that would work with fluorescents.
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Old 12-30-2011, 11:08 PM   #9
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Use 2-wire occupancy switch with fluorescents?


A capacitor in parallel with the fixture should work to provide enough "bleed through current" to operate the occupancy sensor if the ballast doesn't already. However, there are a few things to consider:

- If it's a magnetic ballast, it will already provide plenty of current leakage and shouldn't require an additional capacitor to operate the sensor. Adding capacitance in parallel with a magnetic ballast will initially DECREASE the current flow, because it will provide power factor correction, canceling the inductive reactance of the ballast. You would have to add lots of capacitance (enough to completely cancel the inductive reactance, plus more to provide excess capacitive reactance) in order to increase the current. Again, this should not be necessary - the sensor should work fine with a magnetic ballast alone.

- Make sure your capacitor is suitable for the application. The peak voltage will be about 170VDC, so use a 250V rating or higher. Consider the current flow, too. Many capacitors are not intended for any significant AC current, and have high dielectric losses. They will fail spectacularly. Many ceramic caps are in that category. Metalized film/polyester/mylar would be best.

- You will really have to size the cap by trial and error. There's no way to know in advance what the impedance characteristics of the sensor are when it's in the "off" condition, what its bare minimum current requirement is, or what the impedance characteristics of the ballast are, so you can't know what impedance you need the cap to have without trying it. I'd start with something that will pass about 10mA when the light is on, which is 0.22uF. Work up from there in large increments until it works, then fine-tune it.

To those who jumped all over this guy for being "clueless": He clearly has a solid understanding of the fundamentals of electronics at a level well beyond basic wiring, and well beyond what is usually discussed on this forum. Just because you don't understand him doesn't mean he's a moron.

Note to Micromind: many electronic ballasts (and switching power supplies in general) already present a capacitive reactance to the line, so adding more capacitance doesn't correct their power factor, it just makes it worse (which is good in this application). PFC for switching converters usually requires active approaches internal to the converter itself rather than just hanging reactance across the line. A resistive load would work with less fuss, but wastes power and makes heat. Adding capacitance is a bit more elegant solution, IMHO.

Last edited by mpoulton; 12-30-2011 at 11:17 PM.
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Old 12-31-2011, 01:02 AM   #10
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Use 2-wire occupancy switch with fluorescents?


Learn something every day....I didn't realize they had 2 wire occupancy sensors....makes sense now....on an incandescent, you can use a small amount of current to power the sensor.....much the same way a self powered 4-20ma loop sensor works.

mpoulton has some good info......0.22uf sounds about right for a starting point...might help to see what the min current is for the occupancy sensor is. One note on the cap....use non-polarized......man...I've been away from the component end for so long.....off the top of my head I'm thinking that you do NOT want an electorlytic type cap...I'm thinking 'thin film'? mpoulton...chim in...
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Old 12-31-2011, 12:48 PM   #11
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Use 2-wire occupancy switch with fluorescents?


Definitely not electrolytic, even if it's a non-polarized electrolytic. They do not handle high ripple current well at all. Ceramic is questionable because many have high dielectric losses and will heat up with any significant AC voltage. You want a plastic dielectric. That means mylar/polyester film or similar.
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Old 01-01-2012, 11:31 AM   #12
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Use 2-wire occupancy switch with fluorescents?


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Originally Posted by mpoulton View Post
Definitely not electrolytic, even if it's a non-polarized electrolytic. They do not handle high ripple current well at all. Ceramic is questionable because many have high dielectric losses and will heat up with any significant AC voltage. You want a plastic dielectric. That means mylar/polyester film or similar.
I feel better now....I do remember some of it.. thnx m
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Old 01-01-2012, 05:37 PM   #13
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Use 2-wire occupancy switch with fluorescents?


The safest type of capacitor to use in mains aplications is
"CLASS X" These are metal polyestor caps specially designed
and rated for mains applications.
Being a poly cap they have the lowest losses.
And have a built in fuze link,
so if they short out for any reason
then the link blows out.
http://i883.photobucket.com/albums/a...ilter_caps.jpg

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Old 01-02-2012, 01:56 AM   #14
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Use 2-wire occupancy switch with fluorescents?


Just to clarify my question, as some of the early replies seem to have missed my point completely:
I have a good knowledge of electronics. I took courses in High School, and later in tech school, which covered well beyond what any electrician is required to know.

The purpose of the capacitor with the home automation switch (X10, Plug N Power, etc) was to provide the "leakage" current required to operate the switch, which does not have a neutral.
These switches (which are also dimmers) use a semiconductor device, usually a Triac, and not a relay to control the current to the load, and as such are not intended for use with anything but resistive loads, ie; incandescent lamps. With that in mind, I do agree that a resistor would be a better choice than a capacitor. The sharp edge of the waveform caused by the firing circuit in the semiconductor controlled switch might really screw up the switch and the lamp.

I had an old fluorescent fixture I wanted to control with one of these switches, although not to dim the lamp; it was an old type with a magnetic ballast.
One issue was that the standard dimmer switch (which is what the home automation switches are), control the intensity of the lamp by using phase-angle firing of a triac, which in effect cuts off anywhere from the entire waveform, to a small portion of the AC sine wave, causing a sharp edge as it fires.
You can actually hear this in an incandescent bulb. It's a buzzing or singing sound, which is especially noticeable when the lamp is very dim.

As for occupancy sensors, I suspect that the 2-wire ones also use a semiconductor (triac) to control current, rather than a relay, and this is just one reason they will not work with fluorescent fixtures. The other is that I mentioned; there is not enough leakage current through the light fixture to operate the electronics of the switch.

I do agree that a resistor instead of a capacitor would be a better choice.
Those poly type caps that are suitable for the purpose I discuss are expensive (compared to ceramics for instance)
But the capacitor would not generate the heat a resistor would, and that is why I was thinking of using a cap.
BTW; I am fully aware of what type of capacitor I would need. I have worked in electronics for 30 years, and with high voltage power supplies, which use oil filled capacitors with glass casing. Extremely expensive!!

When I was a teenager, and in my early 20's, I did a lot of experimentation along these lines. I learned a lot, never got shocked, and never caused any fireworks. I think I did know pretty much what I was doing!

Thanks to those of you who stuck up for me.

Now, what about NEC? Would modifying a luminaire be a violation?

This might be a classic case of building a new bridge to cross the river, when a bridge already exists a few miles away! Just pulling new cable, with a neutral, and using the correct switch is probably the best option.
I suppose that if it were feasible to do what I suggested here, then someone would have come out with a fluorescent fixture that would work with 2-wire electronic switches. This would cover occupancy sensors, as well as electronic timer switches, but NOT DIMMERS, as I understand why standard fluorescent lamps cannot be dimmed.

FW

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