"Hum Bars" due to dimmer


  #1  
Old 12-08-02, 06:23 AM
RickJ6956
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"Hum Bars" due to dimmer

I have a friend who just finished a beautiful media room. Unfortunately, his $2,500 pro-series TV projector is displaying slow-moving vertical banding when the lights are dimmed. This is usually caused by a bad ground or ground loop, but that's all been verified.

The lighting circuit is separate, but on the same leg in the panel.

He's tried a dimmer with RFI Supression. It helps, but not much. Someone suggested installing one of the old-style (resistive?) dimmers, but he can't find one.

The problem seems to be caused by the switching frequency of the dimmer. He can't eliminate the dimmer because the room serves as a game room as well as a media room. He can't run any new circuits at this stage of the game.

The projector is mounted in the ceiling. The video cabling runs about three feet away from any electrical wiring or fixtures. It does cross the electrical feed to the projector at one point, but it's at a 90-degree angle.

Interestingly, I just finished my own media room and have no issues at all with three dimmers in that room. (I'm using a standard TV rather than a ceiling-mounted projector.)

Any suggestions? Many TIA.
-- RJ
 
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Old 12-08-02, 05:06 PM
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Is the problem apparent when the dimmer is full on or full off or both? Try to change the switch to a standard on-off (as a test) and see if the problem still exists.
What type of lighting is the dimmer dimming?
 
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Old 12-09-02, 05:57 AM
RickJ6956
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The first dimmer was suspect, so he temporarily installed a switch. The problem went away. He then tried a dimmer with RFI Supression. It helped a bit, but the problem is still there whenever the dimmer is not at full on or full off.

The lighting is 120 volt Halogen.
 
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Old 12-09-02, 06:18 AM
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The following is information regarding RFI interference and dimmers from a good manufacturer of dimmers, Lutron.
RFI is a buzzing noise which may occur in some audio and radio equipment when solid-state dimmers are used nearby. Although every Lutron dimmer contains a filter to suppress RFI, additional filtering may be required in some applications. Typical examples of RFI-sensitive equipment are AM radios, stereo sound systems, broadcasting equipment, intercom systems, public address systems, and wireless telephones.

RFI can be transmitted in two ways:
Radiated
Conducted

Note: The suggestions in this application note will help minimize RFI: however, they do not guarantee that RFI will be completely eliminated.

Radiated RFI
Any sensitive equipment that is in close proximity to dimming equipment can pick up the RFI and generate noise into its system.

The following are three possible ways to minimize the radiated RFI:
Physically separate the RFI-sensitive equipment from the dimmer and its wiring.
Run dimmer wiring in its own metal conduit.
Use a lamp debuzzing coil (available from Lutron) to filter the RFI. See below for more information.
Conducted RFI
In some cases, RFI is conducted through the building wiring and directly into the AC power supply of the sensitive equipment.

To minimize the conducted RFI, follow these guidelines:

Feed sensitive equipment from a circuit without a dimmer on it.
Add a power-line filter to the sensitive equipment.
Add shielded wire for all microphones and input cables. Also, use low-impedance balanced microphone cables, which are less susceptible to interference than high-impedance types.
Make sure all the equipment is grounded. Connect all shields to the ground at one point. Ground lighting fixture metal housings properly.
Use a lamp debuzzing coil (available from Lutron) to filter the RFI.
Lamp Debuzzing Coils
Lamp debuzzing coils (LDCs) are the most effective way to reduce RFI. One LDC is required for each dimmer. Select the LDC according to the connected lighting load. The LDCs may be wired in series on either the line side or the load side of the dimmer. For maximum RFI suppression, keep the wiring between the LDC and the dimmer as short as possible.

Since the LDC itself make an audible buzz, mount in a location where the noise will not be objectionable (e.g., an electrical closet, a basement, or above a drop ceiling). LDCs are designed to easily mount onto a standard 4"x4" junction box. They are UL listed and thermally protected.

The following LDCs are available from Lutron:

Model #
LDC-10-TCP
LDC-16-TCP Rated Capacity:
600-1200W
1201-1920W
Note: For more details on LDCs, contact the Lutron Hotline and ask for Application Note #3.
 
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Old 12-09-02, 08:20 AM
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If both breakers are on the same side of the panel, did you try putting them on different legs (sides) ?
 
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Old 12-09-02, 08:31 AM
J
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"Side" and "leg" are different. Some breakers on the same side are on the same leg, and some on different legs.
 
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Old 12-09-02, 01:26 PM
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I know....but I could figure out how to say it.
I ment to say different leg. Although I'm not sure that will make that much difference, the RF is most likely generated and picked up in that room by the long projector cables. How is the projector fed? Regular RCA cables? S-Video? Are they shielded? Perhaps puting those cables in a metal conduit would help. An UPS device would solve the problem if the RF in on the voltage line, but that's probably not an option.
 
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Old 12-09-02, 03:12 PM
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A regular UPS wouldn't help, since they are generally off-line units. Off-line is utility power till an outage then 1/4 cycle trnasfer to battery and inverter power.
Different signal cable might be the way to go. As recommended by trintro, metalic conduit might help.
 
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Old 12-10-02, 06:31 AM
RickJ6956
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Thanks very much for all the replies.

He used Quad RG6 for the composite and Y/C (S-Video) feeds, and foil shielded 22/9 awg stranded for his component/VGA feed. (He's a video professional too, which is why this thing is stumping us.)

Metal conduit is not an option now because the room is finished.

I believe that he did try to swap one of the breakers to the other leg. I'll ask.

There is a guaranteed solution: A video "humbucker", or isolating transformer. Problem is, they are $120 each and he would need one for every wire of every feed. All to counteract a $15 dimmer. Maybe he should just buy a couple of torcheire lamps and put low-wattage lamps in them.

I will pass on the LDC post. Thanks again.

BTW, I put an O-scope on a dimmer's line at my house. I cannot believe the high-frequency spikes that thing is generating! When the dimmer is turned on, that lovely 60Hz sine wave starts to look like a mountain range. The RFI is present to some degree at every other outlet in the house.

There has to be a better way. (Well, there probably is, but not for $15 a dimmer.)
 
 

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