Question about TV hum

[punter16]

Hello,

We are on a job with Lutron dimmers/lighting control. The client says there is a hum on the TV (not through the fixtures) when the lights in the area are turned on. We investigated and:

No hum in area on TV when lights are turned off.
Hum in area on TV when lights are turned on.
No hum in area when lights are turned ON or OFF when we plug the TV into other outlets through the house using an extension cord.

This points toward an electrical issue to me. Thoughts? Any ideas how the guys that put in the Lutron can troubleshoot? They of course, are insistent that the TV is the issue which makes no sense to me as it has no problems when plugged into many other outlets and has no problem when plugged into the local outlet with the lights turned off.

Thanks in advance!

 

[petersonra]

The way this works is the FCC requires that devices worked normally with a certain amount of noise coming through the power lines. Devices are allowed to emit a certain amount of electrical noise and other devices are required to accept them without it affecting their operation. If the lutrons are within spec they are correct that the TV is the problem.

You might be able to get a plug-in filter and plug the TV into the filter and then plug the filter into the outlet. Might be the simplest solution.

 

[punter16]

The way this works is the FCC requires that devices worked normally with a certain amount of noise coming through the power lines. Devices are allowed to emit a certain amount of electrical noise and other devices are required to accept them without it affecting their operation. If the lutrons are within spec they are correct that the TV is the problem.

You might be able to get a plug-in filter and plug the TV into the filter and then plug the filter into the outlet. Might be the simplest solution.

Would this still be the case if there is no noise when you plug the TV in elsewhere and the lights are turned on OR off? I would think that if the TV was the issue, you would have the issue no matter where the TV was plugged in. Am I wrong on that?

We were going to try the filter (as this would be simple) but I literally can't hear the hum so I can't tell if it would work or not. Some people can and some can't apparently.

 

[retirede]

Would this still be the case if there is no noise when you plug the TV in elsewhere and the lights are turned on OR off? I would think that if the TV was the issue, you would have the issue no matter where the TV was plugged in. Am I wrong on that?

We were going to try the filter (as this would be simple) but I literally can't hear the hum so I can't tell if it would work or not. Some people can and some can't apparently.

The TV likely won’t hum in my house, either. That doesn’t mean it meets FCC requirements.
Plugging it in elsewhere could have reduced the excitation sent to the TV to the point it was inaudible even if the TV isn’t compliant.

See if the branch circuits for the light and TV are on the same leg. If so, you might be able to solve it by moving a breaker.

 

[synchro]

As retirede mentioned above, if the receptacle for the TV and the dimmer are on the same branch circuit check if they can be put on separate branch circuits to isolate them from each other.

Another thing is that most dimmers use "forward-phase control" of the AC waveform, as shown in the picture linked below. Especially for loads like LED lights that have capacitors inside of their driver circuitry, the fast rising voltage when a TRIAC device turns ON can cause the LED light to draw current spikes from the branch circuit. Such spikes could cause a TV on the same branch circuit to buzz from the resulting interference.

The reverse-phase waveform below shows the voltage dropping during a later portion of each half-cycle. That is due to a transistor turning OFF the current, and so it will not cause a large current spike into a capacitive load like an LED light. As a result, interference with a TV might be substantially reduced. The only way to know would be to try a reverse-phase capable dimmer. Legrand Tru-Universal dimmers are one example, and they can be programmed for either forward or reverse mode.

 

[punter16]

Thanks for the help!