Hum interference and two-way light switching.

[Graham G3ZVT]

For a while now I have been plagued with 100Hz hum on my radios.
The stronger the signal, the more noticeable is the hum, and in my case it shows up most on Radio 5 from Moorside Edge, twenty odd miles away.

Because of the correlation with signal strength, and the fact I was using my DAC90, I was calling it “modulation hum” but I have recently noticed that my TR82C exhibits the 100Hz hum on Radio 5 running from its battery.

Obviously I have been going around the house switching things off, including the lights, and not making any progress. With the DAC90 it involved walking back to the conservatory each time I switched something on or off to try and hear any change.

With the TR82 I could take it with me, and that’s when I noticed something odd about the landing light. When I switched it off, the hum stopped, then came back after a few seconds.

The three lamp fitting has the same kind of LED bulbs used almost everywhere throughout the house, GLS style BC base bulbs with a bridge rectifier and capacitive dropper.

I have deduced a couple of things, neither of which is surprising,
Firstly the rectifier diodes being the only non-linear elements, are generating the interference,

And secondly, the topography of the two-way switching is performing as a magnetic-loop antenna.

Additionally, I surmise that the capacitance between the “strappers” and the CPC is sufficient to drive the diodes into conduction even when the light is off, but this is inhibited by the reservoir capacitor until it discharges, for reasons I haven't fully thought through.

I should say that similar bulbs live quite close to my radios and cause no issues, the landing light affects reception throughout the house.

As expected, modifying a bulb with 4x 1000pF capacitors across the diodes affects a complete cure, in fact with just one modified and the other two not, it’s hard to hear much hum, but I’ll do all three.

Two other things in case anyone was going to ask, no dimmers, and no “borrowed neutrals”!

We must all suffer from interference to a greater or lesser degree, and what I’ve described here is the most annoying affecting MW, greater than my computers, plasma TV, and all my SMPSUs put together.

It might be worth thinking about where you have two-way switching and checking for similar issues.

[1100 man]

Hi Graham,
There is a small section in the 'on site guide' to the wiring regulations which specifies the best way to wire two way switch systems to minimise interference.

There are several different ways in which cables can be run to achieve the same two way switching circuit. Some are better than others for radiating interference. Google has some information.

Out of interest, roughly when was the house wired?

It's quite surprising how much capacitance there is in twin & earth wiring. My insulation tester has a discharge facility, so when the test button is released after applying 500V DC to the wiring, it bleeps & shows the voltage as it discharges.

On a reasonable size ring final circuit, it takes quite a few seconds to discharge the voltage to zero. I've never actually thought about the value of the capacitance before, but I'll have to measure it next time I do some testing.

Good detective work on your part to find the cause!

All the best
Nick

[martin.m]

With a two way switching arrangement, the capacitance of the cable going from the upstairs switch to the downstairs one is enough to apply a reduced voltage to the light, even when switched off. Sometimes if a CFL bulb is used, it can be seen to light up dimly or flicker. This wasn't an issue with filament bulbs of course. Thank you for the tip.

Post crossed with Nick's

[Graham G3ZVT]

Thanks for that Nick & Martin, I imagine it was rewired 73 to 77.
Using the prevailing method of the time, L1 & L2 linked by strappers, live via lamp to one COM and neutral on the other switch COM.

I thought my post was long enough but I thought of mentioning our strange affinity for final ring circuits in the context of radio interference.
I was wondering if radials were preferred in recording studios or theatres?

As it happens the conservatory (my den) has recently been converted to a 20A radial circuit. Maybe that's a good thing for my radio activities? (It wasn't the reason though)