Basic question from one rusty on passives

Thread Starter

Baker Steve

Joined Feb 21, 2016
12
I am constructing a filter unit that is intended to allow visualisation (on a scope) and measurement of high-frequency interference on 240v UK mains by means of a high-pass filter to remove the 50 Hz component.

What would be the best (and safest) circuit to isolate the unit from the mains while allowing such interference signals through? I have had vague thoughts of resistive dividers, decoupling capacitors and pulse transformers so far.
 

Ian0

Joined Aug 7, 2020
5,082
2.2nF class-Y capacitor to Live (or neutral), then 50Ω to earth, and then connect your scope or spectrum analyser across the 50Ω resistor. It will show any interference above 1.5MHz.
Use a higher value resistor if you just want to see it on a scope and it will give a lower cutoff frequency, but a spectrum analyser will probably want a 50Ω input.
 

Thread Starter

Baker Steve

Joined Feb 21, 2016
12
2.2nF class-Y capacitor to Live (or neutral), then 50Ω to earth, and then connect your scope or spectrum analyser across the 50Ω resistor. It will show any interference above 1.5MHz.
Use a higher value resistor if you just want to see it on a scope and it will give a lower cutoff frequency, but a spectrum analyser will probably want a 50Ω input.
Thanks Ian, most helpful. I should have said, though, that I'm most interested in interference around 100-200 KHz. The isolator/dropper network will feed into a high-pass filter with a cutoff at around 200 Hz, or higher if harmonics of the mains frequency prove to be an issue.

I don't have a spectrum analyser per se, but I do have a scope that can do a simple FFT display, as well as a Digilent Analog Discovery gizmo, which can do low-medium 'frequency analysis'.

The background to this is that I am nursing an aged but vital X-10 network, and something is injecting sporadic interference into the wiring that is causing unreliability. Its intermittency makes tracking this down a challenge. There is currently no adequate replacement for the X-10 kit in the UK that I know of, as the rollout of Insteon here seems to have failed here. (X-10, as many will know, controls remote units by injecting a coded signal onto the zero-crossing points of the mains at around 120 KHz.)
 

Ian0

Joined Aug 7, 2020
5,082
Try it with 1k to ground, which gives a cutoff at 75kHz.
2.2nF is the largest value that keeps the leakage current to ground within the 700uA legal maximum for what is deemed to be safe to touch.
 

Ya’akov

Joined Jan 27, 2019
5,654
In the 80s I used a lot of X10. I think I might still have a couple of the plug-in relay switches around somewhere.

I did find these guys, looks like pretty pricey kit, and I can’t tell if they are UK certified or provide 240V versions, but the currency’s selector includes GBP, so that’s promising. They appear to be widely used and good quality. Worth a look anyway.

https://pcswebstore.com/
 

Thread Starter

Baker Steve

Joined Feb 21, 2016
12
Thanks. These guys seem to use a proprietary protocol that's not supported by my control software (Indigo). However, there is an X-10 bridge component. I will have to look at this in more detail later. It all ships from the US and is costly, but a bigger snag is the apparent lack of UK-plug-style appliance controllers. The whole system seems to be centered on lighting, whereas my X-10 systems is all power control. The advantage of Insteon (were it available in the UK) is that it's instantly X-10 compatible. It is possible to buy Insteon moduies in the UK from EU-based suppliers, but the shipping costs are now huge (probably due to Brexit).
 
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