Hello

We have a panic alarm in our house which is a programmable receiver plugged into the telephone system. My elderly Mum carries a small radio transmitter and if she needs help she can press a button which causes the panic alarm to phone for help. This all works fine, however...

The panic alarm causes some background interference on the telephone line which noticeably reduces the quality of our voice calls and broadband speed.

The problem is the panic alarm's external PSU which is a small regulated supply, and looks identical to a mobile phone charger, except the output is 12V 0.5 amps.

If I unplug the power supply and run the panic alarm off batteries there is no interference on the telephone line.

Therefore how can I reduce or eliminate the interference, is there perhaps something I can add to the power cable from the PSU to the alarm?

Thanks for your advice

 

The first thing to try is another power supply. All you need is one that is 12V, has a compatible plug, and can supply at least 0.5A (you just need one that can supply the actual current needed, which I am assuming is less than the rating on the present supply). You can get universal supplies from lots of places, though they tend to be a bit pricy.

Some wallwart supplies are just very noisy. My wife has a PC supply that if it is plugged in anywhere in the house (even if nothing is plugged into it), I can't listen to AM radio on any mains-powered radio anywhere else in the house.

 

That is one reason that I prefer linear regulators to switching supplies. You didn't say which you are using, but if it's very small and light weight, it's probably a switching supply. Look for another supply that is physically larger and heavier (because it contains a transformer); the output needs to be the same voltage as your existing supply, and with a current rating that is the same, or higher, than your existing supply.

 

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Therefore how can I reduce or eliminate the interference, is there perhaps something I can add to the power cable from the PSU to the alarm?
...

You can buy a "12v automotive noise filter" from most electronics stores for a few bucks, they are a little gizmo with a balanced inductor filter and a couple of caps inside, and maybe a semiconductor spike absorber.

One of those will probably fix it, and if not, will only cost you a few bucks to try out. However, most types come with wires no plugs so you will have to patch it into the output wire of your 12v 0.5A DC supply.