Isolating cause of mains interference

The *root* cause is plainly the set of fluorescent lights in the garage being switched on or off (or both) via a simple relay controlled circuit of mine . But what I'm really seeking advice about is how to establish why that has suddenly started causing malfunction in *another* circuit.

In my lounge I have an infra-red remote control unit I made about 25 years ago. Push buttons in the battery powered remote toggle various lamps, TV, hi-fi, etc via relays. (Not the garage lamps, which use an independent circuit.) It has its own DC power supply derived from the mains. Very reliable previously. But now, every time I return from the garage after using the light circuit, chaos reigns. Various units are switched on and don't respond to the remote. Removing mains power from the master unit for a few seconds restores normality (sometimes after two attempts).

The garage unit, powered from the mains, switches 4 parallel wired fluorescent tubes of various types if any of several buttons are tapped.

Access to the lounge unit is difficult and so before doing that and opening up the master unit and trying to understand my 25 year old circuit notes I'm seeking practical tips on how I can optimise the forensic work please.

For example, which of these is more likely?

  1. The source noise from the garage lamps has suddenly increased

  1. The DC supply to the remote control receiver and relay circuitry has suddenly become noisy.

What simple tests can I make to help me isolate the cause please? My 'scope (barely portable) is in my now rarely used shed/workshop, so I'd prefer not to be dependent on that in this context.

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Terry, East Grinstead, UK
Reply to
Terry Pinnell
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Time to replace the relay with a 'cheap' solid state switch. For all you know the chatter from that aged relay has gone out to more than twice what it used to be. And solid state relays switch at crossover and are much more quieter, electrically speaking.

Reply to
RobertMacy

I'm voting for relay problems as well- it's an old system with relays that's starting to act weird, plus the lamps are an inductive load.

burned contacts can cause impressive arcing, and it downward spirals pretty fast in relays and contactors.

Reply to
Cydrome Leader

Thanks both, I'll investigate solid state relays. But presumably they lack that one key advantage of relays: complete electrical isolation? I have used triacs and SCRs in circuits and can't recall any significant problems, but that was always one factor that made me a bit uncomfortable.

Meanwhile I'm happy to report that I fixed the immediate problem. For some reason all those years ago I'd not added a couple of important components to the power supply section of the circuit. It's a standard

4-terminal bridge rectifier delivering 21 V followed by a 7815 15V output but I'd left them off. With those now soldered in, the noisy garage circuit no longer causes havoc.

After 25 years of largely trouble-free use, I'm guessing that the failing relays (plus some old tubes) are now causing more noise than the previously adequate circuit can cope with. And now that I think about it, the relays themselves (surplus stuff I used to buy) are probably now at least 40 years old.

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Terry, East Grinstead, UK
Reply to
Terry Pinnell

Don't waste the time. Often an ounce of filtering at the source of the noise is worth a ton of effort at the affected circuit.

A snubber across the switch will likely get it. 100 ohm resistor and .01 uf cap (in series) is a good bet. Cap has to be low inductance ac rated or DC X3 times the line voltage and not an electrolytic - the resistor is there to lower the Q of any resonant circuit that may be present otherwise - so the transient doesn't "ring" and become an RF transmitter.

That's what I use on the fluorescent's since it fits the switch housings - and another on the audio amp for the computer because switching it off would reset the computer.

In the case of my electric cooking range/oven it was easier to just wind a pi filter for common mode rejection. It prevents all of the range element switches and oven switches from interfering. (interfere with the modem when I was on dial up). Large ferrite core, with a pair of coils of 2 turns #8 awg stranded around it, and a 1 uf (what I had in the junk box) 300 vac cap across the 240 VAC line, and mounted close to the power cord connections.

Reply to
default

The rectangle puck style ones are isolated. There can be small amounts of leakage on the load side when "off", and they act funky if you run them way under voltage. I just think of them as wall switches that light up when they're "off".

SSRs off ebay will probably be cheaper than even surplus replacement relays.

Reply to
Cydrome Leader

Be thankful to have managed as long as you did without the R and C.

I should be so lucky...

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)
Reply to
Fred Abse

At a guess, you'd benefit from replacing the ballast in the fluorescent fixture, preferably with a good modern electronic unit. It'll change the frequency of any interfering signal, at the very least.

Reply to
whit3rd

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