/ Home & Energy, Technology

The energy-saving LED bulb that switched off the radio

An LED bulb lighting up the darkness

We get sent some weird and wonderful tales of products going wrong, but one story piqued our interest so much that we just had to send it to the lab to test it out. Can you help us shed more light on the mystery?

Last year we received this intriguing message:

‘I recently changed six halogen down-lighters to more energy efficient LED bulbs. Unfortunately when the lights were switched on, the DAB signal on my radio was wiped out!’

To try and figure out this conundrum, we sent a batch of cheap, generic 12V LED bulbs to our lab and found that when a digital radio was placed within a few metres of the switched-on bulbs the signal went fuzzy. When the radio was placed within a few centimetres of the LED bulbs, it cut out all together.

The plot thickens

LEDs are ultra energy efficient light bulbs that can last up to twenty years and have been hailed as the future of home lighting.

It seems our members are not the only ones who have had this problem. There are other accounts of LED bulbs affecting radios, with AVForums also collecting stories. Nick Tooley shared his experience:

‘I had the same problems with LED bulbs wiping out DAB reception and tried several types of bulbs, but to no avail.’

And it seems that the issue may not just be limited to digital radios – TVs may also be affected. After fitting LED down-lighters in his kitchen, Jackord noticed the following problem:

‘While the lights are much better, we then by accident noticed that the digital TV would not work (I was complaining that we had no reception at all, did not make any sense, began to think that there had been some sort of catastrophic disaster which stopped the TV stations from broadcasting…lol) then someone turned off the ceiling lights in the kitchen and, hey presto, on came the TV.’

Shedding light on cheap bulbs

So what bulbs are affected? We tested three 12V generic LED bulbs and we also compared them to branded 240V GU10 LEDs and some halogens. We found only a very minor interference with our radio signal. So at this stage, the issue seems to be limited to cheap knock-offs rather than branded goods.

We’ve only done preliminary tests on this problem, so can’t make any concrete conclusions on why this is happening or how widespread this bizarre problem is.

That’s where you come in. We need your help – have you had this problem? If so, please enlighten us in the comments below, including what model bulb you were using and where you bought it.

Comments

I have three ‘corncob’ mains-fed LED lamps in conventional fittings in a bathroom and they are cold white, none too bright, and they cause a strobing effect. If you watch the water from the shower at night you can see the individual water droplets and if you make a sweep motion with a splayed hand across your feild of vision the hand/fingers appear to be moving in a jerky fashion. I suspect that the mains is processed by some sort of half-wave circuit and the lack of thermal inertia in the light source (compared with a thermal filament) accentuates this ‘flicker’. We sacrifice smoothness of DC supply for cost and efficiency it seems.

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This effect is not seen with most LEDs because the driver operates at too high a frequency and the phosphor used to produce white light from blue LEDs helps provide a smoother light output. The strobing effect will be seen if they operate at 50Hz with half-wave rectification or 100Hz with full-wave rectification. Some ‘corncob’ LED lamps have been reported to be unsafe because of exposed live parts. Despite the strobing effect and lower efficiency, I don’t think you will have problems with radio interference.

Some LED rear lights used in cars obviously operate at low frequency because strobing can be seen if you wave your hand in front of them. I’ve seen the strobing effect with traffic lights but I don’t think they all do this.

Yes, Wavechange, I first noticed the strobing effect when driving on a motorway at night back in the late 1990s or early 2000s. Because an alert driver is constantly scanning between mirrors and the road scene, transverse movements across the field of view inevitably cause tail lights to pass rapidly across the field (relatively), and seeing multiple images from up-market cars (in those days) was almost alarming. The reds from leds seem to have a higher blue content than incandescent lamps, and those cars like my (now ancient) BMW Z3 that have an LED high level centre brake light and filament main brake lights give the illusion of separate circuits for each. This is because the LED goes from OFF to full ON instantly to the human eye, and the filament has a perceptible ‘rise-time’ and decay that they almost appear independent.
I wonder whether those people who dislike fluorescent lighting now find LED office lights yet more headache inducing than even their hated strip lights?

I’m convinced that some of us are more affected by flickering than others, Motco. Peripheral vision is most affected. In the days when CRT monitors were standard I could look into a room and spot the one at the default refresh rate of 50Hz. Most people did not see this but I was not alone. I was also aware when fluorescent tubes with a choke ballast were nearing failure, when they stop emitting as much light on one half cycle. I’m unaffected by LED lighting running at high frequency. A portable radio on the LW band held next to a LED bulb will show if it’s operating at high frequency.

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I am seeing more examples of overpriced products on sale. Even out of print books are sometimes offered at ridiculous prices, and I’m referring to ones that are fairly readily available rather than rare ones.

It would be interesting if you can find any significant differences between emissions from different brands of LED bulbs, Duncan.

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It might be worth looking at the newer ones. I have a house full of LED lamps now and don’t have problems with radio interference, and yes they were made in China, including the ones branded Philips. The one that did cause a slight problem was made in a European country.

Back on topic, I have just replaced my old Denon kitchen mini-system in which the DAB function had died, with a Panasonic similar. The FM (stereo) reception is still hissy when the lights are on, but on DAB it works perfectly even on a randomly tossed wire aerial.

Being unable to listen to your favourite radio station due to interference is bad enough but I had not appreciated that it could be a safety issue. In another Convo, Duncan mentioned that the US Coast Guard had issued a safety alert about the danger of LED lighting interfering with marine band VHF radio: https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC/INV/Alerts/1318.pdf?ver=2018-08-16-091109-630.

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I hope that real safety issues are systematically investigated and resolved. Thinking back, I remembered earlier mention of potential safety issues being mentioned by amateur radio groups. Here is a page that mentions this Convo: https://www.ukqrm.org.uk/lighting.php

Ultimately, I guess international standards bodies will have to play their part in reducing the impact of LED lamps on matters such as maritime and aviation safety.

The standards are there and what is lacking is independent testing to ensure that products comply.

wavechange – are you saying that some manufacturers and retailers are telling lies about compliance with applicable standards?

I’m not making any such claims, Derek. From posts in Which? Convos and elsewhere, there is evidence of a problem and some amateur radio enthusiasts have investigated the problems in some depth. I suspect that the problem is because brand owners do not always test compliance with standards when they buy from other manufacturers.

Measuring radio-frequency interference requires specialist equipment and perhaps one of the easiest examples of non-compliance is the lack of adequate isolation between mains voltage and low voltage circuitry in many phone chargers etc. There are YouTube videos that give examples of this problem.

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Would it be this site, Duncan? If using some browsers you will have to apply an advanced security exception to see the site.

Hopefully the relevant authorities will deal with safety related radio frequency emissions problems and building products to a price is less of a driver than with household products.

The reason that household LED lamps contain electronic components and operate at high frequency is that this is more efficient than using passive components such as resistors or capacitors as droppers, even though the latter would not cause interference. Having a house equipped with LED lamps and no problem with radio interference, I know that radio interference need not be a problem.

Thanks for the information about the relevant standard.

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Quite a few browsers are now simply throwing these warnings and prohibitions at anything that isn’t https, sadly.

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I believe the main purpose of https is to prevent “man-in-the-middle” attacks.

blog.mozilla.org/internetcitizen/2017/04/21/https-protect/

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I now have 3 Morrisons’ “pound shop” GU10 400 lumens LED lamps installed in my bathroom.

I am pleased to report that they aren’t interfering with my analogue FM radio.

Interference mainly affects DAB radio, though I have a lamp that affects FM but not DAB. My impression is that problems with radio interference are less common than with early LED lamps.

Thankfully, I have no need for DAB radio.

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Duncan – thanks, some interesting stuff there.

PS – on the black graphs the units are dBm (power values relative to 1mW) and the values displayed are negative, but values higher up the graph still seem to represent greater noise. E.g a value of -60 dBm indicates more noise than the lower value of -80 dBm.

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Thanks for the links, Duncan. dBm is explained on Wikipedia, with examples of positive and negative values: https://en.wikipedia.org/wiki/DBm

As can be seen from the article showing dismantled lamps, LEDs can either use passive components or contain electronic components and inductors as drivers. The latter system is more efficient in converting power to light but can generate RF interference. The one GU10 lamp that I attempted to dismantle had the components embedded in some potting compound and I could not establish anything about the circuit design.

I don’t read German. 🙁 DARC is the Deutscher Amateur Radio Club and it’s necessary to be registered to access all the information on their website. We can at least see that some lamps pass and others fail a test.

Many people are using LED lighting and still able to listen to their radios, but the best advice still seems to be to test a single example of a lamp before buying lots of them.

Duncan, I think it is quite simple arithmetic to say that -80 is a lower number than -60.

For example, believing that I have £50 in my bank, I write a guaranteed cheque for £20.

To my horror, I discover that I’m already overdrawn by £60, so when my cheque is paid, my balance reduces from £ -60 to £ -80.

Back on topic, if switching on an LED lamp increases the ambient r.f. power at some frequency from -80 dBm to -60 dBm, that is an increase of +20 dBm or a one hundred fold increase – and could readily “jam” the reception of any lower powered signals there.

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I’ve just bought 3 240v LED ‘Candle’ bulbs, 5.5W 470 Lumens made by GE. They interfere with my hearing aids and give me a constant buzzing in my ears. Is this avoidable?

It’s worth experimenting with different LED bulbs, Robert. You or your neighbours may have other types that don’t cause a problem. Best of luck.

LED bulbs come in dimmable and non-dimmable types. Sometimes these are clearly marked on the bulb but not always, as is the case with the 1521 lumen Philips bulb I found in the bulb drawer. That’s annoying when the lower powered lamps of the same style are dimmable.

It’s necessary to use a dimmer designed for LED bulbs because one designed for old incandescent bulbs including halogen ones may result in the LED not dimming, flickering or possibly failing.

Looking at the rating in watts with the lumen output, non-dimmable bulbs tend to be more efficient in lumens per watt. I have also noticed that 1500+ lumen lamps, which are supposed to be equivalent to 100W incandescent bulbs are rarely dimmable, and guess that the less efficient circuitry produces too much heat.

If the circuitry in dimmable and non-dimmable LED bulbs is different, I wonder if this affects radio frequency interference.

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It was your recent post that set me thinking about this, Duncan. I cannot find any information about the circuitry in LED lamps currently on same in the UK. I’ve only attempted to dismantle one LED – a GU10 dimmable one – but the circuitry was potted and not accessible. I no longer have an oscilloscope to look at RF interference. I certainly appreciate the potential of drivers operating at high frequency to create interference.

I presume that digital hearing aids are becoming more common and if they are more susceptible to interference than the older analogue type this may become a more common problem.

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Thanks again, Duncan. I am fascinated by electronic circuit design for household products. 🙂

I suspect that Infineon is producing designs and selling components rather than manufacturing LED lamps, and maybe for high quality products rather than inexpensive household lighting – but I could be wrong. The commentary in the article is interesting too.

As I read it, the idea is for NEMA to produce standards for performance and testing of dimmable lighting rather than standardise on the circuitry.

I suspect that different manufacturers of dimmable LEDs use different circuitry, as mentioned in the second article. I have a bedside light with a built-in dimmer and that behaves quite differently with different dimmable LED bulbs. With a Diall (B&Q) lamp it works fine on the three brightness settings but flickers or does not work on some settings with any of the other dimmable bulbs I have tested.

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For everyone new to the conversation – here’s the which? page on common problems with LED lights including interference. We had similar issues with our light fittings recently. Our landlord wouldn’t believe us until we showed him this page. We now have newly wired light fittings which solved the problem.

https://www.which.co.uk/reviews/light-bulbs/article/how-to-fix-common-led-problems

Thanks Abby. It might be useful to put this link in the introduction or maybe start a new Convo on LED lighting, since quite a lot has happened in the six years since this Convo was launched. One useful piece of advice that I’ve not seen anywhere else is to avoid mixing conventional and LED bulbs because the latter could be damaged by overheating.

In your linked article and in the magazine it is reported that Which? did not find interference problems but from this Convo, some have had quite serious problems. I wonder if Which? ever contacts anyone who reports problems on Which? Convo. Some of us would be happy to help and feel a little more engaged with our organisation.

It would be useful if Which? asked the industry for their comments on LED interference.

From the document mentioned by Abby: Our LED research In 2017, we asked 1,728 Which? members to tell us about any LED problems they’d experienced in their homes. We then consulted industry experts, and our top Trusted Trader electricians, to shed some light on what might be causing each issue – and how to fix it. According to our survey, 90% of members who have LEDs in their home are happy with their bulbs and the quality of light they produce. More than half haven’t experienced any problems at all when fitting their LEDs. Early failures (28%), flickering (12%), buzzing (5%) and radio interference (2%) were the main problems encountered. For more advice, see our guide to buying the right light bulb.

This suggests that radio interference might be a less serious problem than it used to be, but if 2% are still experiencing problems that is still not good enough. A lot depends on whether users were asked if they had experienced radio interference or had a problem with recent LED bulbs.

I was thinking more of getting an industry view on what standards must be met, how they mitigate interference to meet them and to what extent they find and test non-compliant imports. The Lighting Industry Association in the UK and the European Lamp Companies’ Federation might be starting points

I recall that you have mentioned the LIA before, Malcolm, and I would welcome their input. It does not surprise me that some of the cheap lamps sold online are non-compliant but a fair number of the posts refer to well known brands – for example the recent post about GE bulbs causing problems with a digital hearing aid. If there is a problem with fake lamps being sold under established brand names, the industry will be aware of this and should able to offer useful guidance.

I wish the lighting industry would encourage companies to provide useful information so that consumers have the information that they need at the point of sale. For example, although packaging shows which lamps are dimmable it should state that a compatible dimmer is needed. Using the wrong dimmer may be responsible for many of the cases of flickering mentioned above and possibly some of the failures.

Neil says:
23 April 2019

Just fitted 2 4ft LED battens in the garage and they totally block DAB and FM signals on my radio

I suggest you return them to the retailer for a refund, Neil. Can you tell us the make and model?

Robert McCormick says:
4 October 2019

Didn’t realise there was a problem with these bulbs when I changed one of our kitchen spotlights for a new LED, by Bell Lighting. It immediately cut the DAB radio, which was about 4m away. Had to change back to a standard bulb. This is ridiculous and wished I had checked this out sooner – I have bought 5 of these bulbs. What a waste of money, and a new addition to the landfill!!

When I was on holiday recently I was advised to switch off the wall lights when using the DAB radio. On investigating, I found that all but one of these lights completely blocked reception. The bulb that was OK had obviously been replaced because the colour temperature was different. None of the other LED lights caused a problem. I have seen a few examples of LEDs causing slight interference before, but now I have witnessed the problem that others have described.

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I look forward to learning more about developments on LiFi. China will produce good quality products and – as we know only too well – a fair amount of dodgy rubbish, but that depends on the specification required and how carefully the products are checked for compliance with appropriate standards.

With new technology such as LiFi, initial prices are likely to be high.

Malcolm Albert Tilley says:
7 November 2019

Hello All
I also purchased a led light 50watt and when plugged in the EVOkE radio promptly switches off.I tried several sockets same thing happened also with another evoke dab radio all in my outside garden workshop.
Mu question is can a inline product be purchased to stop the interference.
I have been an avid radio listener for over 70 years now and also produced the radio dials and scales for PERDIO RADIO IN SUNDERLAND – and later circuit boards,alas I am not technical on the radio side
Any help would be welcomed.

Malcolm Tilley

Hi Malcolm – I suggest you simply ask the seller to refund your money or replace the lamp with a different make, explaining the problem. Your lamp obviously does not comply with requirements for products to emit little radio interference.

I remember Perdio from when I used to repair radios in my younger days. 🙂

Malcolm Albert Tilley says:
7 November 2019

Hello again
you mentioned: Your lamp obviously does not comply with requirements for products to emit little radio interference.

Is this a legal requirement…or it would be nice… if it did comply ?

I like the lamp very much so I may try batteries…don’t know if that would work?
Thanks for the reply and nice to know you remember Perdio a great company,sadly the market from abroad put it out of business with modules and became cheaper to buy from abroad.
Lord Suirdale who was involved was a very nice person to all the staff and a real gentleman,
malc

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Here is the relevant legislation: http://www.legislation.gov.uk/uksi/2016/1091/contents

Products are required to conform to all relevant regulations including ones related to safety in order to be CE marked. If the manufacturer has ignored the rules on electromagnetic compatibility, it’s by no means certain that they have bothered much about product safety.

If you plough through the 19 pages of this Conversation, you will see other examples of DAB radio interference caused by LED lamps. With any significant interference the radio will just go silent, as you have experience. Although I had not witnessed the problem until recently I saw an example on holiday, where three wall lights blocked DAB radio, whereas another with a different LED bulb caused no problem.

Trying the radio on batteries will establish if the interference is carried by the mains cable rather than picked up by the aerial. It’s always best to have radios at a distance from a possible source of interference, so that having a lamp and radio sharing a bedside table might not be a good idea.

The loss of our British companies was very sad. I still have a Bush transistor radio dating from the 60s and a Hacker from the 70s. The radios I use most often are a Pure Evoke 1 and an Evoke 3. I don’t know where they were made but Pure is still a British company as far as I know. I shall now read up about Perdio. I had no idea the company was based in Sunderland.

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Malcolm Albert Tilley says:
7 November 2019

Thanks Duncan
The light is a 50 watt led and is brilliant as it only cost £11 on ebay it is too much hassle to return etc,,,I will us my other strip light and bite the bullet

Thanks for the info about Ferrite Suppressors but after reading all the above comments it seems unlikely as the answer is really get another light ;-)…

Yes Perdio transistor was the new market ..I ran the Screen printing dept and radios were all transistors,I still have one Perdio radio..fond memories, The days of pirate ships was good music and the Caralux was made especially to pull the pirate radio in… loud and clear
Thanks

malc

Malcolm, as far as I know the relevant EMC (Electromagnetic Compatibility) standards that apply to low voltage LED lamps are:
EN 55015 – EMC Emissions
and
EN 61000-3-2 EMC limits for harmonic current emissions.
I know nothing about these standards but presume that for an LED lamp to be legitimately CE marked ( to comply with EU regulations) they will need to have been tested and shown to fall within the limits of these standards. If they do not comply they should not have been placed on the market.

However, checking such products seems to be neglected. This link refers to Germany – where the same regulations will apply – and indicates a sorry state of affairs – https://www.elektormagazine.com/news/led-rumpus

You could buy a lamp from a known reputable brand and see if that makes a difference.

It seems that Perdio was there in the early days of British transistor radios: http://www.historywebsite.co.uk/Museum/Engineering/Electronics/history/TransistorEra.htm

Malcolm Albert Tilley says:
7 November 2019

Thanks to one and all for the sound advice (No pun intended) 🙂

Bottom line is.. keep the “Whacky” light . For the jobs where more light is needed

Reinstall on another line for the old LED which enabled me to listen to LBC and classical radio when pottering about.

Many thanks chaps for the help and advice so freely given.

Kind regards and bye
malc

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It is 90% full of people like Mr Tilley, Duncan – it’s just that the other 10% cause all the trouble and get all the attention.

One day I must get a chance to top up your glass to well over half full.

My first radio was a Perdio Town and Country. Lovely tone. Xmas pressie from parents shortly before my fifth birthday – heard “The King is dead” on Luxemburg on that one.

It’s weak spot was a brass pinion and too thin a shaft for the tuning mechanism.

Malcolm – you may even have made the dial.

My first radio was a crystal set I made and listened to with an earpiece under the bedclothes at night. I think it was Radio Luxembourg or would it have been Caroline? Can’t remember now.

Could have been either station, Alfa. Radio Caroline started offshore broadcasting in 1964 but would close down between 6:00 and 8:00 pm to avoid competing with Radio Luxembourg. I preferred the other popular ‘pirate’ radio broadcaster Radio London.

My first radio was a two transistor regenerative radio that I built when I was at school, That was in the late 60s when schoolkids did not have computer games or even X-stations and Playboxes for amusement, I remember the pirate stations because of my interest in radio but had little time for pop music.

Like some modern LED lamps, the pirate stations could cause interference with radio broadcasts, but the greater problem was that they were usurping the authority of the BBC, which believed that the Light Programme provided perfectly satisfactory entertainment for young as well as old. In 1967, the service was replaced by Radio 1 and Radio 2.

I guess we are a bit off-topic. 🙂

Big L 🙂 Yes I did a few crystal sets, but I was 7 or 8 when I played that game – a bit older than 5 (when I had my Perdio)

Well chaps, I have just ordered a replacement 15 watt LED panel for my kitchen because the existing one has begun to fail. Since my posts on interference to FM radio we have replaced the radio (another failure!) with a similar type but different brand. Unlike the last unit the DAB section actually works (at the moment) and because the FM reception is still rendered unusable by the EMI from the lights, we use DAB. Surprisingly perhaps, DAB works with lights on or off, but FM is still hopeless. On ordering the replacement panel I was faced with the decision as to whether I wanted ‘waterproof’ at a £2 premium, or non-waterproof as before. It’s from the same supplier whom, I assume, buys from the same Chinese supplier so I opted for ‘waterproof’ since the PSU may be a better quality. We shall see…

My house is technically a bungalow so the kitchen ceiling has the roofspace immediately above rather than an interstitial space as in two storey houses. The roofspace is, as with many well insulated roofspaces, suffers from condensation on the underside of the sarking felt despite being ventilated to current standards. But that is another beef of mine! Anyway… The light fittings suffer from ‘loft-rain’ drips and an IP65 unit would be better anyway. I’ll test the new unit when it arrives and report.

Welcome back, Motco. My 20 year old house has 200mm insulation in the loft and the recommendation when I bought the house was to increase this. I have not done so because I don’t want to risk problems from condensation in the roof space.

Regarding power supplies, I’ve seen different ones used in the same product. I recently fitted an emergency light and ordered some more of the same model from the same supplier. The power supply is different and so is the battery. In my time dabbling with electronic repairs I have seen other examples of the same product containing different components. I assume that using different components in manufacture is one of the reasons why recalls only affect a range of serial numbers.

At least the power supply is likely to be away from the heat generated by the LEDs in a panel, so might be more reliable.

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It’s not something I have studied. Often it is cheaper products that are inferior but the only LED bulb I’ve bought that produces radio interference (FM) was the most expensive one. I expect that the well known manufacturer provided their overseas supplier with the correct specification but failed to check that this was achieved.

I must correct what I said about emergency lights in my post above. They are in fact two different brands, although the friend who ordered them says that he ordered the same product from the same supplier, and I believe him. Not specifying the brand obviously allows the company to supply alternative products. Nevertheless, I have seen examples of products where manufacturers do include different components. Many will be familiar with the fact that computers are sometimes supplied with different hard disks, RAM, power supplies, etc.

This is the second failure of the six fittings. The first was the power supply and I suspect that this one is too. The luminaire comes on and flashes at about 10Hz for a few minutes then it stabilises. LEDs, in my experience, either dim with age or fail abruptly, they don’t tend to go into a paroxysmal flashing mode of failure so the power supply is the prime suspect.

Something puzzles me, and forgive me if I raised this previously, is the short life of aluminium capacitors these days. When my company was making security equipment throughout the eighties and nineties electrolytic al. capacitors lasted for donkey’s years provided they were not cross polarised – that was five seconds and then BANG! I have a Casio keyboard of some forty years vintage and I recently dug it out and powered it up. A buzzing noise was all it could manage and catastrophic failures usually point to something right at the front end – the d.c. power unit perhaps. I won’t be surprised if swapping a few capacitors in the power supply (built-in) fixes it. My amateur knowledge doesn’t help here but does anyone know if a straight value swap of tantalum caps instead of al. would work? My vintage (well, just old!) HiFi includes a couple of ReVox units – a turntable, tuner, and open reel tape deck – and none of those have had capacitors fail and none is less than thirty years old. Before that we had a Heathkit open reel tape machine built from a kit – I wonder whether that still works? It’s around somewhere…

I’m interested in why electronic circuitry fails and assume that it is the way that electrolytic capacitors are used that contributes to premature failure. For example, rapid charge and discharge and high external temperatures may be factors that are less obvious than components used up to their maximum voltage rating or simply poor quality. They may simply dry up, resulting in malfunction of the circuitry.

Most tantalum capacitors have a low voltage rating and are still relatively expensive compared with aluminium electrolytics.

There are various YouTube videos on failure of LED lighting, and here is one example showing diagnosis of a fault: https://www.youtube.com/watch?v=UerFD3AgJHE

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Ah yes, Wavechange, I hadn’t thought of the voltages because all my manufacturing experience was at no more than 12V and mostly regulated down to 8V. I have no idea of the voltages present in the Casio product and the LED drivers may be at as much mains levels. Thanks for the reminder.

It seems that the more you learn about electrolytic capacitors, the more there is to learn. 🙁

Some things I made at work used high voltage (low-k) ceramic disk capacitors. I think they were 2.2nF rated at 2kV. I had a few problems with my design – took me quite a while to work out that the 2.2nF was fairly accurate down at low voltage, but when biassed up at ~1.8kV it was down to only a few hundred pF. Obvious when you think about it thanks to piezzo effect – but it caught me out for some time. I proved it definitively by putting two in series on a Wayne Kerr bridge (with a bleed resistor across the pair) and a resistively-fed power supply from one end to the middle junction. Sure enough as soon as you get volts up above 1kV the capacitance dropped predictably and consistently, recovering as the volts came down again.

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There weren’t too many stable small discs that could be used in a Cockcroft-Walton stack (I wonder if that will get through the profanity filter first time..). I had some great fun actually exploiting these reducing charge with voltage characteristics.

But yes they wouldn’t be much use in filter stages!
Edit: Yes – first time!

My replacement flush fitting LED panel arrived and I have tested it for FM interference against the failed one (which in full compliance with Sod’s Law, is now working once more) and found that the smart new potted power supply is just as noisy! To recap (no pun here) the first units were noisy so I complained that they didn’t comply with the EU directive on EMI and could not legally bear a CE mark. That scared the supplier into send me a whole new ‘modified’ set FOC and offered to reimburse me for the noisy set. That would leave me with two sets of six and him with no payment at all for either. Perhaps foolishly I declined the offer of a refund and accepted the second set FOC. At least he was paid for one set and apart from the RFI they are decent lights. So I have twelve fittings one of which failed and another that threatens to fail. I need only six so all in all I am slightly ahead of the game but it means that the ‘Today’ programme has to be heard via DAB instead of FM.

Thanks for the update. I wonder if providing a better aerial would eliminate the interference problem. In my previous home I had two FM aerials – on on the roof and another in the loft.

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That would be easy to build, but without any means of tuning I’m not sure of the value. If you do have a problem with radio interference, a portable radio is a useful tool for investigating the problem.

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