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Assistance with powering LED station lamps


young37215
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  • RMweb Gold

I recently purchased 10 LED lamps on Ebay for use as lamps on a station platform. They are cheap Chinese imports but look like they will do what I want. I started by reading the limited instructions which state:

 

Operating Voltage: 12V~18v (must connect a resistor in series), AC or DC Compatible.

Operating Current: 20mA.

10 complete lamps & 10 compatible resistors (for being used on 12v~18v).

 

As there 10 of each it appears that a resistor is required for each lamp. In an effort to test the theory I connected one of the supplied resistors to an LED positive cable and attached power to the resistor and negative cable. Nothing. I tried a second light, again nothing. The power source is a 12v DC regulated power supply which powers my Megapoints network which has plenty of spare amps to power the LED's. Can anyone put me right and explain what I am doing wrong?

 

FF4D534A-88B0-4341-890E-68AC580AF5F0.jpeg.fb25c5970be388736affe68566007da3.jpeg

 

 

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  • young37215 changed the title to Assistance with powering LED station lamps

Yes, you want a separate resistor for each LED.

 

It will only light up if you have the correct polarity, so are you sure you connected the black lead to the -ve of your power supply?

Failing that, I agree with Pete's suggestion that it could have been wired with the colours the wrong way round.

 

If it wont work at all no m,atter which lead you connect as the negative, could you have inadvertently shorted out the resistor momentarily, thus applying 12v directly to the LED and burned it out?

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  • RMweb Gold

I hope you don't mind me throwing in some of my own random thoughts.

 

I'm also suspicious about how the LEDs are wired:

  • I know that some companies sell LEDs that actually are designed to be connected either way round - they're often panel indicators, designed to plug into sockets on control panels of heavy machinery. They actually contain two LEDs, connected in inverse parallel, with a resistor in series. I don't think they'd be a lot of use to you - and they don't look like what you've got.
     
  • Saying that, if they came ready fitted into those mounts, they might still be panel indicators - but more conventional - probably coloured and not particularly bright - and probably only designed to be connected one way round.
     
  • In other words, I'd probably use a DC supply - perhaps 12V.
     
  • Now we come to finding out what you've got. When in doubt, I prefer to use an LED tester. I got mine from Component Shop a few years ago (and I find it very useful) - it's shown part way down this page - it's a convenient way of keeping the "currents" and "voltages" supplied to an LED within safe limits.

    To use it, you connect both LED wires into the lines of sockets along the top - one above the other - the top wire is what you think is positive - below it, you connect what you think is the negative. You then press the button - if the LED lights up, all is good - if not, try reversing the wires.
     
  • One thing I would recommend when connecting LEDs (either to a tester or when you finally install them) is to keep the current low - 10mA is good - 5 or even 2 mA is better. On my tester, the lower current connections are to the left.
     
  • Keeping with the low current theme, when you install the LEDs, I'd suggest going for a higher resistance than is sometimes suggested. If you're using a 12V supply, you might want to use a minimum of 1K (brown - black  - red colour bands, reading from the end of the resistor) perhaps even 2K2 (red - red - red).

Whilst I think of it, if you don't have a tester, you could always use a 9V battery - with a series resistor. When testing LEDs, you're not looking for bright light - you're just checking that the things actually light up - so I'd play safe and go higher with the resistor - perhaps anything 2K2 or 4K7 (yellow - violet - red).

 

Well, that's what I'd try, anyway - but then I've always been cautious - also, my HND is in electrical / electronic engineering, so stuff like this is basically my line.

 

What you do is clearly up to you - whatever you do, I hope it works.

 

By the way, if you've got questions I feel I know something about, I don't mind answering them.

 

 

Regards,

 

Huw.

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19 minutes ago, Huw Griffiths said:

 

  • Keeping with the low current theme, when you install the LEDs, I'd suggest going for a higher resistance than is sometimes suggested. If you're using a 12V supply, you might want to use a minimum of 1K (brown - black  - red colour bands, reading from the end of the resistor) 

 

 

Isn't that what the photo shows?

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  • RMweb Gold
1 hour ago, Michael Hodgson said:

Yes, you want a separate resistor for each LED. Thank you for confirming

 

It will only light up if you have the correct polarity, so are you sure you connected the black lead to the -ve of your power supply? I was unaware of the polarity issue, reversing the supply caused the lamp to illuminate.

 

Thanks to everyone for their speedy and helpful input. I am embarrased to say it was as simple as me not appreciating that LED's and polarity matter. At least I am now familiar with what anodes and cathodes are! 

 

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  • RMweb Gold

  

8 hours ago, Michael Hodgson said:

Isn't that what the photo shows?

 

Sort of - it certainly looks like a 1K resistor - but this wasn't the only point I was trying to make.

 

I could have worded my comment a bit better - referring to that value resistor being shown connected in series with an LED (as I hinted, I think this value of resistor is a good choice for 12V DC supply).

 

(The fact that I didn't notice the reference to 12V DC might tell you that I had a number of other things on my mind when I posted my earlier comments. I suspect that a number of us must have made that mistake.)

 

However, in addition to mentioning the colour code, I was also trying to point out that I might be tempted to go for a higher value resistor than some people would (and a higher value still, for 18V supply - perhaps start with 2K2, 3K3, or even 4K7).

 

I was assuming that some people reading this thread at a future date might not be used to the resistance colour code - so, with me also referring to a 2K2 resistor, I thought it might make sense to say what colour bands to expect on that value.

 

Rightly or wrongly, I took the line that it wouldn't make sense to give the colours for one value of resistor, but not another.

 

Another issue (that I didn't mention - but perhaps should have) is that the band colours on some resistors can sometimes be hard to read in certain types / colours of light. I don't know if the pigments in the paint used might not always be the best - or if certain light sources might have gaps in their spectra - or even if it might just be down to handling / dirt sometimes getting onto component bodies - but this can sometimes be an issue.

 

If photos of resistors are shown on a computer (or phone) screen, you might also get additional issues due to the colour balance and reproduction of different cameras, screens etc.

 

If in any doubt about a resistor value, I'd use a digital multimeter to check.

 

 

Incidentally, I understand that the issue of gaps in the spectra of light from certain types of lamp was one reason why some photographers and film and TV studios didn't exactly jump to embrace first fluorescent and then LED lighting, despite them potentially being cheaper to run than filament lights. (Apparently, they have improved over the years - but this can cause other issues, like matching light sources and not wanting to be stuck with early designs which might not be as good as the current offerings.)

 

In case anyone's wondering why I've raised the issue of (sometimes) indistinct band colours on resistors, this has nothing to do with my colour vision (which I know to be excellent, by the way).

 

In a lot of electronics-related jobs, employers want to know that potential staff can read colour codes etc, so a number of them routinely test job applicants' colour vision. I've never had any problems in this regard.

 

 

At a trade show, a few years back, one of the exhibitors was promoting their ability to supply accurately colour-matched batches of plastics. To do this, they had a "Farnsworth-Munsell D15" colour arrangement test set on their stand - and invited visitors to have a go at arranging the colour samples in the correct order. Needless to say, I had a go - and got a perfect score.

 

 

I then explained my reasons for having a go. I wasn't testing my colour vision - no need. I wanted to see if the test results would be skewed by the (odd coloured) hall lighting - the fact that the results weren't skewed suggested that the colour samples used differing proportions of the same (high quality) pigments.

 

(In case you're wondering why I was able to identify which sort of colour vision test this company had on their stand, electrical / electronic engineering wasn't the only line of work I considered when I was at school. I also considered optometry. Even though I enjoy electronics as a hobby, I often wonder if I made the wrong choice. Some people might suggest that's why I wonder if I made the wrong choice - they might well be right.)

 

 

Returning to electronics, it's noticeable that, on surface mount resistors, they don't bother with colour codes - instead usually giving the first 2 digits of the nominal value (in Ohms), followed by the number of zeroes - so a 47K resistor would be coded as "473", or a 1M8 would be labelled "185".

 

Anyway, that's enough from me. Let's get back to the trains.

 

 

Regards,

 

Huw.

 

Edited by Huw Griffiths
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  • RMweb Gold
3 hours ago, Michael Hodgson said:

If you run it off AC you don't need worry about the polarity - it will only be on half of the time but the human doesn't notice that, it will just appear a bit dimmer than it does on the same voltage DC.  The resistor is important to limit the current.

 

I'm not trying to be awkward - but I don't agree with that. I agree about the resistor being needed to limit the current, however ... .

 

A lot of LEDs are very limited in what reverse voltages they can take - often in the region of 5V. If you've got a standard LED connected - in series with a resistor - across 12V AC, when it's not conducting, it's blocking the full reverse voltage coming from the supply. Because AC is usually quoted in terms of RMS (root mean square) values, the peak reverse voltage isn't 12V - it's actually closer to 17V!

 

When I was studying electrical / electronic engineering at (what's now a) university, I encountered this on a group project - one member of my group had a LED connected in series with a resistor, across 12V AC - and the LED immediately stopped working.

 

Someone else had a LED connected across a DC supply, with a series resistance that was far too low - the LED glowed like the sun for a few seconds, then took up smoking. The effect on that LED was also terminal.

 

Please note that I'm not mentioning any of this as a criticism of the (then) students concerned. Even if I could remember who made these mistakes, it was a deliberate part of the learning process that they made those mistakes then - with a few components that wouldn't cost a fortune to replace - rather than getting things wrong after they qualified, with something on which the consequences could have been a lot more serious.

 

 

To be honest, though, a standard dodge when powering LEDs with AC involves having a diode in inverse parallel with them (+ to - and - to +). This works because, when most diodes conduct, the voltage drop across them is below the reverse voltage limit of a typical LED.

 

In fact, you can sometimes get pairs of LEDs connected this way, in one package - if both diodes are the same colour and type, they're likely to be sold as "AC" LEDs - if they're different colours, they're more likely to be sold as "bicolour" LEDs (with 2 wires).

 

 

I could add that some circuits take advantage of this - circuits like a number of track wiring testers, with LEDs arranged to either change colour, or change which LEDs light up, with changing polarity. Ready-made examples of these have been available in model shops for a number of years - but it isn't difficult to make your own. I know this because I've designed and built a couple over the years.

 

My first one became the subject of a magazine article - unfortunately, the magazine concerned messed up the circuit and pictures, rendering it unbuildable - they also missed out any bits of text which might have anyone a clue how to build it. The following issue, they published a correction - which they'd lost a few years later when they reprinted the article.

 

I responded by designing a more foolproof version - emailing them the details, images etc - and suggesting that they use this new version instead of the old one if they decided to print it again. Since then, they haven't published either version again (which I see as a victory, of sorts).

 

This was all a number of years ago - the magazine in question now has a different editor (probably a whole new team) - so I have no plans to name the magazine concerned. I just don't think it would be right for me to say.

 

What I can say is that, since then, I haven't offered any potential articles to any magazines - and I'm in no rush to do so. Having been "burnt" in the past, I don't wish to risk repeating the experience.

 

 

Huw.

 

 

Edited by Huw Griffiths
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3 hours ago, Michael Hodgson said:

If you run it off AC you don't need worry about the polarity - it will only be on half of the time but the human doesn't notice that, it will just appear a bit dimmer than it does on the same voltage DC.  The resistor is important to limit the current.

Agree on both points BUT the LED will be reverse biased on one half cycle of the AC and therefore should be protected with a small diode like a 1N4148 placed across the LED in reverse parallel or in series with the LED and resistor so that the LED only conducts on the positive half cycle of the AC waveform. I see Huw has responded whilst I was typing this).

Regards

Mike

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  • RMweb Gold
58 minutes ago, Izzy said:

Quick and simple way to test the polarity of LEDS without danger. Use a 3v coin battery sans resistor.

 

Izzy

 

No.

 

A number of single colour LEDs start to conduct well before the voltage across them reaches anything like 3V.

 

Without a resistor to limit the current, if they conduct, they're likely to glow like the sun - get very hot and take up smoking.

 

By now, I suspect most of us know what happens when "the magic smoke" comes out of electronic components.

 

Not recommended.

 

 

Huw.

 

 

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49 minutes ago, Huw Griffiths said:

 

No.

 

A number of single colour LEDs start to conduct well before the voltage across them reaches anything like 3V.

 

Without a resistor to limit the current, if they conduct, they're likely to glow like the sun - get very hot and take up smoking.

 

By now, I suspect most of us know what happens when "the magic smoke" comes out of electronic components.

 

Not recommended.

 

 

Huw.

 

 


Sorry, are you saying 3v LEDS can’t be run at 3v, even temporarily, or that there are many commonly used ones that are rated sub 3v? Obviously resistors are useful to knock down the desired brightness level once set up but I’m a just a bit bemused - it’s common for me - as I have used this basic test method for quite some years without issue with LEDS down to small single colour SMD of just 1x0.5mm. 
 

Izzy

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  • RMweb Gold

Sorry about any confusion. I'm actually trying to say a number of things.

 

When any semiconductors (diodes, light emitting diodes, transistors etc) reach a forward voltage at which they start to conduct (and light, in the case of LEDs), there's a tendency for the current through them to skyrocket - unless you limit the current through them.

 

This current through them causes them to heat up - if the current reaches a point at which the component can't safely get rid of the heat generated, this is when problems start.

 

To keep the current within safe limits, resistors are added in series with LEDs. Some LEDs are even sold with resistors already built in.

 

Meanwhile, if you connect semiconductors (in this case LEDs) the other way round, they don't tend to allow current through them - so you need to reckon on them having the full supply voltage across them. However, if this reverse voltage across them exceeds a safe limit, you run the risk of the component breaking down. Just adding a resistor in series with a LED connected the wrong way round won't limit the voltage if the supply voltage exceeds the level at which the component breaks down.

 

Usually, though, the safe reverse voltage for these components is slightly more than the forward voltage at which they conduct (OK the polarity's different ...), so one way of protecting LEDs is to have another LED connected across them, with the connections reversed ("inverse parallel").

 

 

As for the rated forward voltages of LEDs (and also the infra red versions, sometimes known as IREDs), these vary - a lot. In the past, things used to be simpler - well sort of.

 

LEDs usually came in coloured packages - about 5mm in diameter, or sometimes about 3.5mm diameter - with 2 axial leads, a tenth of an inch apart. They weren't generally very bright. Red LEDs conducted (and lit up) at about 1.6V (infra reds a bit less - not that you'd see them) - orange, yellow and green ran at slightly higher voltages - perhaps about 2V to 2.5V.

 

That was pretty much how things stayed for a number of years. Eventually, LEDs started to appear with higher brightnesses - often in clear packages - while very low current panel indicator types also appeared. I'm pretty sure you'd still be able to find components similar to all of these, even now.

 

 

After a number of years, blue and then "white" LEDs started to appear - essentially in one of two basic "flavours". These needed higher forward voltages than the older types.

 

Some actually had a number of LEDs, of different colours, in the same package (which, by now, could also be surface mounted).

 

Meanwhile, others were really blue, violet or ultra violet LEDs, with light being converted to more usable colours using phosphors.

 

These days, the forward voltages of LEDs tend to be harder to guess than those of 20 or 30 years ago - and quite a few (but by no means all) tend to be higher.

 

Higher brightness types often run at significantly higher currents than the "indicator" types you might fit to a panel or a circuit board to tell you if something's switched on.

 

Another issue concerns which wire is which. A lot of people assume that the short wire is likely to be negative - likely, but by no means certain.

 

 

All of this means that, if you're handed a "mystery" LED, you can no longer be certain what you've got in front of you. As a result, if you buy LEDs, you're more likely to need to check the available data on the type you buy.

 

It also makes sense to have some safe means of testing LEDs before you fit them in a circuit - either to check which wire's which (and stuff like that) - or to match the colour and brightness of LEDs, if you intend to use a number of them together. This is where LED testers come in very useful - as they help to eliminate at least some of the guesswork.

 

The model I linked to earlier in this thread is sold by a number of places - and probably easier to use and more useful than a number of other designs. I don't doubt that other good ones exist - but I know about this design and have used one for a number of years. That's why I'd recommend them.

 

 

I hope I don't sound like I'm having a "pop" at anyone - I'm definitely not. What I haven't mentioned is that I'm currently having to deal with a lot of very stressful stuff - with the result that I seem to be constantly tired.

 

I also hope that what I've been saying doesn't sound like an advert - for LED testers or anything else. I'm not on commission, or anything like that - far from it. The only reason I've brought up the topic of LED testers is that I find them extremely useful - and I suspect that a number of other people would also find them of use.

 

 

Anyway, that's more than enough from me. Let's get back to the trains.

 

 

Huw.

 

 

Edited by Huw Griffiths
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16 hours ago, Huw Griffiths said:

 

No.

 

A number of single colour LEDs start to conduct well before the voltage across them reaches anything like 3V.

 

Without a resistor to limit the current, if they conduct, they're likely to glow like the sun - get very hot and take up smoking.

 

This is the David C Broad method of driving LEDs and you seem to be lacking the same crucial data as he.

 

The point about coin cells is that they have such a high internal resistance the current is limited.

 

I do not, however, recommend this as a long term LED driving method unless you really understand what you are doing.

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  • RMweb Gold
5 hours ago, Crosland said:

This is the David C Broad method of driving LEDs and you seem to be lacking the same crucial data as he.

 

The point about coin cells is that they have such a high internal resistance the current is limited.

 

I do not, however, recommend this as a long term LED driving method unless you really understand what you are doing.

 

The high internal resistance of coin cells really shouldn't surprise anyone. After all, the things are sold for use in equipment that draws very low current over long periods of time - perhaps with occasional bursts of still rather low current - digital watches - pocket calculators - stuff like that, for which high internal resistance isn't likely to be too much of an issue.

 

I could add, however, that I've always been somewhat risk averse (and, as I mentioned earlier in this thread, I saw LEDs emitting "the magic smoke" when I was a student). I was always going to make a point of advising anyone who'll listen to use current limiting resistors in series with LEDs.

 

As I see it, there's likely to be another issue here - anyone looking for advice on how to drive LEDs isn't particularly likely to understand all the relevant details about this stuff - so I think it prudent to suggest they "play safe".

 

If a correctly wired LED has a higher than ideal resistance in series with it, the worst that's likely to happen is that it either lights dimly or doesn't light at all. Neither of these is likely to do any damage to a LED.

 

 

Regards,

 

Huw.

 

 

 

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  • RMweb Gold

Having managed to get one led to illuminate, I foolishly thought I could wire the 5 for platform without further problems. I was wrong!

 

in an effort to reduce wiring I created a bus out of copper tape. I connected power and a single led with resistor in series. It worked, hooray. I added a second led with resistor in series exactly the same way but nothing worked! I disconnected the power , removed the second led but still nothing. At that point I decided to seek advice. 

 

Picture shows the first led connected which originally worked. The second led was connected exactly the same way about 100 mm along from the first. Can anyone tell me what I did wrong?

 

D6855621-2B57-4FD7-A552-E7AA90AFDEBE.jpeg.89f647d341cdf95764b9c6f8348cfc13.jpeg

 

 

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  • RMweb Gold

I like the idea of the rails for platform lamps; mine will be under the platform surface.

To get the replies going:

What is supplying the power?

What resistors are you using?

Are the LEDs the same way round?

Regards

 

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Hi

If you have lamp 1 working correctly but lamp 2 doesn't work, why not unsolder the wire on the end of resistor for lamp 1 and connect lamp 2 lead to that resistor and of course the other lead of lamp 2 to the negative tape too. Does it light then?  If not and with the resistor still connected reverse the lamps 2 two wires and retry. Does it light now?   If still no light it may be a defective LED or lamp?  

If it does light, then rewire it exactly the same way back in place 2.  If it now fails either your power supply is inadequate or there is a break in the copper tape somewhere between lamp 1 and lamp 2 position?

 

BTW. I would reduce the resistors lead length by at least 50%-75% each side to prevent accidental cross connection to something else. 

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3 hours ago, BMS said:

I like the idea of the rails for platform lamps; mine will be under the platform surface. Can I assume that the concept should work?, i.e. resistor connected to power supply and LED

To get the replies going:

What is supplying the power? A Fusion PS101, 12V regulated output up to 8amps. It supplies other things most notably my Megapoints network of servos. I believe that it has lots of headroom to supply the few mA required for the station lights.

What resistors are you using? The ones supplied with the lights, they are from China and have no obvious writing on them

Are the LEDs the same way round? Both were wired with the resistor to the red wire which I believe is the anode. On the basis that the initial light illuminated when first connected, this appears correct.

 

 

Thanks for your input.  I do not have any working lights at present. The first light worked when initially connected but when I connected the second, neither light worked. I reversed what I had done with the second light but the first one still would not illuminate. There are no obvious signs of damage to lights or resistors.

 

E8FEC4AF-8266-4351-89BC-098CF392D829.jpeg.afa7063108b4fe6e2842aa79f531e5e2.jpeg

B8F9185A-37A5-48D6-AFEA-A885AFFB99B8.jpeg.5a2db3cb2a50408bc02d1893c763f670.jpeg

 

 

 

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  • RMweb Gold

Thanks for that.

I assume that you have some sort of multimeter and that the voltage measured across the lighting tracks is sensible to the supply voltage, eg close to 12v? If no multimeter then a 12v bulb with leads should show a live power supply.

I assume the other reply to your question has ignored the statement that nothing is working.

I'm suspecting that something has either got disconnected or alternatively short circuited the supply and so I would like to know whether the other equipment connected to the power unit is still functioning normally. Suggest a browse for short circuits and/or unplugged power supply :o

If the psu is not working try switching it off and on again. If that does not work then disconnect it completely and check that it produces 12v on its own, then gradually introduce the other connections and see if that highlights the cause of the "short" - incidentally I think you possibly have rather a lot of bare wire - potential for shorts.

When the led 1 was lighting was it intense or dimmer ?- I assume it is white rather than red or green etc for the next thought - which is to think about the resistor value as the resistor has to drop approx 10.5V. I need to check the likely current - I'll get back to you on this. (Different LED colours need different resistors typically.

There is, IMHO, nothing fundamentally wrong in your approach.

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That power supply produces 8 Amps. That is way above most needs. In fact if your lighting needs more than 2.0Amp its probably too much!   With 8 Amps available I would recommend you split the output into around induvial 1.0Amp sections, each protected by a 1.0Amp fuse or a resettable 1.0Amp circuit breaker.   Thereby offering 8 x, 1.0Amp protected outputs.

 

If you have a multimeter (and they are essential tool for fault finding and can cost less than £10) disconnect all lamps and then power up the supply. Set the meter to its DC volts range of 20v check that you're getting 12v to 13.8 volts on the copper tapes. Noting which tape has the Positive supply on it from the meter leads and its display. Check volts all the way along the two tapes and everywhere needs to be the same reading.

 

Once the above is proved, then connect one end of a resistor to the now known Positive tape.  Onto the other end of the resistor connect one lamps wire.   The other wire from the lamp connects to the Negative copper tape.  If the lamp illuminates correctly all is good and repeat exactly the same for all other lamps.  However, if the first or subsequent lamps fail to light,  initially try that failed lamps two wires the other way around on the end of the resistor and negative tape. If then it still fails to light consider it probably dead and replace it for another lamp and then retest.

ALWAYS ensure that a resistor is in one lead to the lamp.

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  • RMweb Gold

Thanks gents, I really appreciate your counsel. I don’t possess a multi meter but I have checked that a different 12v light works when connected at various points all along the copper strip.

 

I will try again tomorrow with previously unused resistor and LED and see what happens. 

 

The reason I bought the LEDs was because they are a good representation of what I am modelling.  My eyes were wide open to the downside of buying cheap Chinese 

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On 13/11/2020 at 13:44, Crosland said:

 

 

 

This is the David C Broad method of driving LEDs and you seem to be lacking the same crucial data as he.

 

The point about coin cells is that they have such a high internal resistance the current is limited.

 

I do not, however, recommend this as a long term LED driving method unless you really understand what you are doing.

Found in Home Bargains today, LED Christmas lights with a solar panel and 1.2 volt single AA rechargeable cell, about £4.99 for 50 lights.    I expect they will be a quid after xmas.   But 1.2 volt, most old school pre 2018 clears don't light at all on 2 X AA cells say about 3.2 volts

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