LED Transformers

[cessfordalan]

Good Afternoon Everybody from the windswept Outer Hebrides

 

I must confess that as a new member I maybe reopening a previous topic so please forgive me if I am. I am considering lighting some of my buildings on Dell Bridge with yellow LED's and thought of using mobile phone chargers as transformers. They appear to have a 240 volt input and an output of 3 ish volts and i need to know what.if any resister value to use.  I am planning to leave the island to visit the Glasgow show at the end of February and i am preparing my  annual shopping list for this event.as model shopping locally is very difficult.

 

As an aside are there any railway modelers on the Isle of Lewis? if please get in touch as it is very lonely     furrow I am ploughing.

  

 

Alan

[Brian]

Hi

You need to know the LEDs spec for its forward voltage and the current you wish the LED to operate at.

E.g forward voltage = 2.2volt current to be 10milliamp (or whatever).

Use this LED calculator web site to gain the resistor value needed.... http://www.led.linear1.org/led.wiz

Do not run the LED without a series current limiting resistor

 

Edit to add URL.

[Penlan]

They appear to have a 240 volt input and an output of 3 ish volts 

I would use a multimeter to check what the actual output voltage is, certainly don't rely on any printed information on the transformer.

This information will also help determine the resistor(s) needed.

[stewartingram]

Pedant mode on - sorry.

 

A transformer is a device to convert one AC voltage to another, i.e 240v AC to 12v AC. It will not work on DC supply.

If you need a DC supply, further work has to be done to rectify (diodes) and smooth (capacitors) the AC o/p. You may even go further and regulate the voltage with further components.

Once you have gone beyond the transformer stage, it becomes a power supply unit.

 

 

Pedant mode off.

 

Stewart

[stewartingram]

I would use a multimeter to check what the actual output voltage is, certainly don't rely on any printed information on the transformer.

This information will also help determine the resistor(s) needed.

Also be aware that if using a digital multimeter, and the meter is set to read DC, you will get erroneous readings if there is any AC present. Even if switching the meter to AC, again the reading could be wrong if is not a pure sine wave AC.

[jdp298]

A mobile phone transformer is one thing, but you won't get a lot of lights out of it. The total power output isn't that high. Something like this http://www.maplin.co.uk/p/power-pax-84w-switched-mode-acdc-fixed-voltage-12v-power-supply-n97ju is what I plan to get. I've got loads of LEDs to power and at first I was getting transformers and regulators and fuse holders and stuff. This is higher power, lower maintenance, and cheaper than the individual components, what's not to like?

 

It'll do 7A output, which when you think that a LED takes typically about 35mA (0.035A), that's a lot of lighting. I tend to run all mine in parallel with a resistor on each, and the resistor often a larger rating which drives down the current, but also stops the streetlight looking like a local supernova. I've found most modern LEDs, even the surface mount and 3mm ones to be brighter than you really need.

[corax67]

A nice simple route is to pick up a second hand DC controller, stick a resistor of 1k onto the LED and then slowly turn up the juice to a comfortable brightness.

 

I did this to light a load of buildings on a diorama for a friends daughter (a school project) using a bag of random 5mm LEDs of unknown rating. Total cost was £8 for a Graham Farish controller out of an old N Gauge set and a few pennies for the resistors. If the LEDs are too dim then replace the 1k with a 470 ohm and repeat.

 

It's not scientific but it works and is cheap plus there is the added excitement of "will the light pop?" If you turn up the volts too quickly

[DCB]

I have been using two 3 volt 300 ma AC-DC adaptors with LEDs for years, certainly from before 2007 with no ill efects and no resistors, one is a model D803030A input AC230V 50Hz 3w output DC3V 300ma 0.9VA the other is a LS8191 Primary 230 volt 50 Hz 4.9 w secondary 300ma 0.9VA and both started off powering around 5 X 3mm LEDs and now have around 18 Red Green, Yellow Amber, 2MM 3MM LEDs each. [ Note Clear or three colour LEDS need more like 4.5 volts so they won't light on 3 volts.]

 

Even powering one standard 3 MM LED the 3 volt adaptors dont exceed the max forward voltage of 3.2 volts or the mnax reverse of 5 volts so you can connect the LEDs either way without damage, obviously they only light one way round, but unless you want to run only a few LEDs continuously, as in an exhibition scenario you simply don't need resistors, you may shorten the life of the LEDs slightly but they last for ever, over 10 years continuous use so life is not critical, With 15 LEDs on 300ma you are down to 20ma each anyway and that is the rated continuous current for most standard LEDs.

 

I thought they were 2.6 volt until I checked earlier. I think they were for Game Boys or similar rather than phone chargers but their provenence is lost in the mists of time.

 

Not using resistors simplifies the wiring, I simply have positibe and negative bus bars across the ceiling of the signal box and station building with LEDs strung between, one leg soldered to each, The small signal box wires have lengths of copper 13 Amp wire attached which poke into small diameter brass tube with pieces of PCB or copper washers soldered on to stop them disappearing through the baseboard and wires soldered to the underside making the unit completely removable. The others are not arranged so elegantly.

 

Anyway that is how my set up works, cost, about 10p each for LEDs from Maplin when they have an offer on, about £2 max for a 3 volt transformer from a charity shop, free wire from dead christmas tree lights, and I just unplug them to turn the LEDs off.

[Crosland]

I must confess that as a new member I maybe reopening a previous topic so please forgive me if I am. I am considering lighting some of my buildings on Dell Bridge with yellow LED's and thought of using mobile phone chargers as transformers. They appear to have a 240 volt input and an output of 3 ish volts and i need to know what.if any resister value to use.

 

 

Whatever you do, ignore any advice whatsoever form anyone who advocates using LEDs without a resistor.

 

Andrew

[Crosland]

Even powering one standard 3 MM LED the 3 volt adaptors dont exceed the max forward voltage of 3.2 volts or the mnax reverse of 5 volts so you can connect the LEDs either way without damage, obviously they only light one way round, but unless you want to run only a few LEDs continuously, as in an exhibition scenario you simply don't need resistors,

Do you know what current you would need through a standard LED to get a Vf of 3.2V?

 

Do you understand a typical LED data sheet and how the maximum Vf and If is specified?

 

Perhaps you would like to post a link to the data sheet you are taking these figures from, so we can go through it.

 

What do you think might happen if you used a 10Amp 3V power supply directly across a LED?

 

you may shorten the life of the LEDs slightly but they last for ever, over 10 years continuous use so life is not critical, With 15 LEDs on 300ma you are down to 20ma each anyway and that is the rated continuous current for most standard LEDs.

How do you know the current is shared equally?

 

The problem with your advice that "it works for you" is that you have shown no understanding of why it works for you, and what the pifalls might be for others trying to emulate your setup. If you do know why it woirks for you, and what should be avoided, then please explain. you have actually been given plenty of hints

 

Andrew

[marc smith]

A nice simple route is to pick up a second hand DC controller, stick a resistor of 1k onto the LED and then slowly turn up the juice to a comfortable brightness.

 

I did this to light a load of buildings on a diorama for a friends daughter (a school project) using a bag of random 5mm LEDs of unknown rating. Total cost was £8 for a Graham Farish controller out of an old N Gauge set and a few pennies for the resistors. If the LEDs are too dim then replace the 1k with a 470 ohm and repeat.

 

It's not scientific but it works and is cheap plus there is the added excitement of "will the light pop?" If you turn up the volts too quickly

Yes, I use a 2nd hand old Hornby DC controller for powering my LED lamps too

This has worked well for me, and I use one on several layouts

I picked it up for a song, and it is fine and fit for purpose

You can also control the LED brightness easily

But as others have said, you must ensure sufficient resistance for each LED

 

I bought a small stack of resistors from Maplins a while back

and I experimented with different values, to get the brightness I wanted for each LED

 

Most of my yard lamps came with resistors, with the manufacturers recommended values

However, I still found most of them were too bright in relation to others...

So I added further resistors to get the relative brightness looking the way I wanted

 

Good luck with your project

 

Marc

 

PS Let us know how you get on

 

[SwissRailPassion]

I run my colour light signals (CR signals) from an old mobile phone charger with output 4v.

[marc smith]

Here is one of the layouts I use LEDs on
This one is an OO micro layout of a steelworks
called "Wales, Rails, Rain & Steel"

I find the LEDs are particularly effective
and I like using several different coloured LEDs
.... and BTW the layout is set in the rain!

[cessfordalan]

Good Afternoon everybody from a wet and windy Outer Hebrides 

 

Wow, thank you all for your instant and knowledgeable  response to my query on lighting up L.E.D.S it has certainly given me plenty of food for though and confidence to have a go. Once again thanks everybody

[Gordon H]

 With 15 LEDs on 300ma you are down to 20ma each anyway and that is the rated continuous current for most standard LEDs.

 

I think this statement sums up nicely the lack of understanding of what is actually happening.

Ironically it would be technically correct (if perhaps a little high per LED) if the writer was using a constant currrent source for the supply, but we know that is not the case.

Like Andrew, I would suggest it best to steer clear of any advice on electronic matters coming from this particular source.

[DCB]

I spent an hour this evening checking LEDs using an On Track controller as variable voltage supply with a, 3mm yellow LED or two., The On Track reads right down to 1 volt with just the voltmeter across it, it is not a resistance controller but fine control down in the 2 volt range is fiddly,

 

Resuts  voltage 2.75 current 21ma. Nice colour 

Voltage 1.93 volts current 9ma  Bit dimmer but not too bad

Voltage 1.45 volts, couldn't measure current, LED did not light.

Voltage 4.9 volts current 119ma the LED was not happy! angry orange colour but it survived.

4.9 volts was enough to power 2 LEDs  in series

 

Max speed =18 volts, thats what nominal 12 volt controllers usualyl deliver, with an LED plus 1.6K ohm resistor sold "To go with the LEDs", in series I was going check the current and voltahe across the LED, but while the LED was fine but the resistor was getting uncomfortably hot to hold after a few seconds so I abandoned that line of experiments..

 

Tested at several points along its 20 foot length the number one 3 volt supply actually read 2.29 volts pretty consistently, I counted 18 LEDs alight on that circuit.

 

The number two was running at 2.54 volts at the engine shed. 

 

So the conclusion is that the LED was safe at well over a volt more than my 3 Volt power adaptor can deliver, and the LEDs far from being over driven are running at less than their optimum light output, but it's pretty good as steam age lights and signal lights. 

[SHMD]

Hi David,

 

I like to encourage people to experiment and show their discoveries and triumphs.

After all, modelling railways is a multi-disciplinary hobby and there are always aspects that we know little about.

 

You have discovered/stumbled upon a lighting scheme that saves 1 resister and 1 solder joint per LED.

(I buy resisters by the hundred for a pound.)

(I'm also impressed by the way you have contorted data sheet analysis, maths and the conclusions that you have reached.)

 

This works for you. No arguing with that.

But please don't advocate this as a new/better way of doing things.

There are just to many things that can go wrong.

 

Do you really want to be responsible for damage caused to other people's layouts?

 

 

In the future can you please say that you have found a new way of doing something - but I don't know how good it is or if it's recommended or not but it works for me.

 

 

Kev.

(In a completely unrelated issue, have you done any DIY on your home's electrics?)

[SHMD]

Resuts  voltage 2.75 current 21ma. Nice colour 

Voltage 1.93 volts current 9ma  Bit dimmer but not too bad

Voltage 1.45 volts, couldn't measure current, LED did not light.

Voltage 4.9 volts current 119ma the LED was not happy! angry orange colour but it survived.

4.9 volts was enough to power 2 LEDs  in series

 

Max speed =18 volts, thats what nominal 12 volt controllers usualyl deliver, with an LED plus 1.6K ohm resistor sold "To go with the LEDs", in series I was going check the current and voltahe across the LED, but while the LED was fine but the resistor was getting uncomfortably hot to hold after a few seconds so I abandoned that line of experiments..

 

 

Hi David,

 

I do like your methodical approach to quantifying the behaviour of the LED with "Cheap" low voltage PSUs.

 

Again I will say that it does work for you but.. anyway lets continue.

 

Your results table is good but you do need to be familiar with the PSUs you have as this will affect the 'shape', 'smoothness', 'spikiness', 'quality' of the voltage output.

Are the PSUs electronic (light in weight) or are they linear (transformer and heavy) based? Smoothed or regulated?

Also, most meters will give different answers to different waveforms in. I'm guessing you use a Digital meter as they are now the most common type but I can not guess if it is a 'good' one or not. (For 'good' read a True RMS meter from a well know brand.)

 

But for these low currents the output voltage will probably result in a very smooth 'flat' output.

Some Switch Mode PSUs also have a minimum load requirement, and small (cheap) Linear PSUs have a poor Q factor resulting in higher voltage outputs at lower current demands which drops dramatically with output demand.

 

Both these items together already give you enough headaches when ensuring that the connected circuit (Bare LEDs!) is to be used.

Don't forget that the LED may work (now) but what are you doing to its MTBF figures - how long before failing. (I know you have said that yours have been in use for some years.)

What about when the supply needs to be replaced? Just get another low voltage PSU and hope that is does not blow all the LEDs, (no doubt in awkward places)?

 

 

However, I particularly liked your "LED stressing" experiment. Warm glow LED anyone?

Although the LED was

 

 

Be careful when connecting "Bare" LEDs to Controllers intended to run Trains - even at apparently low voltages!

PWM controllers will give out a short ON pulse at full voltage and then an OFF pulse. The ratio of these two ON/OFF pulses will give you an average voltage (which happens to equal a True RMS voltage.

(A million volts for a millionth of a second is, on average and True RMS, still only 1 volt!)

 

 

Re the "hot" resister. Warm is OK...

 

With a controller voltage output of 18v,a resister of 1k6, and an LED Vf of ~2v then the power in the resister would be ~0.16W.

Not overly hot and most Carbon and Metal Film resisters will be happy.

 

But the smaller the resister then the more concentrated the power and the hotter the surface!

Very small resisters (1/8W types) will be hot!

Another way to think of True RMS voltage is that is the equivalent voltage that will produce that amount of heating effect in a pure resister - regardless of waveform shape!

 

 

For your circuit what happens, to the brightness of a single bare LED when you connect a 47 ohm resister in series with it?

 

 

Kev.