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MOH

3v transformer

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Hi,

 

Just took possession of lots of single leds plus street lamps and platform lamps to try and illuminate areas of my layout, for which i believe I can use nothing higher than 3 volt transformer/s.

 

Question is which type of transformer and how many do I need?

 

Is there a ratio or limit to how many leds one transformer will power, will using more than the prescribed amount of leds per transformer result in their being dimmed somewhat, which may not be a bad thing for house lighting for example.

 

As you can see electrics is very far from my strong point so any info in a dumbed down mode would be really appreciated, thanks.

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6 minutes ago, MOH said:

Hi,

 

Just took possession of lots of single leds plus street lamps and platform lamps to try and illuminate areas of my layout, for which i believe I can use nothing higher than 3 volt transformer/s.

 

Question is which type of transformer and how many do I need?

 

Is there a ratio or limit to how many leds one transformer will power, will using more than the prescribed amount of leds per transformer result in their being dimmed somewhat, which may not be a bad thing for house lighting for example.

 

As you can see electrics is very far from my strong point so any info in a dumbed down mode would be really appreciated, thanks.

 

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As well as limiting current LEDs should be protected from reverse voltages above about 5V.  If driven by a transformer over about 3V RMS a simple half wave rectifier and capacitor will stop the LEDs seeing a potentially damaging reverse voltage. If you are driving multiple LEDs from one transformer you can drive some on one half of the waveform and the others on the other half.

If you want a dimmer light use a higher resistor in series. Another way is to have the LEDs in series.  LEDs take about 1.7V for red to 3V for white at their rated current.

If your not sure what resistors to use get a selection, start with the highest an reduce until you get the required brightness, a digital multimeter will help you in this.

If LEDs have too much current or reverse voltage they may not fail immediately but would do so over a week or so rather than last for years.

 

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You could also run your lighting from a 12v bus using little Buck converters, something like this will do the job

 

https://www.ebay.co.uk/itm/LM2596-DC-DC-Buck-Converter-Adjustable-Power-Supply-Step-Down-Module/303310252711

 

If you have a few of them dotted about your layout you could also have the levels of the connected LED,s adjusted to give the best lighting effect for the area they are in.

 

I have them on mine and they work well.

 

Paul

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1 hour ago, MOH said:

Hi,

 

Just took possession of lots of single leds plus street lamps and platform lamps to try and illuminate areas of my layout, for which i believe I can use nothing higher than 3 volt transformer/s.

 

Question is which type of transformer and how many do I need?

 

Is there a ratio or limit to how many leds one transformer will power, will using more than the prescribed amount of leds per transformer result in their being dimmed somewhat, which may not be a bad thing for house lighting for example.

 

As you can see electrics is very far from my strong point so any info in a dumbed down mode would be really appreciated, thanks.

 

 

Keeping it "dumb",  then ignore the post by Paul on the Buck Converter.  

If you use a DC power supply, then you can ignore reverse voltages stuff.  

 

 

1 - your LEDs need a resistor, one per light.   Calculator suggested by trevora is OK,  or,

if you have a 12v supply, you could start with 2kOhm (2000 ohm) and drop to 1kOhm if that's too dim. 

if you have a 5v supply, start with 470ohm, and drop to 220ohm if too dim. 

In both cases, increase the values if too bright.

 

2 - power supply.  Go with something DC, between 5v (common) and 12v (common).   A simple "wall brick" will do, such as an old phone charger or similar - read the voltage off its case, 5v is very common for chargers.  

 

3 - how many per power supply ?   A typical LED is maximum 20mA =  0.020A  .    So, a 500mA power supply would be theoretically able to run 25 at full brightness, or a 1A power supply could run 50 at full brightness.  In practise, add a bit of "headroom", so 40 on a 1A supply.   If running at lower brightness, then you can run more. 

 

 

 

- Nigel

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For yellow/ amber/ orange building and platform light  LEDs I use any old 2.5/ 2.8 /3 volt transformer from long dead electronic devices.    If they run one LED with out signs of distress  they should be fine, if it glows like a Super Nova and fizzles out then you blew a 5p LED and the voltage is too high, so find a different transformer.   They will run hot if overloaded, I keep to 50% of rated load as a rule of thumb .   Wall warts are good.  Most yellow/ orange etc LEDs light by 2.2 volts  Some modern White LEDs work on 3 volts. Most don't.    Trimming the light intensity can be an issue, some are too bright on 3 volts, my lights are set so they work well in a darkened room and can barely be seen in daylight.   Reds  generally light at  around 1.8 volts so are painfully bright on 3 volts so resistors  are essential.  Use one resistor per LED, but resistor values are down in the 10 ohm range not the 1000 ohm we are used to with 12 volts powering LEDs, so there should be no issues with overheating resistors. 

Overload protection is important, something like a 1/4 amp Polyswitch, a dirt cheap self resetting thermal cut out, is a good idea on a half amp power supply, they cost about 10 p a throw.  

Obviously the LEDs are wired in parallel, you can get away with series but  I wouldn't. Even if the LEDs are the same colour from the same batch you can get uncontrollable variations in brightness.   If you want to dim the LEDs you could always use a lighting dimmer between the socket and the Wall wart. 

 

 

Edited by DavidCBroad
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On 27/07/2020 at 10:12, MOH said:

Hi,

.....................

 

As you can see electrics is very far from my strong point so any info in a dumbed down mode would be really appreciated, thanks.

With the greatest of repect to the prev post, the last sentence of the OP's entry is relevant.

NigelCliffes post preceding that one gives a complete and text book answer to the question.

If dimming is required it can be dealt with as a seperate query.

 

Use of most domestic lighting dimmers to dim a mains powered transformer is potentially dangerous on several levels and should be avoided.

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Many thanks to all respondents, looks like I need to swot up a bit more than expected, my knowledge and grasp of electrics is really poor and unfortunately much of the terminology leaves me blank, sad thing is that the likes of Maplins going out of business cuts out a reliable source of direct conversation with knowledgeable staff and a ready made source of supplies.

 

As I am not UK resident and ordering of parts will have to b done by phone/email would the likes of Eileen's Emporium be a good alternative from which to glean one on one info and advice as well supplying whatever is needed by way of parts?

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LEDs are easily powered from a 5V USB charger. The USB-C types should be able to supply 3A which will be sufficient for quite a lot of lighting. You should be able to obtain a USB-C charger locally at reasonable cost, or from online sources with a plug that fits your local socket. A 3A USB charger should be good for at least a hundred and fifty LEDs, probably much more - if you have less than fifty almost any USB charger will do, you might have one lying around. When using a 5V power supply you will not risk damaging the LEDs if you wire them the wrong way round - they just won't light.

 

Since LEDs vary in their forward voltage from around 1.8V for most red LEDs all the way up to nearly 4V for some of the white ones you will probably have to experiment a bit with the series resistors required to get the brightness you want. Running the LEDs at the typical 20mA will often result in a lot more brightness than you really want so expect to need to use a much bigger resistor than the 68R - 150R that LED resistor calculators would suggest you need.

 

1K resistors are usually a good starting point. Get a big box of them and you can play around and see how you get on. Initially just put one 1K resistor in series with the LED and see how bright it is. If too bright put another in series, if too dim wire two resistors together in parallel and put them in series with the LED.

 

When you want to buy more resistors you can be a bit more targeted and see what values you have from this table:-

  • 2x 1K in parallel = 500R (470R nearest E6 preferred value).
  • 3x 1K in parallel = 333R (330R nearest E6 preferred value).
  • 5x 1K in parallel = 200R (220R nearest E6 preferred value).

 

  • 2x 1K in series = 2K (2K2 nearest E6 preferred value).
  • 3x 1K in series = 3K (3K3 nearest E6 preferred value).
  • 5x 1K in series =5K (4K7 nearest E6 preferred value).

 

Rapid are a good source of components mail order in my experience.

 

Edited by Suzie
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3 hours ago, MOH said:

As I am not UK resident and ordering of parts will have to b done by phone/email would the likes of Eileen's Emporium be a good alternative from which to glean one on one info and advice as well supplying whatever is needed by way of parts?

 

Model railway suppliers are expensive for electronic parts - they have to buy them from other sources and then divide them, making a profit.  And I doubt Eileen's carry these things.  

 

You'll be much better dealing with an electronics supplier in your country.  eg. Conrad in many countries connected with Germany.   Or via eBay/amazon/etc.. 

 

Resistors are simple, they come in packets of at least 100 for a penny (or less) each. 

Either as a bulk pack of different sizes (but you'll never use 90% of them)
https://www.rapidonline.com/truohm-cr25-carbon-film-resistor-kit-1000-pcs-13-0200

or as single types (100 per packet - you need about six sizes to cover most LED situations)
https://www.rapidonline.com/truohm-cr-025-1k-carbon-film-resistor-0-25w-pack-of-100-62-0370

 

DC power supplies come from similar sources, or any general electrical retailer. 

 

 

- Nigel

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There is a tendency to think that electronics is a precision discipline. It definitely is not. Resistor values is a case in point. You pick the nearest preferred  value to the one you have calculated, and that probably has a 10% tolerance. (By the way if you buy a bag of 1000 1KΩ resistors at 10% tolerance NONE of them will be close to 1KΩ because they will have been removed and put in the 5% or 1% bags). A lot of it is trial and error, don't get too bogged down in detail. Experiment and you may burn an LED or two, but you will learn from experience, the best way.

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On 27/07/2020 at 12:04, Nigelcliffe said:

 

 

Keeping it "dumb",  then ignore the post by Paul on the Buck Converter.  

If you use a DC power supply, then you can ignore reverse voltages stuff.  

 

 

1 - your LEDs need a resistor, one per light.   Calculator suggested by trevora is OK,  or,

if you have a 12v supply, you could start with 2kOhm (2000 ohm) and drop to 1kOhm if that's too dim. 

if you have a 5v supply, start with 470ohm, and drop to 220ohm if too dim. 

In both cases, increase the values if too bright.

 

2 - power supply.  Go with something DC, between 5v (common) and 12v (common).   A simple "wall brick" will do, such as an old phone charger or similar - read the voltage off its case, 5v is very common for chargers.  

 

3 - how many per power supply ?   A typical LED is maximum 20mA =  0.020A  .    So, a 500mA power supply would be theoretically able to run 25 at full brightness, or a 1A power supply could run 50 at full brightness.  In practise, add a bit of "headroom", so 40 on a 1A supply.   If running at lower brightness, then you can run more. 

 

 

 

- Nigel

Or keep it really dumb and ignore Nigel's post as well :nono:

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1 hour ago, cliff park said:

There is a tendency to think that electronics is a precision discipline. It definitely is not. Resistor values is a case in point. You pick the nearest preferred  value to the one you have calculated, and that probably has a 10% tolerance. (By the way if you buy a bag of 1000 1KΩ resistors at 10% tolerance NONE of them will be close to 1KΩ because they will have been removed and put in the 5% or 1% bags). A lot of it is trial and error, don't get too bogged down in detail. Experiment and you may burn an LED or two, but you will learn from experience, the best way.

I know they used to make/find resistor values like this. But isn't the manufacturing process now sufficiently good, that they just make the required value in batch form? Even computerised testing of each one, would blow the price to unacceptable levels, for the average user.

Military specs would be different.

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Again many thanks to all respondents, lots of clear advice in those and previous replies, I can feel my confidence growing already, time to get ordering in those resistors and search the house, or more likely the kids houses, for suitable transformers.

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On 29/07/2020 at 14:50, kevinlms said:

I know they used to make/find resistor values like this. But isn't the manufacturing process now sufficiently good, that they just make the required value in batch form? Even computerised testing of each one, would blow the price to unacceptable levels, for the average user.

Military specs would be different.

Actually each metal film resistor is laser trimmed to the required resistance and tolerance 

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Apologies for the delay but I have been busy building a timber shed from scratch for my son nd it took rather longer than anticipated so no railway work done for a while,

 

Can anyone please explain the difference between wiring the leds in parallel as against wiring them in series,and what exactly each term means for a complete electrics dunce like me. 

 

Also can I use the likes of speaker cable to run a bus wire round the layout for the leds, I have used a bus wire for point motor wiring, a lot stronger than speaker cable let me add,  so I at least have some knowledge there!

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4 hours ago, MOH said:

Apologies for the delay but I have been busy building a timber shed from scratch for my son nd it took rather longer than anticipated so no railway work done for a while,

 

Can anyone please explain the difference between wiring the leds in parallel as against wiring them in series,and what exactly each term means for a complete electrics dunce like me. 

 

Also can I use the likes of speaker cable to run a bus wire round the layout for the leds, I have used a bus wire for point motor wiring, a lot stronger than speaker cable let me add,  so I at least have some knowledge there!

If you run a bus around the layout (an excellent idea), then you're going to want to wire your LEDs in parallel with a series resistor each. Resistor calculation is as previously posted, which will depend on your actual voltage.

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22 hours ago, MOH said:

Can anyone please explain the difference between wiring the leds in parallel as against wiring them in series,and what exactly each term means for a complete electrics dunce like me. 

 

Not sure if this helps but, in a simplified form:

Series means you take one wire from the first LED and

connect it to the power source (transformer/battery).

You then connect the other lead from that LED to one

lead of the next LED and so on till you get to the last LED.

You connect the second lead from the last LED to the

other connection on your power source,

completing the circuit.

 

Parallel means you connect one wire of the first LED

to each output of the power source, making a circuit.

You then connect one wire from the second LED

to each wire on the first LED, and one wire from

the third LED to each wire on the second LED and so on.

 

If you want to get into the complicated side and

calculate the total resistance for all the LEDs:

 

Series is easy, you add the resistance values of

all the LEDs together, or if they are all the same,

multiply the value of one LED by the total number.

So if for example you had 5 LEDs each of 10 ohms,

the total resistance would be 10 x 5 = 50 ohms

 

Parallel is a bit more complicated, you add together

the reciprocal of each value (1/value) and this gives

you the reciprocal of the total circuit values.

So if you had 5LEDs of 10 ohms in parallel, you

take the reciprocal of the resistance, 1/10 or 0.1 and

multiply that by the number of resistors 5 x 0.1 = 0.5.

The total resistance is then 1/0.5 or 2 ohms.

 

Confused, if not you should be!

 

( I'm sure someone will soon come along and

correct my maths, or point out that LEDs don't

exactly follow the resistance theory but I did say

it's a simplified explanation).

 

Edited by rab
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Thanks again, I think I have the grasp of the different wiring methods and will most likely opt to use a bus wire, any thoughts on the strength of cable to use as the bus wire, I hope to source some from Screwfix but the choice is bewildering (to me at least!).

 

I copped some reasonably priced drums of 2 core speaker cable and was hoping that might do the job, the wires coming from the leds themselves seem very fine so presumably a stronger cable is required for the bus wire, any minimum sizes I should be looking at?

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Your twin speaker cable should be ok to use for lighting LEDs.  You will need to ensure the positive feed wire is marked throughout - this may be by the insulation colour e.g. red etc or at times by a stripe/line on the one wires insulation. Just ensure you use the same wire throughout for the LED Anode (Positive) feed.

 

Next comes the question of the need normally to add a series resistor. This resistor can be in either the positive feed to the LED or the Negative, it really doesn't matter. But do try and maintain a common standard i.e. all resistors are in the Negative (Cathode) LED lead etc.  The only time a series resistor is not used is where the LED is supplied rated at the same as the supply voltage e.g. 12 volt rated LED, here the LED has a factory built in resistor.   An additional resistor is then not needed, unless you wish to reduce the brightness of the LED then just add a series resistor of a Ohm value to reduce the light level - say in the 1K to 5K region. 

 

Power supply voltage hasn't been mentioned since Suzie suggested a 5v USB supply and the heading suggesting 3 volt.  3v IMO this is too low a voltage.  I would use at least as a minimum 5 volts DC and possibly 12 volts DC.  Both coming from a totally separate power source that is regulated too.  Personally I use 12 volts DC feed from a power supply rated at 1.0Amp to 2.0Amp,  There are many of these sold relatively inexpensively, often under the sales heading of "CCTV power supplies".  eBay and Amazon have plenty and this is a typical example...  12 volt power supply example

Normally all these so called CCTV 12v Power Supplies (PSU) are supplied with a moulded 2.1mm x 5.5mm DC plug.  You can either cut off the moulded DC plug and connect the two wires as required or IMO it is far better to leave the plug intact and use a matching female connector, either one of these which allow very easy onward wiring  Example connector or a chassis or In line style mating Female 2.1mm socket  Note; the link I've placed for the PSU above that the same supplier sells these female connectors too.

 

Series resistor.  IMO for ease of use, use one resistor per LED.  On a 12 volt power supply I always use at least a 1K0 (1000) OHM 1/4 watt or 1/3 watt type. This runs the LEd at a lower current and typically around 10milliamp (0.01A) per lit LED. They sell for pennies each on eBay etc  Resistor example  Resistors can be installed either way around as they are not polarity conscious devices.   But LEDs are and they MUST be connected correctly to the supply. New LEDs have their longer lead as their positive (Anode) connection.

 

All the links above are examples and there are of course many other suppliers too.

Edited by Brian
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Thank you,

 

I quoted a 3v transformer as the leds were advertised as only to be used with 3v, I have test wired three leds to a mobile phone charger and those are working with no resistors added and no blow outs so far.

 

Really no idea what voltage that mobile charger is rated as it's quite old and the details are blurred on the casing.

 

I will plough on so and order that speaker cable and see how that goes.

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remember LEDs are designed to be supplied by a constant current , not a constant voltage , hence any power source thats not current limited runs the risk of destroying the leds . Hence with a  voltage source like a traffo , resistors are always needed 

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