SMD LED 12v through 1mm

[meadowhog]

Im sold on SMD LED rope lighting. I bought some as a back light for some stairs but tried it out under the kitchen units and Im amazed to say the light is just like Halogen!! I love the light and as its only 10mmx 4mm its extremely versatile and dissapears into tiny places, it can even be placed just by the door under kitchen wall units instead of by the wall.

I divert. my question ; has anyone any experience of using this 12v rope light as a main source of light. I would like to use SMD 5050 with a dimmer but at 12w per metre, the 1mm lighting circuit cant take a tranny to convert 230v to 12v and include all the lighting I require.

Is there a tranny that can take a 230v input from a standard dimmer and vary an output to drive SMD5050. Or is there something else OR are the SMD 3528 bright enough. Their wattage is that much lower that the 1mm wire can take the current I require for each room.

I know there are dimmers with remote control that would work but it would mean 2 remotes per room which Im trying to avoid.

 

[ban-all-sheds]

Check with the makers what you need to do to dim it.

 

[ColJack]

i don't get what you're asking..

12W per meter means you can have 5m before you get the same as a single 60W bulb... just how big is your kitchen?

what 1mm cable are you on about?

 

[ericmark]

To get extra bright LED's it is common to overdrive then high speed switch the supply. Even when used as plain LED the LED is to a great extent self regulating and unless you have some special type designed to dim there is very little chance of dimming LED's. Check with manufacturer.
LED's run at around 1.2v so where a 12v supply is used you already have some internal control. Each strip at max length needs 5A and this as a DC supply likely would need a switch mode power supply for each strip costing about £10 per unit to get a supply which allows a variable output both size and price become a problem I paid over £100 for a 1A unit giving variable volts and amps for a 5A unit it would be very high cost and controls are on the big square metal box not on a wall mounted switch. I have looked at adverts and see no reference to dimming so would think they can't be dimmed. Although there may be some control with the tri-colour type but again ask manufacturer.

 

[bernardgreen]

Need to be pedantic

The term LED = Light Emmitting Diode. The LED is a semi-conductor device which generates light when current passes through it. The amount of light generated is proportional to the current passing. That current has to be controlled by components external to the LED.

A lot of light fittings that used LEDs are called LED when in fact they consist of one or more LEDs together with the neccessary control gear to control the current through the devices and hence the brightness of the light.

To get extra bright LED's it is common to overdrive then high speed switch the supply.

More precisely the LED is driven by short pulses of current to create pulses of light. Provided the pulse does not exceed the maximum pulse rating of the device and the average current is less than the rated average this can be a very energy efficient way of getting light. It does have one drawback in that the light is not continuous even though it appears to be. The eye retains the image long enough the bridge the dark time between pulses. The image retention however can create the appearance of several images of a moving object when the only light is pulsed LED light.

Even when used as plain LED the LED is to a great extent self regulating

NO ! ! the basic LED is not self regulating. It requires that the current through it is controlled. Simplest way is a resistor in series

and unless you have some special type designed to dim there is very little chance of dimming LED's.

True for many lights using LEDs but not true for the basic LED device.

The LED is about the only light source that can be dimmed easily and without any of the problems that are there when under-running other types of lamps.

But the light fittings with LEDs as the light source have control gear and it is the control gear that makes many LED lights difficulty or impossible to dim.

 

[ericmark]

I do concur with most of what you say. However the LED current is also dependent on threshold voltage it is after all a diode not sure if avalanche or zener effect, not sure of voltage that separates the two effects, but I have used the effect to raise the voltage of a 7012 chip to 13.2v plus give a warning lamp at the same time.

However the flexible printed circuit refereed to in the post is supplied by 12vdc and it must therefore have within the strip some form of current control and we have no idea as to what form of control is used.

If we consider that a micro processor could be used then reducing the voltage could cause the processor to fail and if on the "Mark" rather than "Space" part of the cycle it could burn out the LED's.

Also 5A DC at 12V can suffer from volt drop and although plugging directly into a local power supply is unlikely to cause a problem running long cables with the DC would require some consideration as to the volt drop.

My quick calculations I stand to be corrected for 1mm cable is 0.22 volts per meter. So I would consider 5 meters would likely be the max which leaves the supply at 10.9v.

I realise my account was not absolutely correct but the point that to try to dim lamps not designed to dim is likely to cause problems which may not be apparent until the lights have failed.

Having tried to return items to China I know it is not simple and for me it cost nearly 3/4 of the price of goods in postage and customs charges. So I would not recommend distance buying from China. Hong Kong very different even now. All the adds I found seemed to be linked to china.

 

[bernardgreen]

What was the current in your calculation.

I will accept that the vast majority of avialable "white" LEDs are manufactured with one or more current sources built into the silicon chip in order to correctly match the three or more single colour emitters needed to create "white" light. ( as opposed to the UV LEDs which produce "white" light by UV excitation of phosphers in the package. ).

But when buying a ready made fitting one is at the mercy of the manufacturer and his profit margins so cheap and inflexible go together.

 

[DetlefSchmitz]

However the flexible printed circuit refereed to in the post is supplied by 12vdc and it must therefore have within the strip some form of current control and we have no idea as to what form of control is used.

I have some of these leds under my kitchen counters. The ones I have, at least, are repeated patterns of three leds plus one resistor all in series, with this circuit then being paralleled up with all the other such circuits across the 12V dc supply. The associated 'transformer' is just a 12V dc switched mode power supply.

A dimming 'transformer' is also available, which is clearly just pulse width modulated to change the brightness.

 

[Steve]

Im sold on SMD LED rope lighting. I bought some as a back light for some stairs but tried it out under the kitchen units and Im amazed to say the light is just like Halogen!!

So what's so good then? Halogen light is awful for lighting worktops.

 

[foxhole]

i don't get what you're asking..

12W per meter means you can have 5m before you get the same as a single 60W bulb... just how big is your kitchen?

what 1mm cable are you on about?

Thats the wattage used , does not relate the the light output.Which is measured in Lumens.

 

[ColJack]

didn't say it did..
I was refering to the wattage of the lamp, not the light output..

you need 5m of that "rope light" to draw the same current as a 60W lamp.. ( minus whatever losses the power supply uses of course.. )

since the average house has several 60W lamps on one 1mm T+E radial ( or at least used to ) then 1mm would be ok for several 5m runs of the "rope light".. that's what I was trying to get at and hence the question..
what is this "1mm" that the OP is asking about?
supply cable to the circuit? (ie the lighting circuit in that area), or output cable from the power supply to the "rope light"..

the whole thing has got me questioning myself..
cable ratings, are they so many amps at so many volts ( ie a "power" rating in reality ), or is it just so many amps irrespective of the voltage?

it is after all about heat dissipation so a wire that's rated for 10A at 230V is in effect capable of ridding itself of the heat generated by 2300W running through it ( in a given time frame ) before suffering damage.. so then at 12V should it's current carying capacity be higher?

 

[bernardgreen]

cable ratings, are they so many amps at so many volts ( ie a "power" rating in reality ), or is it just so many amps irrespective of the voltage?

The energy dissapated in the cable in watts is amps times voltage drop along the cable. The number of amps and the resistance of the conductor determines the voltage drop involved.

W = Vd x I

The voltage drop is determined by amps times resistance of the cable.
V = R x I

The resistance is for practical purposes constant.

Therefore watts per metre of cable = R x I x I where I = amps and R = ohms per metre

it is after all about heat dissipation so a wire that's rated for 10A at 230V is in effect capable of ridding itself of the heat generated by 2300W running through it

watts do not run through a cable. only amps run through a cable

The current rating is the maximum current the cable can safely carry.

The voltage rating of a cable is the maximum design voltage the cable can be used for safely. It is the "goodness" of the insulation.

( in a given time frame ) before suffering damage.. so then at 12V should it's current carying capacity be higher?

No because as explained above the operating voltage ( voltage between the two wires ) is not involved in the generation of heat in the cable

 

[bernardgreen]

Single colour LEDs can be dimmed by reducing the current through them.

It is not so easy to dim "white" LEDs

White LEDs are not a single colour LED but are at least 3 single colour LEDs in the same package with the three colours mixed to produce white light. Each single colour LED has to be the right brightness compared to the other two to get the correct white result. The current in each single colour LED is set by a resistor for that colour.

LEDs of different single colours have different voltage drops which complicates the matter. Changing the supply voltage changes the ratio of currents in the diodes and so the "white" colour changes to a non white colour.

It is simple to set a fixed supply voltage for "white" LEDs and choose the three curent setting resistors to suit that voltage.

 

[aptsys]

White LEDs are not a single colour LED but are at least 3 single colour LEDs in the same package

BS! I have a high flux white LED sitting right here on my desk and it is made from a single die. One of the projects we worked on in the past needed essentially a point white light source. If they these LEDs had more dies in them then it wouldn't have worked!

High flux LEDs do not have resistors in them either. And PWM dimming from a current source eliminates the problem of the LEDs spectra changing in a mismatched way until the average current is very low. You definitely don't change the brightness of LEDs by changing their voltage

 

[echoes]

I do concur with most of what you say. However the LED current is also dependent on threshold voltage it is after all a diode not sure if avalanche or zener effect, not sure of voltage that separates the two effects...

It's neither avalanche nor zener since it operates in forward-bias (although it will break down at some point in reverse-bias due to one or both of these effects). IIRC, the relationship between current and voltage is an exponential function of Voltage, Temperature, and the dark saturation current.