Hi everyone,

I have a project I'm attempting with LED strips...Please be aware, that I have no real understanding of electronics what so ever, only the very basics but I'm looking to go on a course sometime soon, but hope some of you may be able to give some assistance on what I am trying to achieve and explain it in a very basic way. I have included an image below which should give a better representation as to what I want to do.

The main project is held within a 6" x 8" wooden frame (imagine a picture frame come light box). I am inserting a small LED strip on the inside of the frame facing upwards (only one strip on one side). the frame will have a glass front to it, and a backing board to the other side, to close the casing...There are no more than 9 LEDS's within the strip I want to use, and measures about 17cm length. It may be reduced to 6 LED's on smaller frames...I have cut this strip from a 5 meter length LED strip and this was provided with a DC 12W (1.5a) mains adaptor to power it. However, for a smaller strip like I want to use, what sort of power would be required, since surely this is to much power for such a small strip?

Secondly, I want to insert a DC power socket panel into the side of the wooden frame if a DC power supply is required, but don't really know where to start with this and how I would fix this into a frame. Would this be the preffered method or is there another way of powering such a unit? As can be seen in the picture, next to the socket panel, I would also like a button so I can turn the power on/off...

I'm sure this type of set up is fairly simple, but wondered what the best way of doing this would be, and exactly what parts I would need to give this a professional finish.

Thanks for any help in advance...

 

Hi daveym,

Welcome to the fourm!

So, a few questions:

1) All the LEDs from the strip are blue? If not, what color(s) are they?

2) What is the make and model of the strip you're using?

3) What is the output voltage of the mains adapter? Is the current 1.5A or something else?

4) Can you look at the entire strip and see how the LEDs are wired together? Are the all connected in series, parallel, or a combination thereof?

5) How many LEDs came with the strip total?

6) How do the LEDs appear to be wired in the cut 17cm strip (series, parallel, or combo)?

It shouldn't be a problem to do what you want to do with a smaller mains adapter and switch. You'll probably need to add a resistor to limit the current to the LEDs, but we can help you figure that out once the above questions are answered. We can deduce the voltage requirement of the LEDs by knowing the color. The big question is how the manufacturer wired the strip.

In essence, you want to get about 20mA to each LED with the correct voltage.

LEDs wired in series will give you even light output from each and they will each draw the same current, however, the voltage required depends on the LED and the total number wired in series. Blue LEDs are generally rated at 3.5VDC each. So, if you had six blue LEDs wired in series, you'd need 3.5 x 6 = 21VDC at 20mA. You probably won't find a 21VDC supply, but more likely a 24VDC. Therefore you take the supply voltage minus the LED voltage and divide by the LED current to get the current limiting resistor you need. In this case: (24-21)/0.02 = 150Ω, just one. Then you need to factor in the fact that DC mains adaptors usually aren't precise, especially with tiny current draws, so a bigger resistor may be needed. Then we need to be sure we get one with a high enough wattage rating . . .

LEDs wired in parallel can still give a reasonable light output, but you need to use a resistor for each LED. So, again say we have six blue LEDs wired in parallel. Your voltage for all six is the same as it would be for one since they're in parallel, but you need to supply 20mA to each LED now. So, voltage needed would be 3.5VDC at 6 x 0.02A = 120mA. Let's say you get a cheap 6VDC mains adapter rated 200mA or more. Again: (6-3.5)/0.02 = 125Ω resistor is needed for each and every LED.

Okay, more than you wanted to know about LEDs, but that is why we need to know how the LEDs are wired in the strip.

 

Hi,

thanks so much for your first reply...it was very in-depth, and I've tried to include as much info from the spec of the led strip as possible. The LED strip was purchased as a 5 meter roll from a reputable chinese supplier that sells on ebay. As far as can read, nothing needs to be added to the LED strip apart from a mains adaptor. If you cut any piece off the roll, every 3 LED's there are soldering points (apologies for my basic knowledge).

I hope everything below is ok and you might be able to assist. My main issue was if using a mains adaptor how to get this sitting permenantly (and looking professional), into the side of a frame.

To give you additional info as to the purpose of these, I have someone that wants to hang these frames in a bar like a sign board, and they were enquiring how difficult it would be to add this LED strip, so this frame would be hanging off a wall, hence why I need them looking as professional as possible.

Specification:

• color: Blue
• 60 SMD LEDs per meter, total 300 LEDs for 5 meters
• Flexible, every 3 lamps (approx.per 5cm) can be cut apart
• Use 3528 SMD LEDs
• Flexible strip and can be bent at any angle
• Self-adhesive back
• Maintenance free, easy installation
• Long life span: 50,000+ hours
• Input volts: DC 12V
• Current: 1.5A
• Brightness/m: 260 lumen
• Thickness: 2mm
• width: 8mm
• Flexible ribbon for curving around bends
• Completely smooth and even light spread, solving the uneven luminous problem
• Ultra-bright but running at low temperature
• Certification: CE & RoHS
• Low power consumption
• Perfect for your house, garden, auto, bar and Christmas decoration!
• Perfect for accent lighting and enhancing certain areas around your home, Shop, decorating your home
• Easy installment, with 3M high temperature resistance double-sided adhesive on the back
• Wide viewing angle (160 degrees) ensures large illuminated surface

Please note: In order to be used at home, you need to buy one 240v AC to 12V DC power adapter. (More information in reference picture)

Wide Rage of Application:
● Widely for home decoration use, hotels, clubs, shopping malls, car industry
● Architectural decorative lighting, boutique atmosphere lighting
● Extensively applied in Backlighting, concealed lighting, channel letter lighting
● Emergency & security lighting, advertisement sign lighting
● Decorative lights for holiday, event, show exhibition
● Applicable for automobile and bicycle decoration, border or contour lighting
● Soft texture, can be bent, can be arbitrarily fixed in the uneven places.
● Easy installation, behind the strip, are free to paste.
● Each group of three lights to form a loop, low current, low power, energy and beautiful. Three LED can cut an incision.
● Mainly used for car decoration, home decoration, lighting, signs, advertising signs, wine, jewelry cabinets, entertainment and other decorative lighting.
specification

Installation Note:
• Please note this strip light can not be plugged into the mains directly!! There are two bare wire at the end of each led strip. For home or garden use, you need a power Adapter/ Transformer to convert AC 220V to DC 12v (not included in package). Below is a link for a power adapter.

• The voltage of car is 12v DC. So you can install it directly, no need for an additional adapter.

 

These appear to be parallel sets of 3 LEDs in series. But that doesn't quite add up, because only 3 in series would be damaged at 12V without a current limiting resistor, or some other way to drop the supply voltage to around 10V. This discrepancy needs to be resolved. Can you see any resistors included with each loop of 3 LEDs?

The max current per the LED's spec is 30mA, so it's correct to design for 20mA continuous current. If you want two, 3-LED strings to give you 6 lights total, you'd need just 40mA at 10V, or just 0.4 watts. So a very small adapter, but again it must be regulated and we need to figure out how they're applying 12V to just 3 LEDs in series. By regulated I mean the supply can't be a traditional wall-wart that might be 16V at low current.

 

I'm not sure what you mean when you say drop the power supply to 10V, but there will always need to be a current limiting component, and that is almost always a resistor. Mere voltage regulation will not/can not work, as LEDs do not work that way. I suspect you misspoke there.

For the OP, LEDs are current devices, the voltage they drop has to be calculated as part of the design, but it is incidental.

LEDs, 555s, Flashers, and Light Chasers

 

Daveym,

If possible, take a good, up-close picture of the strip, say 3-6 LEDs. We need to verify the manufacturer included a current-limiting resistor for each set of three LEDs. We'd also like to see how these connect to one another. If the resistors are present, then you should be able to use the following for each box:

Power Supply, 12VDC, 100mA:
http://uk.farnell.com/multicomp/mj-15sr/chassis-socket-psu-panel-mount/dp/1737252

Power Jack which will work with the above supply (if you can't make this work in your box, let us know how thick it is and we'll find one with an appropriate mounting option:

http://uk.farnell.com/multicomp/mj-15sr/chassis-socket-psu-panel-mount/dp/1737252

Switch - there are tons on Farnell, I'll leave that to you, but let us know if you need help. If so, then we definitely need the thickness of the box as well as the style you'd like. In the picture, it appears you want a push button - there are also rockers and toggles.

 

I suspect you misspoke there.

Nope, at 3.3V this particular LED will draw a hair over 20mA, well within the safety margin. Three in series with a regulated 10V supply would be fine. Applying one across the poles of 2 AAs in series for 3V would drop current to below 10mA, and going over 3.4V will risk destroying it (>30mA). That's why I asked the OP to look for the resistor - it virtually has to be there if the supply is even a regulated 12V, moreso if it's a wall-wart.

You're right that attempting to control LEDs by voltage gives a narrow target, just 0.5V from dim to destroyed. Controlling current is more reliable.

 

Obviously the LED strip has a current-limiting resistor for each set of 3 LEDs in series.

The LED strip spec's mention being powered in a car so an unregulated wall-wart might produce a voltage that is too high and burn out the LEDs. I have a 12V/100mA wall-wart that has an output of 23V when the load current is low.
Use a power supply with a regulated 12V output voltage.

 

thanks for all the help guys...I'm currently away for 3 days but will upload an actual close up picture of the strip sometime early next week, and that might be able to help.

 

Daveym,



Power Supply, 12VDC, 100mA:
http://uk.farnell.com/multicomp/mj-15sr/chassis-socket-psu-panel-mount/dp/1737252

Power Jack which will work with the above supply (if you can't make this work in your box, let us know how thick it is and we'll find one with an appropriate mounting option:

http://uk.farnell.com/multicomp/mj-15sr/chassis-socket-psu-panel-mount/dp/1737252

Switch - there are tons on Farnell, I'll leave that to you, but let us know if you need help. If so, then we definitely need the thickness of the box as well as the style you'd like. In the picture, it appears you want a push button - there are also rockers and toggles.

I've looked at similar in a local electrical shop (more expensive than farnell though), and my main issue is how to insert this into the frame. The frame is wood with a thickness of approx 1.5cm, and the trouble is how to get this sitting permenantly into it like I have in the picture and connected to the LED's as displayed.

 

Obviously the LED strip has a current-limiting resistor for each set of 3 LEDs in series.

Agreed, but it doesn't hurt to have the OP check beforehand.

The LED strip spec's mention being powered in a car so an unregulated wall-wart might produce a voltage that is too high and burn out the LEDs. I have a 12V/100mA wall-wart that has an output of 23V when the load current is low.
Use a power supply with a regulated 12V output voltage.

Also agreed, but finding regulated supplies isn't always easy and as the OP is in London, I'm not sure where he'd go to get one. I couldn't find any on Farnell that were called out as such.

Here's an example of a U.S. one though:
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_162996_-1

But, the key to using a standard, cheap wallwart for this case is picking one with a current output close to the circuit's requirement, hence the 100mA in the link I posted previously. If the OP powers 6 LEDs, then he needs 40mA (20mA per strip of three). If he wants them a little brighter, he should be okay giving them 25-30mA, making the total draw 60mA max. This should be enough to pull the voltage output close to the supply's rated voltage. If he powers 9 LEDs, then he'll pull 60 to 90mA, which is even better.

An additional current-limiting resistor may be needed so we'll have to keep that in mind, but may be easier than locating a regulated supply. That was my thinking anyway. If anyone can suggest a source for a regulated wallwart for the OP in London, please let him know.

Daveym, you mentioned making multiple boxes - will these be located throughout the bar (each plugged into an outlet of course) or will they be along the same wall close to each other? If the latter, you could get away with the single power supply you already have and daisy-chain each box by wiring them in parallel. You could still add connectors so they disconnect easily. This would also require only one of the boxes to have a switch (or put a switch directly on the power cable coming from the supply). This also depends on the total number of LEDs you plan to power, but you could power up to 225 LEDs with 1.5A, so that should be sufficient.

 

Another option with an unregulated wall wart or power supply, constant current sources similar to this...

 

Nope, at 3.3V this particular LED will draw a hair over 20mA, well within the safety margin. Three in series with a regulated 10V supply would be fine. Applying one across the poles of 2 AAs in series for 3V would drop current to below 10mA, and going over 3.4V will risk destroying it (>30mA). That's why I asked the OP to look for the resistor - it virtually has to be there if the supply is even a regulated 12V, moreso if it's a wall-wart.

You're right that attempting to control LEDs by voltage gives a narrow target, just 0.5V from dim to destroyed. Controlling current is more reliable.
View attachment 33845

I will have to disagree strongly at this point. I have in the past argued against thermal run away on LEDs using resistors for current limiting, but you have found the scenario where it is a virtual certainty. There is NO circumstance where you can regulate current via voltage on an LED, even if you were to measure the current and vary the voltage to match there would be the shunt resistance measuring the current. Vf does vary according to temperature, with a limiting resistance the resistor still controls the current.

 

Bill,

You truly are a wealth of knowledge. I learn something new here nearly every day. I may be asking for a world of confusion, but my curiousity has the better of me: how does the transistor LED driver work? I looked at your LED page and saw the circuit, but don't quite follow how it works.

I have a poor understanding of transistors for uses other than switches, so bare with me. I assume you're limiting the current going to the base of each transistor which in turn is only allowing a limited current to pass between the collector and emitter.

What do the non-LED diodes do? It almost looks like a voltage divider on the left side of the circuit, but I know (or think) you're limiting the current. Is this correct: [Vcc - 2(V_diode)]/1000 = current to transistor base?

If so, how do you determine how much current to put through the transistor base?

Lastly, how did you calculate 36Ω for the resistors? Are you using the standard current-limiting resistor calculation, (Vcc - V_LED)/I_LED, or something else?

Thanks!

 

The transistor with the two diodes and two resistors is a constant current sink.
Each diode has about 0.7V across it so both form a 1.4V regulated voltage.
The base-emitter voltage of the transistor is about 0.65V when its collector current is 20mA.
Then the 36 ohm resistor has a voltage of 1.4V - 0.65V= 0.75V across it which sets the LED current at 0.75V/36 ohms= 20.8mA.

Different LEDs with different voltages will have the same amount of current.
The current does not change much when the supply voltage changes a lot.

 

I didn't understand that circuit when I first started electronics. Basically you have a fixed voltage on the bases of the transistors. This translates as a fixed voltage across the emitter resistor. It doesn't matter at this point what voltage is on the collector, the collector current is very, very close to the emitter current. Since the emitter voltage is fixed, the collector current is fixed.

 

Thank you both! That really is a neat circuit.