LED power supply

William_H said:
The 5050 LEDs are an interesting device. They have 3 diodes in the package which collectively excite a phosphor to provide the white light. The LED diodes seem to emit a blue green. The diodes are each accesible on the back but are usually wired in series for 12v with a current limiting resistor added. The device is clearly made for providing lighting as opposed to Annageek sugesting some are made for SMD indication. Regarding getting too hot and insufficient heat dissipation well this may be so. Heat is a function of current so any additional series resistance to reduce the voltage/current will reduce the heat and light output. But improve reliability. However I suspect that some reliability will be lost in the corrosive environment on a boat.
Just as an excercise in time wasting fiddly things. I had one of those little candle lanterns used for Christmas decorations. I got 4 x5050 chips. Removed them from the base printed circuit and rewired them in parallel. The 4 face outwards in a square mounted on copper wire. With a suitable series resistor they run on 3 x1.5v cells and provide as much light or more as the candle. But no fire risk. Definitely fiddly. A winter project for the insane. olewill
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Perhaps indication LEDs is the wrong term, but certainly, the 5050 type LEDs were never developed for normal illumination. I think they were originally used for things like backlighting in TFT screens. It just so happens that because LED backlit TFT screens are so numerous these days, they have driven the 5050 and similar diodes in to huge levels of mass production - reducing their cost below that of 'designed primarily for general illumination' type LEDs. As a result, they've been the cheapest option for the LED strip makers and cheap LED replacement conventional bulbs, and so they've found use for these applications too on teh grounds of low cost, rather than technical suitability.

I believe the 3 diodes (RGB) in the 5050's are the same design straight out of the PLCC2 package, which contains only a single diode, and these were certainly developed for indication over illumination. That said, I am sure the big players like Cree, Philips and Osram have been working hard cramming higher luminous flux and better heat managed dies into these same packages.

For completeness, its not a great idea running LEDs of any type in parallel (without a current regulating shunt resistor in series with each string). No two diodes are equal, and so the current running through each parallel diode will be different. Furthermore, diodes display (for a period) negative temperature coefficient, so as the diode that is taking the largest amount of current heats, its effective impedance increases, and so it takes more and more current, resulting in a sort of thermal runaway. In practice, this may not happen immediately, and providing your source impedance is such that even if you only had a single diode in circuit then it should still operate within it's safe operating area (SOA), then you should be fine. You will find though, that over time, there will be a very noticeable difference in brightness of each LED. Simple fix, though - 4 resistors of 0.25X ohms (one in series with each diode), rather than 1 resistor of X ohms.
 
Hi Anna, did you mean " ...effective impedance decreases ..." in the third line of your final paragraph, or have I misunderstood what you're saying? David.

ps: If anyone buys/tries any of the supply units discussed (or others) please do report back. I'll do the same. D
 
Ok, I've got the 5050 LED strip and the ally channel. I've made up and fitted the light strips and they look rather neat. I'm just waiting on the power supply units to arrive from China. http://www.ebay.co.uk/itm/DC-Buck-Step-Down-Converter-DC-5A-Regulator-Voltage-Current-Adjustable-WT-/151311737680?tfrom=161303594673&tpos=top&ttype=price&talgo=undefined
Question: When I get the power supplies (and I suspect they will probably arrive with incomprehensible instructions) will I be OK to attach them to a 12V supply with no load? I don't want to switch on and find I'm outputting 30V into my new LEDs. Equally I don't want to blow up the new power supply. Can anyone tell me (in simple terms please) what the relationship will be between the voltage and current controls?
Also, does anyone have a preferred source for cheap small enclosures to hold the power supply? I don't yet know the dimensions.
 
OK, here's my update.
I decided to try this component
http://www.farnell.com/datasheets/1855682.pdf from Farnell, with this heatsink http://www.farnell.com/datasheets/1680141.pdf.
Built it today and powered it from a switchable mains-DC power supply. Here's the results, driving 2 x 2 -metre strips of 3528 LEDs (i.e. 4 m)

Input volts 15.0 V 13.5 V 11.9 V
Output volts 12.05 V 12.05 V 11.65 V
Output current 1.40 A 1.40 A 1.24 A

Points of interest:
Input current was about 30 mA greater than output current at all voltages.
Output current is about as expected for 3528 LEDs (around 350 mA per metre). It should safely power a full 5m strip of 3528s
The device runs happily with an input of less than 12 v and still provides power to the LEDs with only a 0.25V volt drop. The input voltage was supposed to be 12V for this test - I suspect the current draw brought the input down to the measured 11.87 V.
Left running for around an hour, the heatsink became quite warm but not too hot to touch

Here's the setup showing the output current at 15 V in (current shows 1.34 A here)
863c64fb-24ff-44a9-8e7e-4c482c0bec0d_zpsb66db4f7.jpg
[/URL]
(Hope that link works)

Interestingly I then tried it running a full 5m length of waterproof 3528 LED strip, which is also rated at 0.4a/metre. The total current showed around 0.8A. Any ideas why the current should be so much less than expected).


The device and the heatsink are around £1 each ex-VAT from Farnell. They also do a 3-Amp version of the device, which I have bought but haven't tried yet.

Hope this is useful to someone.
David
 
Another attempt at posting the second pic

3fe1d12c-e633-4a1c-9393-4da357a7ccae_zps926056cd.jpg
[/URL]

Ignore the fact that it looks as though I had wired the LED strip with inverse polarity - I haven't, it's just the colours of the wires in the connector I was using!
 
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DRA said:
Interestingly I then tried it running a full 5m length of waterproof 3528 LED strip, which is also rated at 0.4a/metre. The total current showed around 0.8A. Any ideas why the current should be so much less than expected).
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Interesting. When you drove the 4 metres were they in series or in parallel? Possibly voltage loss over the full 5 metres length means the LEDS at the end of the strip were rather less bright than the ones at the beginning. You could try driving it from both ends and see if the current increases.
 
Good suggestion, Alahol. The two 2-m strips were in parallel. I set up the waterproof one to drive from both ends and the current draw went up to about 1.2A, with a visible increase in brightness of the LEDs at the newly connected end. With both ends connected there was a small difference in brightness between the LEDs at the end of the strip and those in the centre (though you probably wouldn't notice it unless you put the two parts of the strip close together).

This shows that if you're driving long strips you need to give some thought to where you connect the supply. On any length more than a couple of metres I guess connections at both ends and in the centre would be the best bet.

I just went back to compare the relative brightnesses of the two and realised that the waterproof strip is "cold white" while the other is "warm white". This probably explains the different current draws. I don't want to cut the waterproof strip yet so I'll leave a comparison of shorter but similar lengths of each until another day.
 
GHA said:
you could always make your own constant current driver, just use a higher power mosfet instead of the transistor in this circuit...:cool:

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I knocked up one of these just for interest. A bit dissapointing as the voltage drop was quite a lot 3 volts or so before current fell. The little regulator seemed to need more voltage than spec to actually start regulating. I feel the difference is that you could only run 2 LED in series rather than 3 in any string using a resistor.
Makes me think though I havn't experimented that a shunt regulator might be better to run LED strips or individual LED using a ressitor on a boat with a smart regulator ie 15v at times.
A shunt regulator would require that some of the series resistance for the LEDs be common to all strings. At 12v input the ressitor would along with the individual series resistors give a correct LED current. The shunt regulator is a circuit which does not draw current until the voltage rises. A 12v zener diode is a simple example. The regulator starts to draw current thus increasing the voltage drop on the common current control resistor so limiting voltage to 12v regardless of the input.
It might waste power and get hot but this onl;y when the battery is on charge so no problem. The advantage is that it may be possible to run 4 LED in series with loss of light as battery gets lower than 12v but an extra LED for no more current must be a good thing.
The little regulator as in the U tube can be adjusted witha trim pot to exactly 12v or perhaps a bit more. It can also drive a power transistro to provide the current load capability for higher power. Just theorising so if any one interested in experimenting PM me olewill
 
Thanks DRA that's a useful bit of research. Both my main strips will be just under 2 metres each so I'm hopeful. I'll try driving from one end but if it leaves a noticeable difference between the first and last diodes I'll lay in a couple of extra wires to drive from both ends.
 
As I said in the other thread, my prefered LEDs are now the ones based around the WS2812 drivers. Just need a good stable 5v psu and a means to control them.

http://www.aboardmyboat.co.uk/projects/files/2015/WS2812.pdf

Doing a corporate project this week with 7200 of these things :) they are also 5050 package, but it seems with the fairly large internal metal areas they dont seem to run that hot. Time will tell but they are great fun to play with.
 
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Ko
William_H said:
I knocked up one of these just for interest. A bit dissapointing as the voltage drop was quite a lot 3 volts or so before current fell. The little regulator seemed to need more voltage than spec to actually start regulating. I feel the difference is that you could only run 2 LED in series rather than 3 in any string using a resistor.
Makes me think though I havn't experimented that a shunt regulator might be better to run LED strips or individual LED using a ressitor on a boat with a smart regulator ie 15v at times.
A shunt regulator would require that some of the series resistance for the LEDs be common to all strings. At 12v input the ressitor would along with the individual series resistors give a correct LED current. The shunt regulator is a circuit which does not draw current until the voltage rises. A 12v zener diode is a simple example. The regulator starts to draw current thus increasing the voltage drop on the common current control resistor so limiting voltage to 12v regardless of the input.
It might waste power and get hot but this onl;y when the battery is on charge so no problem. The advantage is that it may be possible to run 4 LED in series with loss of light as battery gets lower than 12v but an extra LED for no more current must be a good thing.
The little regulator as in the U tube can be adjusted witha trim pot to exactly 12v or perhaps a bit more. It can also drive a power transistro to provide the current load capability for higher power. Just theorising so if any one interested in experimenting PM me olewill
Click to expand...

Did you use a tlv431? I made one on a piece of very board and it behaved exactly as per the video, very accurate tracking of 100mA with whatever the resistor size was, using a power fet. For memory input was down towwrds 11v before the 100mA started tailing off. Made no difference how high the input voltage went, can't remember what buch/boost power supply went, but well over 15v probably. Sounds like somehow our circuits were different.

#edit. all coming back to me, that was driving 3 x cree high brightness led's in series on a bit of breadboard, tracked current very accurately.

edit2:

Found a circuit breadboard sketch. FET is a STP16NF06, shunt is a TLV431. Running 3 x C503D-WAN-CBBDB152 LED's in series.

leddriverstripboard_zps0749b89d.jpg




Selection_003_zps55617c0f.png
 
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GHA said:
Ko

Did you use a tlv431? I made one on a piece of very board and it behaved exactly as per the video, very accurate tracking of 100mA with whatever the resistor size was, using a power fet. For memory input was down towwrds 11v before the 100mA started tailing off. Made no difference how high the input voltage went, can't remember what buch/boost power supply went, but well over 15v probably. Sounds like somehow our circuits were different.

#edit. all coming back to me, that was driving 3 x cree high brightness led's in series on a bit of breadboard, tracked current very accurately.

edit2:

Found a circuit breadboard sketch. FET is a STP16NF06, shunt is a TLV431. Running 3 x C503D-WAN-CBBDB152 LED's in series.

leddriverstripboard_zps0749b89d.jpg




Selection_003_zps55617c0f.png
Click to expand...

Thanks GHA I have found the problem. I was using a TL431 which has a 2.5v reference as opposed to the TLV431 which has a 1.25v reference hence the added volt lost in the circuitry. I should have checked rather than assume they were the same. olewill
 
alahol2 said:
Ok, I've got the 5050 LED strip and the ally channel. I've made up and fitted the light strips and they look rather neat. I'm just waiting on the power supply units to arrive from China. http://www.ebay.co.uk/itm/DC-Buck-Step-Down-Converter-DC-5A-Regulator-Voltage-Current-Adjustable-WT-/151311737680?tfrom=161303594673&tpos=top&ttype=price&talgo=undefined
Question: When I get the power supplies (and I suspect they will probably arrive with incomprehensible instructions) will I be OK to attach them to a 12V supply with no load? I don't want to switch on and find I'm outputting 30V into my new LEDs. Equally I don't want to blow up the new power supply. Can anyone tell me (in simple terms please) what the relationship will be between the voltage and current controls?
Also, does anyone have a preferred source for cheap small enclosures to hold the power supply? I don't yet know the dimensions.
Click to expand...

Have you got anywhere with these yet, if so whats the minimum input voltage for 12volts output (ie. drop across the module?)
 
mlines said:
Have you got anywhere with these yet, if so whats the minimum input voltage for 12volts output (ie. drop across the module?)
Click to expand...
OK finally received the modules today and have done a bit of experimentation with very limited resources. (http://www.ebay.co.uk/itm/DC-Buck-Step-Down-Converter-DC-5A-Regulator-Voltage-Current-Adjustable-WT-/151311737680?tfrom=161303594673&tpos=top&ttype=price&talgo=undefined&clk_rvr_id=779550112685&afsrc=1&rmvSB=true)
The voltage drop across the module appears to be about 0.7V, so to get 12V out you need to supply 12.7V input.
The maximum current I managed to produce from the unit was about 1.7A. Running for a while it got very slightly warm.
I tested the module with a 1.9metre strip of 5050 LEDs at 60LEDs per metre.
The 5050 LED strip stays usefully illuminated down to around 9Volts. Driving the strip at 11Volts and then 12V I couldn't really notice any difference in brightness. At 10V the strip consumed 0.44A, at 11V it was 0.75A and at 12V it consumed 1.04A.
I intend running my strips on the boat at 11V to try to extend their life so this module should be OK to limit the input when the engine is charging or the battery is high.
One thing that surprised me, but is probably well known to the electronics experts, is that the apparent 'Resistance' of the LED strip went down as the voltage/current increased. At 9V the 'Resistance' was 45 Ohms, at 10V 22 Ohms, at 11V 15 Ohms and at 12V 11.5 Ohms. Completely the reverse of a filament bulb.
Anyway, hope that helps, is interesting, to someone.
 
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