Electrical Circuit for DIY LED, need input

[sacy]

u need to have heatsink for the 3Wled you bought. it will be hot. without heatsink your 3W led will not last long. ^^

[limsteel]

IS LED DRIVER A MUST FOR HIGH POWER LED? I planning to connect them as if they are normal low current LED setup. 12VDC (Laptop charger with a few Amps) + resistor + LED = Light

I bought 10 x 3W White LED... got the shipment and I tested using a 9V batt with 100Ω... With this I should get around 40mA running through the LED. 2 out of 10 are dead. When I talked to the supplier, they replied they will not replace the LED as I use Batt+resistor to power them. They claimed that this will burn the LED. trying to BS me!

These 3W LED runs on 3.5V and 700mA. How can a mere 40mA killed it! what logic?

[sacy]

9v is too much of voltage for the LED. think it's not the current that kill your led. and batt are raw power. batt cant regulate Constance power.

A led driver is needed if u don't have a good source of power supply. that can produce Constance power. normally a switching power supply will do the job.

If u use a normal 12V step-down adapter, u need the LED driver.

just my 2 cent^^

do remember a 9V batt can kill a human being. if u know the right way.

[Shadow]

Actually should be ok, if there is no resistor than it will kill the LED for sure.

(9V-3.5V)/100 = 55mA

[limsteel]

Actually should be ok, if there is no resistor than it will kill the LED for sure.

(9V-3.5V)/100 = 55mA

9V batt is quite old one. Measure with Multimeter only have 7.5V. Hooked it up with 100Ω Resistor measure the current, my multimeter shows ~45mA.

NO WAY this current can blow the LED. Unless I directly connect withour resistor.

Sacy:
How about those Nokia HP Charger... they are use to charge HP should be quite precise since it is used to charge sensitive equipment. I am doing up one to light my Aquarium cabinet.

[sacy]

erm.. Hand phone charger is a simple step-down transformer only. the regulating part are done by hand phone.

humm. if u wan to do . u need 1XLM337 and some other component, to help u make Constance output voltage and current.


and do remember ,power the led at near max rating , don't over power it .^^ like led rating(3.5v@700mA) don't power it at 4.5~9v this will cause your LED to draw more Current and Burn the Semi-conductor Chip of your led. unless u got very good heat dissipation device mouth on your led

my 2 cent

[firethorn]

erm.. Hand phone charger is a simple step-down transformer only. the regulating part are done by hand phone.

I am unsure of where you have obtained this conclusion. A hand phone charger is regulated, smoothed and filtered to some degree.

Perhaps you mean that the "charging circuit" is "usually" in the handphone. i.e. The charger puts out 'smooth' dc but the actual circuitry to charge the battery is within the phone.

Also, it should be noted that not only is a hand phone charger NOT a SIMPLE step-down transformer, it is not even a transformer. It is a SMPS unit.


You are right however to say that a popular choice for a voltage regulator in a DIY electronic circuit would be devices in the LM series such as the LM317 (the 337 is a negative voltage regulator). This is truly only needed if you are DIYing the entire power supply yourself. If you already have a relatively stable DC source (those 'newstar' chargers will NOT cut it) and just want to make minor adjustments, you can use a Zener diode and a resistor to regulate the voltage, for a cheaper and much simpler solution.

The LMxxx series though has a lot of cool features including "overcurrent protection (of the IC)" and can typically handle up to 1.5 Amperes of current. You can also adjust the voltage while the device is running, as compared to the Zener diodes where the voltage is fixed (although with regards to lighting a LED, you shouldn't adjust the LEDs brightness by playing with the voltages. Instead, you should play with the current or if you want, with something known as Pulse-width modulation).

If you do decide to DIY the entire power supply, there are quite a few things you need to know before hand, including converting the AC to DC. It's fun and interesting but it's lengthy to explain, especially if you are new to this. But it is fascinating and relatively easy to learn. Just lengthy to explain, and it's not something you can skip because it involves safety.

[limsteel]

Googling around, I found this Is this as simple as the instruction stated? Power Supply + LM317 + Resistors + LED.

LM317 got a Vdrop of 3V using a 12Vdc power supply, I can only hookup 2 more LED Vf of 3.5 to 3.8. with a Imax of 1.5Amps, at most I can only hookup 2more (parallel) Seem like I gotta buy lots of LM317 If I intend to fully replace my Light set.

As for stable power supply, can Computer PSU be used? Computer are filled with sensitive electronic components. Don't tell me that the MB, Hard Drives, Roms, etc have they own Power Stabiliser...

firethorn:
So if the Charger has a stable Voltage, smooth and steady... Can I forget about the LMXXX. I want to hook up a single 3W LED inside my Cabinet so I can see while I work inside the cabinet like adjust my Sump, moving things around..

Also need to setup Single or Twin 3W LEDs for my 1ft guppy tank no plants. this will be powered on for longer.

[sacy]

oh. the LM337 is a Adjustable Regulators. so u can adj the V/out from 1.2V to 37V (depend on your V/in and (R1/R2)) and the Imax/out is 1.5A (depend on your I/in)

about the hand phone charger. i will confirm again. i will try to get a hand phone charger and open it out to double confirm.^^

sorry if i miss lead anyone

[firethorn]

Googling around, I found this Is this as simple as the instruction stated? Power Supply + LM317 + Resistors + LED.

LM317 got a Vdrop of 3V using a 12Vdc power supply, I can only hookup 2 more LED Vf of 3.5 to 3.8. with a Imax of 1.5Amps, at most I can only hookup 2more (parallel) Seem like I gotta buy lots of LM317 If I intend to fully replace my Light set.

Once you've got the hang of it it's relatively simple. For power devices, you just have to be mindful at all times of where everything is.

For your situation, you will need to include a transformer, 4 shottky diodes and around 2 capacitors estimated at about 10000uF. This is to 'rectify' the AC to DC. (or you could google 'making an SMPS')

What is the maximum voltage and current rating for your LED?

The LM317 typically has a forward voltage drop of around 1.5V (at least in my uses for them). By putting the LEDs in parallel, you 'spread' the current around while maintaining the same voltage across the LEDs. By putting them in series, you 'spread' the voltage around while maintaining the same current.

i.e. As a very simple analogy, if you can supply 12Volts and 1Amp and the LEDs need 4 Volts and 200mA. If you put it in series, you divide the voltage meaning you can fit only 12V/4V=3 LEDs in. If you put it in parallel, you can fit in 1Amp/200mA=5 LEDs in.

However, it would be better to use a combination of both so as to maintain the correct voltage(parallel) while keeping the additional component count low(series - LED series only needs 1 regulation circuit per series)

i.e.
4 parallel rows of 10 serials.

As for stable power supply, can Computer PSU be used? Computer are filled with sensitive electronic components. Don't tell me that the MB, Hard Drives, Roms, etc have they own Power Stabiliser...

You could. I wouldn't say it's perfect but it will work sufficiently. If you have those cheapo brands though I'd suggest you use it only as a last resort.

firethorn:
So if the Charger has a stable Voltage, smooth and steady... Can I forget about the LMXXX. I want to hook up a single 3W LED inside my Cabinet so I can see while I work inside the cabinet like adjust my Sump, moving things around..

Also need to setup Single or Twin 3W LEDs for my 1ft guppy tank no plants. this will be powered on for longer.

If it's smooth, stable and steady, yea you can forget about the regulator. However, do note that the charger was probably not meant for such prolonged high power usage so it could fail in some way which would blow your LED so for a permanent setup, it would be best to add some sort of safety mechanism such as a Fuse or similar device to disconnect the power upon failure. For the first purpose you described though, it seems alright.

At 3Watts(Power) and XVolts, you will need YAmperes. Check out XVolts on your LED. Then use Power=Volts x Ampere to determine how many amps(or milliamps) you will need. Then look at the back of your charger to see if it can supply the needed current.

[limsteel]

I read from the site as linked earlier. Author indicated that LM317 has a Vdrop of 3V.

The constant current source has a drop voltage of 3 V, so the supply voltage should always be 3 V higher than the LED voltage and can be up to 37V which is the maximum input voltage of the LM317.

LED I got are rated 3.5 - 3.8V, 700mA ops current.

Using a 12V Computer PSU of better brand:
If 1.5V as you stated then I hookup 3 LEDs in series operating at 3.5V
(3.5 *3 + 1.5 = 12V)
If 3V as the site stated then only 2 LED in series can be connected

So if the calculations given are correct

"1,25 volts between ADJ and Vout" Ohm's law says that U/I=R, which means that Voltage divided by Ampere makes resistance.

R = 1.25 / 1.4 (parallel connected to 2 row of LED 700mA for each row) 0.8 to 0.9Ω

I can use
One 1Ω (>1¼W) resulting - 1250mA
Ten 9Ω connect in parallel to get 0.9Ω (¼W each)resulting - 1389mA

Seem like I need to bring out my Bread board and multi meter liao... If PSU not the best power supply then what can I get
(lowest cost / effective, efficient, reliable)

BTW, I am a Mechanical Eng guy... So don't go too deep in the electronic/electrical term...

[sacy]

humm. the next best thing is just buy a switching power supply from sim lim tower ^^
something like this--->http://www.haitaik.com/detailshow.asp?ID=227

it should not cost a lot. should be cheap ^^. less headache ^^ just find the right output u need.

but don't go for 12~24v@12A~24A type. it will cos $100~$200 plus

[limsteel]

found this Switching Power unit 12V 5A = 60W $30
12V - connect 7 Parallel 3 LEDs.

Buy 2sets totaling 42LED!

Another 12 to 13.6V at 12Amp @ $52++

with 12Amp I can hookup 17Rows of 3 LEDs. buy 1 set totaling 51 LEDs

[firethorn]

Dang. I went on and on about how it would be more cost effective to get an SMPS for your needs. Then you went and found one. haha..

Check out sim lim towers. I do believe they do sell these units also.

But in the same way customers tend to rely on 'better PSUs brands', be careful with these brands. =)

[tawauboy]

IS LED DRIVER A MUST FOR HIGH POWER LED? I planning to connect them as if they are normal low current LED setup. 12VDC (Laptop charger with a few Amps) + resistor + LED = Light

I bought 10 x 3W White LED... got the shipment and I tested using a 9V batt with 100Ω... With this I should get around 40mA running through the LED. 2 out of 10 are dead. When I talked to the supplier, they replied they will not replace the LED as I use Batt+resistor to power them. They claimed that this will burn the LED. trying to BS me!

These 3W LED runs on 3.5V and 700mA. How can a mere 40mA killed it! what logic?

if you tested the leds with the series resistor, then you will not kill the leds. as long as the current drive is low and very little heat is built-up, you will be fine.
however, if you tested the led at much higher current drive >350ma, the led can be damaged due to heat buildup.

[tawauboy]

9v is too much of voltage for the LED. think it's not the current that kill your led. and batt are raw power. batt cant regulate Constance power.

A led driver is needed if u don't have a good source of power supply. that can produce Constance power. normally a switching power supply will do the job.

If u use a normal 12V step-down adapter, u need the LED driver.

just my 2 cent^^

do remember a 9V batt can kill a human being. if u know the right way.

a direct 9v across the led will damage the led due to overcurrent. however, a 9v battery with a resistor in series with the led will limit the current. this arrangement will not damage the led unless the resistor value is too small (causing a overcurrent) or there is not heatsink for the led (when driven at high current levels).

a led driver is nothing more than a constant current source. like a normal power supply, the constant current source has output current as the main control variable instead voltage for a normal power supply.

[tawauboy]

you can use any type of 12v power supply (smps or linear or pc). smps should be the most compact for a given current rating.

if you plan to run the leds at 700ma, you'll need a series resistor of 2.2 ohm (2 watt) for each string of 3 leds. and you'll need to mount both the leds and resistor on a big heatsink. you can select appropriately sized heatsink by calculating the thermal resistance of the whole system which you are designing.

[sacy]

^^. beware of the hot cable u will get. If u run at near max current on the 12amp supply .

[limsteel]

^^. beware of the hot cable u will get. If u run at near max current on the 12amp supply .

I'll use the HiFi Monsta cable, should I run on 12 Amps! Or I can connect parallel to divide the current up.

[firethorn]

If your cable gets hot, you are using the wrong cable. You could divide them up but it will look messy.