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Proper way to supply power to a LED


ulf

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I will below discuss the properties of LEDs and how to setup a power supply driving them.


Current.

LEDs are diodes and the main limiting factor when driving them correctly is the current.

Too much current will break them.

 

All semiconductor diodes, including LEDs need to reach a voltage before they conduct any current.

When that forward voltage is reached the current will increase rapidly. Too much voltage applied to a LED will cause an over-current destroying the LED.

Driving power LEDs close to the maximum current avoiding a breakdown is best done with a controlled constant current.

The forward voltage is dependant of the type of semiconductor material used and that depends of the wavelength the LED is designed for.

The forward voltage also varies with the temperature.

 

Cooling.

The second very important limiting factor, especially for power LEDs is their chip temperature. If proper means for cooling is lacking they will quickly overheat and break.

As the LEDs active chip is rather small even a relatively low power can rise the internal temperature of the chip quickly.

To avoid that a power LED should be thermally well connected to the environment for cooling.

 

A good example is the driving module including the LED in the Convoy S2 UV flashlight.

The module is a massive metall unit that screws directly into the flashlight's body.

I estimate that that solution safely can dissipate as much as 1-2W.

 

High power LED arrays in industrial UV-curing units are often water-cooled just like processors in extreme gamer computers.

Then they can handle power losses of several hundred Watt.

 

The heat that has to be cooled away is the supplied power minus the emitted power in the light.

The efficiency converting electrical power to optical power is normally lower than 50%.

 

Polarity.

Another important thing to observe is the polarity of the voltage supplied to the LED.

LEDs are quite sensitive and will be destroyed by reverse polarisation, with even modest voltages.

Many types of LEDs can only tolerate blocking up to five volts, when reverse biased.

The positive voltage must be connected to the LED's anode and the negative voltage to the cathode.

 

This is especially important when dealing with LED-arrays where several LEDs normally are connected in series.

Then the voltage needed to drive the series coupled chain of LEDs is the sum of all forward voltages.

 

Multiple LEDs.

If several LEDs are to be connected together, the safest and most efficient way is to connect them in series.

Then the same current will pass through them all.

 

If LEDs for some reason must be coupled in parallell, each of them must have a series resistor to balance out the individual differences of the forward voltage-characteristics always present.

Such balancing serial resistors waste energy and adds unnecessary heat to the design.


Power sources, their setups and connections.

Methods setting up a power supply for LEDs depends of the type of power supply and it's capabilities.

A summary:

Start from low or zero settings of voltage and current.

Adjust the current limit upwards to desired current, to below the maximum rated current of the LED.

Then carefully increase the output voltage until the current limit is reached. activating the CC.

Then increase the voltage settings little it further to allow for temperature related changes of the LED's forward voltage.

 

An advanced variable Lab-supply can often have the parameters set before activating the output.

Those supplies always have some kind of indication of set and/or active voltage and current.

The current limiter is just set at some level below the rated maximum current of the LED.

I normally also start with the voltage setting to zero or below the expected forward voltage before activating the output.

After activating the output, I carefully increase the voltage until the current limit (CC) kicks in while observing the voltage changing.

There is a rather small voltage range where the current increases from almost nothing up to the set CC-limit.

 

A power supply-module intended to be setup to the proper operating point for driving LEDs in a fixed setup can be a bit more challenging to set up.

They often have small multi turn potentiometers without any indication of rotation direction for increasing or decreasing the parameters.

The potentiometers are often marked CV and CC on the PCB. (Constant Voltage and Constant Current)

If that information is lacking you first have to experiment a bit, preferably without the LED connected. You will need a multimeter for this setup.

These modules sometimes cannot handle a CC-situation into a direct short circuit of the output. They need some load to create some voltage over the input.

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Brilliant Ulf,

thanks for taking the time to write this, it is most helpful.

I have struggled with the smaller drivers & the confusing potentiometers, as you explain above.

I now have a 48v 20a 1000w power supply from China, but as usual it has no instructions.

Cheers

Col

 

 

post-31-0-01620400-1582794208.jpg

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Hi Col,

 

I think this unit also have problems setting the current limit shorted.

That is not important, only making it impossible to setup the current-limit in advance, without some kind of load, but otherwise it can be OK.

1000W!!! :smile:

 

Just gradually increase the voltage keeping the current below rated maximum.

If the current is limited just follow increasing that control.

I would carefully alternate adjusting current- and voltage-settings.

When I had made the correct current limit setting I would observe what voltage that gave.

After switching off and disconnecting the LEDs I would then turn it on and adjusting the voltage setting slightly (1V?) above the voltage found with current limit.

 

Good luck.

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I bought a different lab supply for my LED-experiments:

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=WANPTEK+NPS1203W&_sacat=0

It is not a very high quality unit, but hopefully good enough for driving some different types of power LEDs

I wanted to get up to 70-80V and this had specs fulfilling the needs.

At lower voltages it is awkward to adjust, but it gets better at higher than say 10V.

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Can I get some feedback about the first post's text, please.

Does it feel complete, understandable and clear, or is something missing or confusing?

 

I am a bit tempted to remove the text parts in BOLD.

Done

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If you want to run multiple LEDs at the same time, you should put them all in series or in parallel rows composed by multiple LEDs in series (if they are all of the same type, power, wavelength, etc.), because little differences can and will exist between seemingly identical chips. If you want to do the same thing, but using different LED types (such as different wavelengths), you must run all of them in a single series row, because this way you will have 100% control of the currents flowing through each LED.

 

Also, high reverse voltages can damage them. It's pretty obvious, but you should check the polarity before powering them. If the (reverse) voltage is low enough, they will simply not turn on. However, keep in mind that LEDs have unusually low breakdown voltages (sometimes just a few volts), so you have to be a bit careful.

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Stefano, thank you for the points.

What you write is correct and I had most of it in my mind when writing, but in the end I felt saturated and tired of writing.

I will add the information to the first post to keep it all in the same place.

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If you like to power high power LEDs you better use a pulse-width modulation LED driver controller.

If you like to use a DC power supply then put some silicon diodes in conduction direction in parallel, they protect the LED from too high voltage. Use resistors too otherwise you will blow the diodes and the LED.

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You should not put leds directly in parallel, even if they are the same type. Minor variations between the leds will cause some to hog more current than others. In such a case, a small resistor in series with each led should be used.
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I power them with a CC CV (constant-current constant-voltage) DC power supply. I'm not a huge fan of PWM, I don't like flickering LEDs. You can put a small capacitor in parallel or a small inductor in series (or both) to reduce flicker. The higher the PWM frequency you use, the smaller the capacitor and/or the inductor needs to be. Also, white LEDs are a little different, because they use a phosphor to produce white light. That phosphor is not instantaneous, so it will "lag" noticeably at very high frequencies (usually in the KHz region).
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If you like to power high power LEDs you better use a pulse-width modulation LED driver controller.

If you like to use a DC power supply then put some silicon diodes in conduction direction in parallel, they protect the LED from too high voltage. Use resistors too otherwise you will blow the diodes and the LED.

PWM is only meaningful to control and dim the light out from the LED, but the maximum current during the on periods must still be respected.

Modern DC power supplies use a PWM technique to control their output both in constant current and constant voltage mode.

DC-supplies also filter the voltage after the PWM.

 

Some light regulating can use PWM directly to save costs on filters, but with an added risk of emitting electronic noise during the switching.

 

I do not agree with what you say in your last statement. The silicon diodes will likely not have a forward voltage to protect any LED in a good way and

if you drive the LEDs with constant current, the serial resistor is meaningless.

This is a driving method is normally used only for low power LEDs like indicators.

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You should not put leds directly in parallel, even if they are the same type. Minor variations between the leds will cause some to hog more current than others. In such a case, a small resistor in series with each led should be used.

I agree.

This was added to the top post while you were writing this.

If several LEDs are connected in series in different strings to be connected in parallell the difference in the strings will be statistically averaged out, but many LEDs directly in parallel is not a good idea.

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I think this unit also have problems setting the current limit shorted.

Not sure. If it's build the same way as any regular bench power supply then 20A is hard set and it won't go past it. That way you can just short it, set the desired current and then connect your LED(s) or other things.

That, or it simply has short circuit protection and it shuts off to prevent damage when current goes over 20A.

If it was my unit I'd try shorting it at the lowest current setting and see what happens. If a simple short would be able to damage it I wouldn't want to continue using it anyway :D

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My power supply is "idiot proof". It will deliver max 5 A, no matter what. You can't damage it with a short circuit, but you can if you apply a reverse voltage on the terminals (I once connected a lead-acid battery the wrong way and blew up the safety diode inside. This diode is in parallel to the output, oriented so that it doesn't conduct. If a reverse voltage is applied to the terminals, it will conduct and usually fail, becoming a short circuit and thus short-circuiting the output of the power supply. By "sacrificing" itself, it protects the internal circuits. You just need to replace it with a new one and you are done).
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I now have a 48v 20a 1000w power supply from China, but as usual it has no instructions.

I'm not sure if you need them but here goes:

Terminals 1, 2, 3 are mains live, neutral and ground

Terminals 4, 5, 6 are DC -

Terminals 7, 8, 9 are DC +

The +V ADJ potentiometer should be for calibrating the voltage. Get a good multimeter and adjust the potentiometer so the voltage displayed on the power supply matches that on the multimeter. It's best to do this at the voltage setting you're going to use most.

HOWEVER This could also be a power supply that's normally meant for fixed voltage and that has been modified with added circuitry and potentiometers. The position of the dials is weird since the current dial blocks the screws for terminals 2 and 3.

On fixed voltage power supplies the ADJ V+ dial is meant to calibrate the fixed output voltage. So I'm not sure if this potentiometer can be used to just calibrate the displayed voltage or if it changes the output voltage as well. Just try and you'll see.

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Thanks Nisei

The voltage & current dials are off-set from the three, mains live, neutral and ground fixing screws, there is no problem with the screwdriver access.

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I have run a 100w UV led with these instructions & everything went well, thanks.

One thing I don't understand how it works is the 50A spade fuse that comes with the power supply ?

As I understand it, with the power disconnected, & the out put volts on zero, the fuse is placed between the + & - terminals, The power is applied & the current is adjusted to near what the LED is rated at, am I correct ?

 

I have just purchased 9 x 10watt UV star LEDs that I want do setup with the power supply each star LED is 10watts, Vf 12-15volts & Cf 700mA.

I want to run this as three parallel rows with three in series each. So that the power supply needs to be set at say 40 volts & 2.1 A....?

The seller has suggested in addition to place in each parallel row a 1A fast blow fuse & a 1 Ohm resistor. The fuse I understand. The 1 Ohm resistor needs to handle 30 Watts, so I will need 3 x 1 Ohm 10 Watt ceramic resistors...?

I hope I have got this right ?

Cheers

Col

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I have run a 100w UV led with these instructions & everything went well, thanks.

One thing I don't understand how it works is the 50A spade fuse that comes with the power supply ?

As I understand it, with the power disconnected, & the out put volts on zero, the fuse is placed between the + & - terminals, The power is applied & the current is adjusted to near what the LED is rated at, am I correct ?

 

I have just purchased 9 x 10watt UV star LEDs that I want do setup with the power supply each star LED is 10watts, Vf 12-15volts & Cf 700mA.

I want to run this as three parallel rows with three in series each. So that the power supply needs to be set at say 40 volts & 2.1 A....?

The seller has suggested in addition to place in each parallel row a 1A fast blow fuse & a 1 Ohm resistor. The fuse I understand. The 1 Ohm resistor needs to handle 30 Watts, so I will need 3 x 1 Ohm 10 Watt ceramic resistors...?

I hope I have got this right ?

Cheers

Col

 

You are correct about the spade fuse. It is because the CC circuit cannot be adjusted with a dead short. The fuse is a kind of better controlled short-circuit connection that allows a CC adjustment without involving the real load. You can later readjust the CC with the LEDs connected.

 

For the resistors and fuses it should be to add them in series for all three separate series strings of LEDs.

You first build the series strings. They can be run independently of each other. Then connect the strings together only at 0V and at the driving voltage.

 

The resistors can be much smaller than 10W, even if it is good to have some margins. Power resistors are designed to be very hot.

The power heating the resistors are dependent of the current passing them and their resistance . P = I2 x R. In your case each resistor 0.7A x 0.7A x 1Ω ≈ 0.5W. I would get 2W resistors, but 1W is OK too.

 

The reason for the resistors is to balance the current even better between the three strings of LEDs. As the Vf already is 12-15V there are LED chips inside each of the units connected in series and parallell.

 

Good luck

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Thanks very much again Ulf, I am very appreciative of your help & knowledge.

The added resistors had me tricked, the guy said they would help balance the strings, but didn't elaborate on the wattage to get, so I could only guess 10 watts x 3, that seemed massive to me, but you have explained it & I will get some 2 watt ones....thanks.

This LED is interesting as it has in each 10w star 4 LEDs, one each of 365, 375, 385 & 395nm.... :smile:

I will post when I have it together in a few days....

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Looking forward to seeing your result.

 

That is an interesting configuration.

Can you please let us know where to buy them?

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Thanks Ulf

These were bought in Australia, I got the only 9 he had, but I will ask for more & let you know when he will expect them.

 

post-31-0-96489000-1583852699.jpg

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That would be an interesting UVA light source. What is the diameter of the unit?

I wonder if you took out the modual from the Convoy S2+, if you could add this unit.

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Nice LED, you will have broadband UV without the sun shining. Colors should significantly improve. Having all the LEDs close together is very important, otherwise shadows will have colored borders. I suggest you to place the LED far from the subject or to use a diffuser to mix all the wavelengths well. It's like RGB LEDs.
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That would be an interesting UVA light source. What is the diameter of the unit?

I wonder if you took out the modual from the Convoy S2+, if you could add this unit.

If you can adjust the maximum CC current setting by turning the screw of a potentiometer and raise the output voltage enough (you should multiply it by four, since Convoys should have only one chip if I'm right), the answer is yes. It looks like a 20 mm star base.
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