High Power LEDs – an alternative light source?
In some of my posts and comments you may have read me banging on about how I use primarily LED lights for my photos. I like LEDs – they’re quite different from your conventional light bulb. Being an awkward little bugger I decided I only wanted to use such systems for photography and put up with any of their draw backs. I’ve been using standard off the shelf LED bulbs for a while now – the ones with clusters of many LEDs.
The biggest disadvantage of these LED bulbs is their directional and relatively low light output – compared to a standard and equally priced halogen/florescent bulb. So in order to get more light output I decided to investigate high power LEDs – plus it’s a good excuse to put my electrical engineering skills to the test again. Ultimately though it’s worth remembering it’s how you use the light that makes a picture; I just thought I’d share with you my in-development high power LED lighting set-up. Who knows maybe you too might give LEDs a try, or maybe it sparks an idea that you might want to try.
It’s also worth stating I’ll be going into a bit of detail about the circuit I’ve assembled. If you don’t have a clue what voltage and current are you’ll probably not have a clue what I’m blabbing about. I’ll try my best to keep things simple, let me know in the comments section if you followed.
So a brief introduction to the LED, or Light Emitting Diode. It’s basically a semi-conductor light source which emits light provided the correct voltage polarity* is applied across it. The intensity of output light is dependent on the amount of current passing through as well as other semi-conductor related properties. LEDs come in many colours now and are typically used in low power, low voltage applications – such as indicator lights on your TV for example (yes I’ve seen your TV :shock:). However that’s not to say they cannot be used for home lighting – especially given recent developments in semiconductor technology. Not really related to LEDs, but your camera uses similar diode constructs, only instead of emitting light they sense it. My current job actually involves the characterization of such systems, but now we’re veering off-topic :P.
(Hmm, is that a clue for my next figure review(s)…)
When we say ‘high power’ LED what we mean is an LED that can take hundreds of mA to A worth of current without being damaged. A standard LED can only typically take mA to tens of mA worth of current for comparison. So basically what we’re doing is using more electrical power to get more light (Electrical power (W) = voltage (V) * current (A)).
Advantages / Disadvantages of LED lighting systems
(No pictures here, just the Great wall of Text™)
The big advantage of LEDs over standard light bulbs is the amount of electrical power required to get an equivalent light output is much lower. This is due to their electrical power to light output efficiency, or better put the mechanism used to generate light. This is also the reason your standard light bulb gets so hot, more electrical power = more heat – and since it’s light we want from light bulbs and not heat we’re wasting energy through heat emission. This is why you’re encouraged to replace your 60W bulb for a 10W energy saving bulb. You shouldn’t notice much difference in light output (energy saving bulbs are typically fluorescent light bulbs), instead you see it in your electricity bill.
Another advantage of the LED is once it’s turned on the light emission is almost instantaneous. Switch on your energy saving bulb and you’ll notice it takes a bit of time to reach maximum light output. LEDs make great flash bulbs for this reason.
The actual light emitting part of the LED is also considerably smaller than any other light source. Energy saving bulbs are big chunky beasts, and standard light bulbs have a filament that needs to be air tight. This means you can compact more LEDs into a smaller space or even situate them in space constrained places. In the case of high power LEDs this may not be as true a statement since you’ll need to take into account other factors – but if we’re talking standard LEDs this holds.
Coloured lighting is also much better with LEDs. No need for colour filters, and as a result no wasted energy. If you put a red filter over your light bulb, the bulb still emits white light, it’s the filter that only lets red light through making you see red light only. With a LED you emit red light and that’s it – no need for filters or wasted energy. This also means it’s dead easy to get white bulbs with different colour temperatures – something you’re probably more aware of from photography.
The biggest disadvantage of high power LEDs, and probably the reason you don’t see many on the market, is their light output efficiency is dependent on temperature. The hotter the LED becomes the less light it outputs and the more its stated colour output drifts. And as already stated more power/current = more light = more heat. This means you need a sizable heat sink to get rid of the excess heat, and as a result the packaging required to include a heat sink becomes more complex and potentially bigger. Now you may remember I was saying ‘oh LEDs are so much better, they like use less power and such’ and now I’m telling you heat is a problem. Well if we take a look at the amount of light we’re outputting and the amount of electrical power we’re consuming you’ll find it’s much lower than your standard light bulb and even your fluorescent light bulb. It’s just LEDs are more susceptible to temperate changes than these other light bulbs*.
Another disadvantage is LEDs emit a relatively narrow cone of light. That’s fine if you want a spotlight, but typically you want to spread as much light as possible. This is especially true for me when photographing figures – so I have to use some means to diffuse the light, or consider adding a lens system.
And let’s not forget cost. LED bulbs as it stands are much higher priced when compared to other lighting. If you want the best bang for your buck stick with energy saving bulbs for now.
Okay so enough with the ‘LEDs are so uber’ spiel, what have I been up to with them?
A couple of things you may want to know before starting.
- When an electrical circuit is connected in series the current flowing through each component is the same, the voltage dropped across each component will be different.
- When an electrical circuit is connected in parallel the voltage across each component is the same, the current flowing through each branch is different.
- (Circuits can be a mixture of series & parallel just to keep things confusing ;).)
- A voltage regulator will provide a constant voltage and a variable constant current, provided the input conditions are maintained and the output current does not exceed the rated maximum.
- A current regulator will provide a constant current and a variable constant voltage, provided the input conditions are maintained and the output voltage does not exceed the rated maximum.
We’ll be combining these ideas to create a controlled LED lighting system.
So here’s the main idea – to drive a constant current through each LED and not have to worry about the required voltage. For an LED to work you must apply the correct voltage: too much and it’ll be damaged, too little it won’t switch on. To make things more annoying each LED colour variant has a different voltage required to operate, making it difficult to switch different colours in and out, or mix different colours together. So you can see why we want to have something that deals with this annoyance for us. This should then allow us to only worry about how much current we want to pass through each LED, and thus their brightness. So how do we do it?
Basically it’s AC Mains -> DC Voltage Regulator -> DC Current Regulator -> LED.
AC Mains to DC Voltage Regulator
I’m going to be using a mains socket to power everything. We take 240V from a mains socket, convert it from AC to DC (AC is a varying voltage which is no good for the components we’ll be using), step it down to 24V (240V is too high for the components), and regulate the voltage at this level (so it doesn’t vary). This 24V Voltage regulator will be used to power a bunch of current regulators – our means of powering LEDs.
I picked a 24V regulator with 1.88A max output because it was the only one I could find a suitable DC jack for. If I could I would have picked a 32V regulator since the maximum input to the current regulators I have is 32V. This would have allowed me to get the maximum out of my current regulators, but oh well.
The voltage regulator looks like your standard laptop charger (the big black box below) – but you probably won’t want to plug this into your laptop ;).
DC Voltage Regulator to DC Current Regulator
For current regulation I use a BuckPuck from Luxeon Star. The only condition these current regulators require to operate is an input voltage greater than the output voltage. So with 24V at the input we can have up to ~23V of voltage across any output – in our case LEDs.
The current regulators I have are 700mA (I also have 350mA versions) – this means they’ll output 700mA provided the above stated voltage conditions are maintained. 700mA will be going through the LEDs, and since it’s a current regulator we don’t have to worry about the voltage each LED requires (remember constant current, variable voltage output). However, we still need to remember not to exceed 24V, so we can’t attach an unlimited number of LEDs – we just don’t need to worry about which LEDs are connected.
This big strip of green screw terminals basically allows me to connect the LEDs in series. So if I was to go and buy single LEDs I could use this board to power 9 of them. Screw terminals are great because they allow me to swap parts in and out without any solder. Although you could use some molex connectors etc if you wanted.
High Power LEDs
The High power LEDs I’m using are known as Rebel Tri-Star LEDs – flashy name. These are again from Luxeon Star. Each hexagon contains 3 high power LEDs. The light emitting part is the tiny domes you see below. Because we’re using so much power a heat sink is required to get heat away from the LED to prevent it overheating. The heat sinks I have are actually no good for 3 LEDs so I need to use a fan alongside them. A bigger heat sink would have been better – or single LEDs instead of groups of 3. I’ve soldered these LEDs such that they are always connected in series.
I’ve got quite a few colours available to me. 5000K white, 3000K white, 627nm red, 655nm red (near IR), 470nm blue, 530nm green, and 447nm blue (near UV). Wow wow wow, hold your horses, what are all these numbers. Basically we’re talking about the primary wavelength of light emitted from each LED. When we mix different wavelengths of light together we get different colours – similar to mixing paint. So if we were to mix Red and Green light we would get Yellow light. Or you could buy a yellow LED which emits this equivalent wavelength ;). To get white light you mix Red, Green & Blue – RGB. The numbers beside white are known as colour temperatures and basically state how red or blue your white light will look. White of 6000K will look slightly blue whereas light of 4000K will look slightly red.
Consequent groups of 3 LEDs are then are connected in series – ie the current through each LED will be the same. This is exactly what we want – but remember in series the voltage across each component will be different. So let’s look at the 2 extremes we have. Blue LEDs are buggers, they require a large voltage to operate – in this case 3.3V. Red LEDs are cool, they only want 2.1V. If we were using a voltage regulator instead of a current regulator we’d have to take this mismatch into account if we were to mix Red and Blue LEDs (you could do it using resistors for example). However we are using a current regulator, the only condition we need to ensure is we don’t exceed 24V. So that means we can have 3 Red and 3 Blues sitting quite happily @ 16.2V with each one receiving 700mA. Oh, what’s that, you want another Red LED? No problem, connect it in series and our voltage is now 18.3V and we’ve got 7 LEDs on the go.
It’s hard to give an example of how bright these are using a camera – since it’s not an ideal imaging system. Let’s just say if you glance directly at them you’ll be seeing 3 little circles for about 5-10 minutes afterwards.
So the whole point of this lighting setup was to get more light for photographing figures. For my last 3 figure reviews I’ve been using this LED setup. One annoyance of my room is it’s painted bright green. My standard LED bulbs are not bright enough to cancel out these green reflections – and using natural sunlight is even worse because it lights up the whole room, not just the figure. I’ve had to use a softbox to block these reflections previously – I no longer have to now. I have so much light any reflections are no longer bright enough to be picked up. This means I have more space to do things such as different viewing angles, closer macro shots, more elaborate backgrounds.
Arsene, the light to her left is a 5000K white LED.
Penguin Parade Naoe used a 3000K LED.
Alter Naoe, the red in the background was the 655nm red LED.
It’s worth noting I still use my LED lamps, they’re just no longer my primary and only light source.
But wait, there’s more!
Those who are still following you may have noticed 3/4 of the ‘things you may want to know’ have been used. We’ve not used the circuits in parallel rule. See the diagram below. 24V from the voltage regulator connected in parallel means we can power more current regulators because the voltage when connected in parallel is the same. More current regulators means more LEDs. We’ve gone from powering 6 LEDs @ 700mA to 12 @ 700mA simply by using circuits in parallel. If I was to then get a voltage regulator with more current output I could power even more LEDs @ 700mA. Or get 4 350mA current regulators – 24 LEDs @ 350mA (which is still very bright).
I’m still working away at this lighting setup. Other things I’ll be adding are fuses for each current regulator. If for whatever reason the LEDs go short (derp soldering from me) or open (an LED blows) circuit we have a problem, and the current regulator could get damaged. A fuse would hopefully prevent this. I also still need to build the voltage regulator board. At the moment I simply have a DC jack floating around so getting that secured in place would be a good idea. I’ll also add a switch to this voltage regulator board so I can shut off power more easily – no need to pull the plug all the time.
Other things you can do with these current regulators include PWM (switching something on and off quickly) and adding variable resistors which would act as dimmer switches. PWM is also a good way to save power, thus less heat. I’m running these at constant current so there should be no flicker – however that means 700mA will always be flowing. If you switch the LEDs on and off you draw 700mA when on but 0mA when off, and thus heat builds up slower making it easier to cool.
So that’s what I’ve been up to with LEDs. Feel free to leave any questions if you want to know more. Any suggestions/corrections are welcome – such as “zomg where’s your 24uF cap!”. Also, my soldering is super awesome, oh yeah sarcasm doesn’t work on the internet. Run! Don’t walk from… The Blob. Regular figure review next, promise.