LED Lighting for High Power Microscopy
This page will explain how to use a Luxeon Star III LED for powerful microscopy illumination using a simple AC/DC power adapter and cheap resistor.
Warnings - Please Read
Never stare into the LED light. The LEDs in
this project are extremely bright. In some countries they are classed as 'Class
2 laser products'
This lighting project is intended for microscopy magnifications of 1500x or more with a minimal condenser aperture. For lower magnifications or other conditions these LEDs might be too powerful without effective dimming
The resistor and LED used in the project can get HOT and must be placed safely.
Some LEDs can produce Infra Red or Ultra Violet light in significant levels. Check the specifications of the LED you choose to ensure these are not present at potentially harmful levels. Some commentators believe that blue light can be harmful to the eye even when it is not obviously too bright. If in any doubt - use a camera's LCD display or video capture for observing with the microscope.
Connecting an LED in reverse polarity can destroy the LED.
An alternative to using a resistor is to use a simple LM317T voltage regulator - see: LM317T regulator for LEDs
Introduction
The LEDs I have used are Luxeon Star III's. I have found powering them with 5 volts at around 600mA more than sufficiently bright. The difference in lumens output between 700mA and 600mA is barely noticable and the LED will run cooler.
Luxeon Star III LEDs come in a range of colours. This project refers to the Cool White, Green, Cyan and Blue colours. The coloured LEDs have a narrow wavelength range that can seem quite strange to view at first but can produce excellent contrast. I particularly like the Cyan and Green for high power microscopy. For DARKFIELD microscopy I find the Cool White excellent.
LEDs use far less power than incandescent lamps and are therefore much cooler - a great benefit when positioned beneath the microscope stage. The LEDs I have used far exceed the brightness I could achive with 20w a halogen lamp.
It is common in LEDs that they direct light predominantly in a narrow beam straight ahead of the lens with a slight dip in brightness in the middle. This is called a 'Lambertain' radiation pattern and is ideal for high-power microscopy. Additional collimating lenses and holders are available. (see links below). The picture is from Lumileds Technical Datasheet 46.
LEDs require a constant current power source (as opposed to a constant voltage source) to function properly. This project requires a constant voltage to produce a constant current. Dedicated power adapters are available including dimmable modules (see links below). This project uses a simple resistor and commonly available power adapter to run the LED. The following schematic shows the layout:
DC Power Source
I have used an AC to DC voltage regulated power adapter which runs off mains electricity. It is rated at 5 volts and 1.5 Amps and is ideal for running a single Luxeon Star III. Using 5 volts is a good choice for these Star III's as their forward voltage is around 3.7v at 700ma. A 6v supply may also be suitable but going up to 9 or 12 volts means a lot of extra power must be dissipated by the resistor so it could get very hot. The DC power supply voltage must be higher than the LED forward voltage so a 3 or 3.5 volt supply is not suitable.
The power supply Amps rating should be comfortably higher than the LED current. For a 700mW LED (i.e. a Luxeon Star III - 3 Watt LED) a minimum 1 Amp supply is preferred.
You must be sure which of the leads on the power supply is positive (+) and which negative (-).
Unless you want to use a line or chassis socket that matches
the power supply plug, the simplest method is to cut off the power connector
(pictured above), strip about 2cm of cable to show bare wire and secure these
in a electrical connecting block. These are available from many DIY and electrical
stores. The resistor and power leads to the LED can also be connected to the
block in the following arrangement:
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Resistor
A resistor is required to limit the electrical current that can flow to the LED. Increasing the value of the resistor reduces the current, dimming the LED. However, such dimming effects are not very predictable so some experimentation may be required.
Getting a precisely matched resistor is not necessary but in general choose the next HIGHER value of resistor to prevent damaging the LED.
To select the correct value of resistor you need 3 details:
1. The supply voltage - from the
power adapter.
2. The LED 'forward voltage' - the Star III forward voltage is given as 3.7
typical.
3. The desired LED operating current - I suggest trying around 600mA to start
with. The difference in brightness between 600mA and 700mA is negligible.
There is a useful resistor calculator at: http://www.luxeonstar.com/resistor-calculator.php
Simply type in the figures above and it will calculate the required value of resistor for you. Ensure that the resistor used has an adequate power rating, always choose a higher rather than lower wattage. The resistors are low cost wirewound ceramic types (pictured above) and are low cost. The calculator automatically recommends a commonly available resistor value. It also recommends the required wattage (W) rating.
Maplins (see links below) stock a range of 3, 7 and 10 watt wirewound ceramic resistors.
Choose a resistor with Wattage ratings higher than the dissipated power.
Resistor lookup table for Cool White, Green, Cyan and Blue Luxeon Star III LEDs
It can be seen in the above chart that for a Luxeon Star III a good choice of resistor would be:
5 Volt DC supply -
a standard 2.2 ohm 7 watt resistor
6 Volt DC supply - a standard 3.9 ohm 7 watt resistor
I suggest the 7 watt resistor as their larger size allows for better heat dissipation and they only cost a few pence more than 3 watt types. Even these will get HOT. If they are mounted inside a box a few holes should be made for ventilation.
The suggested resistor
values will allow the LED to work at brightness that can barely be distinguished
from its full rating. They allow for minor inaccuracies in the AC/DC converter.
Even a 0.1 volt increase above the converters given voltage rating can produce
an additional 46 milliamps!
Heatsink
LEDs must be kept within their operational
temperature range to work efficiently and last a long time. The Luxeon Stars
require mounting on the flat area of a heatsink which can be purchased from
LED and electronics sellers. I made my own heatsinks out of a 2mm thick aluminium
sheet cut to 30cm x 8cm and folded as in the pictures below.
I have held the LEDs in place with 2 screws as I might want to remove them while experimenting, but this is a rather tricky solution. The problem is that the head of the screws must not touch the solder pads which could short the power, so a small countersunk screw is best. As the screw is threaded into soft aluminium a course-threaded screw is needed.
When the LED is mounted with screws a small blob of Thermal Grease (Thermal Compound) should be smeared over the base of the LED. This helps to conduct heat from the LED to the heatsink. Thermal grease (or Thermal Compound) is available from LED and electronics sellers - see LINKS below.
A simpler solution to using screws is to stick the LED in place. Dotlight (see LINKS) sell double sided adhesive pads called 'Cooltape' for attaching LEDs to a heatsink. Cooltape has good thermal conductivity and strong adhesion.
When my Luxeon Star LED's arrived I found that their aluminium base was not perfectly flat. The edges had a definite lip in places which could prevent close contact with the heatsink over the whole surface area. I gently smoothed the base on a piece of fine emery paper placed on a flat surface.
Update on Heatsinks. July 11th 2007: I have used the heatsinks sold by 'Dotlight' (see Links below) and they work fine. My only reservation is that I often keep the LED switched on for days at at time and the heatsink gets decidedly hot. This is not ideal for the life expectancy or efficiency of the LED. I would suggest using the slightly larger heatsink they supply for the Luxeon K2 Star LED.
Connecting the LED
Do not touch the lens of the LED and keep it covered when not in use to prevent contamination.
The power cables to the LED must be soldered in place. The solder joint should be made quickly to prevent overheating the LED. It is a good idea to attach the LED to the heatsink to aid heat dissipation before soldering.
Use a flexible wire rated for at least 1 amp to connect the LEDs. The LED light needs accurate positioning beneath the microscope and adjustments are made more easily if the connecting wires are quite flexible. (see Maplins, XR40T)
| The Luxeon Star III's are clearly marked for Positive (+) and Negative (-) connections and have solder pads on either side that can be used. |
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The wires to the LED must be secured (away from the LED) so that if they get pulled the solder joint is not stressed. I screwed a small terminal block to the heatsink and ran short wires from this to the LED, then the power wires go to the other side of the terminal block.
Here is my setup for experimenting with different resistors:
The rivets seen on the heatsink are holding an extra aluminium plate to give depth for the screws holding the LED, this is not essential and will not be needed if adhesive is used to secure the LED. The terminal block used here is a 'plug in' type that has pins and matching sockets. This is useful for quickly switching units. An alternative would be a chassis power socket with the leads attached to a matching plug.
White Star III LEDs
'White' LEDs use a low wavelength lightsource surrounded by a phosphorescing material that produces the additional wavelengths giving the impression of a white colour. The dominant wavelength in the graph for a white LED is around 440 nm. I do not use this LED for direct observation with the microscope - only via a camera.
Links
http://www.lumileds.com/ Luxeon LED Data Sheets, news and information.
http://www.luxeonstar.com/about-us.php Luxeon retailer in Canada. They also provide the resistor calculator mentioned above.
http://www.dotlight.de/shop/ Luxeon and retailer in Germany. Sells range of Luxeon LEDs and accessories including: power source, dimmer module, heatsink and LED adhesive tape (Cooltape), etc.
http://www.ultraleds.co.uk/ LED and Luxeon seller in the UK
http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/4/ The LED Centre. A simple circuit for regulating up to 1500mA current to an LED using an LM317T voltage regulator IC.
http://www.maplin.co.uk/ Sells a large range of electrical and electronic components. They have a reasonable range of ceramic wirewound resistors, power adapters etc.
http://www.esr.co.uk Sells electrical and electronic components. Good range of 2.5w silicon resistors.
Email pfkempATSIGNtalk21DOTcom
UPDATE
Luxeon K2 Star
I bought one of the Luxeon K2 Star LED's and wanted to provide a few notes:
The K2 Star is not physically identical to the Star III but the Carlco collimator-lens holder is easily modified to fit with a little cutting on the base.
The K2 is rated to run at up to 1 Amp, but check the specifications as some have lower ratings. I got mine from Ultraleds (see Links) and it is full rated for 1 amp.
The K2 Star will operate with the same setup as the Star III and provide more illumination. A slightly smaller resistor can be chosen to give even more brightness. I changed from a 2.2ohm to a 1.5ohm.
The K2 runs HOT at higher power. I found that even a substantial copper heatsink got too hot for my purposes. To overcome this I bought a CPU heatsink and fan and mounted the LED on this. It works very well indeed. The 12v fan works fine with a 9vDC mains adapter and is not too noisy.
I am not yet convinced that the K2 gives any advantage over a Star III for microscopy purposes. Given its cost and additional cooling needs I think it might prove an unnecessary change. Unless I can gain some little improvement in visibility/contrast of microscopic subjects I will probably use the K2 for something else. Will write more on this later
Dimled A1W
This tiny unit is a PWM dimmer for LED's. Simply, it dims the LED by chopping tiny sections out of the current.
It does NOT work with Luxeon Stars and a 5 volt supply. I presume this is due to the pwm cirtuit causing a voltage drop of almost 4 volts! In this case a minimum 9 volt supply would be needed and I have some doubts that even this will work.
Last update: April 24th 2008
