Introduction: How to Measure LED Temperature (and Make Sure Its Not Too Hot!)
This instructable will show you how to attach a Type T thermocouple to an LED temperature measurement point so you can measure the LED operating temperature.
First: LEDs heat up when you drive them with an electric current. The more current, the more light they produce and the hotter the LED chip (the active part that produces the light) becomes. There is a limit to the amount of power, and heat, that an LED can take. As they heat up, they become less efficient at producing light, so you have a situation of diminishing returns when you ramp up the current to get more light out of an LED.
Why is this important?
LED temperature is very important to the light output lifetime of an LED. This is usually called 'lumen maintenance' or, if you are talking about how fast an LED fades with time, 'lumen depreciation'. LEDs are capable of providing light output for a very long period of time, in the tens of THOUSANDS of hours, but only if they are driven properly so that they don't get too hot.
Many low power LEDs used for pretty blinky lights will not get hot enough to suffer large lumen depreciation because of temperature, and these will probably do just fine for a long time as long as you don't overdrive (and kill) them. Other factors, like defects in the little LED chip or changes to the transparency of the LED lens (part of the LED package) may still lead to light output losses, and so even LEDs that are properly managed with respect to temperature can still fade over time.
Higher power LEDs, particularly those used in lighting, are expected to produce hundreds or thousands of lumens (a lumen is a unit of light output) and these LEDs are often driven at higher power levels to get as many lumens as possible out of the LED. It is imperative that these LEDs are attached to adequate heat sinks to dissipate as much heat as possible. It is also imperative to keep the LED chip temperature (called the LED junction temperature Tj) at an acceptable level. LED data sheets list this information, and LED LM80 data (google it for your LED) will give you lumen maintenance estimates for your LED running at a particular temperature.
What you will need:
You will need Type T thermocouple wire and the appropriate connection plugs, a meter that can measure Type T, a decent set of electronics wire cutters, a soldering iron, and (maybe) a heat gun. A flux pen with no-clean flux may be helpful as well.
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Step 1: Find Your Temperature Measurement Point
Where do you measure the temperature?
There are a few different names for the spot you are trying to measure. Solder point temperature (Ts), case temperature (Tc), or temperature measurement point (TMP) are equivalent for our purposes. This represents the closest physical location you can get to the LED chip itself.
Most LEDs have 2 or 3 solder connections to the PCB. Two are electrical leads (anode+ and cathode-) and the third, if it's there, is the heat sink connection. The anode and cathode leads carry electrical current. These leads are sometimes visible on the sides of the LEDs or they may on the underside where they are not visible. The cathode side is marked on the package with a dot or a diagonal mark. The heat sink lead is for heat dissipation and is typically (though not always) on the underside of the LED where you can't see it.
An LED has several possible configurations (these are the typical configurations, there are others):
1) The LED cathode is also used as the heat sink and the LED has two leads.
2) There is a separate heat sink lead and this is electrically (and thermally) connected to the cathode and the LED has three leads.
3) There is a separate heat sink lead, it is electrically isolated from the cathode, and the LED has three leads.
This information, as well as the location for the temperature measurement, should be on the LED data sheet.
The LED is mounted to a PCB. If properly designed, the PCB will have a large heat sink pad that dissipates heat away from the LED, either laterally (to the side), through the PCB to the back side, or both. You want to attach the thermocouple somewhere to this pad as close to the LED as possible. Some PCBs have a spot on the board that is designated specifically for this attachment, and some do not. If there is no TMP pad, you will need to scrape off some of the solder mask to reveal the copper underneath.
Step 2: Prep Your Thermocouple
You can buy pre-made thermocouples or you can buy a spool of thermocouple wire and make your own. A place like Omega has all the connectors you will need to plug the thermocouple into your meter. Its much cheaper to make your own, this section shows you how.
I like to use 36 AWG (gauge) wire for my thermocouples. This wire is very thin, however, and may be difficult for some people to strip and use. I like the thin stuff because it's quite flexible and can be attached to very small pads with little risk of changing the thermal properties or mechanically stressing the attachment point.
A thermocouple is two wires of different metals, connected at either end, that exhibits a voltage potential differential when one end is at a different temperature than the other. This is called the thermoelectric effect. I use Type T wire, which is copper/constantan, because it is easy to solder and this is critical to a successful attachment. Type T has a temperature range of -270 to 370C which is fine for LED measurement. I much prefer soldering to using a thermal epoxy because it is quick (no waiting for the epoxy to harden), non-destructive (epoxy tends to be messy), and can be repositioned, repaired, or replaced.
What to do:
Use your cutters to gently grab the wire about a quarter of an inch (1/4") from the end. Do not cut through. While applying a little pressure, pull the insulation off of the wire (just like you're stripping it). This takes some practice and if you accidentally cut the wire, no biggie just try again. This is why you have a spool! Eventually you will get it. If you notice, you will see that there is an oversheath on the wire pair and individual insulations on each wire. When you get it right, though, you will definitely see two bare wire sticking out.
You then need to twist the wires together. I like to use a pin vise to grab them, but a decent pair of needle nose pliers or tweezers will also work. The goal is to form a tight twist. If a wire breaks while doing this, cut the whole thing off and start again. This is why you have a spool!
When you have a tight twist, cut the end off close to the insulation so that only a few turns are left in the twist. Congratulations, you just made a thermocouple!
Step 3: Tin Your Thermocouple and Solder Pad
Tin the thermocouple
Adding solder to the surfaces to be joined is called tinning. This covers each part with a thin solder layer and can help make soldering the two surfaces together much easier. I highly recommend tinning parts prior to soldering them together whenever the solder joint is challenging. Always make sure your surfaces are clean. For the thermocouple wire, you just stripped it and so the wire should be fresh and not need cleaning. For the mounting pad on the LED board, use isopropyl alcohol with a q-tip (or a toothbrush) to scrub the surface and remove any old flux or grease. Soaking in alcohol for a few minutes can help too. If you have any liquid flux (from a flux pen, for example) you can add a little to your surfaces.
When you tin a wire, it is often easier to start with a small drop of solder on the soldering iron tip. Always start with a clean tip by wiping on a damp sponge or brass wool pad (I use both). When you have a small drop of solder on the tip, hold it to the wire and let the wire sink in. Then add a little more solder to add some fresh flux.
Tin the circuit board pad
The thermal pad is designed to carry heat away from the LED. As such, it can also make this spot very difficult to solder. You will want to remove any external heat sinks connected to the board, if you can.
It will probably be necessary to preheat the board with a heat gun prior to soldering. Always start the heat gun away from the board (about 6 inches is good) and bring it slowly closer as you heat the PCB. I always heat the back of the board when possible to avoid melting the LED or other adjacent components. If you have to heat from the LED side, be careful not to overdo it - you only need the board warmed up and are not trying to reflow your solder with the heat gun. Go slow and be patient, this is the one step where you can damage the LEDs or PCB. It's OK to see a little smoke from left over flux, but any excessive smoke, or bubbles in the solder mask on the board, means its too hot and you need to back the heat gun away from the board.
After you warm up the board, move quickly to tin the pad. Use the biggest soldering iron tip that will fit, but don't turn up your iron as this can damage the pad. Your soldering iron tip will be hot enough to melt the plastic of the LED package, so be careful not to touch the LED package or lens.
Tinning the pad prior to soldering will tell you whether or not you are going to be able to attach the thermocouple, so this is a very important step. Try to leave a little extra solder on the pad when you are done tinning.
Step 4: Attaching the Thermocouple to the LED Board
This is the hard part. The objective is to get a good solder joint between the thermocouple tip and the pad.
Preheat the board with a heat gun as outlined in the previous step. As you can see in the video, I'm holding the board with a set of forceps so I can heat the metal core board from the back. The forceps are held in a vise on my workbench. When the board is warm, try to solder the thermocouple to the pad. Having a little solder on the tip, as before, is usually helpful. And since both sides already have solder, you probably don't need any more (though it can be helpful to add some flux from your flux pen before you go for it).
I my first attempt I had a good joint but I wanted the thermocouple deeper into the solder bead. My third attempt worked, and while it wasn't pretty it was a solid connection and will give a good temperature reading.
You can reheat and try again if your first attempts fail. As always, be patient. When you think you have it, give the thermocouple a small tug to make sure its attached firmly. After this, try not to pull on it or bend it at the joint, as this can break it (and you will have to start again).
Step 5: Make Your Measurement and Interpret Your Results
When the thermocouple is attached, you are ready to assemble your light and make a measurement. If the LED is inside an enclosure or under a lens, you want to reinstall these components so that you are simulating the real world conditions that the LED will see to ensure that your temperature measurement represents the actual temperature that the LED will experience in service.
Power the LED and let the system reach thermal equilibrium. This may take some time depending on how big the heat sink is and how much power you are putting into the LED. You can monitor the temperature rise, and when it stabilizes you will have your final temp.
1) Some drive circuits can be electrically noisy, especially switching drivers which pulse the LED current. The thermocouple may be electrically attached to the LED chip (depending on whether or not the LED heat sink pad is electrically isolated from the chip) and this can pick up the noise. This may be a problem for some meters, and you may notice erratic temperature readings as a result. Switching off the power should remove that noise, and if you see a noticeable jump in the meter reading when you do this you probably are seeing the noise.
2) Many handheld meters can read two or more thermocouples. If you are measuring multiple LEDs in an array, this may cause a problem with your readout unless your temperature channels are isolated from each other inside the meter. Many handheld meters do not have isolated channels and so you will need to measure the LEDs one at a time.
3) Some LED datasheets and LM80 datasets list case temperatures, and some may list junction temperatures. If you need to know the LED junction temperature, you will have to calculate it based on the case temperature. I wrote an explanation of this, along with other LED thermal design considerations, in a Technical Note that can be found here.