drive them to their (safe) peak of current and luminosity."

Within the same batch they will be similar but different for each batch depending on the quality of manufacturing. As already noted, cheap grab bag LEDs will be all over the place compared to good quality manufactured to specification ones. The real question is how much luminosity do you need? Then design the circuit to meet that specification. Making them brighter than necessary, depending on their specification and the circuit design may result in premature failure i.e. short lifetime per specification and will waste power in today's "Green World" if you believe in that. There are lots of types of LEDs with various luminosity specifications. From small indicators in darkness ones to hi lumen illumination ones used for headlights. There are illumination standards if you really want to get into it.

 

All I have is a single cheap multimeter, that I dont like how is build, but is the only one that I have that has a temperature symbol on its dial and also I test it and is working. I dont get any real temperature from it, though, I only get a number that rises or fall, depends how I warm up the sensor. The sensor I managed to scrap, is from a good medical electronic thermometer. The thermometer was very limited in temperature range, but I dont think the sensor itself is limited. I didnt test it fully yet.
Some light into this subject will be very much appreciated !
Thank you.
This is how the scrapped temperature sensor looks like:


These temperature sensors are called thermistors and I can search for them on the components markets.
- What temperature sensor can you recommend ? As a single component or as a full device. Your recommendation.
I will wait for your feedback and after that I will buy some spares. I think I want a linear termistor, and also for low to medium to high temperatures. Low as 1dgr C and high as 500 dgr C. I never deal with terminators until now. Im very new to them. Or any temperature measurement device for electronics.

 

The thermometer was very limited in temperature range, but I dont think the sensor itself is limited. I didnt test it fully yet.

Now would be a good time to read the spec sheet for your multimeter and the sensor used. When measuring temp, and in this case it would only be the outer skin temp, it is helpful to use a thermal conduction paste between the sensor element and source. Try and measure as close to the source of light as possible. You don't need to worry about shielding the source of light for this measurement. IF a heatsink for the LED is to be used, it should be in place for the measurement.

 

Thank you mister @SamR
I didnt jump into measuring LEDs temperature yet. Thats advanced topic at this point. I just managed to scrap the termistor today, it was actually molded in plastic, and I melted all the plastic to take it out, then I searched for a device and I found this extremly poor designed I dont give 2 cents on it, but its all I got that can measure temperature and show me on its screen some data if I connect the leads of the DMM to the sensor wires. This DMM I took it very cheaply from bazaar, and the dot that usually hint you about 0.1 or 0.01, doesnt appear, I suspect the ribbon to the LCD is damaged or even the processor itself or the LCD itself is damaged for this dot element. So, I almost never used this DMM, but now, its the only device I can use and read temperature with. It is formidable.
So far I play around and these are the results I gathered. (I just tested it right now to get the data right)
(im not sure if its a dot anywhere or not since I have this dot handicap on the DMM screen)
In ambient air, when I am not influencing the sensor is around 630 ish.
If I blow air with my mouth into the sensor, DMM is showing me about 615
Then, if I hold the sensor between my fingers or in my closed palm, I get 650
If I hold it 3mm distance from my fully heated iron solder (which is at 250-300dgr C) I get 705 on DMM
I believe this (medical) termistor is not having a very large range.... Im not sure, I am playing for the first time with this things.
Is this a normal termistor range? or there are others more sensitive and wider range?

 

Most of my temp sensing was done in liquid or vapor state so the material was enclosed in piping or tankage. Type of sensor depends on requirement of what you are sensing. Here it is surface temp. I have used quick read, non-contact, Infra Red "guns" but my recollection is they are not as accurate as contact surface temp methods. But, the question here is how accurate do you need? I once worked with a fractional distillation expert who after having his specified sensors installed where he recommended them on a ~100' fractional distillation column and my configuring his sensors into our distributed control system, we were controlling to within 0.001°C. I didn't believe it was possible and at best maybe ~0.02°C. So, how accurate do you need these measurements? That will determine the method (contact or noncontact) and the sensor type, range, and repeatability accuracy.

 

I get your point. I already say what range I want up in the previous comment.
At this point, any temperature reading is gold for me. I am inclined to be happy with tens of degrees measurements.
I do like (very much) the point contact sensor because I can put it directly ON the smd LED or transistor that (I will) measure. Also I like it is like a tiny blob and not as a case, Ive seen already they look like transistors. Those models cases are too big for my target measurments. Im measuring very tiny components here. So the tiny blob is perfect for the job. BUT if other types of termistors are better than the blob ones, do please enlighten me.

 

You need the correct probe to use the temperature range. Mine uses a type K thermocouple. I doubt that a thermistor is the right thing.

Edit: For something as small as an SMD LED, the probe is going to act as a heat sink and drastically change the temperature.

Bob

 

 

Google contact surface temperature sensor. Most links will be to industrial or lab suppliers and these will just be for sensors. But first you need to find a temp display device or if you are going to use your multimeter then find out what its specs are to let you know what thermocouples to use. Take a look here D38999 Thermocouple Contacts | MILNEC to see some basic standard information and they do have tech assist contact info to ask questions.

 

Edit: For something as small as an SMD LED, the probe is going to act as a heat sink and drastically change the temperature.

All of the surface contact ones are going to act as a heat sink so they need a bit of dwell time to settle on the temp reading. You might also look at the noncontact IR ones as I'm sure they have improved in the 20+ years since I had one in my hands. One note as to noncontact is I would advise using them with some kind steady mount such as a tripod maybe. Ones that also use a laser pointer to mark the reading spot would also be a plus.

 

I listen to your advice and I read the DMM instructions paper and indeed they say it is showing the real temperature in dgr C.
I believe they are referring to a thermic probe that this DMM should have been delivered with, but it didnt had one. Or I am not using the right contacts, although I played around with its probe plug holes and I did see a 40 and a 50 displayed but I am sure my ambient room temperature is somewhere at around 20 dgr C right now.
The good thing is that I have a reading and a range of values (from 600 to 700 whatever it is) that are somewhat consistent. So I am ok with this reading device so far.

I am curious what other temperature (cheap) devices you are using practically.

Talking about non-contact devices, I do have this temperature gun, but it is having a very widespread area of reading, it even show on its label. It is a nameless ebay device, that is working fine so far, but not good for my project. Or at least I dont know how to properly adapt it yet. It is very good for palm size area temperature reading. Not too small and not too large area. It still doing a good job, as nameless as it is, if Im using it properly.

 

The medical thermometer itself was having a small range - I only test it very briefly - it was from around 30 dgr C to 45 dgr C. You know, some low and high body temperature, but after those ranges was showing LO or HI, informing me that is outside its range. Very shitty that they made it so specific and not a bit more larger temperature range like the old mercury thermometers. I think they were ranging from 0 dgr C to 60 dgr C, I know my mother was checking some water temperature with one of those. My bad luck with this electronic medical thermometer, although it was a good idea.

 

For something as small as an SMD LED, the probe is going to act as a heat sink and drastically change the temperature.

Its a good point and I did thought of it. Im not at this stage yet, but when the time will come, of course I will leave the sensor touch and sink into component temperature for some time and after that gradually rising up the component current and checking the temperature.

 

If your DMM has a °C scale, the manual for it should specify which type thermocouple it uses unless it was proprietary, which I doubt it is. Likely a type J or K thermocouple?

EDIT: Even the old mercury and later spirit thermometers used in medicine you had to wait several minutes for them to settle. Same with the drugstore electrical ones or the ones I used in industry. The noncontact ones I used were fairly quick if they were held steady, but the reading was for a much larger area than an LED.

 

Very interesting idea mister @SamR, I looked right now and it say: "K-type thermocouple wire".

To be a bit more specific.... I believe, I will need a 1 degree Celsius step increment in about 100 steps range from ambient temp 25dgr C to 100dgr C thermistor sensitivity. I am guessing what I need, but Im trying to ease what you should recommend. Im interested what did you used in some general and normal temperature readings in your past experiments.
Edited:
I believe, any component that will reach 100 dgr C will be very solicited electrically and structurally. So this range (25 to 100 dgr C) I believe is the most sensible range I will need to cover with my thermic sensor. So find for me a termistor or whatever name they have, that is fitting in this range, + or - some degrees, its not written in stone.

 

As a parantesis to the subject, I was curious on the difference between the J and K thermocouple (that you mentioned) and I found this very to the point explanation:

 

A thermistor and thermocouple are two completely different things. You cannot use them interchangeably.

As your manual says, you need a type K thermocouple.

Bon

 

A thermistor and thermocouple are two completely different things. You cannot use them interchangeably.

I didn't know this detail ! Thank you ! Very good information.

 

It occurs to me that, if you think your time is worth even minimum wage, the time you are spending on this could buy you thousands if LEDs with known, reliable specifications. Saving money can be very costly.

Bob

 

To mister @BobTPH - Its not ONLY about LEDs. Its about how to make this thing work. Im learning new things, new components, new methods of reading and experimenting. This LED tester project is a very basic way to lay down a practical idea on how to measure ANY component power stress under controlled current input. It is more than leds, it will be a bit more universal and in the end you will want to make one for yourself. I will be very glad if it will come out that good, haha.