brightness / temp

[aljsk8]

leds...

brightness decreases with temperature increase

but with increased temp comes lower vf so current goes up and led gets brighter

ok example lets say you have a cree xre with a vf at 350ma of 3.2v
and its nice and cool and lets make up a figure of 100 lumen's

now take cooling away temp goes up so vf goes down now lets say 3.2v is 450ma brightness should go up because of current but down because of temp

can anyone explain this for me?

also once you heat up an led is the vf permanently reduced?
im really hoping to use this in my favour for a direct drive application

 

[Genes]

As Vf goes down, the current can go up, unless the led is being driven by a constant current device. This causes further heating. The change in luminosity is not a function of forward voltage, but of junction temperature. The change in luminosity with temperature is more a semiconductor issue than current vs voltage change. Luminosity, wavelength and forward voltage are all functions of junction temperature.

 

[CzarDestructo]

Vf is a product of temperature, as a result if you run the LEDs until they get really hot it will NOT permanently reduce Vf. It will normalize once it cools off.

At the Vf goes down, assuming you keep a constant current, the power decreases since its a product of V * I. This will contribute to lower lumens. If the power is kept the same but the Vf drops because of temperature it will still loose efficiency, however it's not significant. The reason for this, I'm fairly certain but not 100%, is because as the voltage drop changes the wavelength emission of the LED changes. Since the LEDs aren't white and need to be converted to white light via phosphor the conversion becomes less efficient as the wavelength deviates from the expected norm.

Hope this helps.

 

[uk_caver]

also once you heat up an led is the vf permanently reduced?

There does seem to be a permanent reduction in Vf (at a given current/temperature) compared to the 'birth' value for LEDs that have been run for a burn-in period.

It's possible that the effect varies in strength between device types.

Search for 'burn-in vf' in the search box.

 

[HarryN]

leds...

brightness decreases with temperature increase

but with increased temp comes lower vf so current goes up and led gets brighter

ok example lets say you have a cree xre with a vf at 350ma of 3.2v
and its nice and cool and lets make up a figure of 100 lumen's

now take cooling away temp goes up so vf goes down now lets say 3.2v is 450ma brightness should go up because of current but down because of temp

can anyone explain this for me?

also once you heat up an led is the vf permanently reduced?
im really hoping to use this in my favour for a direct drive application

Hi, I have used these effects in conjuntion with a pseudo direct drive (resistored) light. There is the added curve which is the battery voltage vs current curve.

The idea is that you take all of these curves:
- Vf vs temp
- Vf vs current
- Battery current vs voltage as it discharges
- resistor ohms vs temp
- brightness vs current
- Vf change with time

and you move them around with each other until you obtain a relatively flat output. It is quite an interesting exercise and learning experience.

You can use the published curves as a guide, but your individual LED results will matter, especially if you are pushing it towards its limits. If not, then you can do a lot.

The idea does work reasonably well though, and I have used it to build a 3 resistor light with a rotary switch to produce 3 different brightness levels. I like the approach - so simple and reliable, just don't expect to use it to push an LED to its limits.

 

[HarryN]

In some ways, it is harder to do this Vf vs current game now than it was 5 years ago. The reason, is that power leds used to be run where there was a fairly steep climb in the Vf vs current curve, and the LED mfgs have "improved" the product to lower Vfs, and make them have flatter curves. The effect, is that the Vf changes relatively little with current compared to the past.

This makes the Vf vs current control parameter really a hair trigger compared to 5 years ago.

What you can do, it just use the resistor as your control mechanism. You can buy resistors with various resistance changes vs temperature, and this is easier to tune.

The LDO method of control is also quite handy. More or less, the LDO chip can be made into a feedback loop that self regulates at a current maximum. (AMC / National / others make these). If you buy these, try to buy them from a reputable distributor, as many on the secondary market are ones that didn't meet spec and were recoverd from the trash.