Thermal Compound - Results Posted

Indicative of a good situation.

No, because when the thermal path is good, the LED will get less hot and have better efficiency and thus produce less heat.
The heat that it produces either way will eventually end up in the body, so with a bad thermal path the body will get hotter anyway.

If he is measuring the body temperature of the flashlight then the temperature should be greater because more energy is being transferred to the body.
The better the thermal path, the higher the body temperature should be and the lower the MCPCB temperature should be.

It is not possible to have the two be the same temperature, there will always be a delta T between them.
Better thermal path = lower delta = less energy in MCPCB = more energy in body.
More energy in body => higher temperature => larger delta T between body and ambient air temperature => more heat dissipation.

When the LED produces more heat, the body will get warmer too, because the heat has nowhere else to go, albeit through a bad thermal path.

Of course.
The higher the delta T, the more heat will be transferred.

Ditto the rest, there should be no measurable difference in the body temperature. The exact voltage that the cells are at and the exact resistance that the switch had that time around, among other things will play a bigger role if I had to guess.

When I tested thermal paste, I found that as long as it is there and squished reasonably well,the performance is goning be too close to notice.

You’re right that the delta T will be higher with the poorly applied thermal compound. But so will the thermal resistance.
Those that predicted 113 degrees know that the flashlight as a whole will eventually drift towards thermal equilibrium, and they are right. Energy into the light HAS to equal energy shed. That energy is the sum of heat and the energy of photons emitted.

Take it a step further and as the LED gets super hot due to the poorly applied compound, it will draw MORE current. Also as it gets hotter it will become less efficient. An even greater percentage of the energy input will be heat.
As I type this I can’t remember if the driver is constant current or DD.
For the better thermal path, my prediction would be temperature goes down, slightly.

neutralfan, did you press down on the star and gently twist side to side (45 ish degrees,) when applying the new set of thermal paste?

Assuming it is DD, the energy will be the same in both cases, regardless of good or bad thermal path.
The difference is that when it has a bad thermal path then more heat remains in the MCPCB and less transfers to the body.
The total energy dissipated is still the same, but there is more energy stuck in the MCPBC than in the body (compared to a good thermal path, where the energy can easily transfer from the PCB to the body.)
Since the MCPCB has a lot more energy and doesn’t transfer as much to the body, that results in a higher temperature at the PCB.

In the opposite case where the thermal path is good, the temperature difference becomes less, so the PCB and body are closer to the same degrees.
That means the MCPCB is cooler and the body is hotter. (but the MCPCB will always be at a hotter temperature than the body)

Enderman,
IF it is assumed that the energy input is the same for each case, and we disregard the decrease in efficiency of the LED at higher temperatures, then the body of the flashlight will be the same.
You are right, there will be a higher delta T, but there will also be a higher thermal resistance, so the heat flow will be the same! (Double the Voltage and Double the Resistance in a simple circuit, and electron flow is the same)

What the heck are you talking about?
This has nothing to do with voltage or current.

If you have 1kj of energy on a piece of copper with a bad thermal path to a chunk of aluminum, the copper will be much hotter than the aluminum.
If you have a good thermal path, the copper will only be slightly hotter than the aluminum because the energy will transfer between the pieces easily and equalize.
That means the copper temperature decreases and the aluminum temperature increases.

Think about it as two buckets with a tube running between the two.

But the heat will be more because when badly cooled it will produce more heat and less light.

Only for a short while, until delta T is large enough to ‘force’ heat dissipation to the body. (it’s got nowhere else to go)

So in this case delta T is comparable with voltage. make it high enough and the ‘heat current’ to the body will be high too.

Well, first off, where did NeutralFan take the measurements? I don’t see where in the OP he states the location. If it is the outside body, close to where the shelf contact is, then with better heat transfer you would think the outside body would take on more heat, keeping the MCPCB cooler in the process. I’m guessing it will be slightly hotter but I doubt it will be noticeable.

Cool project though, you sound like me. I always want to do things as efficiently and effectively as possible, the first time.

Makes sense, either way I still think it’s not enough heat to see a measurable difference, unless hes driving that LED at 6A+ or something like that.

@ rdoc613:

The LED produces heat and light.
The heat has nowhere else to go then to the body.
So, the efficiency of the LED basically determines the heat going to the body.
The efficiency suffers when there is a poor thermal path (high delta T) because the LED will then operate at higher temperatures.
At a certain point (saturation) the delta T is high enough to stop the LED from getting even hotter and thus all of the generated heat will be transferred to the body.

I don’t know how big the difference in efficiency between a properly cooled LED and a badly cooled LED can be.

When I put the LED MCPCB back on the shelf over the thermal compound, I first slowly pressed down evenly and gently. Eventually I pressed hard and slightly rotated the MCPCB clockwise and counter-clockwise by approximately 5-10 degrees.

Well unless you push an LED pretty far, it seems like the difference isn’t very significant:

I took the temperature measurements using an infrared thermometer. I pointed it at the body of the flashlight and found the hottest spot. The flashlight was tail standing in turbo mode. The hottest spot was near the top of the head, just below the bezel.

You should use a thermal probe attached directly to the copper MCPCB to get a useful reading without all of the thermal variables discussed above.

I agree with this and that was my rationale for the experiment. Assuming a better heat transfer from the LED MCPCB to the shelf using the dot method versus the spread method, I would expect the temperature to increase faster near where the shelf is located.

The driver was in direct drive turbo mode and the same battery was fully charged before each set of measurements.