Read the last part of Mik’s post above mine, after the picture.
Greg
TonyWalsham said:That's what I meant. I think. Intuitively it looks like the extra heat - especially if it is enclosed in say a smallish tender or road switcher carbody - might eventually adversely effect the performance or shorten the life of the decoder, no, not as fast as simply overloading it would. But shorter than if both were house separately or with fans. A different analogy might be to say if you run your 40 watts through the decoder you're throwing it in the fire, if you stick the other thing under it in a tight space means you put it in a pan on a burner - neither would seem to keep it cool.
Greg, I think Allen was using the starter motor analogy in the sense that headers from an exhaust system that are run too close to the starter motor, may affect the performance of the starter motor because it can get too hot from the headers.
I was thinking along the lines of how long most computers continue to work (or work well) after the fan fails… if you say that’s apples to oranges, then OK
It’s not extra heat, it’s “instead of” heat… the Revo board will run cool as a cucumber since the booster will do all the work.
Also, again, the heat radiated from the booster board is nowhere near the temperature of the junctions of the transistors, so it will not affect the Revo.
The 40 watts is through the booster, the Revo will draw virtually nothing at all, again the booster will do all the work.
It’s not about being warm or cool, unless I misunderstand your question, which was (to paraphrase) “won’t it be bad for the heat from the booster heating up the Revo?”… the answer, as I have explained several times is no. You have to remember the air is cooler than the heat sink which is WAY cooler than the transistors.
So heating the revo to even the temperature of the heat sink on the booster (which is impossible) is no big deal.
Greg
thanks greg, that’s why I ask, it’s how you learn 
Yep, the cooling of semiconductors and the various temps involved is almost a science. I worked in consumer electronics where passive cooling was a paramount goal, to eliminate cost of fans and to avoid fan noise.
Regards, Greg
Greg, I’ll take your word for what you say, but it still seems counter-intuitive. that heat-sink is sitting directly under the decoder… heat rises, there-fore it would be like placing the decoder over a stove burner… its gonna get cooked.
You talk about the junction of the transistors, but thats not a good comparison either… the booster is going to be handling up to 12amps… so that means it needs to dissipate that heat, which would be far greater than the decoder was designed to take.
I’m not trying to say you are wrong, just trying to explain why is seems counter-intuitive. Is there any chance you could show some math to show that the heat sink isn’t going to radiate enough heat over time to cook the decoder? At least if there was a fan blowing air through the heatsink, I would think the air directly under the decoder wouldn’t get as hot.
Please pardon my confusion
The carrying capacity of the board(s) is 12 amps, but relatively little of that current is dissipated as heat by the on-board components.
Most of the (up to) 12 amps (actually the wattage resulting from the equation P=I x E) is dissipated as work and heat energy at the motor.
Think of a switch (the controller board(s)) controlling a 100 watt light bulb (the motor). The light bulb is dissipating most of the energy as light and heat. Very little energy is dissipated as heat by the switch. Not a perfect analogy for solid state circuits, but close enough to support an understanding.
Happy RRing,
Jerry
Watts is Watts. It does not matter what is dissipating the watts as heat, it’s the same amount of heat. So if you do 24 watts on the Revo, it will most likely make the same heat as 24 watts on the booster, since they are both using FETs.
As an aside, not a lot of heat comparatively, that is why they use PWM. The FETs don’t make much heat because they are basically off, or full on, in the low resistance state. Cannot be compared to a linear amplifier.
But back to the question, suppose they are running twice as much heat from the booster… it’s the air temperature and whatever radiant heat (which would be small) more convection from the heat sink heating the air. The air temp, given the much larger heat sink, would not be as high as the air temp off the Revo. Remember that the Revo is not running any load to speak of now, so the heat does not matter.
I could explain in more detail, but just take this to keep it simple.
Greg