80
votes
Accepted
Why don't we make CPUs with 1000s of layers to make use of space in the third dimension?
The two killer reasons are yield, and heat.
Yield. Every time you do a process step, you get less than 100% perfection. Let's say you get 99% perfection per step. In a process with 20 steps, you would ...
- 169k
50
votes
Why hasn't my LED burned out by now?
Properly designed, built and used, today's LEDs have incredibly long lives and the wearout mechanisms are not catastrophic in nature. So instead of using incandescent lamp MTTF statistics, a ...
- 2,004
46
votes
Accepted
Why am I cooking MOSFETs?
Your problem is the gate drive voltage. If you look at the datasheet for the STP16NF06, you'll see that the 0.08 Ω Rdson only applies for Vgs = 10 V, and you're driving it with only (a bit under) 5 V, ...
- 2,631
41
votes
Accepted
Is DC heating faster than AC heating?
When you say 120V @ 50Hz AC you are implicitly saying 120Vrms.
The RMS voltage is qualitatively defined as the voltage which will give the same resistive heating (averaged out over time) as a DC ...
- 56.9k
37
votes
Why aren't CPUs cooled from below as well as above?
They aren't cooled from below because they have pins on the bottom, and FR4 below that.
Due to having a much lower thermal conductivity,
$$
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\text{Copper:} & 385\phantom{.25} &...
- 85k
35
votes
Accepted
Heating a wire with DC current; why is it hottest in the middle?
There are two effects going on. The heat sinking effect of the connections and the temperature coefficient on the wire.
Initially the wire is all at the same temperature.
You turn the power on and ...
- 6,912
33
votes
Using CPU heat to generate electricity
The issue with thermoelectric generators is they are horrendously inefficient.
For a CPU you HAVE to get rid of the heat they produce or they melt down.
You could hook up a peltier module and ...
- 46.8k
33
votes
How to 'fix' this in an enclosure without glueing?
You can buy (or 3d print) fasteners made for such a case. Search PCB mounting feet.
These include a standoff to help cooling and prevent shorts.
- 431
32
votes
Accepted
Why does only the tip of the electrode melt when arc welding?
The electrode's resistance isn't what's heating things up – it's the resistance of the ionized air in the arc!
Hence, things close to the arc get hot, and things farther away don't.
- 96.9k
30
votes
Accepted
Why do BLDC motor (1 kW) controllers have so many MOSFETs?
The reason to use multiple MOSFETs is to lower power dissipation resulting in a cheaper design.
Yes one MOSFET can handle the current but it will dissipate some power as it does have some resistance, ...
- 81k
28
votes
Accepted
Are there still practical modern uses for large through-hole resistors?
You can get 1W (and more!) SMD resistors for situations where you need a lot of power dissipation. The reasons for needing that power dissipation are varied, but a common case is shunt resistors for ...
- 10.8k
27
votes
Accepted
Does a resistor block or dissipate electricity?
You are getting your terminology all muddled up. Firstly, V = Volts which is a measure of potential difference (voltage), not power. So saying "extra 7V of power" is incorrect. Secondly, the resistor ...
- 67.9k
27
votes
Accepted
How to 'fix' this in an enclosure without glueing?
Cut a piece of pcb stock (or any sheet plastic) to fit properly in the housing, including screw holes to match those bosses.
Then attach your components to this base plate either with double sided ...
- 1,479
25
votes
Why is it dangerous use a coiled extension cord?
Here's another illustration.
Used to charge an EV at 'only' 10A... MW
- 259
25
votes
Why hasn't my LED burned out by now?
Best guesstimate I have for an indicator LED life span is more than 5,000,000 hours.
I don't think anyone would be surprised if a micro controller lasted 10-15 years. More likely one would EXPECT ...
- 7,457
23
votes
Accepted
Heat output from a 200W electric radiator?
Yes, electric heaters are basically a resistor.
A resistor converts electric energy into heat, it does that with 100% efficiency.
That might sound weird but think about it this way: if a resistor ...
- 81k
22
votes
Accepted
Why is 10W resistor getting hot with only 6.5W running through it?
First let's do a quick number crunch:
6.528W/10W = 65% (of 10W)
Referring to the datasheet:
There is about a 165C rise in ...
- 5,182
22
votes
Why aren't CPUs cooled from below as well as above?
Cooling isn't important, it's crucial. A modern CPU can easily put out something between 15 W and 200 W, from a die that's a few cm². If you're not transporting that heat away, that chip has to stop ...
- 96.9k
21
votes
How to 'fix' this in an enclosure without glueing?
I would try something like that:
You assemble like that, put crazy glue on the legs, let it dry in the container. Then you can unscrew when fixed to pick up the device.
20
votes
Accepted
Setting heat on electric stove
The hob power is typically controlled using a thermo-mechanical duty-cycle controller.
Figure 1. Part of a hob power regulator.
There are three parts to the control.
A small heater element that ...
- 177k
20
votes
Accepted
Can unconnected inputs make an IC get warm?
Yes, CMOS circuits can get hot when there are floating inputs. You should always connect unused CMOS input pins to a defined voltage, usually GND or Vdd, unless the datasheet tells you otherwise (see ...
- 768
20
votes
Accepted
One of four MOSFET in parallel heating too much
You need to use an opamp for each MOSFET to ensure balancing. Each MOSFET should have an independent sense resistor feeding its own opamp for feedback.
The opamps would all be driven from the filtered ...
- 33.8k
20
votes
Are there still practical modern uses for large through-hole resistors?
Power resistors aren't going away, at least not any time soon. There are many places where you just can't return some power but must dissipate it in situ instead.
A dramatic example is braking ...
- 39.9k
20
votes
Accepted
Why are coils of wire used in heating vs a straight wire?
Three benefits to coiled resistance wire used in heating:
Thermal expansion. There is no strain on the terminals.
Easier to tune the resistance to the required voltage and power requirements for ...
- 17k
19
votes
Using CPU heat to generate electricity
For generating electricity, you want the hot side (processor) to be as hot as possible for maximum efficiency. The thermal generator slows down the movement of heat as it extracts energy from it.
For ...
- 46.9k
19
votes
Using CPU heat to generate electricity
Surprised that nobody else has mentioned this:
Generating electricty from the waste heat from some process that burns fuel can make sense. Generating electricity from the waste heat from a system ...
- 3,168
19
votes
Is there a fluctuation effect between heat, resistance and current?
This could be analyzed in the same way as a control circuit with feedback. From a practical sense, the heating will be much slower than the other effects, so that will dominate the loop equations. ...
19
votes
Why don't we make CPUs with 1000s of layers to make use of space in the third dimension?
Heat removal is the issue.
Already some chips have higher energy density than a nuclear reactor.
Consider a hair_drier ---- 1,500 watts with a air_blast fan to cool the tungsten coils. And the coils ...
- 34.6k
18
votes
Accepted
MUR460 diodes get extremely hot - what am I missing?
First of all, paralleling diode doesn't work very well. Unless the Vf are matched current will prefer one of the two. It sort of work, but not expect a 50% load sharing.
Fast diodes will work ...
- 8,536
17
votes
Why is it dangerous use a coiled extension cord?
Last week (1st week in Jan 2017) we almost had a fire from a coiled extension cable.
It was connected to an electric urn that does draw high power, and the only reason it did NOT start a fire was the ...
- 179
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