How does a USB heating pad work?

Question

Here is a picture of two USB heating pads:

As far as I can understand, 5 V comes from the USB and goes into the curly wires, which produce heat since they are like a long resistor that dissipates power.

Will it produce more heat the shorter the curly wires (up to a certain limit where it would melt)?

These pads come with a button that allows you to switch between low/medium/high heat. How does this work? Does that big on/off button hide 3 big resistors that each limit the current to low/medium/high?

I would like to make something similar. Would it be better, in order to control the heat more efficiently, to have some kind of pulse width modulation?

I think I can do it with a tiny Arduino, but is it possible to achieve this with more basic components? Like a low and a high MOSFET and something to controle the on/off rythm aka duty cycle, but what?

Answer 1

As far as I can understand, 5 volts come from the usb, go into the curly wires, which produce heat since they are like a long resistor that dissipate power.

Yes. Likely a specific wire like nichrome designed for heating.

Will it produce more heat the shorter the curly wires? (until a certain limit where it would melt?)

Yes, relatively. As the total voltage is spread across a lower resistance in a smaller package, the heat from that smaller section will be relatively higher. Like pouring a gallon of water into a small cup vs a large bucket. The total power will be the same though. Just concentrated.

These pads come with a buttons that allow you to switch between low/medium/high heat. How does this work? Does that big on/off button hide 3 big resistors that each limit the current to low/medium/high?

Maybe. Using resistors to divide the power source is one way. Others are linear regulators or switching regulators. And others use pwm or similar methods. Not enough evidence to say what it uses.

I would like to make something similar. Would it be better, in order to control the heat more efficiently, to have some kind of pulse width modulation? I think I can do it with a tiny Arduino, but is it possible to achieve this with more basic components? Like a low and a high mosfets and something to controle the on/off rythm aka duty cycle, but what?

You can control them with any timer circuit if you use the right mosfet. You can find 555 timer examples online. But that's an exercise in self punishment. No reason not to throw a attiny or other microcontroller at it other than "I don't want to"

Answer 2

If you can use 2.4A USB charging ports that do not require communication, you can get 12W which is less heat than what is lost on a hot cup of tea.

The wires only show one fixed loop and the upper one has a sensor. That would indicate it has some temperature sensing.

With only 1 resistor and multiple settings, it must already use PWM, so there is no reason to re-invent the wheel.