I have problem with the model of given power limiter (below are the images). Problem is in the red rounded resistor on the last image – that resistor overheats for some reason (I've put 3 ohm one instead of 2.2 ohm).
What should I do?
Electronic – Resistor overheating
heatresistors
Related Solutions
The problem with using the container as a resistor is that it's actually a very good conductor. Your wires will have more resistance, and will therefore get hotter than the container.
What would be amazing is if somehow you could deliver a very high current to the container, using a lower current flowing in the wires. Well, you can!
A very efficient way to heat an iron or steel object is by Induction Heating. It's used a lot in industry for heating bearings and things. It's also quite possible to do it yourself. Many people do.
It works by inducing an alternating current in the object to be heated. Essentially you're creating a transformer, where your object is the (single turn) secondary coil. Much more current can be made to flow in the secondary coil simply by using many turns in your primary.
An induction heater can get things really hot if you want:
And you can even use it to heat large containers:
This can be easily calculated.
The power supply is 5 V and the LED drops 2 V. That leaves 3 V accross the resistor. 3 V / 150 Ω = 20 mA, which is a typical max current for small LEDs. That means the LED is driven correctly.
Now look at the power dissipation. 20 mA x 3 V = 60 mW. That's well within the capability of what looks to be a "1/4 W" resistor in your picture. Again, everything is fine.
Dig out a datasheet for the resistor and see how hot it is expected to get if you actually were to have it dissipate 1/4 W. That would probably be in the 150-200°C range. Even at 150°C for 250 mW, and assuming 20°C for ambient, you have 130°C / 250 mW = 520°C / W. 60 mW would therefore heat the resistor 31°C, which you can definitely feel. If starting at 20°C, then the resistor would be at 51°C, or 124°F. So it makes perfect sense that it would feel "warm" or almost "hot" to you.
Best Answer
What power does the parts list specify for R4? The output current is up to 2 A, then a 2.2 Ω resistor will dissipate 8.8 W. A 3 Ω resistor will dissipate even a lot more: 12 W. Even at 1 W a small resistor will get hot.
Now the thyristor only conducts for short periods, in order not to have a higher voltage on the smoothing capacitors than necessary. And here Wouter brings up an important point. If the current is pulsed it will cause a lot more dissipation. At a constant 2 A current the 3 Ω resistor dissipates 12 W. If we pulse 20 A at a 10 % duty cycle we get the same average current, but when we calculate the dissipation:
\$\mathrm{P = 10\% \times I^2 R = 0.1 \times 20^2 \times 3 \text{ W} = 120 \text{ W } !! }\$
Note that the circuit was designed to reduce power dissipation in the voltage regulator's power transistor.