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# Maximum heat and non-burning resistance

#### electronium

##### Member
Hello guys... have a good time...
One question seemed interesting to me, so I thought I would share it with you. What is the tolerance threshold for resistance 0.5w and 1w?
How much heat can pass through a 150k 0.5w resistor without burning?

It depend on how its mounted and how much airflow there is around it.

The resistor may survive it near its limit, but it will be very hot!
Heat causes things to degrade and age far faster, so always try and avoid any component running hot if at all possible.

I'd always used resistors with ratings at last twice as high as the expected worst-case dissipation, or more so if it would be under continuous load.

Typically, I de-rate and expect a resistor to dissipate only 25%-50% of rated power. So, I would keep a 1w resistor to about 0.5W.
If the resistor is only expected to draw max current for fractions of a second, I may use a 1W resistor if it is dissipating 1w for 50% of a second (or less). No need to risk anything when making only a few items. Any resistor drawing more than 1/4 watt gets special attention (like mounting a few mm above the board and controlling air flow ).

Probably a bigger issue is burning yourself on a hot resistor - they will burn you well before the resistor burns up.

If we consider the base and index on the ambient temperature of 25c Celsius, we should have a resistance of 150k 0.5w and a current 2mA of with a voltage of 311 volts dc will pass through it, or is there a relationship and formula that can be used to calculate the temperature?
Does the next article affect the temperature with the type of dc or ac current?

Last edited:
Does the next article affect the temperature with the type of dc or ac current?

I did the test with a 120k 0.5w resistor with city electricity and peak city electricity, the result was interesting
As a result, I applied 230v ac directly to the resistor with city electricity, the current was 1.95mA ac and the temperature of the resistor was 40 degrees Celsius.
Another time, I gave the same resistor with the city power peak of 320v dc, the current was 2.7mA dc, the temperature of the resistor was 110 degrees Celsius.

The more interesting thing is that even though the resistor was heated with a voltage of 320 vdc, it did not change color and did not burn... I went ahead with the calculations, according to this current and voltage, it should be 120k 1w
ip=i rms*sqr2=1.95*1.4142=2.7
pr ac=(1.95 mA)^2 * 120k=0.45w
Pr dc=(2.7mA)^2 * 120k=0.87w

It's down to temperature rise, rather than absolute power. With the resistor in open air rather than mounted flat to a PCB, it should be able to stand rather more power as it can dissipate the heat better.

For info:
The formula for power dissipation from voltage (AC RMS, or DC) and resistance is:

V^2 / R

(230 x 230) / 120000 = 441mW

(320 x 320) / 120000 = 853mW

If you know the current instead of the voltage, the formula is then
I^2 R; I x I x R

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