Resistor smell like its burning?

[EEC]

I have a 10 ohm 25W resistor that smells like its burning. The applied voltage is between 12V & 24V Ac or DC. The resistor still measured out @ 10 ohm after the smell of burning. What happening?

 

[ggunn]

I have a 10 ohm 25W resistor that smells like its burning. The applied voltage is between 12V & 24V Ac or DC. The resistor still measured out @ 10 ohm after the smell of burning. What happening?

At 24V, P = V^2/R = (24)^2/10 = 576/10 = 57.6W. Too much power through the resistor.

Four of those resistors in series-parallel would give you the same resistance at 4X the power rating.

 

[gar]

110104-0902 EST

Do a little calculation and you should be able to answer your own question. Measure the actual voltage before the calculation.

.

 

[EEC]

Wattage rating stamped on resistor

Wattage rating stamped on resistor

Is the wattage rating on the resistor the same as the rating of wattage on an incandescent lamp? The lamp will have a voltage rating, the resistor does not have voltage stamped on it. I take it, regardless of absence of voltage rating on resistor that calculations need to be performed.

 

[n1ist]

Maximum voltage ratings on resistors are only to prevent flashover. The power rating is the maximum power dissipation with the manufacturer's recommended heatsinking and airflow, though you should derate it for safety.

You need to perform the calculations at your maximum supply voltage (or current) to determine what power resistor you need.
/mike

 

[gar]

110104-0957 EST

EEC:

Provide us with some information on your circuit theory background.

A power resistor such as an Ohmite 25 W model 210 is a wire wound resistor on a ceramic core with a vitreous enamel overcoat. Maximum surface temperature is going to determine the wattage rating.

Further you are basically limited to a maximum current thru the resistor. In the case of the 210 resistor, an adjustable type, you are current limited rather than power because you are concerned with the maximum temperature in any small area of the resistor. The current limitation will be the current thru the resistor based on its nominal power rating when the full resistor has current thru it.

I do not understand your question about an incandescent lamp bulb relative to a resistor. The lamp's voltage rating is the nominal voltage at which the bulb is design to work. The power rating is the nominal power consumed at that voltage rating. Bulbs I have measured are quite close to their rating. Because of the large temperature change of the lamp filament from room temperature to operating temperature the bulb resistance is quite different between cold and hot. For example: A 100 W bulb may be about 10 ohms cold, and 144 ohms hot.

Ordinary power resistors are relatively constant in resistance from room temperature to full rated power. Wire temperature is much lower at full power rating than is filament temperature at full power (voltage) rating.

Using an IRC PW5 (5 W) nominal 350 ohm +/-10% with an actual resistance at room temperature of 308 ohms measured 312.5 ohms at 5.12 W dissipation. The room temperature measurement was with a Fluke 27 on its resistance range, and the hot measurement was calculated from V and I.

The change of resistance from room temperature to hot was about 1.5%.

.

 

[ggunn]

Is the wattage rating on the resistor the same as the rating of wattage on an incandescent lamp? The lamp will have a voltage rating, the resistor does not have voltage stamped on it. I take it, regardless of absence of voltage rating on resistor that calculations need to be performed.

The wattage rating on a resistor and that on an incandescent lamp are two different things. A lamp will have a specific voltage rating, and the wattage is the power it will consume at that voltage. The power rating on a resistor is the maximum power it can handle, and you must calculate at what voltage that limit will be reached. For a 10 ohm 25 watt resistor, V^2/10 = 25, V = sqrt(250) = 15.81V. That's why it smells funny at 24V.

A series-parallel array of four 10 ohm 25 watt resistors will have a resistance of 10 ohms and a power capacity of 100 watts. The power at 24V is well within that limit.

 

[Jraef]

Just FYI, the wattage rating of a resistor and an incandescent light bulb differ in this way.

In a light bulb, heat (and therefore light) is the desired effect. The resistance of the filament at the rated voltage causes the filament to heat up and glow white hot, and we get light. The wattage rating then is telling us that when the rated voltage is applied, the given resistance will cause the filament to put out XXX lumens of light.

In a resistor, heat is an undesirable waste byproduct of the task of adding resistance to a circuit for other reasons, such as dropping a voltage. So the wattage rating of the resistor becomes an expression of the maximum amount of waste heat the resistor can handle without self destructing.

Your resistor is in self destruct mode.

 

[gar]

110104-1256 EST

I suggest the wattage rating on both a power resistor and an incandescent lamp mean about the same thing. Do not operate that device much above its power rating very long. The device's life will be shortened. Operate below the rating and probably life will be increased.

Also the design life of different devices likely will be different. Incandescent bulbs at rating are usually in the ballpark of 1000 hours. But there are some that operate at a lower temperature that may be 5000 hours, and others at an even lower temperature may have a 50,000 hour rating. Typical LEDs might be in the 50,000 hour range. A power resistor might be in the 10 to 50 year range at rated power.

The real point is that you have to understand the basic characteristics of any device with which you work, and how those characteristics relate to any ratings.

.

 

[ggunn]

I think we all agree that the reason the resistor smells like it is burning up is that the resistor is, in fact, burning up.

 

[hbiss]

I have a 10 ohm 25W resistor that smells like its burning. The applied voltage is between 12V & 24V Ac or DC. The resistor still measured out @ 10 ohm after the smell of burning. What happening?

Other than to say that yeah, it's getting too hot there is no way to answer your question without YOU telling us EXACTLY how you are using this resistor. Considering that you are looking for a voltage rating on it I can't wait to hear the answer. (Sorry, I had to say that.):roll:

-Hal

 

[LarryFine]

Incandescent bulbs at rating are usually in the ballpark of 1000 hours. But there are some that operate at a lower temperature that may be 5000 hours, and others at an even lower temperature may have a 50,000 hour rating... A power resistor might be in the 10 to 50 year range at rated power.

What is the ratio of hours to years? (Seriously)

 

[LarryFine]

The simple test is to measure the voltage across the resistor while it's cooking, then apply simple Ohm's Law.

I think we all agree that the reason the resistor smells like it is burning up is that the resistor is, in fact, burning up.

Well, yeah.

 

[gar]

110104-1759 EST

There are approximately 365.25 days per year. This is without the 400 year correction. Thus, hours per average year is 8766 and 10 years is 87,660 hours and 50 is 438,300 hours. Average is done over 4 years. Three non-leap-years and one leap-year.

Somewhere, at least a few years ago, there was an incandescent bulb still burning that had been put in service around 1900. I do not know where to find this information.

Did a Google search --- http://www.centennialbulb.org/facts.htm --- so there are several long lasting bulbs.

.

 

[LarryFine]

There are approximately 365.25 days per year. This is without the 400 year correction. Thus, hours per average year is 8766 and 10 years is 87,660 hours and 50 is 438,300 hours. Average is done over 4 years. Three non-leap-years and one leap-year.

I could have done the math. :roll: ))

I meant is there a standard hours (other than 24-per-day) per month, year, etc., figure used to simulate average real-world use, duty cycles, and such?

 

[dbuckley]

Also the design life of different devices likely will be different. Incandescent bulbs at rating are usually in the ballpark of 1000 hours.

I believe that the 1000 hour quoted life is the time for 50% of the bulbs to fail...

 

[steve66]

Is the entire input votage applied and dropped across the resistor? Or is the resistor in series with another load?

 

[gar]

110105-0918 EST

Larry:

I believe the average life figures for incandescent bulbs are based on continuous on conditions. This is an easier test to run and provides a longer life figure.

I used a 757 bulb in a smoke detector as the light source, and at a slightly reduced nominal voltage. Initially as I remember the GE spec for average life was either 25,000 or 50,000 hours. Later this was reduced to 5,000 hours. Why the spec changed I do not know. Possibly because many of these would be used in DC circuits and then the life would be shorter.

I use a 757 at 12 V in our bathroom as a night light and never have to change the bulb. Even though it is a small bulb and operated at somewhat less than half voltage it is adequate light.

.

 

[LarryFine]

I use a 757 at 12 V in our bathroom as a night light and never have to change the bulb. Even though it is a small bulb and operated at somewhat less than half voltage it is adequate light.

Years ago, when my son was little, I connected a diode across the hall bathroom light switch, so it was bright/dim instead of on/off, and was a great nightlight, too.