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I recently took apart a combination VCR/DVD player. Along with other questionable quality inside including a green LED display viewed through a red acrylic front panel and a DVD player assembly using only 3 brushed DC motors, I found that there were 3 mystery components marked as 27KΩ 2W resistors in a form factor that I have never seen resistors in.

enter image description here

As you can see in the picture, I decided to cut one open and found that it was a 1/2 watt carbon composition resistor potted in a plaster-like material. It had also failed short for an unknown reason.

What is the reason for potting the resistor in a larger package like this?

Update: I cut up another one of these resistors in the name of science and it appears to have a metal film or wire wound resistor inside. The core is too hard to cut with diagonal cutters.

enter image description here

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    \$\begingroup\$ To increase the wattage. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 17 '15 at 2:20
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    \$\begingroup\$ No he's serious, not about marking it differently, but encasing it increasing the wattage. \$\endgroup\$ – NathanielJPerkins Dec 17 '15 at 2:22
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    \$\begingroup\$ No, but adding a bunch of cement around it does increase its thermal mass. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 17 '15 at 2:22
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    \$\begingroup\$ Then the manufacturer cheaped out on it. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 17 '15 at 2:29
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    \$\begingroup\$ that core looks a bit bigger than a half-watt resistor to me. \$\endgroup\$ – Jasen Dec 17 '15 at 5:38
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This is possibly a metal film resistor encased in a ceramic case, filled with high alumina cement. For higher wattage, wire wound resistors are used in a similar but larger casing. This type of construction is common as you can see here, here and here.

The ceramic casing increases the surface area as well as the thermal mass. The surface area give higher wattage since it can dissipate more power. The increased thermal mass gives it a higher surge rating.

From the pictures, it seems that the surface area has increased by much more than 4 times, making it possible to be marked as a 2W resistor.

Here's an image from the Xicon datasheet (2nd link):

enter image description here

And one from the Uchi datasheet (3rd link):

enter image description here

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  • \$\begingroup\$ Although the This does not appear to be what it is. The resistor clearly does not have a resistive element wrapped around it (judging by the 27KOhm rating, it should have quite a bit). Also, I am assuming the datasheet means silicone cement instead of silicon cement, and my resistor was potted in what is a very crumbly plaster-like material that exploded into fine dust when I crushed it with diagonal cutters. \$\endgroup\$ – 3871968 Dec 17 '15 at 16:22
  • \$\begingroup\$ Update: I marked this as accepted because it appears to be what the second resistor I opened is. \$\endgroup\$ – 3871968 Jan 4 '16 at 19:18
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You can see the white casing as a heat sink that provides better heat transport to the surrounding air.
That is possible because it has much higher surface area than the original resistor (from the picture I guess at least factor 5) and it is made of a material with high thermal conductivity.

So that really increases the wattage of the resistor.

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  • \$\begingroup\$ Yes the wattage increases+1 but does it go up to 2 watts ? maybe but the surge current rating or peak power rating will be basicly the same as the original uncased resistor. Often people select those big resistors for the surge rating.People are more likely to wrongly estimate surge rating than continious rating .Those tortured wha fong resistors are poisonous. \$\endgroup\$ – Autistic Dec 17 '15 at 9:11
  • \$\begingroup\$ The wattage it can dissipate is proportional to the surface area. It does look like the surface area has increased more than 4 times by the casing. This is assuming the material is of high thermal conductivity. \$\endgroup\$ – Vinod Dec 17 '15 at 9:51
  • \$\begingroup\$ @user1736936: Wattage is affected by surface area, heat conduction to the surface, maximum acceptable internal temperature, and maximum acceptable external temperature. Only if the maximum acceptable external temperature is the limiting factor would wattage increase proportional to extra surface area. \$\endgroup\$ – supercat Dec 17 '15 at 17:39
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cement resistors are common design in power board. Most engineer will use metel oxide film resistors under 5 watts, cement resitors above 10 watts. To use a 2 watts cement resistos is not common, becuse it can not be in auto insertion process.

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