- #1
NTL2009
- 596
- 380
- TL;DR Summary
- 3 W resistor is HOT @ ~ 1.6 Watts; Question on Transistor thermal calculation, Junction to case.
BACKGROUND, skip if you want: I'm repairing the amps (2) and power supply for a 1970's era Rhodes Electric Piano (for my local HS music depart).
Since this is a classic instrument, I found plenty of info, so the rebuild went well, but I'm curious about one thing, and want to check my calcs/assumptions on another. The amps were upgraded from their old/obsolete (and blown!) germanium T03 power transistors, to silicon MJ15025. These were originally biased with an 820/2.7 Ohm divider, with 36V~38V supply at top of the 820, and the 2.7 across B-E (emitter near ground when idle). With roughly 36 V across the 820 Ohm, that's ~ 1.6 Watts. Those old 820 Ohms were burnt and did some charring of the fiberglass PCB, they looked to be 2W or 3W carbon comp parts ( ~ 3/4" L x ~ 5/16" D), not sure of the size for watt ratings for 1970 era resistors? Does not seem like much safety factor to me, and probably is the reason for the failure. For the silicon transistors, the sources say to use a 3W rated replacement for the 820. I did that, all tested out good, but that 820 Ohm was hot, around ~ 140F~145F estimated, I could touch it for ~ 4 seconds. The part is rated for a surprising (to me!) 235°C . Holy Cow, it can withstand 135 C over boiling, that's 455 F! No wonder they talk about flame-proof coatings. I was just not familiar with these sorts of specs. But I was also just not comfortable with such a hot running part.
https://www.digikey.com/product-detail/en/te-connectivity-passive-product/RR03J820RTB/A138495CT-ND/9371896
I found I could add 270 Ohms for 1090 total, so I ordered two 510 Ohm 5 Watt parts, and will use those in series. Now, I know we are still dissipating the 1.6 Watts, so 0.8 watts per resistor, so the same total heat, but I figured this would spread the heat over a larger area, so reduce the temperature. I powered the 3W R and a 5W sandbox R with 1.1 W, and the 3W got just to the point of too hot to hold, while the 5 W just barely got warm.
My question/observation: I'm assuming the temperature difference I'm seeing is mostly due to surface area (maybe some small secondary effect if the relative emissivity is different?). The 3W R is ~ 5/8" L and ~ 3/16" D, for a surface area of ~ 0.423 sq" (include ends, not leads). The 5W R is ~ 3/8" sq x 7/8" L, for a surface area ~ 1.59". The ratio is ~3.75x. Is it correct to assume that if I see a rise of 80 F (70F to 150F) on the 3W R, that I could expect that rise to be 1/3.75 on the 5W? That would be ~ 21F rise on the 5W, raising it from 70F to 91F - which matches my empirical rough estimates by feel.
Second question on Transistor dissipation: I also had to replace a transistor in the voltage regulator, which needs to handle ~ 2W (10V x 0.2A), and since I had spares of the high powered amplifier output transistors (250 W @ 25C case), I just used one of those, without any heat sink. Thermal Resistance, Junction−to−Case, is listed as 0.70 C/W. The case gets warm, but not hot - not above 140 F (60 C). So is my calculation correct - the junction temperature would be just 1.4 C above the case temperature? Spec is Operating and Storage Junction Temperature Range, −65 to +200_C. So I'm well within limits?
TIA - NTL2009
Since this is a classic instrument, I found plenty of info, so the rebuild went well, but I'm curious about one thing, and want to check my calcs/assumptions on another. The amps were upgraded from their old/obsolete (and blown!) germanium T03 power transistors, to silicon MJ15025. These were originally biased with an 820/2.7 Ohm divider, with 36V~38V supply at top of the 820, and the 2.7 across B-E (emitter near ground when idle). With roughly 36 V across the 820 Ohm, that's ~ 1.6 Watts. Those old 820 Ohms were burnt and did some charring of the fiberglass PCB, they looked to be 2W or 3W carbon comp parts ( ~ 3/4" L x ~ 5/16" D), not sure of the size for watt ratings for 1970 era resistors? Does not seem like much safety factor to me, and probably is the reason for the failure. For the silicon transistors, the sources say to use a 3W rated replacement for the 820. I did that, all tested out good, but that 820 Ohm was hot, around ~ 140F~145F estimated, I could touch it for ~ 4 seconds. The part is rated for a surprising (to me!) 235°C . Holy Cow, it can withstand 135 C over boiling, that's 455 F! No wonder they talk about flame-proof coatings. I was just not familiar with these sorts of specs. But I was also just not comfortable with such a hot running part.
https://www.digikey.com/product-detail/en/te-connectivity-passive-product/RR03J820RTB/A138495CT-ND/9371896
I found I could add 270 Ohms for 1090 total, so I ordered two 510 Ohm 5 Watt parts, and will use those in series. Now, I know we are still dissipating the 1.6 Watts, so 0.8 watts per resistor, so the same total heat, but I figured this would spread the heat over a larger area, so reduce the temperature. I powered the 3W R and a 5W sandbox R with 1.1 W, and the 3W got just to the point of too hot to hold, while the 5 W just barely got warm.
My question/observation: I'm assuming the temperature difference I'm seeing is mostly due to surface area (maybe some small secondary effect if the relative emissivity is different?). The 3W R is ~ 5/8" L and ~ 3/16" D, for a surface area of ~ 0.423 sq" (include ends, not leads). The 5W R is ~ 3/8" sq x 7/8" L, for a surface area ~ 1.59". The ratio is ~3.75x. Is it correct to assume that if I see a rise of 80 F (70F to 150F) on the 3W R, that I could expect that rise to be 1/3.75 on the 5W? That would be ~ 21F rise on the 5W, raising it from 70F to 91F - which matches my empirical rough estimates by feel.
Second question on Transistor dissipation: I also had to replace a transistor in the voltage regulator, which needs to handle ~ 2W (10V x 0.2A), and since I had spares of the high powered amplifier output transistors (250 W @ 25C case), I just used one of those, without any heat sink. Thermal Resistance, Junction−to−Case, is listed as 0.70 C/W. The case gets warm, but not hot - not above 140 F (60 C). So is my calculation correct - the junction temperature would be just 1.4 C above the case temperature? Spec is Operating and Storage Junction Temperature Range, −65 to +200_C. So I'm well within limits?
TIA - NTL2009