This is sort of a follow-on question to
a post from last month where I described a circuit to rectify 120 VAC to 48 VDC for dampening a fluctuating input to a relay coil. I've been experimenting with the circuit and have some questions about the temperature rise and heat dissipation of the dropper resistor.
In the schematic below, R1 drops about 60 Vrms and has an average power dissipation of 1 W as calculated by LTspice. The voltage waveform across R1 is half-wave rectified AC. I can confirm these numbers within about 5% on the bench.
In my first design, I used a 2 W metal film resistor (
Vishay PR02) and the surface temperature reached about 100 degrees C steady state. This is within spec and seems to agree with the datasheet's hot-spot temperature rise at 1 W dissipated power. The ambient temperature of my lab is 22 degrees C, so 22 + 75 = 97 degrees C; close enough.
100 degrees C is a little hot for my liking, so I thought I would try a higher-wattage resistor. I replaced the 2 W metal film with a 5 W wirewound (
TE EP "S" Series - "Small Size"). To my surprise, the steady-state resistor temperature was only about 5-8 degrees cooler. The physical package of the 5 W resistor is nearly five times larger than the 2 W (by volume), not to mention its power rating is 150% higher.
The TE datasheet doesn't have any charts for temperature rise, but I wouldn't have expected such similar performance between two seemingly different resistors. In both tests, the ambient temperature was around 20-22 degrees C. I'm measuring the temperature with a FLIR thermal camera.
Now, I did find a temperature rise chart for another 5 W general-purpose wirewound resistor (
Vishay AC Series), which suggests that a temperature rise of ~75 degrees at 1 W is appropriate. I don't know how closely this correlates to the TE resistor, but it's probably close since the construction seems similar (ceramic/cement) with similar power ratings.
As a final "controlled" test, I dropped 60 VDC across each resistor (~1 W) from a bench power supply and observed similar temperatures within a few degrees. So, I think everything checks out.
What would cause both the 2 W and 5 W resistors to reach about the same temperature when there's such a difference in power rating and physical size? Could it be the composition (metal film vs. wirewound)?
Short of using a much larger resistor (7 or 10 W), or putting a few in parallel, I'm not sure how else I might reduce the surface temperature. (I know that this isn't the most efficient circuit, of course, so let's not comment on that
).