I’m using ESPHome to trigger two 40amp relays on a 240v circuit (controlling my air compressor). I want to measure the temperature of the relays and trigger a fan to turn on if the relays get to a specified temp. Does anyone know of a temp sensor that works with ESPHome that specifically measures the temperature of a surface? In this case, a relay heatsink?
I think I just took apart the dht11 and it worked as a surface temp?
The MLX9061 sensor measures an object’s temperature using infrared.
While there is no official support in ESPHome, you can find custom component code here: https://github.com/exxamalte/esphome-customisations/tree/master/mlx90614
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You can thermal epoxy Dallas ds18b20 sensors to a surface. For a more easily replaceable sensor epoxy or friction fit it into an uninsulated ring lug (where the wire usually goes) and screw the lug to a surface. Another option (if you can find them) is a 2N3402 transistor TO-92 heatsink.
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Thanks for the replies! I like Tom_I’s suggestion. In the end, I decided to go with a vibration sensor to see when the compressor is running then run the fans based on that. I figured a binary sensor that only updates when the compressor runs would be better than a temp sensor updating every x seconds when the compressor may only run once or twice per week.
Your relays should not be increasing in temperature, so there should be no need for fans. 
If you mean “relay” and not “contactor”, then it is the wrong device to control that load. For a big inductive load like a compressor pump motor you need a contactor. I use one to switch my air compressor (240V, 5HP, magnetic starter) and the contactor stays cool to touch.
I’m using these relays, and they get hot.
Should I be using something like this instead?
This is the air compressor, it’s a 5hp, 230v, 21.5 amps.
https://www.tractorsupply.com/tsc/product/ingersoll-rand-5-hp-single-stage-twin-cylinder-pro-air-compressor-60-gal
Ah yes, an SSR would definitely get hot because they are in fact a TRIAC which has, though very small, a resistance.
Yes, ideally you would use a set of physical contacts like the link you posted. It may be more convenient for you to get a contactor with a 240V coil and use a smaller relay to switch it rather than finding a source of 24V. That’s what I do.
The nice thing about a contactor vs. a SSR is that a contactor won’t fail closed, while the normal failure mode for an SSR is to fail closed. Plus you get a really satisfying “THUNK!” when it closes.