Thought I'd post a quick share of my DIY dew heaters, since there have been discussions elsewhere about this. SVbony sells various sized, nominally 5vdc, dew heaters for ~$15/ea. They look nice, and are plastic on the inside for heat conduction, but fuzzy on the outside. I bought 3 of these, and built dedicated 12vdc power modules for each, for ~$10 per box.
The power modules include a hygrometer / thermometer (~$3/ea. on eBay for cigar humidors) to monitor the heater temperature and outside humidity (you can find the ambient temperature on your phone); and PWM modules (~$4/each on eBay) inside ABS plastic boxes (5 for $9 on Amazon). The potentiometer on the PWM module changes the duty cycle to modulate temperature and save battery power, and they have a power light. Your temperature at the scope should be just above the ambient temperature (or maybe a bit more if humidity is high).
That's it. It's one box per dew heater, and if it breaks (which hasn't happened to me yet) just throw it out.
Edited by Chris Westland, 05 March 2021 - 12:39 PM.