![]() ![]() I fully expect this will exceed the supply capacity of the garage door opener's power supply. So you're going to be making a lot of heat, and will need appropriate safety caging for the resistor bank. The math isn't overly complicated, but that's a narrow range that I expect will require a fairly enormous resistor ladder. In your application, you need to spec your resistor ladder so your voltage stays between 3.3V and 5V at load draws between 0A and 250ma (or whatever measurement proves out). For any reasonable working range, the vast majority of power will be consumed in the resistor ladder, not the load. You mitigate it by making the upper and lower bypass resistors larger (in watts and physical size, smaller in ohms). This varying of voltage is the Achilles' heel of resistor ladders. The lower resistor, between load and GND, keeps the voltage from floating upward toward 600V if there is no current draw by the load.īut the voltage varies considerably, since the lower resistor is paralleled with between 0 and 15 contactors on a 5-car train. For instance, Westinghouse HL traction drive equipment (the L is for line powered, not battery) uses a resistor ladder to synthesize roughly 80V from 600V line voltage. I've worked with resistor laddes a fair bit. If it's packaged neatly enough, you may not even need a steel junction box to hide it. That has screw terminals and a case, rather than wires sloppily soldered onto a unit meant for PCB mounting. The point of this is, favor a DC-DC that is packaged more like a finished good, i.e. But again, rules are relaxed for low voltage <55W stuff. "Equipment means "not components": difference being, the equipment has labeling/instructions describing its consumer use, and UL tested and listed it that way. Its current draw will certainly be within the range available from the opener.Ĭode requires the use of "approved" equipment which generally means it is UL listed. The steel's thermal conductivity and the size of the box will take care of heat dissipation. The converter will make less than a watt of residual heat, and will be small, so it will fit neatly into a common, cheap steel 4" or 120mm square junction box. Neat and workmanlike, don't be stupid, make an effort to staple it out of harm's way, and don't use mains power wiring as a hanger.) So in a Code context: A DC-DC converter You will need to use proper enclosures and do wiring to the low standards of doorbell or thermostat wiring (e.g. ![]() Fortunately those codes are pretty easy on low voltage (30V) low power (<55W) devices. Since it's part of a building, it must reasonably comply with the electrical codes. AC voltage is a big mess for a DC device, but it works with transformers! A more common control voltage in residential wiring is 24VAC, seen in doorbells and thermostats. The load will vary considerably depending on what the Arduino is doing, and especially whether the WiFi module is sending data or not.įirst, make sure your garage uses 24VDC. ![]() It combines all the disadvantages of linear regulator inefficiency (you're dissipating the voltage difference in the resistors) with an unregulated output voltage that varies depending on the load. With very sensitive devices you might even have to use a combination of a DC converter to drop the voltage down to close to your target, and then use a low dropout linear regulator to get the final stable voltage without losing a lot of efficiency in the linear stage (since it's only a small step down, instead of a large step down). In your case, an Arduino with no sensors, it doesn't really matter that the power rail is somewhat noisy.īut if other cases if you have sensors that depend on exact input voltage, or other devices that are sensitive to ripple it may matter. Whether the ripple matters or whether you can smooth out the DC converter output enough depends on the device you're powering. So, DC converters are very efficient, but noisy, and linear regulators are very smooth, but inefficient (they basically "waste" the voltage difference as heat stepping down 24V→5V a very large amount of heat). Because of how they work DC converters will always have some amount of high frequency ripple on the output while linear regulators are extremely smooth. One point to consider between choosing a DC-DC buck converter or a linear regulator is the quality of the output.
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