A friend asked me to help him with his Ring2 video doorbell, which was already installed and working fine, Figure 1. The only problem was that it was operating from its self-contained rechargeable battery, which doesn’t last that long on a charge. Although the vendor says it will run for 6 to 12 months a full charge, the actual runtime appears to be far lower, and is a function of number of times it is used (of course), and the ambient temperature. (Amazon acquired Ring in 2018.)
The wireless Ring2 video doorbell measures 5.05 in. × 2.50 in. × 1.08 in. (12.8 cm × 6.35 cm × 2.76 cm) and uses the smart phone as its user interface; it comes with everything you need to install it including tools, optional wire leads with spade lugs for external power connection, and even “wedges” so you can angle it up/down and left/right to optimize its viewing orientation. (Image source: Ring)
The problem is that charging the battery is a major hassle as you have to unscrew a tiny “security screw” at the bottom with a T-6 Torx head screwdriver they supply, hope you don’t misplace that tiny screw, remove the battery, put it into a charger, then do the reverse when the battery is recharged. (They say it’s a “quick release” battery, but that depends on your definition of “quick”.)
Recognizing the problem (somewhat), the folks at Ring included a built-in recharging circuit that operates from the low-voltage AC supply from the previous doorbell if there was one; if not, you can hopefully snake a wire through to door frame and add a transformer inside the house. I was called in to help figure out the wiring.
“How hard could this be?” I figured -- after all, we are talking about two wires and low-voltage AC -- there’s not much “there” there, as they say. Once I looked at the instructions, though, I realized the vendor numbers and guidelines did not make sense. But, hey, this is basic voltage and current stuff, so I assumed applying a little “Ohm’s Law” to the problem would take care of things.
Well, I was wrong, The instructions “How to Connect Your Ring Video Doorbell Directly to a Low Voltage Transformer (Without a Pre-existing Doorbell)” are fairly brief and have the usual caveats about being careful about various things. The real core is the single diagram, Figure 2. It’s a very simple schematic: the 8-to-24 VAC transformer charges the Ring2 through a resistor. All simple enough, except that the resistor is a 25 Ω, 50 W unit.
This is the very simple schematic that caused so much confusion and didn’t make sense when I looked at the numbers and did some basic calculations using Ohm’s Law.
Wow, I thought, that must be one big transformer doing a really fast charge if it is driving enough current to need a 50 W current-limiting resistor. That wattage resistor is not one which you could have picked up at your local (although now defunct) Radio Shack, so the Ring folks give two specific resistor models and sources from distributors, here (and a Digi-Key TechForum even says they have received many specific inquiries). Even if I was going to use that resistor, it’s a nuisance to mount, and it needs free air. I even wondered if any less-technical users simply stuffed it in a wall or other enclosed space, where it might overheat.
Or maybe it won’t overheat. I contacted the Ring folks and asked a few questions, such as: What’s the desired charging current? What’s the needed VA rating for the transformer? They couldn’t (or wouldn’t) tell me the charging current, but they did reveal that a 10 VA transformer is OK. Aha..a useful number! Plus, there’s no way a 10 VA transformer can deliver 50 W, that’s for sure.
I still wasn’t comfortable with that 25 Ω resistor, as that seemed to allow a lot of charging current for a modest-size battery (they wouldn’t tell me the battery’s A-hr rating either, but it can’t be that much; the battery is about the size of a deck of cards, and you and get replacements online, of course.
Finally, it was time to do some Ohm’s law analysis. With a transformer at the higher-end voltage of 24 VAC and assuming that a 10 VA transformer is about the same as a 10 W unit, charging through a 25 V/25 Ω resistor leads to a maximum current 24 V/25 Ω, or about 1 A. That’s a lot of current for a small battery, indeed. The maximum power dissipation would be (1 A)2 × 25 Ω = 25 W, so I see where they got that 50 W sizing, large as it is.
Going the other way, at the low end with the 8 VAC transformer, the current is 8 V/25 Ω, about 1/3 A (300 mA), and the maximum dissipation would be about 2 1/2 W -- much less, but still seems way too high for this situation.
My next idea was repeat the analysis but increase the resistor to 100 Ω. At 24 VAC, the current dropped to 1/4 A and the dissipation went down to 6.25 W; at 8 VAC, the current dropped to 80 mA and the dissipation was under 3/4 W. These are numbers with which I felt more comfortable.
Nonetheless, I was still confused. Was there something obvious I was missing? So, I went online to various message boards and found lots of similarly confused users. About half said they were unfamiliar with basic electricity and didn’t know what a 50 W resistor was or how to connect it, which I can understand; the other half (my half) said they know components and Ohm’s law, and the set-up made no sense at all.
So where do we go from here? I am not sure yet; I don’t want to burn out that $200 unit, that’s for sure. I am puzzled, and my friend doesn’t understand why I can’t just make it work and why I am confused. I see that other vendors have addressed the problem, and Amazon has an 18 V/500 mA transformer with associated resistor and designed specifically for this unit, so maybe I’ll go that route. They don’t give the resistor ohm or watt rating, but the numbers work out to 36 Ω and 9 W, which still seems high to me. I might try to work the numbers based on a charging current of 100 to 200 mA, which seems about right for trickle charging this size of battery, but I am not certain that’s what I’ll try with his unit.
So, what do you think is going on? Is there something I am missing here? What charging current and series resistor would you use? And have you ever been in a situation where basic vendor-supplied number just didn’t add up right?
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