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When Vendor Numbers Defy Ohm’s Law

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.)

Figure 1

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 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.

Figure 2

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.

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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6 comments on “When Vendor Numbers Defy Ohm’s Law

  1. AubreyKagan
    October 26, 2018

    Bill

     

    I don't know if this is going to colour your investigations- all transformers used for household purposes like the transformer for the bell are rathed as a “Class 2”. What this means is that the transformer is maximum 100VA and maximum 30VAC and is current limited either through the resistance of the winding or by a fuse so as to be “safe”. The stadard is  UL 5085-3, There is a bit about them here.n

     

    When we qualified a product for Class 2 through UL, “safe” mean that the object with the output in short cicuit would not get hot enough to set fire to anything. Our test involved placing a musln cloth over the device and leaving it in the fualt conditon for 3 days and then checking for charring on the muslin cloth. (3 days seems a bit of an overkill if the device has turned off when overloaded and there is no heating at all)

  2. bearchow
    October 26, 2018

    This from Jerry Steele (aka Pesky Varmint)

     

    They are a little on the large side but not worse than a 50 watt resistor.

    And, no heat to worry about.

  3. HRConsult
    October 31, 2018

    Just a thought: The 50 watt resistor is probably bigger than the unit.

    I am missing the part of your analysis where you include the battery voltage in your current calculations. Your analysis starts with the absolute worst case of the entire 25 volts appearing across the 25 ohm resistor. Giving the battery credit for something other than zero volts would immediately decrease the worst case dissipation.

    My guess is that the internal charging circuit has lots of control over the conditions applied to the battery, especially the charging current, and that current would be largely independent of the applied voltage. The clue here is that the device is supposed to work ok with the same 25 ohm resistor and the input voltage ranging between 8 and 24 voilts.

    I think I would investigate the device a bit with my multimeter. First, I would check the battery voltage. Then I would use an external power supply with current limiting and check the charging current, even going so far as to discharge the battery and measuring the current under that condition. Once I had that information, I would be in a position to determine the right components for the external charging supply.

  4. Bill_Jaffa
    October 31, 2018

    Good points–yes, I used the worst-case scenario of zero V at the battery, which is how I like to start this kind of analysis. As for your other suggestions: they are also good, but it is not my unit,I can't risk damaging it, and I always like to keep in mind the guideline that “no good deed goes unpunished” — so I will step away from helping my friend unless I have a clear-cut answer from someone who has figured out the situation and has a solution that has been tried and verified.

  5. Bill_Jaffa
    November 2, 2018

    I now have the specs on the battery: 3.65 V, 6040 mAh, 22.646 Wh (given with such precision to 5 significant figures!). Measures approx 2.75 x 1.75 x 0.9 inches (70 x 44 x 22 mm) and is in a very custom size/shape enclosure, has mini-USB port for charging via external cable, a mechanical release tab, and 5 small contact bumps. I assume two of the contacts are for internal AC charging input, two are for internal DC out. no idea what the third is– unless the battery is really a dual unit with bipolar outputs. But that's speculation. Again, it is not my brand new $200 unit soI can't play with it too much, plus I can't risk damaging it either

  6. Steve Taranovich
    November 2, 2018

    Here are two battery images to support Bill's reply to “Something missing”

    Battery #1

    Battery 1

    Battery #2

    Battery 2

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