EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Peabody on May 27, 2022, 03:41:26 pm
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Yesterday my Ooma Telo box operated normally on outgoing calls and retrieving voicemails, but started going into reset on answering an incoming call. It turned out to be a bad power brick, and after wiring into the power jack terminals I was able to power it from a good power brick (2A), and it appears to work fine. The new replacement brick is due tomorrow.
However, in an attempt to see what amp rating I needed for the replacement (the old one is 5V 2A), I measured the current drawn by the Telo using an ancient Weston ammeter, model 301, which goes up to 70mA. It's probably from the 1940's, has a ".02 MFD" capacitor across the terminals, as well as a 5-inch length of copper wire, probably 28 gauge, as a shunt. Taped to the face is a hand-written sticker saying "20mA = 1A".
Based on this, I find that the Telo idles at about 1A, and goes up to 1.25A when I place a call. But when I answer an incoming call, there's a sharp, short current spike that appears to go above 2A. But I don't know if the needle actually shows the extent of the spike because of the capacitor, or possibly even overstates it because of the inertia of the needle once it gets moving.
So I thought I would try to display the current draw on my scope (battery powered, non-referenced to anything). But the smallest resistor I have is 0.51 ohms, which would cause too big a voltage drop if the spike is really 2+ amps.
What can I use as a shunt that would be 0.1 ohms or less that wouldn't also introduce inductance issues? Would a foot of 30 gauge wire wrap wire work, or even six inches, if I still have some of that? Is there anything else that's commonly used that I'm likely to have in the junque box?
And I guess I should ask what would cause the big spike on answering a call. I've tried disconnecting all the house wiring and plugging a single phone into the Telo, but I still get the spike. And as I said, I do not get the spike if I pick up the same phone to make a call. It's very puzzling. The replacement brick is 3A, but I'm wondering if I should plan on replacing the Telo too. It just seems this current spike shouldn't be happening, but for all I know it could have been happening from the beginning - about 10 years ago.
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Well I think this is a nevermind. Today, all the phones and the house wiring are working ok. No current spike when answering a call. So either it healed itself, or Ooma updated the firmware overnight.