EEVblog #267 – Voltage Detection Stick TeardownPosted on April 13th, 2012 7 comments
It’s Teardown Tuesday time.
Ever wondered what’s inside those mains voltage detection sticks?
Dave cracks open an IDEAL VoltAware, and giggles at a patent.
Thanks to http://www.triosmartcal.com.au for the stick.
Thanks for the interesting post. I enjoyed watching the teardown as usual.
Do you think the first capacitor between points B and C and pull-up resistor act as a sort of first-order high-pass filter? Maybe they were trying to reduce sensitivity to low frequency signals (below the 50Hz or 60 Hz of the AC mains)…
Dave, to find out the truth on the chip, just desolder it, then hold it in a gas flame for 15-20 min (outside)
after that, rub off carefully the remaining epoxy, and read out the part number on the di with your microscope :))
If you want to see how the voltage probe works, why don’t measure it as it is working? Using a unity gain configured op-amp as a buffer to your scope, you should be able to measure the signal at any of the nodes(even the high impedance nodes) in the circuit.
The first resistor and cap form a high-pass filter, the second cap, along with part of the bias divider forms a low pass filter. Both together bandwidth limit the device. Remember, just because there’s a DC connection to Vcc, that connection is still an AC path to ground.
I wouldn’t be surprised if the second chip with no visible marks is a micro-controller.
What did that detector cost by-the-way?
Many models down to US$2.76 at dealextreme, shipping included. Most seem identical to the one Dave reviewed, of course they’re not Fluke.
Very easy to build one anyway, any CMOS gate would have the necessary high input impedance to make a good detector.
Points “B” and “C” form a differentiator. The output will convert a square wave to a spiked wave. For example if the output was across the resistor, you’d have an integrator and would have seen a square wave.
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