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    EEVblog #1022 – Finding A $50 Oscilloscope On Ebay

    Another look at finding a $50 analog oscilloscope on ebay. Forum HERE Podcast: DownloadBe Sociable, ...

    • Greg

      I said Casio keyboards not Yamaha (but who knows they may have been the same!)
      Greg

    • tlhIngan

      I never knew there was a “standard”. I always assumed the polarity would be evenly split between center positive and center negative. The general advice was to always check polarity always and don’t assume which I always took to mean there was an equal chance it was the opposite of what you needed.

      And curses to those universal adapter makers that don’t make it obvious what the polarity is. Some, like Radio Shack, put nice + and – signs on the plug pack end and symbols like center or some such on the tip end so it’s easy to set. SOme plugpack makers have a switch that says + and -, and it’s not obvious which way is which (what does it me when the switch is on “+”? THe switch merely says “Polarity”)

      As for when the switch happened, I’d say it was a recent thing. Devices that had dual battery/AC operation were probably center negative operation until everyone used managed batteries. AC only devices were center positive (because the barrel would be a good ground option and I think the location of the terminal was convenient on the PCB to connect to the ground loop on the outside of the PCB). This was for reasons shown in the video.

      As battery/AC operation moved to rechargeable battery packs, they too went to center positive because there was no need for cheap switch disconnects – the battery was actively managed and in many devices, was actually formed a part of the power supply architecture itself – the AC adapter was sized to be powerful enough for most loads and charge the battery (barely, in some cases). But the load could be temporarily exceeded so instead of investing in a beefier AC plugpack, they use the battery to supply the brief spike of power. (This is true for things that have very intermittent high loads, like cellphones and laptops)

    • tflynn6693

      Fluke scopemeters use the same negative center wiring with an added feature a long reach connector… I felt it was not for technical reasons but making the fluke charger the only solution when that backward plug breaks…

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