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Question about this NiMH charging circuit - BQ2002 — Parallax Forums

Question about this NiMH charging circuit - BQ2002

W9GFOW9GFO Posts: 4,010
edited 2013-06-04 08:16 in General Discussion
I'm building a 5 cell NiMH charger using the TI BQ2002 chip. Can anyone explain what the purpose of Q1 is in this schematic?

BQ2002DevBoard.jpeg
BQ2002DevBoard.jpeg
885 x 530 - 69K

Comments

  • Tracy AllenTracy Allen Posts: 6,664
    edited 2013-06-04 07:31
    Q1 is forward biased only when the DC pin is greater than the battery voltage, and that appears to turn on (or off) the Vcc supply. On charge only when DC supply is greater than battery voltage by at least 0.6V.
  • W9GFOW9GFO Posts: 4,010
    edited 2013-06-04 08:11
    I don't understand why it would need to do anything with VCC. Is this a means of supplying the BQ2002 with power?

    If VCC is coming from a regulated 5V supply what happens when 9V is on the DC pin and the Batt is at 5V?
  • Mike GreenMike Green Posts: 23,101
    edited 2013-06-04 08:12
    Vcc is supplied by a Zener diode regulator (1N751A) switched on and off by Q1. There's no external source of regulated 5V. Vcc is derived from DC.
  • W9GFOW9GFO Posts: 4,010
    edited 2013-06-04 08:16
    Well that makes a difference. Thanks! Didn't realize it was a zener - no squiggles on the line...
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