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From: Roy McCammon
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Subject: Re: Transistor leaking? :)
Date: Sun, 19 Jan 2003 16:37:04 GMT
NNTP-Posting-Date: Sun, 19 Jan 2003 10:37:04 CST
Organization: Road Runner - Texas
Jim Drew wrote:
> Ok, I have an interesting problem. I designed a simple circuit to allow
> a microprocessor to be turned on and held on with a single push button
> switch (a soft power switch). The problem I am having is that there is
> 1.0xx volts and .47ma leaking out of the transistor all of the time.
> The circuit is simple:
redrawn for compactness
> 9v |---+-------.
> | | PIC <--|>|-.
> E | 100K | |
> \| | | ---
> FZT705 |------+-----10K-----|>|--+-+--O O-gnd
> /| |
> / |
> To 7805 |
> To HOLD pin on PIC >------10K-------+--| 2N2222A
> | |\
> 100K | E
> . | |
> . gnd----+---'
> When the button is pressed, the battery voltage flows through the FZT705
> transistor (which can handle the 1 amp of current needed with the 7805
> voltage regulator). When the PIC powers up, it sets a bit that holds
> the power to be on. The 2nd diode isolates the push button and feeds
> the push button to one of the input lines of the PIC (so it can be used
> for data input after the system is powered up).
> Now, why would I be getting voltage out of the FZT705 when the button is
> not being pressed??
Assuming beta = 100, it would only take about 12 uA
of leakage from the 2N2222A to account for the behavior.
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