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Date: Sun, 19 Jan 2003 20:00:14 -0700
From: Jim Drew
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Subject: Re: Transistor leaking? :)
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Sir Beef Wellington wrote:
> On Sat, 18 Jan 2003 21:42:52 -0700, 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:
>>.. 9v Battery |----+---------+
>>.. | |
>>.. | \
>>.. | / 100K
>>.. \ E \ PIC <-----|>|---+
>>.. \ / | |
>>.. \| B | 10K Diode | -----
>>.. FZT705 |------+----------/\/\/\/--+--|>|---+-O O
>>.. /| | |
>>.. / | |
>>.. To 7805<-----+/ C | |
>>.. | GND
>>.. C |
>>.. 10K B |/
>>.. To HOLD pin on PIC >------/\/\/\/---+------| 2N2222A
>>.. | |\
>>.. / \
>>.. \ E \
>>.. 100K / |
>>.. \ |
>>.. | |
>>.. | |
>>.. GND 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??
> I think it is caused by the "leakage" from the PIC port during
> powerdown. As the voltage collapses, there will come a point when the
> value is too low for the i/o to behave properly.
> Try disconnecting the PIC pin from the 2n2222 and see if prob goes
I disconnected the 10K resistor connected to the base of the 2N2222A
(the 100K to ground is still there), and the problem still occurs - no
change at all, so its not some sort of parasitic load from the PIC when
not being powered.
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