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From: Spehro Pefhany
Subject: Re: Transistor leaking? :)
References: <3E2A2CCC.firstname.lastname@example.org> <3E2AD401.email@example.com>
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Date: Sun, 19 Jan 2003 17:59:03 GMT
NNTP-Posting-Date: Sun, 19 Jan 2003 12:59:03 EST
On Sun, 19 Jan 2003 16:37:04 GMT, the renowned Roy McCammon
>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.
The FTZ705 is a PNP darlington with a beta that is in the
mid-thousands, but it won't do squat until you get over a volt on the
"base"- that's still over 10uA, and a modern silicon transistor leaks
more like 10nA at room temperature. He could go a little lower on the
"it's the network..." "The Journey is the reward"
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