Author Topic: External power switch - What am I doing wrong ?  (Read 651 times)

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Offline usahinTopic starter

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External power switch - What am I doing wrong ?
« on: October 07, 2019, 09:13:41 am »
Hello,

I am trying to add a controlled external power supply to a battery powered device. You can see the schematic in the attachment.

When i turn on the EXT_IN power, Q1 automagically turns on and Q3 latches the gate of Q1, I removed the C2 cap and D1, changed the value of R1,R2 and R7 couldn't find any solution.
In my opinion, during power up R1 should keep Q1 closed.  After first power on / power off sequence every thing works fine. Should I use a fet with smaller gate charge instead of Q1?  What am I doing wrong ?

Thanks in advance for your replies.
 

Offline Zero999

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Re: External power switch - What am I doing wrong ?
« Reply #1 on: October 07, 2019, 12:57:05 pm »
Perhaps the drain to source capacitance of Q1 is causing Q3 to turn on, when the power is applied. Try a 1nF capacitor between Q3's base and emitter.
 


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