What is wrong with this circuit?

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Vance Briggs

Jul 12, 2022, 2:34:48 PMJul 12
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Hi experts :-)


This circuit above seems to short out the +3V3 rail when I push SW4. I suspect it is applying +3V3 to the base of Q10, which gives Vbe of 3V3, probably not desirable!  My best guess is that this causes breakdown of the base emitter junction of Q10 effectively shorting out the 3V3 rail.  If I put a 10k directly onto the base of Q10 would this sort out the issue or introduce more?  Also would that still allow the microcontroller signal OPEN_2 to still switch on Q10 and therefore provide +V_BATT through Q7 and out to OPEN_2_12 ?

Also, its the Electronics Evening on Thursday if anyone wants to come along and tell me where I have gone wrong :-)


Richard Ibbotson

Jul 12, 2022, 2:58:44 PMJul 12
to reading-...@googlegroups.com
Hi Vance,
As you thought the transistor Q10 will try to clamp the Vbe to 0.7V effectively dragging down the 3.3V supply.
I don't know what is driving OPEN_2 but assuming it is a 3.3V totem pole, I would put an additional 10k in series with the switch SW4. This should safely or the switch and the logic input.

Sent via Richard's Phone

On 12 Jul 2022, at 19:34, Vance Briggs <va...@axxe.co.uk> wrote:

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Frank in Woodcote

Jul 12, 2022, 4:43:57 PMJul 12
to rLab / Reading's Hackspace
Yep, I'll second Richard's comment. If OPEN_2 input is held to GND, the switch with a 10k resistor in series will still overide any input, turning on Q10. The voltage between 1&2 on Q10 just needs to get to about 0.7V and it'll switch on. Treat 1-2 like a silicon diode.
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