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Soft switch latch help

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Evilmachinist:
I need the soft switch latch circuit for a machine I’m building.
The circuit diagram on Dave’s video has one of the 3904’s is upside down.
I realize that this might not be a big deal, but to someone who’s struggling to begin with it is devastating!
Could someone clarify this circuit for me?
It needs to run on 12v dc and switch a simple relay on an off using a normally open momentary switch.
Also, what would be the proper P channel MOSFET part number to use?

Peabody:
I don't know what you mean by upside down.  Both transistors look correct to me.  What looks wrong to you?

Peabody:
Here's a circuit that's even simpler than Dave's, but works essentially the same way.  And there's an explanation of how it works.

https://www.edn.com/design/power-management/4427218/Latching-power-switch-uses-momentary-pushbutton

The choice of mosfet depends on how much current your relay coil draws.  Perhaps you can provide information on your relay.

[In this edn circuit, I don't understand why R3 is needed.  Can anyone explain?]

SiliconWizard:

--- Quote from: Peabody on December 12, 2019, 04:49:17 pm ---I don't know what you mean by upside down.  Both transistors look correct to me.  What looks wrong to you?

--- End quote ---

Same here. :-//


--- Quote from: Peabody on December 13, 2019, 03:53:26 pm ---Here's a circuit that's even simpler than Dave's, but works essentially the same way.  And there's an explanation of how it works.

https://www.edn.com/design/power-management/4427218/Latching-power-switch-uses-momentary-pushbutton

(...)
[In this edn circuit, I don't understand why R3 is needed.  Can anyone explain?]

--- End quote ---

What I can think of: when the circuit is in the "latched" state, Q1 conducts, and C1 charges. The moment you will push the button to turn it off, if R3 were just a short, C1 would immediately (ie. very quickly) discharge through the pushbutton and Q1, and Q2 may never get a chance to turn off.

Peabody:

--- Quote from: SiliconWizard on December 13, 2019, 04:15:55 pm ---
--- Quote from: Peabody on December 13, 2019, 03:53:26 pm ---Here's a circuit that's even simpler than Dave's, but works essentially the same way.  And there's an explanation of how it works.

https://www.edn.com/design/power-management/4427218/Latching-power-switch-uses-momentary-pushbutton

(...)
[In this edn circuit, I don't understand why R3 is needed.  Can anyone explain?]

--- End quote ---

What I can think of: when the circuit is in the "latched" state, Q1 conducts, and C1 charges. The moment you will push the button to turn it off, if R3 were just a short, C1 would immediately (ie. very quickly) discharge through the pushbutton and Q1, and Q2 may never get a chance to turn off.

--- End quote ---

Yes, I see that now.  I think that's the explanation.

In the comments over at edn, there are several reports that when power is applied, the latch's initial state is unpredictable.  Specifically, it may come up with the power turned On, which might not be a good thing.  The only explanation I can come up with is that starting with everything discharged, when power comes up, the mosfet gate voltage lags a little because of gate capacitance, and the mosfet may turn on enough to turn on the NPN.  Does that make any sense?  Is there another explanation?  Well, if I'm right about that, it might be prevented by adding a very small capacitor between the source and the gate to bring the gate up to +Vs more quickly than R1+R3 can.  Another way I think of it is the little cap acts like a charge pump cap acts - whatever its charge state, if you change the voltage at its bottom, the voltage at its top instantly changes by the same amount.  Depending on the mosfet, I suspect the cap could be very small, not big enough to materially interfere with the 330nF cap.  What do you think?


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