Re: [N8VEM: 14588] Switch debouncing and related stuff (like CPU reset circuits)

[Paul Anderson <wacky...@gmail.com>]

It seems to me, if you are already using an IC, some resistors, a capacitor, diode, etc, why not just use a 555 to debounce it?

Sent from my iPhone

On 2012-09-09, at 2:50 AM, Sergey <skis...@gmail.com> wrote:

> Hi,
> 
> Electrical engineers among us, please correct me if I wrong :-)
> 
> I was designing an ATX power-on circuit for my next 8088/XT board, and
> one of the issues was debouncing of the power switch (which is a
> momentary switch).
> 
> It must be some kind of conspiracy about switch debouncing, it looks
> like 90% of the related pages on the Internet are referring to this
> "research": http://www.ganssle.com/debouncing-pt2.htm . More exactly
> to the circuit on Figure 3 (the circuit on Figure 1, which also
> frequently referred everywhere, is mostly unusable, since nobody will
> use a double throw switch just to overcome the contact bounce)
> 
> The problematic part there is the purpose of the diode... This diode
> in the configuration shown on the diagram is useless. It will conduct
> electricity only then voltage difference on it is bigger than diode
> forward voltage drop, which happens only during very short time. The
> given explanation doesn't make any sense. The claim that the voltage
> on the capacitor Vfinal will be 4.3V, is wrong... after voltage
> difference on the diode will be less than diode forward voltage drop,
> the capacitor will continue to charge through R2, and eventually will
> reach the power supply voltage (5V).
> 
> The only useful purpose that diode might serve in such circuit is the
> protection of the Schmitt tigger's input. In this case the diode
> should be connected backwards - with the anode connected to the
> capacitor and the cathode connected to Vcc.  This is mostly useful
> then using CMOS gates that have ESD protection diodes on inputs. In
> this case, when power supply disconnected, capacitor might discharge
> through these ESD protection diodes, potentially damaging them. If
> diode is connected as described above, capacitor will discharge
> through that diode.
> 
> Another problem with debouncing circuits (and CPU reset circuits we
> use in N8VEM projects) is that capacitor is discharged through switch,
> potentially passing very high current through that switch, and leading
> to its failure. To eliminate that we used to add a 10 ohm resistor in
> series with the switch, but it looks like 10 ohm is too low to limit
> the current. Most tactile switches are rated for 50 mA. When using 10
> ohm resistor, the worst case current will be 5V / 10 ohm = 0.5 A. Ten
> times more than the switch rating... Now one solution will be just
> increasing the resistance of that current limiting diode, say to 100
> ohm. But replacing resistor in naive way will basically form a voltage
> divider, and depending on the ratio of R1/R2 (resistor that is used to
> charge the capacitor to the resistor that used to limit the discharge
> current), it is very possible that resulting voltage in the "switch
> closed" position will be higher than Schmitt trigger threshold. So
> instead of that, I'd recommend using circuit similar to one shown on
> Figure 3 (or Figure 2) on the page above, but connecting diode in
> reverse between the capacitor and Vcc.
> 
> Any comments?
> 
> Thanks,
> Sergey
> 
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