Shorting a small capacitor with a switch

[Pimpom]

I know it's not a good idea to short-circuit a charged
capacitor, especially a large electrolytic. What about
smaller caps - like a low voltage ceramic disc of 0.1uF or
less? And if the shorting element is a low-power switch like
one of those ubiquitous little square tactile microswitches,
is the discharge energy enough to erode the contacts and
shorten its life significantly?

If it helps evaluate the situation, take as an example two
cross-coupled NAND gates in the classic toggle switch
configuration. As a bias to determine the initial state, say
one input of one NAND element has the small cap to ground
and is tied to Vdd via a resistor of ~100k. The microswitch
parallels the capacitor. Is it advisable to insert a series
resistor to limit the discharge current?

 

[WangoTango]

In article <kf5sqp$8kg$1@news.albasani.net>, Pimpom@invalid.invalid
says...

I know it's not a good idea to short-circuit a charged
capacitor, especially a large electrolytic. What about
smaller caps - like a low voltage ceramic disc of 0.1uF or
less? And if the shorting element is a low-power switch like
one of those ubiquitous little square tactile microswitches,
is the discharge energy enough to erode the contacts and
shorten its life significantly?

It is done everyday.

Pull up resistor, debounce capacitor, mechanical switch.
Every video game in America has one somewhere.

 

[Ian Field]

"Pimpom" <Pimpom@invalid.invalid> wrote in message
news:kf5sqp$8kg$1@news.albasani.net...

I know it's not a good idea to short-circuit a charged capacitor,
especially a large electrolytic. What about smaller caps - like a low
voltage ceramic disc of 0.1uF or less? And if the shorting element is a
low-power switch like one of those ubiquitous little square tactile
microswitches, is the discharge energy enough to erode the contacts and
shorten its life significantly?

If it helps evaluate the situation, take as an example two cross-coupled
NAND gates in the classic toggle switch configuration. As a bias to
determine the initial state, say one input of one NAND element has the
small cap to ground and is tied to Vdd via a resistor of ~100k. The
microswitch parallels the capacitor. Is it advisable to insert a series
resistor to limit the discharge current?

I'll add my vote to those saying put a small resistor in circuit.

Some of the small "tactile" front panel buttons have a silicon rubber disc
with a conductive rubber pad which is resistive anyway, but most have "real"
metal contacts - even a small capacitor in parallel will take its toll
eventually.

A few years ago I made a quick lashup breakdown voltage tester from a
voltage doubling bridge rectifier with a pair of 180uF electrolytics and
slow charge up resistor, it obviously needed a shorting switch to to make it
safe between tests - the switch lasted precisely 1 operation!

For this particular application; an NTC inrush limit thermistor from a scrap
PSU solved the switch contact destruction problem.

 

[Pimpom]

Ian Field wrote:

"Pimpom" <Pimpom@invalid.invalid> wrote in message
news:kf5sqp$8kg$1@news.albasani.net...
I know it's not a good idea to short-circuit a charged
capacitor,
especially a large electrolytic. What about smaller
caps - like a low
voltage ceramic disc of 0.1uF or less? And if the
shorting element
is a low-power switch like one of those ubiquitous little
square
tactile microswitches, is the discharge energy enough to
erode the
contacts and shorten its life significantly?

If it helps evaluate the situation, take as an example
two
cross-coupled NAND gates in the classic toggle switch
configuration.
As a bias to determine the initial state, say one input
of one NAND
element has the small cap to ground and is tied to Vdd
via a
resistor of ~100k. The microswitch parallels the
capacitor. Is it
advisable to insert a series resistor to limit the
discharge current?

I'll add my vote to those saying put a small resistor in
circuit.

I normally insert the series resistor but sometimes, for

convenience in PCB design, it's tempting to omit the
resistor.

Some of the small "tactile" front panel buttons have a
silicon rubber
disc with a conductive rubber pad which is resistive
anyway, but most
have "real" metal contacts - even a small capacitor in
parallel will
take its toll eventually.

That's why I tried to make the distinction in my description

between the two types of switch. I meant the metal contact
type but didn't think of saying precisely so. I believe the
contact resistance of such types are in the region of some
tens to a few hundred milliohms.

A few years ago I made a quick lashup breakdown voltage
tester from a
voltage doubling bridge rectifier with a pair of 180uF
electrolytics
and slow charge up resistor, it obviously needed a
shorting switch to
to make it safe between tests - the switch lasted
precisely 1
operation!

Shorting 180uF electros directly with a switch? Wow, that's
asking for trouble.

For this particular application; an NTC inrush limit
thermistor from
a scrap PSU solved the switch contact destruction problem.

 

[Pimpom]

WangoTango wrote:

In article <kf5sqp$8kg$1@news.albasani.net>,
Pimpom@invalid.invalid
says...
I know it's not a good idea to short-circuit a charged
capacitor, especially a large electrolytic. What about
smaller caps - like a low voltage ceramic disc of 0.1uF
or
less? And if the shorting element is a low-power switch
like
one of those ubiquitous little square tactile
microswitches,
is the discharge energy enough to erode the contacts and
shorten its life significantly?

It is done everyday.
Pull up resistor, debounce capacitor, mechanical switch.
Every video game in America has one somewhere.

The video game controllers I know about usually use printed
carbon contacts that are inherently resistive. I meant
discrete switches with metal contacts.

 

[Ian Field]

"Pimpom" <Pimpom@invalid.invalid> wrote in message
news:kfcoek$no1$1@news.albasani.net...

Ian Field wrote:
"Pimpom" <Pimpom@invalid.invalid> wrote in message
news:kf5sqp$8kg$1@news.albasani.net...
I know it's not a good idea to short-circuit a charged capacitor,
especially a large electrolytic. What about smaller caps - like a low
voltage ceramic disc of 0.1uF or less? And if the shorting element
is a low-power switch like one of those ubiquitous little square
tactile microswitches, is the discharge energy enough to erode the
contacts and shorten its life significantly?

If it helps evaluate the situation, take as an example two
cross-coupled NAND gates in the classic toggle switch configuration.
As a bias to determine the initial state, say one input of one NAND
element has the small cap to ground and is tied to Vdd via a
resistor of ~100k. The microswitch parallels the capacitor. Is it
advisable to insert a series resistor to limit the discharge current?

I'll add my vote to those saying put a small resistor in circuit.

I normally insert the series resistor but sometimes, for convenience in
PCB design, it's tempting to omit the resistor.

Some of the small "tactile" front panel buttons have a silicon rubber
disc with a conductive rubber pad which is resistive anyway, but most
have "real" metal contacts - even a small capacitor in parallel will
take its toll eventually.

That's why I tried to make the distinction in my description between the
two types of switch. I meant the metal contact type but didn't think of
saying precisely so. I believe the contact resistance of such types are in
the region of some tens to a few hundred milliohms.

A few years ago I made a quick lashup breakdown voltage tester from a
voltage doubling bridge rectifier with a pair of 180uF electrolytics
and slow charge up resistor, it obviously needed a shorting switch to
to make it safe between tests - the switch lasted precisely 1
operation!

Shorting 180uF electros directly with a switch? Wow, that's asking for
trouble.

The 2x 180uF in series is actually half the individual capacitances, but
they were in a voltage doubling arrangement - the voltage rise leveled out
just a smidge over 600V - which I presume is the onset of leakage in the
bridge rectifier I used.

It made a fairly impressive "crack" and the switch was done!

 

[WangoTango]

In article <kfcph5$pmq$1@news.albasani.net>, Pimpom@invalid.invalid
says...

WangoTango wrote:
In article <kf5sqp$8kg$1@news.albasani.net>,
Pimpom@invalid.invalid
says...
I know it's not a good idea to short-circuit a charged
capacitor, especially a large electrolytic. What about
smaller caps - like a low voltage ceramic disc of 0.1uF
or
less? And if the shorting element is a low-power switch
like
one of those ubiquitous little square tactile
microswitches,
is the discharge energy enough to erode the contacts and
shorten its life significantly?

It is done everyday.
Pull up resistor, debounce capacitor, mechanical switch.
Every video game in America has one somewhere.

The video game controllers I know about usually use printed
carbon contacts that are inherently resistive. I meant
discrete switches with metal contacts.

OK, but I didn't specify CONTROLLER.