Shorting a small capacitor with a switch

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?

Reply to
Pimpom
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put an R in series with the switch...

We short out large charged loads of 20Vk or more with a 10Watt 25k R and the crow bar contacts last a long time!

When I say large I mean large cables of 10,000 feet of more with lots of dielectric charge using 20kV or more.

Jamie

Reply to
Jamie

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

Reply to
WangoTango

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.

Reply to
Ian Field

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

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.

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

Reply to
Pimpom

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

Reply to
Pimpom

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!

Reply to
Ian Field

OK, but I didn't specify CONTROLLER.

Reply to
WangoTango

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