TACT switch for mains operation

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Hello guys:

 

I've recently came with a dirt cheap (and with that I really mean dirty!) design for a mains operated device

It has a transformerless PS (way dirty) and some stuff on the inside that consumes roughly 20mA as an absolute maximum.

The device in question needs to be woke up with a 220V signal and once it's energized it retains

it's source by means of a relay while doing some stuff. Then it releases the relay powering itself off.

IMHO this device is complete isolated, with no metal parts accessible whatsoever.

 

My question is, since this startup signal must be derived from 220V, if it's advisable to use a tact switch for a production run

(RA switch with 18mm stem and plastic cap). Any other switch (as far as I could find) will add too much to the cost.

 

However I designed the startup circuit with a 4700 ohm limiter resistor in series to the switch and a varistor before the switch, my main concern

comes from the eventuality of internal arcing started with a line transient, since this kind of switches uses to have contact ratings of 50mA@24VDC

 

For now I prototyped the circuit, with such a tact switch and it works like a charm, its permanently connected to the line and I power it 3-4 times at day and it works,

but I can't tell how many time it will work ok

 

Any experience with these switches?

 

best regards

Ignacio

Last Edited: Thu. May 21, 2015 - 03:28 PM
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I'd say that using that kind of switch (24V, 50ma) is a really, really, poor idea. 

 

With a 4.7K limiting resistor, you still have about 45mA potential current at 220V. 

 

In my personal opinion, "dirt cheap" does not belong in the same sentence with "220V". But, then again, maybe in Argentina, people do not sue if they are hurt by your design. In other parts of the world, you could potentially loose far more (in the courts) than what you save by "dirt cheap".

 

Jim

 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Are funerals "dirt cheap"?

 

I also advise against it Ignacio.

 

Cheers,

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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Let's put it into context - i have some cheapy rf switches that use a tact switch to turn the load on and off as well as rf. it uses a reactive dropper (capacitor) along with a zener,electro and some other bits. It has a little micro in there running off 5V and the tact switch. Clearly the switch is only switching 5Vat low current. It also has a long plastic pin operating it for isolation. If you're doing much the same, then no problems, but if you have high voltage across the switch even if it is low current, then expect problems. Exceed the specs, expect problems.

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Thanks for the answers

 

@Kartman : yes, there are 220VAC between contacts when the circuit isn't running. I've tested it and it worked, but probably

ambient humidity is in the end the most critical factor (these switches are not sealed)

 

@Ross and Jim:   Although in Argentina there is  electrical testing and compliance (very much the same of UL), it's not enforced properly. But you have to design for compliance

because your design may be turning against you on the long run. All the components are flame proof and/or fire retardant. But yes, a tact switch with the possibility of arcing

is not a good idea...

 

regards

nachus