# Which push button ratings should be considered due to its data-sheet

I'm planning to use the following normally on push-button switch: http://uk.rs-online.com/web/p/push-button-switches/1336473/ According to the following data-sheet: http://uk.rs-online.com/webdocs/137f/0900766b8137f3b1.pdf it says:

Contact Current Rating: 650 mA @ 30 V dc

But in my application the button will be pushed less than a second where the voltage across it will be 48V and the current through it will be 48mA for less than a second.

Should I look at the power (P = I*V = 30*0.65) in this case, or I shouldn't use it since it says 30V in its ratings?

Edit:

Push-button controls 48V supply and is normally open. Here is the part of the circuit where the switch is in LTspice(I didn't check this in scope):

Points A and B represents the terminals of the push-button. Below green plot is the potential difference across the switch terminals A and B:

And the blue plot below is the current through the switch:

And finally, I zoom to see both the voltage across the switch and the current through the switch:

What I understand from the above plots, the switch is exposed to 48V whenever it remains open. The moment the switch is closed, the potential difference drops quickly in nano-seconds to zero volts and micro-ampere level current passes through it.

What I understand from the first answeers is that, the problem with the following push button: http://uk.rs-online.com/webdocs/137f/0900766b8137f3b1.pdf is that if I use them in my circuit, they will be exposed to 48V all the time when the switch is normally open. So the real problem does not occur the moment I push the button but it occurs when the push button is not pushed at all.

Is that correct?

• You can't just throw a SW (switch) in a schematic in LTspice and expect it to behave phisically correct like a switch. A model for one would encompass far more than what you did. Unfortunately, I can't help you with a model for such a device because I have never needed one, and so I don't know all the gory details, but, if what you want is quasi-real life scenario, then you need a proper model. – a concerned citizen Oct 21 '16 at 17:10
• but besides ltspice, is that right the switch sees 48V across its terminals when it is open? and since the push button is rated to 30V it should not be used? thats what i understood from other commentators. one of the reasons i'm digging it is that these push buttons for 48V rated are very expensive comparing tio 30V ones. – user16307 Oct 21 '16 at 17:13
• Yes, that is correct, using the switch for any other case where the current or the voltage exceed the recommendations, means all bets are off. – a concerned citizen Oct 21 '16 at 17:25

Arcing distance is determined by the voltage. Since the voltage in use is higher than the rating, either the switch may become damaged over time due to the contacts either eroding away or welding closed, or opening the switch may not reliably break the connection since arcing will occur.

• Here is another one: uk.rs-online.com/web/p/push-button-switches/1036280 It says here: Contact rating of 4A @ 250Vac, 8A @ 125Vac, 4A @ 24Vdc Are these ratings for continuous ON operation? In my case current is low enough but voltage is 48V DC. – user16307 Oct 21 '16 at 13:01
• You're considering using a switch with an even worse rating? – Ignacio Vazquez-Abrams Oct 21 '16 at 13:05
• updated my question please see the edit. i would be glad to have you comment. – user16307 Oct 21 '16 at 16:31
• Arcing can occur whenever the contacts are within arcing distance but don't actually touch. This situation exists on both press and release. – Ignacio Vazquez-Abrams Oct 21 '16 at 16:41

A rating of "650 mA @ 30 V dc" is just a shorthand notation that tells you that the contacts can handle at most 650 mA when closed and at most 30 V when open. If you violate either of these specifications at any time, you have exceeded the ratings of the switch.

Multiplying the numbers together is completely useless.

• updated my question please see the edit. i would be glad to have you comment. – user16307 Oct 21 '16 at 16:31

The reason the DC voltage ratings are so much lower then the AC ones is that a DC arc will not self quench, and 48V is more then high enough to sustain an arc across typical switch contact gaps if there is enough current flowing.

You want a switch rated for more then 48mA AND more then 48V DC, either one on its own will not really suffice.

Of course you could try to qualify a switch for operation outside the datasheet ratings, but you are pretty much on your own at that point (With your very current limited load, and the modest overvoltage) I might try it if reliability and cycle life did not need to be very extreme, but it is not something I would do in a production product.

• updated my question please see the edit. i would be glad to have you comment. – user16307 Oct 21 '16 at 16:31