Simple switch question

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Perk

Joined: 05 Oct 2004
Posts: 5

05 Oct 2004 19:03 Simple switch question

So simple I am almost too embarassed to ask. Almost....

So I am looking at buying some switches and I see most of them are rated at 1 amp at 250V. Basic theory says that the switch can take 250 W....so I assume that it is safe to push 12 amps through that same switch at 12 V.

Thanks in advance for your help.

--Chris

svicent

Joined: 11 Jul 2001
Posts: 405
Helped: 20

05 Oct 2004 19:19 Re: Simple switch question

For the contacts, normally, the maximum current and voltage for DC and AC operation is specified. Toggle switch contacts have maximum voltage and current ratings for AC and DC operation. A typical toggle switch can handle less DC voltage than AC voltage at the rated current; for example, a switch might be rated at 5 A for 125 Vac or 5 A for 28 Vdc.
The reason for the difference is that when the contacts are just opening, the current will continue (briefly) to arc across, which tends to burn and pit the contact surface. AC will arc less than DC for the same voltage because AC is going to 0 V twice each cycle.

pisoiu

Joined: 31 Dec 2002
Posts: 889
Helped: 24
Location: Romania

05 Oct 2004 19:32 Re: Simple switch question

Nonono.... Both maximum ratings must be take into consideration. This mean that when the switch is on, the maximum current through it can be 1A, and when the contact is off, the maximum voltage between contacts can be 250V. The maximum current is dictated by the physical dimension of the conductive part of the switch. If you will overload the switch with a current higher than the maximum rated current (but not too much higher), the switch will probably heat, and you risk to get the contacts stuck together because of the electrical arc. If you will pass a very high current (let's say 12A in a 1A rated switch), you will probably melt the contact. The maximum voltage is directly related to the distance between contacts and the type of the isolation material, in order to avoid electrical break off.

/pisoiu

Perk

Joined: 05 Oct 2004
Posts: 5

05 Oct 2004 19:43 Re: Simple switch question

Thanks for your help, but I am still not clear on your answer.

I am planning on using the switches at a much lower voltage, but will be pushing a lot more current through them. The ratings I am reading are indeed contact ratings. Most are VAC ratings. They specify 6 amps at 125v or 3 a at 250 v. Which is what I would expect....p=i*e.

But I am asking if this will hold true even if I want to push 2x as much current through it as they are specifying. I want to use this switch for 12 amps at 12 volts. Seems to me that this will be fine as it is rated for 750 watts and I will only be using it for 144. But in the back of my mind I am questioning if it is ok from the standpoint that I am going to put a lot more current through it than they are spec'ing. And resistance to arcing (higher voltage applications) is different than the heating effect you will get from running 2x as much current through the switch than it is rated for.

But in the end, it seems like it should only come down to overall power rating.

Thanks again for your help, I would still like to get more info related to my question.

Perk

Joined: 05 Oct 2004
Posts: 5

05 Oct 2004 19:48 Re: Simple switch question

pisoiu -
I read your post after my last reply. And that was exactly my concern. However, just looking at my Jameco catalog here, I see them specing the contact rating at (example)

Part Number 76523CL xref JMT113 A(at)VAC 6(at)125 & 3(at)250

which is why I ask if it is simply a power rating. But I wouldn't be asking if I thought it truly was that simple.

Thanks for your help.

--Chris

pisoiu

Joined: 31 Dec 2002
Posts: 889
Helped: 24
Location: Romania

05 Oct 2004 20:00 Re: Simple switch question

Perk wrote:

No, it is not a power rating. Bottom line is: you cannot go with the current above the specified maximum current for your switch, regardless of the voltage. You cannot go with the voltage above the specified maximum voltage, regardless of the current. I hope it is clear Smile

/pisoiu

onemilimeter

Joined: 23 Sep 2004
Posts: 65
Helped: 2

06 Oct 2004 7:29 Re: Simple switch question

Analogy ...

Bussmann Fuse
Rating: 250Vac / 1A
You cannot use this fuse to protect your electrical device if the normal operating current of the electrical device is 110Vac / 10A.

Spectra-Strip 0.05" Flat Planar Ribbon cable
Rating: 300V / 1A
It does not mean you can use the cable to supply your electrical device that requires 12V and 25A.

The voltage rating, for example the 300V ribbon cable, is to tell us the maximum voltage that the insulator (wrapped around the conductor) can withstand before it breaks down. In high voltage application, the cable might be rated as 1KV / 1A, but it does not mean that you can use it for 1V / 1000A application !!!

Perk

Joined: 05 Oct 2004
Posts: 5

06 Oct 2004 16:01 Re: Simple switch question

Thanks one. I do understand the concept. The main reason I was questioning it is because in the contact rating they went to the trouble of specifying two ratings. 1 amp at 250 V & and 2 amps at 125 V.

nicleo

Joined: 06 Sep 2004
Posts: 717
Helped: 60

07 Oct 2004 4:31 Re: Simple switch question

Try to look at the spec of Matsushita/NAIS TX Relays shown in the figure at bottom. Although the Max. switching power is given as 60W, it does not mean you can operate the relay to switch load current higher than 2A, even though the voltage is below 30VDC. It's something like SOA (safe operating area) of mosfet or IGBT. In my opinion, you probably can switch load current of 4A, but the relay's life might not be long.

If you can get this book Engineer's Relay Handbook, 5th edition, probably you could find the answer in the book. Some contents of the book are published at http://www.leachintl2.com/english/english2/vol6/ and you can read it when you're free. If you have any finding, pls share. Thanks.

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