Danny,

On Tue, 17 Jan 2006 22:02:17 -0600, Danny Sauer wrote:

>...
>  The ratings on a mechanical switch.
>  From what was said above, I'd think that a switch
> was rated similarly - the voltage rating refers to the maximum
> difference in potential across the contacts when the switch is in the
> off state, and the current rating refers to the maximum flow through
> the switch in the on state?  So the two should be unrelated, right?

Switches are much more complex than a simple ohms-law device, and the ratings take into account not only the 
two steady states of the contacts (open, no current, maximum voltage / closed, maximum current, minimal 
voltage) but also the transitional conditions during switch operation.  The current and voltage ratings that 
result from having to break a live circuit do not have a linear relationship, but are affected by the 
structure and dynamics of the switch, and the nature of the current (AC is easier to break than DC because it 
goes to zero twice per cycle, helping to break any arc) and the circuit itself (inductive loads can produce a 
high-voltage "kick" when they are interrupted, which may cause an arc after the contacts have separated, hence 
the "condenser" fitted to traditional car ignition points to snub this).  So you will often see ratings such 
as "250V 3A AC, 24V 3A DC" or "13A resistive, 2A inductive" on switches and relays.
 
> I was buying that as a nice, simple explanation and everything was
> right with the world, 

Ah, but simple explanations are often wrong!  :-)  They are usually there to make something easy to 
understand...  to get the whole picture you have to know the current-carrying capability of the 
conductors/connectors when the switch is on, the voltage the whole thing is capable of isolating when it's 
off, and the combination of these with the frequency and circuit characteristics during switching.

> but then I picked up a couple of microswitches
> (I was roaming through my junk drawers looking at the ratings on all
> of my switches.  My evenings are just *full* of excitement, eh!).  On
> these switches (all electronics catalog number MTS-5) is stamped
> 250VAC 3A and 125VAC 6A.  And MTS-1, which is weird given all
> electronics' part number.

I've no idea what this MTS stuff is, but the more current you are trying to interrupt, and the higher the 
voltage it is fed by, the worse will be the arc across the contacts as they separate, so at these two voltages 
the contacts are designed to break those two currents...

> Anyway, that sent me right back to
> wondering again - is the current rating of a switch related to its
> voltage rating after all?  If so, is it a linear relationship? 

Absolutely not!  There will be a complex set of lines (and/or curves) forming areas inside which things are 
OK, and the ratings will likely be at some corners of these areas.

> If I
> plan to switch a 14.4V DC circuit, can this tiny little thing actually
> handle nearly 60 amps?  

No!  Nor could it handle 2500V at 0.3A or 25,000V at 30mA... 250V and 6A are likely to be the maxima 
regardless of anything else because they come from different characteristics, as I said above.

> That doesn't make a darned bit of sense - I'm
> not sure the miniature solder lugs could handle that kind of current,
> let alone the little slidie deal (yay precise terminology!) inside.
> So there has to be a point where the potential across the open
> contacts relates to the current across the closed contacts (and maybe
> the arc when the contacts are closing), and a point where it stops
> being relevant.  

Opening is more of a problem, usually, because the current jumps the gap and forms an arc which then sustains 
the current flow as the contacts open wider.  The closing arc only happens when the voltage is able to ionise 
the airgap and as the arc is forming the airgap is closing anyway so there is a limit to how long the arc will 
be there (when the contacts mate there is nowhere, and no reason, for the arc to exist.  On opening, in the 
worst case, the arc could sustain with the contacts fully open, which is a Bad Thing!  :-)

> Is that a constant across switches, or is it a
> constant like the spring constant, which varies for most every spring
> and can only be found through measurement?

It's a design characteristic.  There will be a linear limit to voltage brought about by insulation 
capabilities of the structure of the switch itself plus the isolation possible across the open contacts, 
another linear limit to current brought about by the cross-section of the conductors plus the carrying 
capability of the closed contacts, and another current-voltage limit which may be linear or not.  You have to 
stay below all three of these to be within the ratings.  If the diagram below makes it through the system 
(monospaced font needed) the simplest graph you might expect could be as follows.  You have to stay in Area X 
to not exceed the ratings, but without the actual manufacturer's graphs, you can only guess what the actual 
limits are.  In your case you can guess that the VA limit is a line of V = 750/A from the markings on the 
device, but you can't be sure.  There may also be a minimum current for reliable operation (the so-called 
wetting current) but I think that's enough!  :-)

^V
!
!          \         
!------------\-------+--- < 250V limit
!              \     !
!                \   !
!                  \ !
!      X             \
!                    ! \
!                    !   \ < VA limit
---------------------+-------->A
                     ^ 6A limit

Cheers,


Howard Winter
St.Albans, England


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