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'[EE] Power dissipation in a co^b^b switch'
2006\01\17@230218 by

Olin wrote regarding 'Re: [EE] Power dissipation in a connector' on Tue, Jan 17 at 17:34:
> A connector is intended to have as low a resistance as possible.  It will be
> designed to carry some maximum current.  The voltage accross the connector
> at any time is the current thru it times its resistance.  Ohms law states
> that V = I * R.  If R is fixed as it is with a connector, you only get to
> pick one of V or I.
>
> The voltage spec of a connector refers to the open voltage or the insulation
> voltage between adjacent pins or to some other point.

This bring to mind something I've wondered about for a while but have
never asked - and I guess prior to now haven't given much though to (I
had the question but never looked for an answer).  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?

I was buying that as a nice, simple explanation and everything was
right with the world, 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.  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?  If I
plan to switch a 14.4V DC circuit, can this tiny little thing actually
handle nearly 60 amps?  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.  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?

Sure, I'm only planning on running current on the scale of milliamps
through this switch for periods on the order of seconds, but it seem
like something handy to know none the less... :)

--Danny

On Jan 17, 2006, at 8:02 PM, Danny Sauer wrote:

> 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 have some excitement during transitions.  Arcing, welding,
oxide burning away, etc.

BillW
Also, the reactance of the circuit comes into play. With a capacitive
load there is a  high current transient on connect, with an inductive
load you get a voltage spike on disconnect. In both cases there is
minimal contact pressure  or gap so the effect is worst-case. (Not to
mention bounce on connnect).
Most switches are rated for resistive operation or for low power
factor - if you are running reactive loads the capability reduces
further.

RP

On 18/01/06, William Chops Westfield <westfwmac.com> wrote:
{Quote hidden}

> -
Danny Sauer wrote:
> On
> these switches (all electronics catalog number MTS-5) is stamped
> 250VAC 3A and 125VAC 6A.

That is an unfortunate way to specify switches and fuses that seems to have
stuck (although it makes a bit more sense for fuses).  You are right that a
static open switch has a maximum voltage it can handle without arcing and a
static closed switch a maximum current without melting (or various other bad
effects), and the two are unrelated.  Your switch can withstand at least 250
* sqrt(2) = 354V when open and 6A when closed.

However, problems arise during transitions, particularly when the switch is
opening.  The contacts don't move apart instantly, so the gap in the switch
grows slowly from 0 to its final maximum state where the 354V can be
applied.  In between the switch is a spark gap with varying arc over
voltages.  Now consider that once an arc has been established (which is
unavoidable immediately after starting to open) the air is ionized and
becomes a conductor, which greatly lowers the voltage required accross the
contacts to sustain the arc.  This will happen even if the circuit as seen
from the switch is a voltage source in series with a resistance.  Now
consider what happens when there is inductance in series too.  A big enough
inductor will provide whatever voltage is necessary to sustain the current
at the time the switch started to open.

Specifying and then understanding all these parameters and those of the
circuit is too much to handle for an electrician installing a wall switch in
your house.  To keep it simple, standards have been developed for various
domestic wiring systems which specify gap distance and a host of other
parameters the switch manufacturers have to deal with.  Then they mark the
switch with "125VAC 6A".  That really means "We built this switch as cheaply
and flimsily as possible to just meet the UL (or whatever) standard for
normal 125V AC house wiring that can carry up to 6A.  As long as you use
this switch in a 6A or less circuit, our butt will be on the line, not
yours.".

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
Olin wrote regarding 'Re: [EE] Power dissipation in a co^b^b switch' on Wed, Jan 18 at 06:45:
[...words, current, voltage, more words...]

So, basically, there's no simple way for the commoner to extrapolate
one from the other, and assuming the worst would be a good policy.
Right? :)

--Danny
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

Danny Sauer wrote:

> So, basically, there's no simple way for the commoner to extrapolate
> one from the other, and assuming the worst would be a good policy.
> Right? :)

I think so. I wouldn't necessarily trust a switch that's rated for 3 A/250
V with more than 3 A, even if the load voltage it has to switch is lower.
And I wouldn't trust it with higher load voltages, even if the current it
has to carry is lower. This is not a power specification...

Also, with most mechanical contacts there's a difference between holding
current and switching current. The switching current is usually lower
(sometimes much lower) than the holding current. I guess you just can hope
that it's the switching current when the only thing you have is "3 A/250 V"
:)

Gerhard

Howard Winter wrote:

> So you will often see ratings such as "250V 3A AC, 24V 3A DC" or "13A
> resistive, 2A inductive" on switches and relays.

Got me thinking... the problem with switching inductive loads (without any
protective measures) is probably that you actually exceed the switch's
voltage rating when switching off. (Just as you are likely to exceed the
switch's current rating if switching on into a big enough empty capacitor.)

Inductive, capacitive or resistive, you should be fine as long as you don't
exceed the voltage and current ratings (including considering switch-on and
switch-off effects)... And the "inductive current" capabilities are just a
simplified way to say that a circuit with an assumed inductivity and
resistance is fine to switch -- but it really depends on the power factor
whether that is ok or not.

Gerhard

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