Switch Confusion

[itsmoked]

Hey would somebody give me a quick confermation of what a pushbutton switch is that is called:


**********************************
SNAP ACTING MOMENTARY PUSHBUTTON

( ) denotes momentary

SPDT ON-(ON)
**********************************

I have been staring a switch descriptions for two days and have now got myself confused.

Of course it is a Single Pole Double Throw switch.
Which means it has a common and two "switched" terminals.

What I need is a switch that is closed when the plunger/actuator is out. And open when the plunger is in. I need this switch to detect when a lid is opened by closing. The same exact function as a car door switch... Light on with door open.

I am looking specifically at:

http://www.ittcannon.com/media/pdf/catalogs/Leaf/SW_push_8020.pdf

Part number: 8121SHZGE

And before you say it.. I can't use a $1 car door switch because this switch is in "Dry contact service" (need gold).

I am confused buy the "ON-(ON)" nomenclature.
Does this mean...

When the switch is just sitting there the plunger is out all the time?
And from the common to one terminal you will have a closed switch?

And conversely when the plunger is depressed the aforementioned closed switch is open and now from the common to the other terminal will be closed?

Hence the function I need is to use the "first" contact set described immediately above?

Thanks for the help!

 

[Comcokid]

Answer to the questions in the last part of your post is - YES

ON-(ON) for a SPDT means that unpressed there is one set of contacts closed and another open. When pressed, the first set opens and the second is closed.

To have contacts made when the switch is pressed you need to use the "ON" terminal and the "common" terminal. The "(ON)" terminal is connected to the "common" only when the switch is pressed.

 

[itsmoked]

ummm...

You said:
To have contacts made when the switch is pressed you need to use the "ON" terminal and the "common" terminal.

Did you mean un-pressed?

Thanks Comcokid for the quick reply..

Now I can route my sheetmetal.

 

[logbook]

itsmoked, there is a little picture on page two of the linked pdf which shows graphicaly how it works.

When the button is out 1 and 3 are linked.
When the button is in 1 and 2 are connected.

I haven't seen that "snap acting" part before. I was thinking it was a typo at first like "latching momentary action" but then I realised that the over-centre action on the contact gave a much higher current rating and a better switch. Very sensible after all

 

[itsmoked]

Dang you are right!! I don't know how I missed that. Sheesh. Very annoying.

Many thanks man.

 

[jraef]

Snap action means that you can't "mush" the contacts closed by slowly pushing the button. There is a spring loaded knee-like mechanism that will snap over quickly once sufficient travel of the plunger has ocurred. It prevents contact bounce, operation by vibration and insufficient force.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Comcokid]

Oops! - That is an error in my post.

Correct statement is: To have contacts made when the switch is un-pressed you need to use the "ON" terminal and the "common" terminal.

I know what it's like. Sometimes you suffer from that intermmittent short-between-the-headphones and you can't make sense of the simple stuff. Or in you rush to reply to the post you make an error!

 

[fsmyth]

jraef, from what I have seen, snap-action switches of any
sort are MORE prone to contact bounce. The rest of what
you posted is absolutely correct, and I would add the added
benefit of a positive feel to the operator. Saves turning on
a lamp.
<als>

 

[Skogsgurra]

Both jraef and fsmyth are correct. Snap action does often bounce more when switching, but it does it in a well defined, short period and is easy to debounce. It also keeps the contacts better closed when operated so it prevents multiple "strikes" when the button is pushed and released. This latter phenomenon is also considered to be "contact bounce" although it should be called "dirty contact" or "low quality contact". The Germans sometimes call it "Wackelkontakt" - I have no good English word for it.

Gunnar Englund

http://www.gke.org

 

[fsmyth]

skoggs, I completely agree, and probably should have gone
into a bit more detail. I tend to treat "snap-action"
switches in much the same manner as relays, although the
quality and construction of ANY switch affects bounce, delay,
and closure rate. I DO like the "Wackelkontakt" term; it is
now in my vocabulary.
<als>

 

[itsmoked]

I'll let you all know when this particular switch comes in just how "snappy" it is. I'm using it to shut off heaters when a door closes, directly in the heater control circuit (logic control side) so I don't care about bounces. It could Wackelkontakt for 30 seconds with no real impact.[lol