I'm looking at relays and there are multiple types of contacts forms, such as: A, B, C, X, Y, Z and much more. Searching around found different schematics for same form. Can someone explain them or show me a picture of the different contact forms?
There is a mesh in symbols because even manufacturer agree on a common language, marketing issues differentiate them frequently.
Even the common assosiation RELAY AND SWITCH INDUSTRY ASSOCIATION I think it is not existe anymore, but they are publish a handbook about relays, the "Engineer's Relay Handbook"
Bellow is en extract of the above handbook (5th edition) the types of contacs are available throught the market.
Normally open, held close. In it's un-powered state the contacts is open. Also referred to as a "Make" relay.
Normally closed, held open. In it's un-powered state the contacts are closed. Also referred to as a "Break" relay.
This is a DPDT relay where one contact is Normally Open and the other is Normally Closed. Basically, a Form A and a Form B in one package.
This is a variation of the Form A relay, that is Normally Open, Held Closed, but the contact is double make.
This is a variation of the Form B relay, that is Normally Closed, Held Open, but the contact is double break.
As I'm sure you've guessed by now, this is a variation of the Form C relay. One side is Normally Open, the other is Normally Closed but as the relay is engaged each contact is double make or double break.
A form A contact is single throw, normally open, closes when relay is operated.
A Form B contact is single throw, normally closed, opens when relay is operated.
A form C contact is double throw - the moving contact is connected to one "output" when the relay is released, and to another when the relay is operated.
A single pole relay will have only one set of switch contacts. Double Pole will have two sets, so can switch two circuits. Relays are available with four or more poles.
The Form A, B, and C terminology is also used with switches. Form C contacts may be described as "shorting" or "non-shorting". In a "sorting" switch, all three terminals will be connected together briefly as the switch moves between positions.
Relay and switch contact arrangements are often given as "SPST-NO" (Single Pole, Single Throw, Normally Open) for a single Form A contact, or "DPDT" (Double pole, Double Throw) for two form C contacts.
A Double Throw (form C) switch or relay can be substituted for either Form A or Form B - just ignore the unwanted contact.
I'm not familiar with X, Y, or Z contact arrangements.
Form X, Y and Z are the same as Form A, B and C, except there are two sets of contacts instead of one. They are usually used for power relays where the contacts have to carry a lot of current, e.g. 40 A at 440 V. The double contacts I presume would reduce wear and arcing when the contacts are separated.
Here are the schematic diagrams for a Form Z and Form x:
The bifurcated contacts of Forms X, Y, and Z are actually designed to reduce noise when switching low power signals for the sake of digital circuitry, as two small contacts bounce much less than one massive one.
Forms U, V, and W are similar to X, Y, and Z, respectively, but the armature itself is doubled, not the contacts. And I'm at a loss as to why that was done...
And of course, GR Tech's detailed chart shows there are still more derivatives available.
There is, however, some debate as to whether or not the bifurcated contacts are really worthwhile, as they tend to be more expensive, and a good software debounce is usually just as good as a hardware debounce.