Magnetic Contacts - Expiry date? - False Alarms years later!

[sparkymarka]

Post picture

 

[PaulUszak]

wish I could find a physical switch that is held shut to complete the circuit

This is not the typical solution to alarming a door. But if you have the bent, electrically determining the position of an object (such as a door) is exactly what limit switches are designed for. You can get girl's and boy's versions. The girl's version goes into your microwave as a door interlock. The boy's version can go into a steelworks. They're all rated for 240VAC but will work perfectly on low DC. These two images are examples. There are many alternative configurations /geometries for the lever arm that will suit any situation. Guess which is which...

 

[Europlex]

......... and so easy to overcome

 

[bernardgreen]

but will work perfectly on low DC.

You need to ensure the current being switched is more than the switch's wetting current. This is the current the switch has to switch if the contacts are to be self cleaning. The lifespan of a switch can be shortened if the current it is switching is below the wetting current

With all this discussion concerning the reduction of switch contact arcing, one might be led to think that less current is always better for a mechanical switch. This, however, is not necessarily so. It has been found that a small amount of periodic arcing can actually be good for the switch contacts, because it keeps the contact faces free from small amounts of dirt and corrosion. If a mechanical switch contact is operated with too little current, the contacts will tend to accumulate excessive resistance and may fail prematurely! This minimum amount of electric current necessary to keep a mechanical switch contact in good health is called the wetting current.

Normally, a switch's wetting current rating is far below its maximum current rating, and well below its normal operating current load in a properly designed system. However, there are applications where a mechanical switch contact may be required to routinely handle currents below normal wetting current limits (for instance, if a mechanical selector switch needs to open or close a digital logic or analog electronic circuit where the current value is extremely small). In these applications, is it highly recommended that gold-plated switch contacts be specified. Gold is a "noble" metal and does not corrode as other metals will. Such contacts have extremely low wetting current requirements as a result. Normal silver or copper alloy contacts will not provide reliable operation if used in such low-current service!

copied from http://www.allaboutcircuits.com/textbook/digital/chpt-4/switch-contact-design/

 

[Europlex]

......... and so easy to overcome

... by putting a screwdriver or chisel between the door and switch to hold shut, or similar.

 

[PaulUszak]

... as easy, or easier, than putting a loose magnet up against the reed switch inside B&Q magnetic contacts?

 

[Nintendofan]

... as easy, or easier, than putting a loose magnet up against the reed switch inside B&Q magnetic contacts?

The magnetic contact I fitted to my shed have 'magnetic interference detection' are you saying a loose magnet will bypass these?

 

[bernardgreen]

are you saying a loose magnet will bypass these?

Some can be held closed by that method, but some sensors have a means to detect a rogue magnet and this will create a tamper or alarm condition.

 

[PaulUszak]

are you saying a loose magnet will bypass these?

Only if you bought it from B&Q, like most people outside of the security community do. Like this perchance:-

Gets 4½ stars. Brilliant. I'd feel very comfortable having Genghis (my hamster) watched over by this.