Magnets and reed switches (what's an AT?)

[jgannon]

See the chart on the second page of this:
http://www.cotorelay.com/RI-01B_Series.pdf

Can somebody tell me what the unit AT is? Amp-tesla? Google doesn't seem to recognize that phrase, and it sounds a little funny anyway. Judging from the way that it's being used, it seems like some unit of magnetic force or field strength. How does this translate into some function of the strength of a magnet and the distance away? I'm interested in using a permanent magnet, but I suspect that AT has something to do with coils.

[Gabe]

Don't really know what it is either, but your coil theory would go best with AT for Ampere-Turn according to Wikipedia.
http://en.wikipedia.org/wiki/Ampere-turn
Honestly, I'm no electrical engineer, but maybe someone out there is an engineer.

[Dave K.]

Originally Posted by jgannon

See the chart on the second page of this:
http://www.cotorelay.com/RI-01B_Series.pdf

Can somebody tell me what the unit AT is? Amp-tesla? Google doesn't seem to recognize that phrase, and it sounds a little funny anyway. Judging from the way that it's being used, it seems like some unit of magnetic force or field strength. How does this translate into some function of the strength of a magnet and the distance away? I'm interested in using a permanent magnet, but I suspect that AT has something to do with coils.

Ampere Turn. Coto's application guide http://www.cotorelay.com/Reed_Relays...n_-_Entire.pdf covers a fair amount of information that you might find helpful.

The application guide for the reed switches themselves is here: http://www.cotorelay.com/Reed_Switch...n_-_Entire.pdf

These documents, and others, can be found under the 'Literature' section of their website.

From that, you should be able to correlate a coil's AT field with the measured magnetic strength of your magnet.


When working with a switch like this, be aware of things like contact bounce when writing software that takes actions on the switch action.

While not as much of an issue with small reed switches, many switches and relays will have a minimum contact current specification. Commonly referred to as the "wetting current", it is the minimum current required to flow through the contact to prevent oxidation buildup from creating a high impedance path.

[jgannon]

On the odd chance that anybody else should ever have the same question, one of the documents that Dave linked to has a graph that indicates that an ampere-turn with a Philips Standard Coil is equivalent to about 0.44 gauss for a permanent magnet. Thank you to Gabe and Dave for their help.