Designing a coil for dry reed switch.

Thread Starter

jitushah

Joined Jun 14, 2013
1
Dear All,
Greetings.
I wish to design a coil/reed swich unit such that when an a.c. current between 4.5Amps to 9.5 Amps flows in a compressor motor including a small
coil which surrounds a small Normally Close contact dry reed switch gets operated and the switch drops when the current is less than 2Amps.Could some one guide me how to do this.
thanks.
jitu
 

LesJones

Joined Jan 8, 2017
4,174
You would first need to find a supplier that could supply reed switches with closely controlled pull in and drop out amper turns ratings. This is a link to one supplier. If you chose a reed with a pull in rating of 30 amper turns then to pull in at 4.5 amps you would need 30/4.5 turns = 6.7 turns so in practice you would need 7 turns. For it to drop out at 2 amps you would need the drop out amper turns value to be 2 x 7 = 14 ampere turns. Most reed switches I have seen are capable of switching at more than twice normal mains frequencies (50 or 60 Hz so the reed would be pulsing at 100 or 120 Hz To avoid this you would need to feed the coil via a bridge rectifier and you would need to put an inductor in series with the coil (On the DC side of the bridge.) with a high enough inductance to smooth the current through the reed coil. I do not think using reed switches this way is a very good solution to your problem as you would have to get the supplier to select reeds that met closely your amper turn ratings. I think using a current transformer would be a better method.

Les.
 
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