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Pickering Electronics Blog

Placing & Driving a Reed Relay Coil

Posted by Graham Dale on Jun 23, 2016 12:30:00 PM

There are a number of considerations when operating with Reed Relays. The following blog post explains how Reed Relays should be placed and the different ways to drive a Reed Relay coil. 


Magnetic Field Interaction

Reed switches are operated by magnetic fields provided by coils and in the case of energize to break (Form B), sometimes internal bias magnetics within the reed relay assembly. For Pickering Electronics Reed Relays the inclusion of a magnetic screen ensures they can be densely packed together. However, it does not make them immune to magnetic fields generated by EMR relays or by other reed relays that do not include magnetic screens (or include ineffective screens). So when reed relays are used on PCB some care should be taken to avoid them being excessively close to parts that might generate a strong field, including disc drives and large inductors.


Transistor Driving

A common method of driving reed relays is to use either a bipolar transistor or an FET to directly
drive the coil using an open collector/source. The coil can have one end connected directly
ground or to a power supply – the most common method used to is to connect to a power supply so that a grounded transistor or FET can be used.

When driving with a transistor a diode has to be fitted to control the Back EMFvoltage spikes
generated when the coil drive voltage goes open circuit.

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Topics: Reed Relays, Relay Coil, Magnetic Interaction