[AMRadio] Relay 101 Reed Relays

Rick Brashear rickbras at airmail.net
Thu Mar 23 16:46:43 EST 2006


I don't know about everyone else, but I truly appreciate the 
information.  I've used reed relays in the past, but certainly didn't  
and don't know much about them.  Your info helped clear up some questions.


W1EOF wrote:

>Well like machanical relays it's very much about the contact materials. In
>the relay business we spend a lot of time working on contact materials,
>plating, etc. So it's hard to illuminate too much without knowing the
>particular device in question.
>What's a reed-relay? A relay made from a reed-switch inside a coil. A reed
>switch has two ferrous blades which have had a contact material deposited on
>the ends making up the contact. When the coil is energized the steel blades
>of the reed switch deflect a very small amount and close the contacts.
>That's it.
>A few thoughts:
>1. Reed relays tend to be fast. Regular reeds close in less than a
>millisecond, mercury reeds about 2ms.
>2. If you don't know what the coil spec is, hookup the coil across a
>variable power supply. Adjust the meter upward slowly until the contacts
>close (determined by a DMM across them). Bring the coil voltage to 0.00 and
>repeat a few times. This is call the Operate voltage. Normal operate voltage
>for a reed relay is 50% overdive. So your typical 5.0vdc relay will operate
>about 3.6 volts or so. Most relays are find at 100% overdrive, it doesn't
>hurt the relay.
>3. Contact life is all about the materials. When designing a generic
>multi-purpose reed relay they choose a reed switch which can handle moderate
>voltages and loads. In very broad general terms if you were interested in
>very very low contact resistance you use a soft contact surface like gold.
>This is terrible for higher voltages or currents though. For those you want
>something really hard.
>4. If you have a mercury or a mercury-wetted relay make sure it's oriented
>properly. These relays have a blob (or at least some small balls) of mercury
>in them. You don't want that mercury floating around and shorting the
>contacts when the relay is supposed to be off. They will sometimes have an
>arrow on them but not always.
>5. Form-B reed relays rely on a magnet to hold the switch closed. When you
>apply voltage to the coil the magnetic force overcomes the magnet and opens
>the switch.
>6. Form-C reed relays are made with a form-c switch. The blade is held
>against the N.C. contact via mechanical force. Applying coil voltage results
>in magnetic force that moves the blade from the N.C. contact to the N.O.
>contact. It relys on mechanical force to return the switch to the resting
>7. Reed relays can have two types of shields in them. One is a magnetic
>shield on the outside of the coil. This helps prevent interaction when the
>relays are mounted very close to one another on a PC board. The other is a
>RF shield inside the bore of the coil. This is to provide a constant
>impedance to the circuit. This is very, very important to many reed relay
>customers. For instance automatic test equipment (ATE) manufacturers are big
>consumers of reed relays. A modern semiconductor tester can use 10,000
>relays. They want to test fast device quickly. So they are very very
>concerned with the RF performance of the device.
>Many years ago I started working for a reed relay company. I was naive,
>thinking "How complicated can these things be?' Ha! Now I'm older and wiser
>and understand that there is a wealth of knowledge and complexity in many
>things we deam "simple". Reed relays incorporate electronics, physics,
>magnetics into their design and use. TO design and test them you work with
>voltages from microvolts to thousands of volts. .001 ohm to 10^12 ohms
>(million-megohms). I'm still learning today after twenty years. Relay
>manufacturers are constantly pushing the envelope to make devices which not
>only perform better but are much smaller and less expensive. I have recently
>seen reed relays that are approximately the same size as a 1/2W resistor!
>I don't know what else to say in general about them. I'll be glad to answer
>any questions on this subject. I hope this gives you some food for thought.
>Mark W1EOF

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