Mark,
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.
Thanks,
Rick/K5IZ
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
state.
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.
73,
Mark W1EOF
