How to select a reed relay

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There are many different brands and types of reed relays for signal switching applications.

The manu's seem to copy the series data for V, A, P, Ron from each other. Yet prices for, say, a Form A SIL 12V reed relay may range from <$2 to $10 a piece. What is the difference in performance or quality that causes the price difference? How do I find the data that distinguishes them - the data sheets are all very terse.

There may be no difference. I look up fluted knobs now and then, and the classic ones from 60 years ago can sell new now for $30 a pop. Are they "better" than other knobs? No, but they meet some mil spec of whenever.

Relay maker might mainly serve OEMs in some field that had specific needs and documentations, and they sell the parts for a lot, then some other relay maker sells mainly to commodity parts markets and is real competitive with lower prices.

That is al hypothetical of course, but unless the contacts in one are gold plated or some other detail, the price doesn;t necessitate there being a lot of difference.

What specs matter to your application? And do the data sheets cover those? Contacts have to handle the current you need, and break the voltage you need, and have the on resistance you need. Chances are the million operations versus 10 million operations won;t be an issue spec. The coil needs to operate on a voltage you have or can easily make, and coil current may be an issue, what will drive your coils?
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Jan, very good question. It gets more interesting when you find that some manufacturers make the glass-enclosed reed assembly for others. Based on years of experience, I've been happy with Coto, though the 9852 'form C' units tend to stick in the NC position if not actuated frequently. The very tiny SMT models tend to be most expensive from any vendor. Experience with Pickering has been mixed. In my previous life we avoided several brands which may or may not be around anymore. Mercury wetted are available by special order in the USA, but may be banned in Boston and Europe.
Guys, thanks for the pointers, I am going to follow that up.
My application is an auto-ranger for audio signals to be put in front of a soundcard for measurements.

The prototype is working well with Pickering series 101 reeds, but they are relatively expensive, and one failed getting stuck in the closed position (which may be just bad luck).

Just yesterday night I spoke with Bruce Hofer from Audio precision at a presentation for the NL AES section, and to my surprise he mentioned AP uses lots of 'regular' armature relays in their analyzers, not reeds! We looked at the inside of one of the analyzers and there they were: nice bright red relays. With the marking removed of course ;-)

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You want a gold bifurcated crossbar contact on a sealed relay. For example a Panasonic TXS2-4.5V-1

These look nice Ed, and reasonably priced. Yet there is a Coto series 9000 Spartan that's 1/4 of the price and specs sort of the same. That's why I think the spec sheets aren't very useful.

BTW How should I interprete a spec 'Minimum voltage/current'?

These look nice Ed, and reasonably priced. Yet there is a Coto series 9000 Spartan that's 1/4 of the price and specs sort of the same. That's why I think the spec sheets aren't very useful.

BTW How should I interprete a spec 'Minimum voltage/current'?


The Coto 9000 series I saw are sealed reed relays not gold bifurcated crossbar contact style. Quite different no matter what the "specs."

Relays get a film on the contacts. This requires a small voltage or current to guarantee that you will break down the film and actually make contact. That is why there is a minimum specified.

Cross bar contacts increase the connection pressure by using a smaller area so they try to mechanically pierce the film. Gold actually in air will turn from gold oxide back to gold. Bifurcated means there is a slit in the contact. So a set of bifurcated contacts touch at four places when activated.

The Coto data sheet I saw had no minimum data. The Panasonic is 10 uA or 10 mV but apparently goes even lower for the Gold plated Palladium version.

The relays aren't really intended for the same use.
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Gold but not bifurcated crossbar would be my guess.

Does that mean if I use relays to switch nA bias currents and uV offsets from op-amps in and out in an ATE environment it won't work? But it does to the tune of 10^6 times a week or more, COTO 2200 series by the thousands.

Jan I'll send you a sampling of the COTO and Teledyne relays we use to test all our products.

Ed I gather you still haven't read it, there are no mV breakdown gaps.

Paschen studied the breakdown voltage of various gases between parallel metal plates as the gas pressure and gap distance were varied. The voltage necessary to arc across the gap decreased as the pressure was reduced and then increased gradually, exceeding its original value. He also found that at normal pressure, the voltage needed to cause an arc reduced as the gap size was reduced but only to a point. As the gap was reduced further, the voltage required to cause an arc began to rise and again exceeded its original value.
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As usual you respond to something, just not the topic.

Breakdown in gas has nothing to do with corrosion or dirt. It is the sulfides that form on the contacts and prevent conduction.

Passing signal so low it is known as a dry contact is known to be a problem to switch, connector and relay manufacturers.

Maybe you should have a chat with the folks who design the test equipment to see if they are actually switching dry circuits or if they are looking for something else like low level changes.

But gee Scott maybe all those manufacturer's data sheets are wrong when they specify minimum switching levels.

Then there is the annual Holm conference on such issues named after Ragnar Holm the guy who wrote the first really comprehensive book on the subject. Or is that just a fictional issue also?

Quite simply you are wrong on intermittent contacts and so short sighted you haven't even looked at the literature. It is a real issue even in the audio gear discussed on these threads. The folks who hear improvements from cleaning their connectors are not imaging things. The systems that stop working due to contact failures aren't imaginary. The A-B test is real clear. No sound clean connector now there is sound.

I have had dozens of system failures where the cure was to clean the preamp mute relay. That is because designers without experience use the relay as a series switch. In real audio designs mute relays short the signal. It is done that way because any signal large enough to cause an issue from a failed mute relay will usually clear the contacts.

Now why don't you show the circuits for the test gear and let us see if they are switching dry contacts or if there is enough voltage or current to cleàr the switch on initial contact.
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Here you go.

A typical op-amp test circuit used in ATE since I started in semiconductor test with Fairchild nearly 50 (!) years ago.

You should do some more reading and open your mind to different ideas.

The whole point is that reeds are sealed capsules in an inert gas or vacuum, with Rhodium plated contacts.

Circuits like these have been /are used to test BILLIONS of components. Easy to measure pA currents and sub mV, with long term consistency and amost zero failure rates.

An inconvenient truth, I'm afraid!


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That's the problem here - you ask two people you get three different answers ;)

Scott thanks, those COTO 2000 look nice albeit relatively expensive, but I will try them out.
Cliff what type are those relays you use?

I'm trying to think about a way to do some meaningful measurements on relays. Running say 10mA signal current through them, sampling it across a resistor and measuring distortion?

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