Monday, April 16, 2012

Understanding Switch Types for Automated Test

Switches serve as the central nervous system in automated test. They interface between the DUT and the test instruments routing signals to and from the DUT. In this post we will look at different low frequency switch types and discuss some of their pros and cons to help you choose the right type of switch for your measurement needs.

The types of switch we will be looking at in this post include: armature relays, reed relays, solid-state switches, and mercury-wetted relays. The following is a quick overview of each.

Armature relays — Because of their ruggedness, cost, and ability to handle higher currents and voltages, armatures are the most commonly used relays. Armature relays usually have low resistance. They generally have slower switch times, and they are somewhat more susceptible to arcing and switch bounce than the other types. Some armature relays are sealed; others are not.

Typical Lifetime: 10M
Typical speed: 250/s

Reed relays — When you need to switch at high speeds, reed relays typically are a good choice. In general, reed relays switch much faster than armature relays, have very low contact resistance and offer the added benefit of being hermetically sealed. They do not have the capacity to carry as high voltages and currents as armature relays.

Typical Lifetime: 10M
Typical speed: 2000/s

Reed Relay
Solid-state switches — Solid-state switches can cover low to high power switching applications, for instance they can be used for switching ac line voltages. The two main advantages of solid-state switches is their speed and no moving parts. Solid-state switches have the fastest switch times of all the switches. Since they have no moving parts there is no arcing or switch-bounce problems. They basically have an infinite lifetime as long as they are used within their power ranges. However, they generally have the highest "on" resistance of the switches and their isolation and crosstalk specs are typically the worst of the group.

Typical Lifetime: Inifinite
Typical speed: 4000/s

Mercury-wetted relays — Of all the switches mercury-wetted relays are the least common. These switches use liquid mercury inside to avoid switch bounce. They also have a long life compared to the other mechanical switches and have very low contact resistance. However, they are position-sensitive due to the liquid mercury, and must be mounted in the correct orientation to operate properly. They can also be expensive. These switches are used in applications were switch bounce cannot be tolerated and a low "on" resistance is needed. Today there are not many test and measurement companies who provide mercury-wetted relay cards or products so you typically have to integrate them into your test system on some type of custom platform.

Typical Lifetime: 10M
Typical speed: 50/s

Mercury-Wetted Relay
The best way to ensure your relays have a long life is by ensuring that they are used within their power ratings. But with mechanical relays, namely armature and reed, the lifetime can also be dependent on the type of load they are used with. This is due to their susceptibility to arcing which occurs from the electric field at the relay when it is opened or closed. Relay manufacturers specify how long their relays will last, but the expected lifetime will vary depending on the loads they are subjected to. For resistive loads, manufacturers’ specifications are typically fairly accurate. On the other hand, if you are using capacitive or inductive load, your relay life span will be shorter than the manufacturers specification. How much shorter depends on the type of loads you are switching. One technique you can use to better manage the lifetime of your switch relays is derating. Derating gives you a realistic picture of how long your relay will last. Loads can be classified into four general groups:

Resistive loads — Relay manufacturers assume you will be using resistive loads when they rate their relays. The load is a simple resistive element, and it is assumed that the current flow through the contacts will be fairly constant, although some increase may occur due to arcing during “make” or “break.” Ideally, a relay with a purely resistive load can be operated at its stated voltage and current ratings and attain its full lifetime. Industry practice, however, is to derate to 75 percent of the relay’s stated capacity.

Inductive loads — Switching inductive loads is difficult, primarily because current tends to continue to flow in inductors, even as contacts are being broken. The stored energy in inductors induces arcing; arc-suppression schemes are frequently used. When you are switching inductive loads, you typically will want to derate relay contacts to 40 percent of the resistive load rating.

Capacitive loads — Capacitors resemble short circuits when they are charging, so the in-rush current from a capacitive load can be very high. Series resistors are often used to limit in-rush current; without a limiting resistor, contact welding may occur. Its common today for switch card makers to integrate current protection resistors into their products. These protection resistors can be switched in or out of the current path as needed. When you are switching capacitive loads, you typically will want to derate your relay to 75 percent of the resistive rating.

Motor loads — When an electric motor starts up, it has very low impedance and requires a large in-rush current to begin building a magnetic field and begin rotating. Once it is running, it generates a back electromagnetic force (emf), which can cause a large inductive spike when the switch is opened. The result is a large in-rush current at “turn-on” and arcing at “turn-off.” When you are switching a motor load, typical industry practice is to derate to 20 percent of the resistive rating.

In this post we looked at an overview of different switch relay types for low frequency automated test purposes. From there we discussed switch lifetime, focusing on armature and reed relay types. If you have any questions on this post send me an email and if you have anything to add use the comments section below.

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  1. This comment has been removed by a blog administrator.

  2. Switches are one of the electronic components and they are so very useful to everyone. Because of your article, I understand more the use and benefits of this component. I learned the 4 types of switches and also the overview of each one. Can you post more new updates here? Thank you!

  3. Thanks for this blog. Also you have written about the effects of mercury as part. by OK agencies, a Biggest Liquid Mercury Dealers in India

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