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Graham Dale

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Recent Posts

Contact Abuse

Posted by Graham Dale on Sep 15, 2016 12:30:00 PM

Relays are sometimes considered slightly mundane and often little thought is given to them, which occasionally causes vulnerabilities. This blog post discusses contact abuse and will help you maximise the reliability of your design whilst using Reed Relays

 

High current or high power inrushes are the most damaging and most frequent cause of contact damage. Reed Relays have specified maximum Current, Voltage and Power ratings. The Power figure is simply the product of the voltage across the open contacts before closure and the instantaneous current as they first make.


We at Pickering have lost count of the number of times that we have heard something like “I was only switching 5 volts at 50 milliamps onto this CMOS logic board” when the user has completely disregarded the current inrush into the liberal sprinkling of decoupling capacitors and several micro-Farads of reservoir capacitance on that board.

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Topics: Reed Relays, contact abuse, Current Inrush

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

Relay Operating Temperature Effects

Posted by Graham Dale on Jun 11, 2015 12:30:00 PM

 

Reed Relays are sometimes considered a mundane component by design engineers and often little thought is given to their operating parameters. One of these parameters is operating temperature and failure to consider its effects can lead to the possibility of the relay not operating at high temperatures.

The relay’s reed switch is operated by a magnetic field generated by a coil which is wound around it using copper wire. Copper has a positive coefficient of resistance of approximately 0.4% per °C and its resistance will increase with temperature at this rate. As the resistance increases, the current and therefore the level of magnetic field will fall.

 

Distribution of Operate Voltages

The industry standard ‘Must Operate Voltage’ sometimes called the ‘Pull-In Voltage’ is 75% of nominal and usually quoted at 25°C. For a 5V relay this would be equal to 3.75V, although in practice it will be lower than this figure. The first graph below shows the actual distribution of Operate Voltages for a batch of 1000 Pickering relays. In the second graph you can see how this operate voltage figure will change with temperature.

 

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Topics: Reed Relays, Operating Temperature Effects

The unique features of Pickering Reed Relays

Posted by Graham Dale on Apr 28, 2015 12:00:00 PM

 

Reed Relays remain an attractive solution for switching applications. Having a metallic path, they do not suffer from a relatively high contact resistance and high off-state leakage current usually associated with solid state relays. Having hermetically sealed contacts, reed relays offer a better low level performance than conventional electro-mechanical relays as they do not suffer from oxidization or films building up on the contacts. They also have the advantage of faster operate and release times, critical in today’s instrumentation and test systems.

Unfortunately though, relays are often considered a mundane component and little thought is given to them by the design engineer which can lead to problems.

In general, the most common construction for a reed relay offers no magnetic screening, involves the use of a hard moulded package and an operating coil wound on a plastic bobbin surrounding the reed switch capsule. Pickering offer a technically superior solution.

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Topics: Reed Relays, High Density Applications

An introduction to Reed Relay basics (Part 2): The Configurations

Posted by Graham Dale on Mar 24, 2015 12:30:00 PM

 

Normally Closed Reeds

 

Normally open reed relays are by far the most common configuration of reed relay. However, normally closed relays can also be supplied where the blade is biased so it is normally closed and the application of a magnetic field opens the relay contacts.

The contact bias is created by adding an internal permanent magnet to hold the reed switch in a normally closed state. When the relay coil is energised it cancels out the magnetic field bias and the contacts open. If the coil voltage is increased substantially beyond its nominal voltage (typically greater than 1.5 times nominal) there is a risk that the contact will reclose.

Not surprisingly normally closed relays are more difficult to manufacture and have higher magnetic interaction due to the bias magnet.

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Topics: Reed Switch, Two Pole Relays, Changeover Reed, Normally Closed Relay