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Pickering Electronics Blog

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

Understanding Reed Relay Terminology

Posted by Kevin Mallett on Mar 23, 2016 12:30:00 AM

 

The relay industry has evolved with a set of its own nomenclature that describes the products available, however, not all of these terms are familiar to users. The following blog post seeks to describe and explain these relay terms.

 

Form A

Form A describes a relay whereby the contact is a simple switch, which is open or closed, and the un-energised position is the open condition. For a single relay this would also be described as a single pole, single throw (SPST) relay with a normally open (NO) contact.


If the relay has multiple contacts in the same package it would be described as having (for example) 2 Form A contacts (DPST).

Pickering has a range of  Form A Reed Relays, for example our Series 109 that are an ideal choice for high density applications, or our high voltage Series 104 for up to 3kV.

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Topics: Reed Relays, Form B, Form A, Form C

Choosing a Reed Relay

Posted by Kevin Mallett on Sep 28, 2015 11:30:00 AM

 

The following blog post describes and recommends the key features an engineer should consider when choosing a Reed Relay.

 

Signal Voltage, Current and Power Specification

All reed relays have specified voltage and current ratings that need to be kept within if the reed relay is to have a long service life. It important to be clear if the application envisages hot switch or cold switching, it can have a substantial impact on the cost and size of relay used. If hot switching is likely to occur note the power rating of the reed relay, the fact a particular relay may be capable of 100V and 1A does not mean it can hot switch a signal with these extremes. At 100V a 10W reed relay will only switch 100mA reliably.

If hot switching is not expected then the user can rely on the carry current rating and the rated withstand voltage across the contacts.

SMD or Thru Hole Mounting

For applications where relays may have to be serviced Pickering Electronics recommend thru hole components are used. Outside of this the choice is driven by user manufacturing preferences and component choices such as footprint area, relay ratings and relay height.

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Topics: Reed Relays, Hot Switching, RF Relays

High Voltage Micro-SIL Reed Relays

Posted by Kevin Mallett on Jul 15, 2015 12:30:00 PM

 

Reed Relays remain an attractive solution for switching applications. Having a metallic path, they do not suffer from the relatively high and non-linear contact resistance together with the high off-state leakage current usually associated with solid state relays.

High voltage reed relays are intended for applications where the voltages are beyond the capabilities of conventional reed relays. They use vacuum reed switches to maximise the voltage rating for a given blade separation and to manage arc duration as the contacts open or close.

Any loss of seal will rapidly degrade the switch operation so reed switches have to be carefully managed as they are packaged into reed relays, using suitable encapsulation materials and Pickering’s SoftCenter® technology. The relay design must ensure there is adequate clearance distance both internally and externally for the rated voltages, something which also needs to be considered for the PCB layout.

 

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Topics: Reed Relays, High Voltage, Dry Reed Relays, Micro-SIL

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