Whenever you make a voltage measurement, one must consider the effect of creating a voltage divider. What is the resistance on the other side of the switch? For instance, if the switch measures 1E10 ohms and it is connected to a 100 mega ohm (1E8) resistor and 10,000 volts is applied to the other end of the switch away from the resistor, and series circuit is set up such that some of the voltage will be dropped across the switch and some will be dropped across the 100 mega ohm resistor. One has a series circuit set up with basically two resistors in series. One resistor is the switch at 1E10 ohms and the other the load resistor 1E8 ohms. When applying 10,000 volts to this circuit approximately 1 µA of current will flow to the open switch and through the load resistor. Simply using ohms law the 1 µA will generate 100 volts across across the load resistor. Now if the insulation resistance across the switch is 1E11 ohms the voltage across the resistor would only be 10 volts. However, if the insulation resistance across the reed switch is 1E9 ohms, then the voltage across the load would be up to 1000 volts.I hope this all makes sense to you. Obviously, the insulation resistance across the reed switch is very important as is load resistance. Hope this explains better what you and the customer are seeing.
Generally thermal compensation is required in a low thermal relay. Alumina and Beryllia are commonly used because they have great thermal conductivity while they maintain electrical isolation.
For a low thermal reed relay at 20°C the reed switch junction connected to copper will generate 1 millivolt and changing the junction by 1°C will generate an additional 60µV.
The higher the coil resistance the less power produced by the relay and therefore there will be less thermal offset voltages generated. By applying a magnetic shield, the contacts see a much stronger magnetic field. This allows the relay designer to increase the coil resistance which in turn reduces relay power and heat generation.
Yes, the coil resistance directly controls the amount of heat generated in the relay. The more heat generated, the more the need to compensate for thermal voltage offsets. Making the coil resistance as high as possible is a clear step in the right direction.
A reed switch is made up of nickel/iron and when connected to copper (a PCB trace), you end up with a thermocouple producing a high offset voltage. Since you have this thermocouple on each end, you need to compensate for these high offset voltages, otherwise they will swamp out any small offset signals the customer is trying to switch. So the key to make a low thermal relay is to develop a compensation technique that will compensate for these high offset voltages. Carefully placed thermal chips get the job done.
Generally low thermal relays switch differential signals that require two single throw relays. A one pole single throw relay is used on the front end of high end multimeters.
Low thermal or low offset reed relays are used in applications where transducers are used that produce very low voltage signals that need to be switched and amplified. They are also used on the front end of high end multimeters and switching thermocouples in data acquisition systems.
A low thermal reed relay is used to switch low voltages in the low microsecond (µV) range and not alter the signal level in any way after it has passed through the relay.
The SIL series can be used up to 800MHz and the MS series can be used up to 1.5 GHz.
Yes, a simple trick which improves the RF characteristics of a relay is to ground the start wire of the coil. Since the coil wire is copper, its first layer can represent the shield for the signal. This may allow the customer, using this technique to switch and carry RF signals up to 500 MHz. This allows us to use the SIL and MS relay series in RF circuits.
The best way to correlate when testing RF is use the same test fixtures. We can loan our RF fixtures to our customer to obtain the same results.
Once our customers receive our RF surface mount relays they need to match the impedance going in and coming out of our relays to his PCB. They do this by adding small amounts of capacitance and/or inductance on each side of the relay at the junction of the relay and the PCB.
A ‘T’ switching configuration is a way to improve isolation in RF circuits. It comprises three reed relays. The relays are arranged in the following manner: one is on the left upper part of a T, the second one in on right side of the T after the junction, and the third relay is mounted on the vertical component of the T. For maximum isolation, the first and second relays are in the open state. The third relay is closed and its bottom end of the T is grounded. With the first relay open any signal that leaks thru to the junction of the three relays will be shunted to ground. Any signal still left at the junction will be further isolated by the open contacts of the second relay. When conducting a signal through the ‘T’, both the first and second relay are closed allowing the signal path. The third relay is open. The ‘T’ configuration will improve isolation, but there will be some signal loss due to the longer signal path.
Our customers should mount our RF reed relays in a surface mount environment assuming he has chosen one of our surface mount reed relays. For best performance, he should mount our relay axially on his PCB. Also, he needs to tune his impedance on his PCB to exactly match ours going into and out of our relay.
To get the best RF performance from a reed relay, its leads should be mounted axially to the PCB. This means that a hole needs to be cut in the PCB for almost half of the relay body to sit in. Here the leads exit the reed relay in a straight line with no turns minimizing signal travel.
To make the best possible RF reed relay you need to make a simple geometric design most preferably a coaxial design with minimal alterations. The design should be as short as possible.
If your customer is using multiple relays in a matrix format and is passing RF signals through the matrix it makes good sense to offer them a multiple relay matrix in the same package. This is particularly true when the relays are in series, because this essentially reduces the signal path length. In this case, the path length in and out of the relays is eliminated, where the signal simply travels out of one relay and into another relay with minimal path distance.
Yes, always strive for the shortest path length the signal will see, going through the reed relay. Also, minimize the number of turns the signal needs to take, going through the reed relay.
Yes, the more consistent the characteristic impedance and the closer to 50 Ω, the better the RF characteristics. Whenever there is the slightest change in impedance, some of the signal will be reflected reducing the insertion loss.
Testing a reed relay for its RF characteristics is not a very simple undertaking. You need a network analyzer, with special RF test fixtures. See the Standex Electronics Engineering Note: Testing of RF switching components.
The isolation for a reed relay in an RF circuit is basically determined by the gap distance. So the only way to control or improve the isolation in the reed relay design is by going to a wider gap reed switch. This means using a higher ampere turn switch which translates to a higher powered coil.
S-parameters are generated by our network analyzer when we make RF measurements. Since they are stored electronically, they can easily be passed along to RF designers and potential customers via email.
The S – parameters are important to the designer of an RF circuit because they are used by placing them in RF software. This software simulates an RF circuit. In this way, the RF designer has an idea of how our relay will interact with other RF components, in their circuit.
A reed relay designed to carry high frequencies will generally use a coaxial design approach. With this in mind the formula to calculate the characteristic impedance is the following: Z = 60/(√(€R) + ln(2h/d)) where Z is the characteristic impedance, √ is the square root, (€R) is the dielectric constant between the shield and the reed switch, ln – is the natural log, h is diameter of the shield, d is the diameter of the reed switch.
A reed relay designed to carry high frequencies will generally use a coaxial design approach. With this in mind the formula to calculate the characteristic impedance is the following: Z = 60/(√(e)) ln((D)/A) where Z is the characteristic impedance, √(e) is the square root of the dielectric constant, ln – is the natural logarithm, D is the diameter of the shield, and A is the cross sectional of the reed blade.
The inductance is calculated using the following formula: L = µo n d A1 where L is the inductance, µo is the permeability constant, n number of turns, d is the length of the signal line, and A1 is the length of the signal line shield
The capacitance is calculated with the following formula: C =( e A)/d where C is the capacitance, e is the dielectric constant, A is the shield and reed switch blades and d is the distance between the shield and the blades.
The characteristic impedance is calculated by the formula: Z = √(R + (XL – Xc)2 ) where Z is the characteristic impedance, R is the DC resistance, XL is the inductive reactance, and Xc is the capacitive reactance.
The capacitvie reactance is calculated by the following formula: XC = 1/(2∏fC), where XC is the capacitive reactance in ohms, f is the frequency in Hz, and C is the capacitance.
The inductive reactance is calculated by the following formula: XL = 2∏f L, where XL is the inductive reactance in ohms, f is the frequency in Hz, and L is the inductance.
At any given point along a signal path, if the capacitance, resistance or inductance changes, the characteristic impedance will change?
When a pulse traveling along a given signal path encounters a change in characteristic impedance, part of its signal strength will be reflected back along the original signal path. This represents a loss in signal strength.
The signal path, the shielding, and the material with its corresponding dielectric constant are the main constituents that make up the characteristic impedance.
The signal path and its length is critical. The shorter the better. It’s best to think of the signal path and the shield as a geometric shape. Maintaining that geometric path as consistent as possible is critical. Any variation will change the characteristic impedance and will produce signal loss.
If a given relay has rise time of 50 picoseconds, a given digital pulse passing through it will have its rise time increased by 50 picoseconds. Now, if they have to travel through a matrix of five relays, its rise time will increase by 250 picoseconds. Now the frequency response after one relay is 20 GHz but after the fifth relay it is down to 4 GHz. So it’s important for the system designer to know how many relays or components its signals will pass through to determine if the components will work in his circuit.
To equate continuous wave with a digital clock running at 2 GHz, you have to consider how many harmonics of the base frequency are needed to construct the digital pulse. Normally at least 5 harmonics of the original frequency need to be considered. So for 2 GHz this represents a continuous wave frequency of 10 GHz. So to pass 2 GHz digital pulses in a circuit it would need to have a frequency response of 10 GHz.
The critical area of a digital pulse is its rise time. If the rise time of the pulse’s leading edge is for instance 50 picoseconds, the corresponding frequency is equivalent to 20 GHz.
S – parameters are supplied for a given frequency and are supplied with a magnitude and direction. They are very useful in supplying information about the characteristics of a component in digital format. They can also allow the RF designer to know how that component will function in his circuit, before the actual component is added to the circuit.
When you pass a digital pulse through a component or a circuit it will enter the circuit with a certain rise time. When it leaves the circuit or component it will have a new rise time. The slew rate is the difference in rise times from the leaving rise time minus the arriving rise time.
Rise time is usually referred to in digital circuits. The shorter the pulses the more important the rise time has become. It is measured as the time from the beginning of a pulse to the 90% point of the pulse height. Circuits need to be able to have good RF characteristics to pass these fast pulses. Rise time is an important parameter that needs to be accounted for. Circuits not capable of handling fast rise time pulses would effectively swamp out the digital pulses.
VSWR stands for voltage standing wave ratio. When signal traveling in a circuit are reflected back, they may reach another component and then be reflected forward again. These reflections back and forth can produce standing waves in the circuit. These waves can create a very lossy circuit.
When a signal enters a circuit or component, some of the signal may be reflected back in the direction from which it came. Return loss is a measure of that signal loss.
Insertion loss is the loss of signal when traveling in and out of a given circuit or traveling into a component and out of the component. If your signal is at 100% going into a component, and coming out there is a loss, its described as insertion loss and is measured in decibels (dB). 3 dB is described as the end point for any component and is equivalent to the signal strength being reduced by 50%.
RF can and does cover over open circuits. The amount of signal that travels from the input to the output of a switch represents that measure of isolation measured in decibels (dB), -65 dB is considered the best for isolation. Generally -20 dB is a workable level.
RF like to travel within a circuit with a consistent characteristic impedance. Any changes in characteristic impedance will produce signal loss. Characteristic impedance Z is essentially a measure of resistance. It has three components that are added vectorially. The components are: the pure DC resistance in the x- axis, the inductive reactance in the y-axis, and the capacitive reactance in the z-axis. The characteristic resistance is calculatd all along a given signal path and any change in any one of the 3 above resistances at any point will alter the resistance. 50 ohms (Ω) is the most generally accepted resistance in most RF circuits.
RF rides on the outer part of the conductor. The higher the frequencies, the farther it moves to the outer edge of the conductor. Many RF characteristics are quite different than DC. It has a whole new set of parameters:
The reed relays have a flat frequency response out to 20 GHz. Their cost is moderate and stable. Their size is becoming smaller and smaller. Quality issues has been their main problem. They are not good at switching higher power, but improvements are under way in this area.
Electromechanical relays can switch up to 20 GHz. They can be very expensive and are very large. Like the reed relay they do have a good flat frequency response. Their large size takes up too much board space and they require a lot of power to operate. They have very good isolation and have the ability to switch higher power RF.
Semiconductors can be used to switch up to 100 GHz. Cost becomes very high over 10 GHz. Semiconductors represent the smallest size when compared to the other technologies. Its frequency response has discontinuities. They have inter modular distortion and need added circuitry to control. They also need added circuitry to improve its frequency response.
Reed relays are very linear over a large span of frequencies, typically ranging from DC up to 20 GHz. Semiconductors need filters and suffer from inter modular distortion. This means additional components need to be used. The reed relay by itself will do the job and are ideal when switching low signal level RF loads. The reed relay’s size is much smaller than the electromechanical relay and comparable in size to the semis.
Generally semiconductors, reed relays and electromechancial relays are used to switch RF. Each technology has its good points and bad points.
RF are waves of electrical impulses that oscillate at very high frequencies. The waves are no different than our 50 cycle or 60 cycle line voltages and currents. Instead of having 50 or 60 cycles occurring every second you can have billions occurring every second. A frequency of 1 GHz is a frequency that is oscillating at 1 billion times every second. In the digital world electrical pulses pass along information. The shorter the pulses the more one can pass added information every second. A computer operating at 2 GHz is capable of processing 2 billion pulses every second. For electronic circuits to process a pulse it has to have the ability to carry 5 times its base. This means that the circuits carrying 2 GHz pulses need to have the capability of carrying 5 times that or 10 GHz on an RF basis. This is because square waves are made of 5 harmonics of the original frequency.
RF energy (a combination of voltages and currents) when traveling through a conductor will tend to travel on the outer part of the conductor. The higher the frequency, the more the RF energy is traveling on the outer diameter of the wire, or traveling on the ‘skin’ of the conductor. This effectively reduces the cross sectional area in which the energy can travel. If it is only signal level the RF energy will pass thru the conductor with a minimal amount of attenuation attributed to resistive loss. However, if the RF energy is significant, where a fair amount of power is being conducted through the conductor. Severe resistive losses may occur. Dramatic lose of signal may occur. Furthermore, major heating may occur that could cause the temperature on the contacts to rise above the curie temperature. In this case, the reed leads will lose their magnetism resulting in the contacts opening. This now can cause complete destruction of the reed switch contacts. This is produced by the contacts reclosing once its temperature drops below the curie temperature and its magnetic properties are regained. Now the contacts will close the full load and heating will begin again until the curie temperature is reached again. Here the contact will open and close until the contacts are shorted or destroyed. In this case, adding copper to the outer surface of the contacts and their leads will reduce and or eliminate the potentially disastereous effects.
Inspect the reed switch to see if you can see any finite cracks. If not, you should send the switch back to Standex Electronics to determine why the switch has lost its vacuum.
In relays which have two switches in series: If one of the switches loses its vacuum, it will have a low breakdown voltage. Two switches in series is used to achieve the additive effect of two 10,000 volt breakdowns adding to give over 20kV. So what has probably gone wrong is one of the switches has lost its vacuum, perhaps due to a small crack or a bad seal. Try to remove some of the epoxy on the end were the reeds are soldered together and then test them individually to see which one may be bad.
If the high voltage is still testing good, it sounds to me like they may have switched too much power and/or carried too much current. Carefully break open the reed switch capsule and look at the contacts to see if there is any sign of pitting or burn marks right on the end of the contacts where they come together when the contacts close. If you see this, you will need to find out exactly what the customer is applying to the contacts and/or what he is carrying across the contacts. There are a few things that the customer can do:
RF are waves of electrical impulses that oscillate at very high frequencies. The waves are no different than our 50 cycle or 60 cycle line voltages and currents. Instead of having 50 or 60 cycles occurring every second you can have billions occurring every second. A frequency of 1 GHz is a frequency that is oscillating at 1 billion times every second. In the digital world electrical pulses pass along information. The shorter the pulses the more one can pass added information every second. A computer operating at 2 GHz is capable of processing 2 billion pulses every second. For electronic circuits to process a pulse it has to have the ability to carry 5 times its base. This means that the circuits carrying 2 GHz pulses need to have the capability of carrying 5 times that or 10 GHz on an RF basis. This is because square waves are made of 5 harmonics of the original frequency.
An RF reed relay is specifically designed to carry high frequencies up to 20 GHz and carry digital pulses in the sub nanosecond pulse widths. Shielding is critical and the geometry of the signal path as it relates to the shield is of utmost importance. The higher the frequencies the more critical they become.
RF relays are typically used in the test equipment market for PCB functional testing and Integrated circuit testing. They can also be used in medical electronics or any market where RF or fast digital pulses are involved.
Use the small copper plated Hermetic Switch in the LI or BE relay package.
Use the ORD2210V switch in the SIL HV or LI relay packages.
Use the HE and/or the HM series with the high voltage copper plated reed contacts, capable of high carry currents.
Depending on how fast the digital pulses use either the CRF or the SRF high frequency reed relay series.
Depending on the size/cost requirements consider the SIL, MS, CRR relay series in that order from a cost and size standpoint.
Use a BT Series special relay designed for high voltage dielectric and capable of switching voltages less than 1µV.
Use a two pole special BE series relay.
Use the HE and HM series reed relays.
Use the BT series or special BT low thermal reed relays.
Use the SRF series reed relay.
Use the CRF series reed relay.
Use the CRF series or the SRF series relays.
Use the 6 Pin SIL series or the MS series relay with the start wire grounded.
Use the CRF or the SRF series relays.
If size in not a critical issue use the SIL(six pin) series or MS series (grounded start coil lead).
All too often customers find their relays failing early in their life times that is often caused by the existence of common mode voltages. Common mode voltages usually arise from line voltages in the area or nearby a given circuit. If there is stray capacitance in the line it can become charged to the peak of the line voltage. If the line voltage is 240 VRMS this translates to potential peaks as high as 400 volts. Switching this voltage even though the stray capacitance is only, say, in the order of 50 picofarads, it will cause metal transfer on the contacts. This will eventually cause early life failures. Better grounding can eliminate common mode voltages. Reducing the stray capacitance will help. Also, adding some resistance in series with the contacts will reduce the inrush. Keep in mind all the damage occurs in the first 50 nanosecond upon contact closure.
Reed relays can be constructed with more than one switch. Standex Electronics typically will manufacture up to four reed switches in a given relay. This could be up to 4 single pole normally open switches, up to 4 single pole normally closed switches, or up to 4 single pole double pole throw switches.
A latching relay is bistable. It can be in the closed state with no coil power applied or it can be in the open state with no coil power applied. It only takes a pulse of 1.5 milliseconds to change from the open state to the closed state; or a 1.5 millisecond pulse will change from the closed state to the open state. A magnet partially biases the reed switch to create the latching states. Generally two coils are used: one is used for closing the contacts and the other is used for opening the contacts.
With a Form B relay, the contacts are biased closed with a magnet. So with no power on the coil, the contacts remain closed. When power is applied to the coil, its magnetic field is opposite the field of the magnet, cancelling it out and opening the contacts.
This is usually a condition that can develop when using a Form B or normally closed reed relay. The contacts are biased closed with a magnet. So with no power on the coil, the contacts remain closed. When power is applied to the coil, its magnetic field is opposite the field of the magnet cancelling it out and opening the contacts. If the coil is too strong, the contacts can reclose. So a reclose voltage is added to a Form B relay that is usually 25% to 50% above the nominal voltage. For a 5 volt relay with 50% safety factor the reclose would be 7.5 Volts. This guarantees to the customer that applying up to 7.5 Volts the contacts will not reclose.
This description is used in radio transmitter and RF applications. The old radio designs used amplitude modulation. The wave basically varies in size with the audio content, but is transmitted using a 30 MHz envelope. So PEP is just an expression that expresses that in very abbreviated terms. The audio is superimposed on the RF. This is what AM music is/was – audio modulation before digital modulation.
We suggest checking the following items:
Best to use a small copper plated reed switch in an application where the carry current is about 3 amps RF. Greater than 3 amps you should use a large copper plated reed switch. The RF will be riding on the outside ‘skin’ of the switch.
Use the Standex Electronics KSK-1A85 reed switch series.
Use ORD228, the ORD211 iridium, or the ORD311.
For a sensor use the ORD228 with iridium or the ORD2210 for a relay.
Small electromechanical relays are not good for switching low levels of voltages and currents. Electromechanical relays need a hefty voltage and/or current to break any film buildup. It is this film buildup that won’t allow very low voltages and currents to pass through the contacts. Reed switches are clearly the best. Using sputtered ruthenium contacts or iridium contacts are the best materials for these low level loads.
Switching and breaking voltages of 250 volts and above is best done with a vacuum reed switch. Up to 4000 volts can be effectively done with the ORD2210V as long as the current levels are not too high. Above 4000 volts use the Hermetic reed switches.
Miniature reed switches less than 20 mm (0.80 inches) glass length can effectively break up to 250 Volts. This depends on the pull-in AT (mT) used. The higher the better. Reed switches less than 10 mm will shrink this value to around 150 volts. Minimizing the current flow at the time of opening will improve this value.
Reed switches whether they are used in sensors or relays all will be asked to switch some load. Generally there are two aspects to this load.
This signature takes into consideration not only the steady state load but also any transient voltages or current that may be present during the first 50 nanoseconds. These transients may be from stray capacitance, inductance in the line and/or common mode voltages. From a reed switch designer standpoint, the signature is all there is. The most important time during the switching of a load is that first 50 nanoseconds. That is when all the damage to the contacts with occur if you are switching the contacts ‘hot’. If a customer is having a problem with early failures, this is the first place to look. Equally important and not to be overlooked is what voltage and current is actually being broken when the contacts open. Any healthy voltage and/or current present will chew up the contacts rapidly leading to sticking reed contacts.
There are several key factors:
® Copyright 2019 Standex Electronics, Inc. ® All Rights Reserved. A Standex International Corporation Company.
