Sections:
- Basics of Relay Coil Surge
- Suppression Using General Purpose Diodes
- Adding Zener Diodes
- Relays with Built-In Coil Surge Absorption
- Varistor and CR Circuits
- Summary
Basics of Relay Coil Surge
In many basic relay circuits, damage may occur when the relay coil is de-energized. When the power supply is removed, the stored energy in the coil (inductor) reverses polarity and seeks a path for dissipation (current flow). During this time, a large and possibly dangerous voltage potential builds at the component junctions.
This may be called Back-EMF (back electromotive force), BEMF, CEMF (counter electromotive force), flyback voltage, etc., depending on the source. In relation to relays, these all refer to the same effect. It is usually expressed as V = -L(di/dt). This voltage spike may damage or destroy adjacent components and may induce electrical noise that can interfere with microcontrollers or other signals within the vicinity. In cases where there are vulnerable points, some type of suppression should be included.
V = -L(di/dt) or e = -L(di/dt) A graphical representation of this induced voltage without any suppression is shown below.
Image source: [click here https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN ].
Suppression Using General Purpose Diodes
A common solution for suppressing relay coil BEMF is to place a general purpose diode across the coil to serve as a shunt. The diode is oriented to block the source voltage during normal relay operation, but also to conduct with the reversed polarity of the induced flyback voltage once the relay power is disengaged. This component may have other names, but if it is used for this basic purpose, the function is the same. Documents may describe it as a flyback diode, freewheeling diode, snubber diode, suppressor diode, commutating diode, clamp diode, or catch diode.
An example of this simple circuit is shown below.
Image source: [click here https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN ].
The previous article discusses contact sticking which may occur more frequently with the addition of a suppression diode. Calculations could be made to estimate the diode’s effect on release time, but it is equally valid to conduct tests on various components to determine the minimum requirements. For this post, we’ll only look at general practices that are common to both field applications (DIY) and manufacturer recommendations.
Using the phrase “general practices” for diode selection is a bit of a disclaimer, and with good reason. There are too many opinions based on individual circuits, cost and/or availability, margins of error, etc. None of these are wrong, but none of them are an exact science, either. They are simply practical. Different options are explained below.
For starters, the lowest limit suggested in some documents and electronics forums is the source voltage (relay coil voltage) and rated current (relay coil current). Using Digi-key part number Z6256-ND (Mfr# LY1N-DC24) as an example, we can see that the coil voltage is 24VDC, and the coil current is 36.9mA. [click here https://www.digikey.com/short/z5zzh8 ]. Those two specific values should always be available in a manufacturer’s documentation. Here is the LY series datasheet: [click here https://assets.omron.com/m/3e54678f7feeb2fc/original/LY-Electromechanical-Relay-Datasheet.pdf ].
To translate that into diode selection, use the filters titled ‘Voltage – DC Reverse (Vr) (Max)’ and ‘Current – Average Rectified (Io)’ from the main Diodes – Rectifiers – Single’ category: [click here https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280 ]. Product Index > Discrete Semiconductor Products > Diodes – Rectifiers – Single. To simplify the choices, select only “Standard” and “Schottky” from the ‘Diode Type’ filter.
More common is the suggestion to raise the diode voltage level to a value of 2x to 3x the coil voltage. Literally, that would be 48VDC to 72VDC for the example part number noted earlier: Z6256-ND (Mfr# LY1N-DC24). Suggested forward current values are still stated as “at least as large as the load current” (coil current), so higher values are acceptable. Some sources will even offer 10x as the correct diode voltage level, but this is either debatable or applies to specific applications.
If the source is a specific manufacturer, they may have their own guidelines. However, even these are not based on any precise calculations. They are best described as “safe practices” with a large margin for error.
Panasonic has this basic information in their ‘Relay Technical Information’ guide.
Image source: [click here https://www.panasonic-electric-works.com/pew/eu/downloads/technical_information_relay_en.pdf ].
The last suggestion (for this post) that appears very often is to use a standard diode from the basic 1N00x designs starting with 1N001 at 50VDC and up to 1N007 at 1000VDC. For the purpose of BEMF suppression, the small electrical spec differences across different manufacturers can be ignored.
The reasons for using the 1N00x designs are simple and practical. For many relays, even the lowest 50VDC, 1A diode will be sufficient protection. The 1000VDC, 1A version should cover most DC relays that won’t encounter high current levels. The 1N00x design in also very common, readily available (in stock), and inexpensive. Many electrical DIY customers will have some of these 1N00x products on hand, so they don’t have to put any thought into calculating specs and purchasing more components for a quick, casual relay suppression fix.
To find single diode products, use this link: [click here https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280 ]. Product Index > Discrete Semiconductor Products > Diodes – Rectifiers – Single.
To find 1N00x products in that list, type 1N00 in the ‘Search Within Results’ area, and press Enter (or click on the magnifying glass icon).
Adding Zener Diodes
As noted earlier in the ‘General Purpose Diode’ section, the addition of a suppression diode can add to the release time of the relay contacts. This can cause damage and reduce the lifetime of the relay. One solution is to add a Zener diode in series with the standard diode. Many forums and manufacturer documents will recommend a Zener voltage close to the level of the coil voltage.
Image source: [click here https://www.panasonic-electric-works.com/pew/eu/downloads/technical_information_relay_en.pdf ].
The Zener diode improves relay switching speed which may be slowed by the single, general purpose diode. Compared to the operate/release graph posted earlier, note the difference in release time in the diagram, below, where the standard and Zener diodes are together in series.
Image source: [click here https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN ].
Here is the main category for Zener diodes: [click here https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-zener-single/287 ]. Product Index > Discrete Semiconductor Products > Diodes – Zener – Single.
Look for the filter titled ‘Voltage – Zener (Nom) (Vz)
There is also an option to add a Zener diode across the transistor in circuits that use this configuration. In this case, the Zener voltage should be slightly less than the Vce (voltage across the collector-emitter junction) of the transistor. For a generally safe power rating, choose a mW/W level that is twice (2x) that of the coil power value.
Relays with Built-In Coil Surge Absorption
If the relay has not yet been purchased or designed into a circuit, there is the opportunity to buy one with surge suppression included. This eliminates the need to include extra components in a prototype, or to add one later where an application would benefit. Models may include diodes, varistors, or CR circuits (capacitor/resistor). (See a note regarding CR circuits and varistors after the photos, below.)
A problem may occur if the diode within the relay fails, though. Most manufacturers don’t construct relays with the idea that customers will repair them in the future, so if the internal surge suppression component fails, the whole relay may need replacement. The manufacturer may not release the specs for the internal surge suppression component, either, so even the most ambitious and skilled DIY person would have a difficult task when looking for a matching part.
These built-in suppression features can be found in our filter options or in manufacturers’ model number structure guides.
Use our ‘Features’ filter to find those options (Diode, Varistor, or RC Circuit (same as CR)).
Product Index > Relays > Signal Relays, Up to 2 Amps [click here https://www.digikey.com/products/en/relays/signal-relays-up-to-2-amps/189 ]
Product Index > Power Relays, Over 2 Amps [click here https://www.digikey.com/products/en/relays/power-relays-over-2-amps/188 ]
Manufacturer datasheets may show model numbers with built-in suppression. You can use this to verify the product features we have listed and also to search for those product codes on our site. If we don’t have a particular model, we may be able to ask the manufacture for a quote on the price, lead time, and minimum order quantity.
Here are some diode and CR circuit options shown for the Omron LY series mentioned earlier.
Image source: [click here https://assets.omron.com/m/3e54678f7feeb2fc/original/LY-Electromechanical-Relay-Datasheet.pdf ]
Here is another example from the MKS series datasheet.
Image source: [click here http://www.ia.omron.com/data_pdf/cat/mk-s_ds_e_6_3_csm1382.pdf ].
Varistor and CR Circuits
Although these suppression options are not covered in detail in this article, they can be useful for both AC and DC relay applications. If the specs are in question, look for recommendations in documents from individual manufacturers.
This ‘Relay Technical Information’ document from Panasonic is the source for the diode and diode + Zener diagrams shown earlier, and it has suggestions for varistor and CR circuits as well: [click here https://www.panasonic-electric-works.com/pew/eu/downloads/technical_information_relay_en.pdf ].
Summary
Relay coil surge suppression may be necessary to prevent component damage during contact release. General purpose diodes provide an easy and inexpensive solution, but they may cause problems due to slowed release times. The addition of a Zener diode improves the release speed, but it adds an extra component which increases costs for larger production runs. Varistor and CR circuits can also be used for both AC and DC relay applications.
In many cases, exact calculations for diode specs are not necessary because many common, inexpensive products meet or exceed the requirements of low voltage/current relays. Calculations can be made where absolutely necessary, but sample testing and performance observations are just as useful for determining the need for, or the values of, component level coil surge suppression.