EYE on NPI – TE SSRMP Series Solid-State Relays #EyeOnNPI #DigiKey @TEConnectivity @DigiKey

Happy new year, everyone! It’s time for the first EYE ON NPI (video) of 2021, and we’re gonna switch this one up…with an adorable little solid state relay from TE Connectivity.

The SSRMP series of SSRs are so petite, you wouldn’t imagine at first that they can handle up to 480 VAC and 25A, but they can! And they do it with style. Compared to most large SSRs, these are quite small, with a body of 22mm x 22mm square, 15mm tall. The heatsink is 35mm long and 3 mm of thick, solid aluminum for good heat transfer. There are nice silkscreened labels all around, so it’s self-documenting.

The two input ports are 0.1875″ quick-connect lugs, outputs are 0.25″ lugs, so you could use every-day spade connectors to attach/detach wires quickly. The input even has an adorable counter-sunk green 3mm LED to let you know when its activated.

Solid state relays are an interesting electronic device. Usually when folks start out turning AC power on and off, they tend to use a relay. Relays are dead simple: inside is a mechanical switch with a ferromagnetic throw. When a nearby coil of wire has current going through it, the coil becomes an electromagnet and pulls the throw to one side. Relays are electrically and mechanically isolated because there’s air between the electromagnetic coil and the switch throw. They’re easy to use, and inexpensive, so they get used a lot and TE makes plenty of options.

Because relays are electro-mechanical, they have to two properties that may be undesirable. First is speed: it takes a few milliseconds (say about 10ms) to energize that coil and move it to the side. The second is mechanical wear and failure: relays will eventually fail from mechanical stress or oxidation. Every time you switch the relay, a little arc is formed from the mechanical chatter, which creates a little burn on the contact. It’s a very small oxidation, but it adds up, and the more you use a mechanical relay, the faster it will fail. The fail rate depends on the load type, activation voltage, contact material, etc., but they’re in the 10s of thousands. Relays not meant for high speed or repetitive switching, which is why a lot of them are plug-in or easily swappable.

That’s where SSRs come in!

SSRs function similar to electromechanical relays except that these relays are contactless, using electronic components such as triacs, thyristors, and power transistors as part of the switching element. An input signal to an SSR switches the output from a non-conducting state to a conducting state, switching the load circuit on and off. Instead of using a magnetic circuit for the intermediate signal to achieve galvanic isolation between the input and the output, as in the electromechanical relay, SSRs use optoelectronics, capacitive connection, and electrical field coupling as the intermediate signal. Therefore, SSRs respond quickly, are highly resistant to vibration and shock, quiet when switching, and unaffected by the presence of dust, gases, and other contaminants.  (From https://www.te.com/usa-en/products/relays-contactors-switches/relays/mil-aero-relays/solid-state-relays.html).

As long as you maintain the voltage levels rated by the electronics, and dissipate any heat generated, you can switch as much as you like! It’s fast enough to ‘PWM’ cycle AC current, such as dimming lighting.

Trade-offs? Yep, there are some. First, you do need to heat-sink SSRs, unlike relays. You don’t get a ‘normally closed’ contact (SPDT) like relays. Instead, you have only one contact that is normally open and you can activate to close it. SSRs are for AC current only, not for DC current, unlike relays that are a purely mechanical connection. And some SSRs are not electrically isolated, you may need to add an optoisolator if you need electrical/mechanical isolation. (In this case, these modules are electrically isolated, but always check the datasheet).

That said, for rapid switching of AC current, nothing beats an SSR. And these SSRMP series modules are good for jobs great and small! Available in different AC and current ratings, they’re a quick addition to your next electromechanical control project.

Digi-Key has them in stock right now for immediate ordering. Search for TE SSRMP to get the options, and if you order today, you’ll have it tomorrow morning! Then check out Digi-Key’s article on using SSRs for their tips and tricks.

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1 Comment

  1. Just wired up a PID controller using one of these! The note on heatsinking is very important. My 40x60x15mm heatsink reached 78C taking a toaster oven up to 100C and then hovered at 70C to maintain it. Running the 25A model anywhere near that limit will need considerable cooling.

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