SSR Research

Looking into what SSRs are available on DigiKey, I found that a few of my assumptions about what I would find were mistaken. It turned out that the ones availble in the "Hockey Puck" package all have internal circuitry which was more complicated than the simple optocoupler example that I was expecting. Also I was surprised to find that nearly all of the SSRs available on Digikey were made by a single manufacturer, which is one I hadn't heard of before, Sensata-Crydom. That seemed to also be true on Mouser. The Sensata-Crydom SSRs that I checked the datasheets for all had fairly complex circuits for the LED side, and also had some kind of circuit to drive the mosfet on the switch side rather than having only a simple mosfet. In retrospect, it may have been too much to expect a "simple mosfet", and additional circuitry obviously helps with ensuring a solid relay switching function. For most SSR applications, such large amounts of current are typically being switched that there would be no reason to care about using a tiny bit of current to make the switch work. Also I learned that there are a few types of SSRs. In particular, some are MOSFET, others are BJT transistors. The BJT ones have a voltage drop across the transistor, as they do. The specs for these do not show an on resistance. Instead the forward voltage drop is listed. I found the following execellent tutorial that covers this:

https://www.allaboutcircuits.com/technical-articles/basics-of-ssr-solid-state-relay-the-switching-device/#:~:text=When%20the%20switching%20device%20of,on%2Dstate%20voltage%20drop%E2%80%9D.&text=Assuming%20a%201%20V%20drop,about%201%20watt%20per%20ampere.

Here are links to the datsheets for some of the SSRs that I found, with pros and cons noted:

An odd looking Sensata datsheet for the Crydom D06D SSR. Shows blocks for a "Control Circuit" on the LED side and a "Trigger Circuit" on the mosfet side. https://www.sensata.com/sites/default/files/a/sensata-d06d-series-panel-mount-datasheet.pdf

The Crydom datasheet for the D06D SRR. https://www.mouser.com/datasheet/2/657/d06d-series-dc-panel-mount-1371031.pdf

A datasheet for a different Crydom mosfet SSR, "DC series", shows a fairly similar internal circuitry. This has a nice contact cover: http://www.crydom.com/en/products/catalog/power-plus-dc-series-60-dc-panel-mount.pdf

Here's a Digikey page for yet another Crydom SSR. Why are there so many models and series available for Crydom SSRs?https://www.digikey.com/en/products/detail/sensata-crydom/D2D40/1771532

This TE SSR has a mosfet on the switch side, and a really simple LED circuit. There is a "mosfet driver" block that probably uses some power from the switched side, and the LED circuit may be an outright lie: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=SSRDC&DocType=DS&DocLang=English

Another Crydom power plus datasheet. Looks like about the same circuits. https://www.mouser.com/datasheet/2/657/power_plus_dc_series_100_dc_panel_mount-1370752.pdf

A diffent series Crydom SSR. This is AC output and is a dual triac type. We don't want this: https://www.mouser.com/datasheet/2/657/gnssr-1063303.pdf

Here is a digikey search of all available ssrs with the hockey puck package, mosfet outputs, proper LED voltage range. I attempted to get a simpler output circuit by searching for ones with an allowed output voltage of 0V. I found that most of the returned results actually required at least 1V. https://www.digikey.com/en/products/filter/solid-state-relays/183?s=N4IgjCBcpgbFoDGUBmBDANgZwKYBoQB7KAbRAGYBWSgTgHZyQBdAgBwBcoQBldgJwCWAOwDmIAL4FYNBCGSR02fEVIUATDTVUQBchq0AGZm06Qe-YWMng1dWfMW4CxSGXIHKEXQBZyMgmBqYN5BOjZgnowB0n5qYXA0vt7xdJRBXuB0AByUBskEagbk5GBZYYXFIeVF5N5lBTV0Rg3FqdW1Bs0gFd4R1ZR0sPANg%2BR2I7CUwyBJQV1qGsGU5d40lFnTaqtZmuVZcMF7B%2BPd9LkZsL2wJ5elySwgHFy8gqISATQ79qiYTiquICMDyeZhelne3TAxW%2BCl%2ByhcZAMAAIAGpIgB%2BSLAnQAdMiUWF8RikYUDHjUYTUcTvLj8ZS0ZjcmS6QQiZjYMyKcDTCAAEpoUQ4ACqQgE7AA8igALI4NBYACufBwEKCtRhjnhqm8SIAtuKABY6rBhSi6g1Gk042Bmw3GgiUHFZG0WqTOu0gOhusJZHGmvW2sI0L0BZH%2Bl3gMDBmxRqEx61h91gT0J6pR9xRmlR3JR2BqGPYmPplMBahRuZlovm90aUNVsJUWsB%2B3Z4sgLKdL3crgisWSmVyxXK6wAWji0DkUH48s1AOWTHE1jHZCwhAwAgAJkisOw0OwcEilRg0ABPYwgeDjjdcVX5R48nsS6WyhVKsLsY%2BsZVmOXIBdAA