Mechanical & Solid State Relays

Hey guys,

I’m busy ordering all the hardware I need for my project and ran into the following issue. The mechanical relays used by Kyle (which are pretty much the only ones you find online) are the SRD-05VDC-SL-C. When looking at the datasheets we find that these relays require a nominal switching voltage of 5V DC. The output from our GPIO pins however is 3.3V. There is a SRD-03VDC-SL-C version which require a 3V DC. I suppose that due to the ubiquitous nature of 5V DC it is hard to find the 3V version.

The solid state relays present a similar problem. When consulting the datasheet they say the nominal operating voltage is 5V DC but that the operating range is between 4V and 6V DC. Have any of you used these relays with 3V DC? I’m a bit weary, especially for the mechanical relays that a voltage that is too low would result in a bad/worse contact and thus presenting a fire hazard.

Any input is more than welcome, thanks guys!

They state 5 volts but I’ve used dozens and they all work with 3.3 volts. This only applies to the products I’ve mentioned. Of course you can find many relays elsewhere that may or may not work at 3.3 volts.

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Consider ULN2803: 8 Channel Darlington Driver

These SSR relays have a range from 3-32v dc.
I´ve had issues with the mechanical relays that they weld themselves into on position when my LED´s went on.
SSR relays dont have such issues and a probably much more reliable overall.
I run them directly on the IO ports without issues, ever.

Mechanical relays are reliable with both resistive and inductive loads, whereas solid state relays (SSRs) must be sized ~10 times greater than the listed current of any inductive loads placed on them. The large surge currents from some inductive loads when starting up, such as motors, can greatly exceed the current listed on the device. Repeated exposure of this to an improperly sized SSR will cause premature failure.

hi Kyle,

can you please clarify this a bit? when would a SSR be appropriate over a mechanical relay?

i am switching my boards to relays and using mechanical relays for higher compatibility but might be misunderstanding here.

SSR’s would save me board space

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It’s going to depend on your application and what devices you’re switching. There are so many variables it’s impossible to say with just a question like “which is better?”. Each will be better under different conditions. Here are some characteristics of SSRs compared to mechanical relays, but this is by no means an exhaustive list. There are many places documenting differences and applications where one is more appropriate than another.


  • Can switch at the AC zero-cross, which mechanical relays can’t (as far as I know). Certain devices operate better with zero-cross, others do not.
  • Are silent (mechanical will click when opening or closing).
  • Are more expensive for the same capacity as mechanical.
  • Have a much longer mean time before failure.
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thank you i will do more research.

these relays will never go more then 12v dc. SSR might be the way to go for a general purpose box with outputs running through relays at 5v or 12v. can save a lot of space
below is a very early screenshot of my new board

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