Help reading datasheet for solid-state relays

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elnjensen
 
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Help reading datasheet for solid-state relays

Post by elnjensen »

Hi,

I'd like to use a solid-state relay in a project where I want a breakout (TPL5111) to be on most of the time, but able to be switched off periodically. I'm looking at using a normally-closed solid-state relay that I can toggle open occasionally using a Feather RP2040. I'd like the power dissipation to be low when the relay is in its (normal) closed state, but I'm unsure about the terminology used in datasheets, which give relay values for the "on" state. Since the relay is normally closed, does the "on" state refer to when it is open? And is the "on-state resistance" measured on the control side (as opposed to the load side)?

Here is an example of a datasheet, for this relay. And here is another, for this relay.

Essentially what I'd like to know is the closed-state resistance and/or power dissipation on the load side, when nothing is happening (Feather is off) on the control side. I'm assuming it's very small, but I can't tell for sure. I'm also assuming that since the relay is normally closed, it will conduct current on the load side even if the control side is floating/unconnected (since there won't be any control current), but correct me if I'm wrong there.

Thanks for any help! (For context, this is related to my query in this thread, about trying to reset a TPL5111 to keep two of them in sync.)

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adafruit_support_mike
 
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Re: Help reading datasheet for solid-state relays

Post by adafruit_support_mike »

'On' refers to the state where the relay conducts current. A normally-closed relay defaults to the 'On' state when no control input is present. The ones you posted have 7 Ohms of resistance (the LCB110) and 15 Ohms (the TLP4227).

When you apply control current (labeled 'If' in the datasheets), the relay switches to its Off state. In that state both relays have about 1uA of leakage current.
elnjensen wrote:Essentially what I'd like to know is the closed-state resistance and/or power dissipation on the load side, when nothing is happening (Feather is off) on the control side.
At 7 Ohms, the LCB110 will dissipate 280mW at its maximum rated current of 200mA.

At 15 Ohms, the TLP4227 will dissipate 338mW at its maximum rated current of 150mA.

Both values are calculated using the power formula P=IIR (current-squared times resistance).
elnjensen wrote:I'm also assuming that since the relay is normally closed, it will conduct current on the load side even if the control side is floating/unconnected (since there won't be any control current), but correct me if I'm wrong there.
You're correct. You have to turn on the control LED to shut off the output side.

BTW.. I don't think much of those two datasheets. Even knowing what I was looking for, I had to do a lot of scrolling to locate things.

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elnjensen
 
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Re: Help reading datasheet for solid-state relays

Post by elnjensen »

Thanks, Mike! That’s very useful. I ordered a couple of these and will try them out in a project this weekend.

I appreciate your help!

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