solid state relay module

[mblyman89]

Hello, I am trying to build a solid state relay module that uses the raspberry pi with shift registers to achieve a 25 channel relay module active high. I have been struggling to understand how these ICs work, and was hoping to get some clarification. I am using a Panasonic 4 pin SMD (part# AQY211EHAX), (LCSC parts library #C129282), SSR spst-NO, rated for 1A continuous current and up to 3A pulses. It is a photovoltaic (I think) power mosfet on the output. I feel confident about my understanding of the input side, I have an indicator LED with the appropriate sized resistor for both the LED and IR LED in the IC while using 3.3v. On the output side there are two pins, a source and a drain I would imagine. I have my load connected on the drain side. my load is a small piece of nichrome filament, used to light a fuse. the nichrome wire requires 1A between 12-18V. I've uploaded a picture of part of my design. on the left is the input, "OUT 201" comes from the shift register. on the left, 12v is connected to the source, and netport 201 goes to to positive lead of the nichrome and the negative of nichrome lead goes to ground to complete the circuit. Will this work in this configuration? 

[bob91343]

I don't see why not, if the isolator can handle the current.

[ledtester]

Here is a page which describes how the back-to-back configuration of the MOSFETs work to create a bi-directional switch:

https://www.homemade-circuits.com/bidirectional-switch/

The relay is bi-directional, so it so you don't have to worry about which end is connected to +12V.


And here is Panasonic's own "high-level" description of how their PhotoMOS devices work:

https://www3.panasonic.biz/ac/e/corp/nyumon/relay/photomos.jsp#content03_03

[mblyman89]

Thank you for the reply! After reading the articles, do I need a Schottky diode if I am only switching in one direction rather than bidirectionally? or am I way off here?

[ledtester]

What Schottky diode are you talking about???

The circuit in your first post should work just fine assuming you get enough current through the LED.

[mblyman89]

Cool beans, my bad. Thank you for the help, I’m still learning how it all works.

[floobydust]

It's not going to work - you only have at most 3.3V from a RPi DO and the PhotoMOS LED uses ~1.25V and then a red LED is ~1.8V so the 1k resistor will give almost nothing for current.
I don't think that SSR is tough enough, 1A is its max rating and cold nichrome has much lower resistance longer than 100msec.

https://www.eevblog.com/forum/projects/my-arduino-powered-firework-sequencer-this-is-gonna-be-a-blast/

[mblyman89]

thanks for the reply! the link you posted is broken. I used a resistor calculator to figure out the resistor value, so it was my best guess. If I remove the 1k resistor, will that provide enough current for both leds? the data sheet says it's rated for 3A pulses, how long is a pulse? the minimum on time for the relays will be .25 seconds, at most they will be on for 2 seconds. I am using talon ignitors, and while using a 12v battery and a regular em relay, .25 seconds was the minimum amount of on time to light the fuse I found. the data sheet also states the min voltage on input is 1.7V I think, or something like that, is that something the RPi can handle, or is it just because I have a 1k resistor in series with the two leds and all I need to do is remove the 1k resistor? I don't really know what I am doing, just stumbling around in the dark!   

[mblyman89]

I went back to the data sheet of the led I am using as an indicator light. it has a minimum forward voltage of 1.7v, normal 2.0 and max of 2.5v. the data sheet of the relay says the min forward voltage of the led is 1.25, max 1.5v. if I take out the 1K resistor, will that be enough to power both leds with a 3.3v supply? or would I be better off leaving the 1k resistor and removing the indicator led?

[ledtester]

Just put the LED in parallel with the SSR each having their own current limiting resistor.

The meaning of the LED changes slightly -- it now indicates if the MCU is trying to activate the relay rather than if current is going through the relay's LED -- but it still will be a useful indicator.