Looking to get a simple PCB fabricated for a brewery controller that will hopefully be nothing more than a glorified set of jumper cables and a place to mount some screw termnnals and RJ9/12/45 jacks.
Looking to drive some external din rail mounted SSRs to handle the mains voltage switching. Sourcing these from Aliexpress, and as per normal the Aliexpress listings don't provide a full "spec". What i am looking at is some slim relay din bases with plug in "6A 240Vac" SSRs - control voltage is 3-32Vdc, but the don't provide a rating for the control i put current. Other similar plug in SSR have a rating of 7.5mA at 12Vdc, so i am assuming (hoping) I'll be fine - but want to confirm before I kill the SSRs LED or an Arduino I/O pin. Can anyone confirm I'm on the right track with my though process below?
Using a 12Vdc supply and a resistor of a known value I should be anle to calculate the internal resistor value of the SSR (or confirm that there is actually no internal resistor)
Estimating the internal resistor is around 1.35kOhm with a single IR LED (1.9V voltage drop). If I add a 470Ohm external limiting resistor I should measure a current of 5.5mA when supplying it 12Vdc;
(12-1.9)/(1350+470)=5.5mA (if I get something different I can back calculate the actual internal resistor value)
If the assumption that there is an internal current limiting resistor is incorrect then the current would be:
(12-1.9)/(470)=21.5mA which shouldn't kill the IR LED.
Does that sound all right? Or have a made some mistake or wrong assumption somewhere?
Bad idea, not only because you never know what you are getting, but also because there have been reports of fake SSR being supplied by some of the resellers that use that site to distribute product.
Yeah I've been out of the hobby for a few years but still remember lots of threads on the homebrew forum I was on with knock-off SSRs failing. However with the significant cost saving I am happy to roll the dice at least once to see what the quality is like since I initially only need one to start with and it will be significantly over-spec'd for what i need. Will be switching a 0.35A pump vs the expected rated 2A (listing says 6A but I don't trust that given the official ones I have seen max out at 2A).
Forget all the "would be maybe" calculations and simply drive the ssr via a decent logic level mosfet like say an IRFZL44.
You have to use interface somewhere to your 12v anyhow.
Since it can go from 3V to 32V, I suspect that there is more than just a single resistor, maybe a zener or transistor. Why don't you just buy it, apply 5V and measure the current. I doubt if it will be over 20mA at 5V.
Because the exact item i am looking to buy is not really relevant to the question i am asking, but here is a link anyway (hopefully it is not dead to soon) https://a.aliexpress.com/_mqLf5U8
And I have seen another datasheet (can't find it at the moment sorry) that includes product codes for ordering their SSRs with or without thebcurrent limiting resistor.
At the end of the day whether the SSR comes with the protection or not isn’t the issue - I just need tonfigure out which situation I am in, if I need to provide it or not.
You're probably right, but I'd still be assuming. I don't need a 5Vdc PSU for the project so my plan was to do similar to what you say but different (test it with limiting the current rather than the voltage). I guess i could hack up a old USB cable to get a 5Vdc supply or use an old Nano that I wouldn't care if it died - if you think that's the wiser choice?
Please don;t go through life not accepting the specifications of component manufacturers. Why are you assuming you can get 5VDC from a Nano. Have you ever purchased any electronic components?
What are you talking about? My original question is regarding the fact that I don't have the full specifications. How can I trust something that I don't have?
And my answer to your question of "why am I assuming that I can get 5Vdc from a Nano" is literally "because i trust the Nano's specifications"
And if in the slight chance that it doesn't work out (i.e. I damage either the SSR or the Nano) just buy another one right? Don't really want to waste time or money with that approach.
The issue is that while most SSR have internal protection circuitry, there are some (not common) that don't. If I happened to run out of luck and this is the uncommon type, connecting it direct to an Arduino will likely end up burning out the internal LED in the SSR or burning up the arduino pin (or worst still the entire chip).
Finally got round to buying the relay and testing it - turns out it was way easier than I was expecting. Haven’t tested the actual SSR but removednit from the DIN base and check the resistance between the input terminals and the SSR pin socket - one side has a resistance of around 600 Ohms. So final answer - no external resistor needed for this SSR/DIN base as the base has the protection (and indication) circuit built in.
