Anyone familiar with this?

Looking at the data sheet, the recommended input current is between 16mA and 24mA so lets use 20mA. If your RAMPS heat bed output is 12V and we subtract the forward voltage of the LED (~1.2V as per the data sheet), we have 10.8V. So 10.8V / 20mA = 540Ω. The next highest standard value is 560Ω as per your first calculations. The power dissipated by the resistor would be ~200mW so I would use at least a 1/2W part. a 1kΩ resistor would give you barely over 10mA of drive current (again this is assuming you are getting 12V drive from the RAMPS) which could cause the SSR to not fully turn on and cause excessive heat. And the second circuit with a second LED in series is even worse which would give you only ~9mA drive current with the 1kΩ resistor.

Edited 1 time(s). Last edit at 10/26/2015 11:55AM by Ryan_M.

Thanks for the fast answer.

where do you find the 16mA and 24mA trigger range?

ok the 1k resistor felt funny so ill keep the 560 but put the led in series

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sungod3k
where do you find the 16mA and 24mA trigger range?

HERE, top of page 8.

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sungod3k
ok the 1k resistor felt funny so ill keep the 560 but put the led in series

I'm not a big fan of adding the 'indicating" LED in series. Electrically its fine but adds a little more unknown. A google search shows that forward voltage drop of a typical red LED can range from 1.8V to 3.3V. If you still use the 560Ω resistor this could result in a drive current between 13mA and 16mA. A 390Ω resistor would give a range between 19mA and 23mA if you go that route. You could always put the 560Ω in and measure the voltage across it and work the math backwards to get a more precise value.

Aha thats right.
I get slightly different values in my ohm calculator, are there other factors you're considering as well?

560 ohm, 10.2V --> 18.21mA
560 ohm, 8.7V -->15.54mA

390 ohm 10.2V --> 26.15mA
390 ohm 8.7V --> 22.31mA

13-16mA would be a bit on the low side indeed and even the 19-24mA would not be enough for the 16-24mA trigger threshold.

Im just playing around with the numbers and if I dont use the led and put in 470ohm I get 25.53mA, the should better for the trigger threshold right?

You're missing the voltage drop of the LED in the SSR. The data sheets list this as 1.2V typical, with a max of 1.4V. So the total voltage drop of the SSR's LED + the indicating LED could be anywhere from 3V to 4.7v.

I have been talking with the supplier in China. He asked me to confirm my wiring method. He agreed that it was defective and was (as we knew) just a switch meant to break one leg of the power supply and is sending a replacement n/c. I already ordered an American replacement but will try both and evaluate. Thanks for the input guys.

Got the new SSR and it works perfectly using it to break the + leg from the power supply. Does lose 1.1 volts. Printing cooling fan bracket for it now. The main goal was to remove said load from the main board. Hopefully this will buy it some time.