0
\$\begingroup\$

I need to control 6 SSR inputs. These are 16Vmax rated on the control side. I need to use them with 24V control voltage out of a PCB which looks like has mosfets for output and all positives of the output are connected together. I assume these mosfets are sinking current. I thought about a simple voltage divider, which should be ok. Are there any other options?

SSR PART NUMBER: SSR-1028ZD1 DATASHEET: https://www.tme.eu/Document/b96df566ad9896bc37fe6c7e432d75ab/SSR120A.pdf

\$\endgroup\$
2
  • \$\begingroup\$ Include the SSR part number, and a link to its datasheet. \$\endgroup\$ Commented Dec 1, 2019 at 18:21
  • \$\begingroup\$ included the datasheet.. What I was really wondering is how the control part of the ssr is made... I gave it a try with only a 2k7ohm resistor in series and at 24V on the terminals it had about 6V. Voltage divider works fine... \$\endgroup\$ Commented Dec 1, 2019 at 19:55

1 Answer 1

0
\$\begingroup\$

Typically good SSRs use something like a constant current circuit at the input. Here is the input characteristic curve for a 4-32V Panasonic SSR AQA221VL from the datasheet:

enter image description here

As you can see, the current changes little from 5V to 32V, drawing what the internal LED needs via a crude linear current regulator and little more.

Cheaper ones (especially those with limited input voltage range) probably use a crappy Zener shunt regulator which just burns the excess current up in an internal resistor. The current will rise faster than with a resistor in this kind of circuit. Yours is probably of the cheaper type.

Since your datasheet is a bit light on information, if you have a multimeter and a bench supply you can easily characterize the one you have and determine a suitable series resistor value that will be well within the normal operating range of the SSR. Of course the manufacturer could change the internal circuit without telling your on future orders.

You could also just put an LM78M05 on each SSR and then it would behave more like the good SSR.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks, I didn't think about measuring the input current.. I will also check that out. \$\endgroup\$ Commented Dec 1, 2019 at 20:29
  • 1
    \$\begingroup\$ Thank you, I measured the current and the curve looked somewhat like the one you showed. Going for the voltage divider works, but I also tried a single resistor in series with the ssr and current and voltage across the ssr where well within specs. This ssr probably has a current limiting circuit. \$\endgroup\$ Commented Dec 2, 2019 at 19:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.