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Problem with High Impedance of SSR in Analog Output of PLC


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I have some problems with analog SSR (Celduc, Carlo-Gavazzi, etc brands) when I try to control from an 4-20mA analog output of OPLC.

For example with the V200-18-E2B the load impedance is 500Ω maximum for current output. The input dynamic impedance of the SSR is ≥ 100 Ω but a drop in voltage ≤ 8 VDC @ 20 mA.

The application doesn't work from 4-8mA, I suppose the dynamic impedance of the SSR will be higger than 500Ω.

I found one solution to use several SSR with an analog output, inserting a Power Suply between the output and the SSR. Do you use it any time? does it secure for the analog output of Unitronics?

Look at the image:

powersuplyforssr.jpg

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Calculating things out for the RJ1P puts the impedance of the SSR right at 500ohm. And you pointed out, the Unitronics is looking for < 500ohm.

500 ohms is pretty typical across all manufacturers for a max impendance for an output like this. I would point the finger more so at Carlo in this case. They shouldn't have to design it with so much impedance.

The power supply is a valid solution, but I think I would put more effort into finding better SSRs.

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OK, I have never had problems with SSR of ERO ELECTRONIC, CONTINENTAL, FUJI ELECTRIC. The end customer have this kind of low cost SSR but they works well with Toho Controllers.

Well, I think the idea of the power suply to solve the drop in voltage of the cheap SSR will be the solution.

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