Solid State Relays

[DL06]

Hello all. I have what may be considered a stupid question, but hey, at least I'm asking before trying it blindly, so here we go...

I am feeding high speed inputs into the first 3 inputs of an AD DL06-DD1 PLC (X0 – X2). To get these inputs, I have tied off of the existing circuitry in the machines. The voltages are all different, anywhere from 5VDC to 12VDC. The switching frequency is up to about 3Hz.



I figure that since my inputs are all on the same PLC common and the voltages are all different, I would clean things up by running everything through some solid state relays. I have the pulses triggering the SSR and the SSR outputs going through the +24VDC from my power supply back to the PLC inputs.



No big deal up to this point. However, the only SSRs that I have are AD-SSR210DC units. 3-32VDC coil voltage, no problem there, but the output is rated at 24-280VAC 10A.



Is there any harm in using these to switch a very small DC current going to the PLC input? I have tested the input circuits to the SSRs, but haven’t energized the output circuit.



If this is a major no no, can anyone suggest where I can get some SSRs for this application? I haven’t come across any that are DIN mountable and switch a DC circuit. Automation direct only has SSRs for switching an AC circuit for some reason.



Advice appreciated. Thank you all once again.

 

[zam]

I don't see why they shouldn't work, but if they for some reason don't, I've used these http://www.crouzet.com/catalogue_web/int/eng/637/ref_pre_ENG_84_130_104.htm

 

[keithkyll]

SSR's need a power supply to operate the internal circuits on the output side. The input side doesn't need power. The input drives an LED in an optoisolator. The output side needs power to drive the SCR's or Triac. It 'borrows' from the load. For an AC SSR, the voltage drop is about 4vac. That 4 volts is used to power the SSR internally. With 120vac and up, stealing 4 volts isn't a problem.
For low voltage DC, the designer needs to do better.
The Crouzet product that zam mentioned looks like the perfect solution.
It only 'steals' 1.6 volts for operation (specified as Voltage Drop). It's only switching transistors instead of SCR's, so it doesn't need much.

 

[bernie_carlton]

The output of an AC SSR is typically a triac. These would certainly turn on but they require the load side supply voltage to pass through zero to turn off, not very likely with DC.

Even if you get a DC ouput isolation device check to see if the output OFF leakage current is high enough that it might keep in PLC input on anyway.

 

[rdrast]

There is no harm, as long as the leakage current through the SSR isn't high enough to trigger your inputs. In the worst case, you may need to put a resistor from the SSR output to ground to keep it loaded some.

 

[DL06]

Thanks for the input everyone. I figured that there may be some issues with the leakage current, but wanted to check here to see if there would be any other problems.

rdrast - if I do go the route that you suggested and need to put a resistor from the SSR to ground, how do I go about sizing this?

 

[paulcs]

The AD-SSR210DC won’t work. It is for switching AC devices only.

You need something like an IDC module, available from several manufacturers. Google on “idc modules” for suppliers.

You should install it like this:

IDC module wiring information

-- OR --

You can use your AD-SSR210DC and install an AC input card in your PLC. You will probably have to add a burden resistor at the PLC so SSR leakage does not cause false activation. If you use this AC method, remember that there are some minor time delays for turn ON or turn OFF since the AC switching SSRs are synchronized to the AC sine wave’s zero crossing mark. These "might" affect your high speed requirement.

 

[kamenges]

I looked at the specs for the AD relay that DL06 listed. bernie_carlton is right. The relay is specifically listed as a 'zero crossing relay'. It may not even turn on if they designed it for true zero crossing operation. It will most definitely not turn off. It also has a 50mA leakage current, which is pretty high if you are driving a PLC input.


Many manufacturers make solid state relays specifically for your purpose. You are probably better off looking into one of those. They will be designed for the lower voltage DC output and have much lower leakage current.

Keith

 

[zmanvortex]

I don't know if this really applies here, but I have used solid state relays for PWM outputs from an AD PLC and it worked fine.

I was able to make a 120VAC solenoid valve act as a transducer by adjusting the duration of the on time 300 times a second. By doing this I was able to place the shuttle valve on the solenoid in any position I wanted. The valve was running a hydraulic motor and it allowed me to have some speed control of the hydraulic motor.

Sorry about the rant, but I was impressed that it worked.

 

[DL06]

Thanks for the help once again gentlemen.

Just to give you an update, I ended up feeding some prox sensors to the PLC (luckily I had two spare conductors to each machine that I could use). Turned out to be cheaper than the solid state relays and less susceptible to dust which is abundant in the area. The sensors that I initially tried to tie off of were photoelelectrics that get dirty quite often.

So, after a few attempts, things are looking up.

Thanks again for all of the input - I have been learning a lot from all of you and it is appreciated.

 

[testsubject]

Just to add my two cents, I have been using this model for many years and it works well in accepting 4-28 VDC switching either 120VAC or 24 VDC depending on the model. We use them for TTL outputs from Strain Gage indicators to the inputs of PLCs.

[url="http://www.ciicontrols.com/products/ioinput.htm"]http://www.ciicontrols.com/products/ioinput.htm[/url]