Flyback diode mesa 7i95t

Hi peeps,

So I have a mesa 7i95t and I'm using two of the ssr outputs to operate a relay board that in turn operates some solenoids.

What i am experiencing is. Occasionally when the relays are opened and closed quickly I'm getting some EMI when the coil collapses causing my monitor to turn off for a second. and the occasional estop.
I know I need to add in some flyback diodes to stop this. But......where do I put them? Usually you would go across the positive and ground. But my relay board has 5 channels. So it has a main positive and ground
 then a separate ground for each channel that comes from the 7i95t output pin. Do I put the diode across the positive and signal from the 7i95t or across the positive and relay ground? I will attach a pic of the relay boards and a very bad sketch of what i mean.


Thanks in advance    

Thanks. I have a load of 1n4148s the problem is between which pair as the relay board has two grounds and one positive. One ground is from the psu and the other is from the ssr on the mesa board.

I have tied going from the psu positive to the mesa ground pin and it hasn't stopped the emi.

Yes, as tommylight says, the solenoids need flyback diodes as well as the relays.
( or if they are AC solenoids the relays need arc suppressors )

Do the solenoids draw more than 2A DC?  If not, they can be driven directly from the 7I95T
 

The solenoids are ac 120v. So I can't use a flyback diode on them. Its the relay coil that is giving me spikes going back to the mesa 7i95t. So I need to stop that.

They are 120v ac so unfortunately can't use flyback diode.

Then you need arc suppressors across the relay contacts.

Possibly something like this:

www.digikey.com/en/products/detail/bourn...OV-10D201KTR/2408223

Note that it looks like the relays have their own drivers, so the 7I95T does not need
any flyback diodes.

The issue here is that the relays are switching a 120VAC solenoid.

Because the solenoid is an inductive load, this will cause massive EMI unless
there is contact arc suppression across the relay contacts. This can be done
with a bipolar TVS diode, a MOV or an RC arc suppressor.

Some do some don't, you'll need to check the data sheets.
The role of the diode is to protect the driving circuit from the reverse voltage generated from the collapse of the magnetic coil. No flyback will eventually cause component failure, a far bigger issue than EMI.
I'd take PCW's advice.
Another thing I would look at is the routing of your wiring and ensuring physical seperation of control and AC, if you haven't already taken care of this issue.
If you do use optos make sure that the input side and output side are powered by seperate PSUs and the ground are not connected, it aggravates me when I see opto circuits with the input and output sides connected to a common ground.