Mesa 7i77 output

I have a problem with the coolant pump circuit on my mill. I am using Mesa 5i25 + 7i77 on a Bridgeport Boss 8 which I finished converting a couple of years ago. I have a push button linked to a field input on the 7i77, then a field out drives a 24v relay which operates my 240v coolant pump. The functionality used to work, but occasionally gave the error shown in the attached image.

I asked on this forum at the time and PCW suggested the relay arcing was causing EMI, and that I should have arc suppression across my relays. Stupidly, at the time I did nothing, then left the machine for a couple of years. When I used it again this morning, after turning on the coolant pump I could not turn it off. Hal Show, shows the signal turning on and off when I press the button, but the output of the 7i77 is 24v constantly. As a test, I moved the wire to a different output on the 7i77 and updated the HAL file. Same thing happened again, and now both outputs are 24V constantly. Assuming these two problems are related, have I damaged the 7i77?? All other pins are still working.

Some quick googling "arc suppression" suggests a small resistor in series with a capacitor, connected across the relays. Can anyone suggest what value components to use?

Many thanks,

There are actually two separate issues here

1. Relay contacts that switch significant inductive loads should have arc suppression devices across the contacts
if you don't have arc suppression you will generate large EMI pulses whenever the relay disconnects the load
this is not like to damage the 7I77 however

2. If the relay is large enough (>60 ma), it needs to have a flyback diode across its coil
You may have damaged the driver circuit on the 7I77 if you switched a larger inductive load
without a flyback diode.

A diode across the coil of the relay will protect the 7i77 from spikes or as you say a snubber circuit on the motor side will protect the relay contacts which sounds like to me has been welded together... .1uf cap and 120 ohn resister works with most cases but there is a calculation that can be done. You need to know the motor inductance.

A solid state relay would work the best.

Cycle the power of the 7i77 and the computer and disconnect the relay. Power up, and check your outputs then if they dont come on I would say you burnt the outputs. PCW can confirm that.

Just my 2 cents.

oops, PCW already answered it....

Hi, and thanks for such a fast response.

This is the relay i'm using

I've attached an image of the wiring diagram, it shows a diode across the coil, so I guess it's already built in?

14ma relay and it already has a diode across it.

looks like to me that relay is getting hammered from the 220. Probably needs a snubber circuit.

Hi.

I power cycled the PC and machine this morning, but sadly nothing changed. As soon as I power the machine on I get 24v on the output pin of the 7i77. I think I have damaged it.

Tom_R2E3 wrote:

Hi.

I power cycled the PC and machine this morning, but sadly nothing changed. As soon as I power the machine on I get 24v on the output pin of the 7i77. I think I have damaged it.

you get 24v only on the one output? all the others are ok? If so, switch outputs and declare that output defective.

Check your relay, maybe it has failed in some way or your wiring might have a short somewhere that it is killing your outputs.

If you have a bad driver output, the whole NCV7608 chip
has got one foot on a banana peel, best to replace it

You should also check carefully if any other loads on that
chip could be missing flyback diodes, I dont think EMI
could damage a driver chip unless it was pretty horrendous

That terminal shows a diode is internal to the terminal from what I see.

I have some outputs on my VMC that energizes a relay and I have diode on it, and when I release the relay, the 7i77 locks up and i have to cycle power. I put snubber circuit on the contact side of the relay and it fixed the issue. It is however a highly inductive load and I am probably lucky the 7i77 lived through it.