Random arc signal lost

More
06 Sep 2020 01:57 #180884 by snowgoer540
Replied by snowgoer540 on topic Random arc signal lost

The 7i96 manual says:

The isolated inputs use opto-isolators with a 4.7K input series resistance. This
results in an approximate current draw of 5 mA at 24V.


So I think we are still short of the required minimum resistance even at 24 volts


I couldn’t find a similar statement about the 7i76. So I resolve to measure out of curiosity ¯\_(ツ)_/¯

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 01:59 #180885 by rodw
Replied by rodw on topic Random arc signal lost

I’d think the same 100mA would apply, the reading I did suggested they pick a standard amperage value. So for 24V I’d need a 240ohm resistor... somewhere... to make the same 100mA draw. I do know my board is rated at 800mA max (7i76). I planned to measure the current amperage draw tomorrow out of curiosity.

I’m sure oxide does form, just not sure how much of an issue it is. If Jim Colt has advised these same relays have been used for years and years, again, you’d think the manual would bring it up so they didn’t see as many field issues.


Ah yes, but in the same post Tom Caudle from CandCNC said that Arc OK circuits usually have some resistance in them (but never said how much). I think the 4.7k resistance is better than the 7i76e but in my calcs (if I'm right), the circuit has about 1 amp and it needs 1.2 amps. I think the minimum current will vary with voltage (higher voltage has more power)

I think that my 1K resistor would put about 4.5 amps through the relay without affecting the Mesa card. That is well within the 10 amp relay spec.

So its possible that the manufacturing tolerances may mean that most machines will work OK but maybe you won the jackpot and were closer to the quoted spec than many others!

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 02:05 #180886 by rodw
Replied by rodw on topic Random arc signal lost
7i76e on page 18

The 7I76E field inputs have a nominal input resistance of 20K Ohms to field power ground.

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 07:48 #180906 by Aciera
Replied by Aciera on topic Random arc signal lost
This is from FINDER relays:
Minimum  switching  load:The  minimum  values  of  power,  voltage  and current  that  a  contact  can  reliably  switch.  For  example, if  minimum values are 300 mW, 5 V / 5 mA:- with 5 V the current must be at least 60 mA;- with 24 V the current must be at least 12.5 mA;- with 5 mA the voltage must be at least 60 V

Now I'm aware that different manufacturers define it in different ways. Some also state that 5V / 5mA means that the voltage has to be > 5V AND current >5mA. So take your pick.
Another thing is the possibility that a contact has been switching a large load sometime in the past and that burned the top layer of the contact thus exposing the lower layers that have then a larger minimum load. But that is of course not an issue if you get a new unit.

cheers
The following user(s) said Thank You: phillc54, rodw, thefabricator03

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 08:35 #180909 by rodw
Replied by rodw on topic Random arc signal lost
Ok, so 60 mA comes from the power formula P = I * V
I = P/V
I = 0.3/5
I = .06 A (60 mA)

So lets design a circuit that has about 200 mA of current after I have dinner!
We don't want to go over 5 watts of power as it will be harder to find an off the shelf resistor
We know it will have 2 resistors in parallel, the 4.7k via the 7i96 and the unknown pull down resistor where V = 24.

See what you come up with while I am off the air.

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 08:57 #180912 by Clive S
Replied by Clive S on topic Random arc signal lost
I think you can forget the 4.7k so 24/0.2 = 120ohms that should give about 200mA
But then you would need about at least a 5w resistor

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 09:54 #180932 by rodw
Replied by rodw on topic Random arc signal lost

I think you can forget the 4.7k so 24/0.2 = 120ohms that should give about 200mA
But then you would need about at least a 5w resistor


Yes, I agree, including the internal resistance which reduces the circuit resistance to about 117 ohms, I make it will generate 4.92 Watts. the resistor specs are typically +- 5% so I think you need to allow at least 20% headroom. RS components list some 6.5 W resistors but its probably cheaper to go for 10 W like this one.
au.rs-online.com/web/p/through-hole-fixed-resistors/1991675/

So there you go, after 6 pages going down the debouncing rabbit hole after the issue was correctly identified, we have a clear solution backed by the laws of physics.

This should be adopted on all builds. It should still work for a 7i76e as the 20 k resistance on that card won't stress a 10 W resistor.

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 09:56 #180933 by Clive S
Replied by Clive S on topic Random arc signal lost
I would use a wire wound R.

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 10:18 #180936 by rodw
Replied by rodw on topic Random arc signal lost

I would use a wire wound R.


Well, you'd drop back to a 7 W resistor but that is enough headroom.
au.rs-online.com/web/p/through-hole-fixed-resistors/2060953/
or
au.rs-online.com/web/p/through-hole-fixed-resistors/4852811/

Here, we can't just go and walk into a local store to buy stuff like that so if I wanted to fix something in a hurry, I'd probably get 2 of these
www.jaycar.com.au/68-ohm-5-watt-wire-wound-resistor/p/RR3270

But mount these resistors proud of the circuit board for heat dissipation. They will get hot!

Please Log in or Create an account to join the conversation.

More
06 Sep 2020 10:22 #180939 by rodw
Replied by rodw on topic Random arc signal lost
Actually by this this definition , the first option I suggested is probably better.

Please Log in or Create an account to join the conversation.

Moderators: snowgoer540
Time to create page: 0.201 seconds
Powered by Kunena Forum