Problem activating relay Mesa 7i76E

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19 Oct 2020 11:38 #186568 by Sneaky
Hi brains trust, I am doing something wrong/stupid/whatever but for the life of me I cannot get my outputs to trigger a relay board (admittedly a chinese one, but still) without throwing an over current error. (error 27)
I have seen other threads on here relating to that error, I do not get it with nothing connected to the board, I have tried several different relay boards, some chinese, some from RS components here in Au. All are opto isolated solid state relays, all work if I connect them to a lab power supply.

A multimeter tells me I am getting 24v on the output when I activate it and there is nothing connected to it, so I must just be missing the point somewhere/somehow....

Am I correct in thinking that I should be able to connect the negative input of the relay to the 24v negative used for the field power input, then connect the output of the Mesa card to the positive input of the relay?

I'm just trying to turn on a coolant relay.
I have inputs for home and limits working.
I have a touch probe and tool setter working.
Why the F#$*@&#%!!!! can't I get a simple relay to work?? My lab power supply does indicate 340ma of current when the relay is activated, is it as simple as I need a low current draw SSR? Does such a thing exist??

Surely plenty of us are activating relays with the Mesa boards, somebody slap me with a fish and point me in the right direction please :-)

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19 Oct 2020 11:53 #186570 by JohnnyCNC
340ma is really close to 350ma limit per pin. You might want to try a relay that draws less current. Also make sure you have a diode across the the relay coil. Here is a helpful diagram from someone on this forum.

https://forum.linuxcnc.org/media/kunena/attachments/3278/7i76_Anschluss_2017-01-02.pdf

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19 Oct 2020 12:45 #186577 by rodw
Some of the optos and ssrs need some current passing through them and some are polarised.
For DC I've been using this one but polarity must be correct.
au.rs-online.com/web/p/solid-state-relays/8886843/

But I've never had any troubles with Jaycar din mount panel relays (be sure to add a flyback diode)

Also Ocean Controls have a good range and I use some of their blue din rail mount AC relays.

And yes take +24v field power and connect it to -24v field power.

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19 Oct 2020 13:11 - 19 Oct 2020 13:12 #186581 by rodw
You are connecting your output to the coil side of the relay aren't you?
Couple more pointers
1. Always maintain a table of I/O connections
2. Always update the table before you add a new wire
3. Always check the pin in the Halshow watch tab (does it need the -not signal?)
4. Then use a multimeter to confirm the physical pin you think you are connecting to lights up.
5. Connect your relay or input and test functionality again

A methodical approach saves a lot of time in the long run, Particularly as you start off.
Last edit: 19 Oct 2020 13:12 by rodw.

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19 Oct 2020 13:53 #186584 by Mike_Eitel
If your relais draws 340 this means at the 24v ca 8 Watt.... I think that is realy a lot. Plus the starting current is eventually higher. I would try a silly test and try with a small resistor in series. Try first with your bench supply how high the r value can be.. But this is only for the test, would never have that as solution..
Mike

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19 Oct 2020 14:39 - 19 Oct 2020 14:45 #186585 by Sneaky
Hi BeagleBrainz,and JohnnyCNC I have that connection sheet, I've used it to great success with everything else hence my frustration here.

Mike_Eitel I will give that a go with the resistor and see what I learn.

RodW I will have a look at those RS relays tomorrow and maybe compare what I see from my lab power supply with one of the relays I'm using now.

Thanks for your replies guys.
Last edit: 19 Oct 2020 14:45 by Sneaky.

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19 Oct 2020 16:02 #186590 by PCW
A normal 24VDC control relay typically draws between 25 and 40 mA
Are you sure you are driving a 24V relay?

The 7I76E outputs will flag an overload if they see a (even very short)
load of greater than 800 mA so I suspect some issue with the relay
or wiring. All 7I76E outputs are tested with a loopback fixture that
loads the outputs with 450 mA.

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19 Oct 2020 17:11 #186594 by tommylight
Also check if it is an AC or DC driven relay, driving AC relays with DC will burn them up very quickly.
And please do not drive big relays directly from Mesa 7i76E.

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21 Oct 2020 01:26 #186776 by Sneaky
So....

Some further investigation reveals that the current draw is NOT 340ma but 34ma, I misread the lab power supply but the multi meter set me straight, so that makes the "over current" error even more of a mystery?
I have tried 2 different boards from 2 different sources and I get the same error.

I have a friend who "knows a bit about electronics" (yes I know..... But desperate times call for desperate measures) who is going to have a look at it for me on the weekend.

I've also been given some opto isolated relay boards designed for use with Arduinos:
Opto isolated relay board
They take 12v inputs so I've made a simple voltage divider circuit and have 12v tapped off 24v working using my lab PS as a source, this is activating the relay as expected. Can anyone see a reason I can't use the 7176 output to provide the 24v to the divider circuit in order to step it down to 12v for the relay board?

I just know it's going to be something dumb that I have done, but if this discussion helps someone else in the future then it's a win.

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