Making auto weigher. Electronics.

[Oldmacdonald]

 

[Pheasant Surprise]

Yeh, so terminals 4 and 5 are the switched relay terminals and 7 is common. Doesn't appear from the instruction you have there, that you can change the default logic from NO to NC, so you would need to use an external relay with its input wired to your weigher - when your weigher closes its contacts, your external relay energises and goes from NC to open.

May as well use a low voltage 12V coil external relay, as it's just that bit safer than switching mains voltage between the two devices. Just make sure the external really is fully mains rated on its output.

 

[Oldmacdonald]

Yeh, so terminals 4 and 5 are the switched relay terminals and 7 is common. Doesn't appear from the instruction you have there, that you can change the default logic from NO to NC, so you would need to use an external relay with its input wired to your weigher - when your weigher closes its contacts, your external relay energises and goes from NC to open.

May as well use a low voltage 12V coil external relay, as it's just that bit safer than switching mains voltage between the two devices. Just make sure the external really is fully mains rated on its output.

But is the output from the internal relay not going to be 240v? So I would need a 240 coil relay?
Or have j got that wrong somehow?

 

[Pheasant Surprise]

But is the output from the internal relay not going to be 240v? So I would need a 240 coil relay?
Or have j got that wrong somehow?

No. It's what's known as a dry (or voltage free) contact.

So there is no voltage present between either 4 and 7 or 5 and 7. There is just continuity between the terminals when the logic goes to 'closed' from 'open'.

It's up to you (within the spec of the internal relay) what voltage you place across those terminals, you could put mains 240VAC or you could just put 12VDC if your wiring it for an external relay.

 

[Oldmacdonald]

I'm gonna have to draw myself a diagram to make sense of all this!

 

[Pheasant Surprise]

I'm gonna have to draw myself a diagram to make sense of all this!

Just think of terminals (4 and 7) and (5 and 7) as two (independent) switches. That's really all they are. You only really need one pair, either 4/7 or 5/7, you choose.

The external relay will have an input coil that is either low voltage, like 12VDC or mains voltage. That is the voltage the relay needs to actually work. I suggested using a low voltage (coil) relay as its less mains AC to be messing about with.

On the output of the relay you wire the NC and Common terminals in series with your motor AC input.

When the weigher hits the magic number, it closes its contacts (switch). This closes the circuit for the external relay, thereby energising it (activating it). Its flips from NC (normally closed) to Open and hey presto your motor stops running.

Don't forger to put some fuses in the circuits, especially the mains side of things.

 

[Oldmacdonald]

So really I've got 3 circuits going on.
A 240v circuit to the control unit.
A 12v external relay circuit
And a 240v motor circuit?

 

[Pheasant Surprise]

So really I've got 3 circuits going on.
A 240v circuit to the control unit.
A 12v external relay circuit
And a 240v motor circuit?

Mmmm yeah OK.

If there was 12VDC, or something like that, going to power the control unit, then I'd say definitely use that lower voltage for the external relay as its a bit safer.

However it probably doesn't warrant having a separate 12V supply just for that external relay.

So all things considered, I'd go for the 240V relay. Fuse the input side of the relay and also fuse the output side , at say 3 times the normal motor current.

 

[Oldmacdonald]

But at the points where I feed my 240 v into the control unit I can take a feed off there, and step down and change to DC to run the external relay circuit?

Or is just taking a feed straight to a 240v relay simpler?

 

[Pheasant Surprise]

But at the points where I feed my 240 v into the control unit I can take a feed off there, and step down and change to DC to run the external relay circuit?

Or is just taking a feed straight to a 240v relay simpler?

Taking a straight AC feed would be the simplest. Just triple check all your wiring and make sure you have inline fuses. I know I keep repeating this, but mains wiring doesn't take kindly to mistakes.

 

[Mursal]

If you can find the 12Volt Aux output it will be able to run the external 12Volt relay for you, reducing the high voltage circuit, always a good thing. Probably just two pins on the connector.
Just an idea mind ........

 

[Oldmacdonald]

Ez

If you can find the 12Volt Aux output it will be able to run the external 12Volt relay for you, reducing the high voltage circuit, always a good thing. Probably just two pins on the connector.
Just an idea mind ........

Interesting thanks.

Hooked the unit up to power supply and wired in a load cell platform cannabalised from an old scales. Unit must need some sort of calibration as it showed 100 with a 2kg weight on the platform.

 

[Mursal]

And if you put 4kg on do you get 200?
Is your load cell running from the box output?

 

[Oldmacdonald]

And if you put 4kg on do you get 200?
Is your load cell running from the box output?

I'll check tomorrow. Yeah it is.

 

[Mursal]

No worries ..........
I see you have the option of a reset button. that connects across pin 8 and 9 wonder would that help, so you start from zero?

 

[Oldmacdonald]

No worries ..........
I see you have the option of a reset button. that connects across pin 8 and 9 wonder would that help, so you start from zero?

Yeah there's a reset/zero button on the front.

 

[Pheasant Surprise]

If you can find the 12Volt Aux output it will be able to run the external 12Volt relay for you, reducing the high voltage circuit, always a good thing. Probably just two pins on the connector.
Just an idea mind ........

That may depend on the variant though?

I think the instructions are rather errr “generic”, catering for several different types of unit. But you never know, the pins may be there but un-labelled, though that would be a bit naughty of them

Would be nice though, if a low voltage secondary supply was there though. Like you I’m not a huge fan of using mains voltage to switch relays. Leave that for the load side only if possible.

 

[Spudie]

Our setup

 

[Spudie]

Meant to say I stops at required weight on scales

 

[tinman]

Meant to say I stops at required weight on scales

dose it trip a limit switch or something to that effect.