stupid question about electrical, nothing relate to plc !

[leonlai]

I saw in a 24 V solenoid valve which has 3 wires (2 reds, 1 green); so 1 red is 24V power, another red is .... a common ??? or the green is the common or ground ??? or 1 red goes 24+, 1 red goes 24-, the green goes ground ???

 

[bce123]

1 red goes 24+, 1 red goes 24-, the green goes ground

 

[OkiePC]

Yes, the red wires are likely to be interchangeable in your case, but be aware that if there is an LED on the coil, the polarity may be important in order for the "light" to work. The coil, however, should not "care".

 

[leonlai]

thanks,

 

[James Mcquade]

i would advise adding a diode to clamp the reverse bias voltage and help save the plc output.

regards,
james

 

[Lancie1]

Now give it the old smoke test and find out if we guessed correctly!

 

[BigChrisDude]

i would advise adding a diode to clamp the reverse bias voltage and help save the plc output.

regards,
james

Hi there - I have a couple of questions...

Not doubting the need of using a clamp diode in the above configuration, but if a PLC output was used to switch a relay, and the relay used to switch a solenoid, that this would add isolation and negate the need of the clamp diode? (The solenoid power supply is also separate to the PLC power supply in my configuration)

Are there any bad repercussions if the ground wire of the solenoid isn't connected to anything at all?

Many thanks

 

[keithkyll]

The red leads are the coil. The valve is AC. The green is safety ground.

 

[Lancie1]

Are there any bad repercussions if the ground wire of the solenoid isn't connected to anything at all?

In the US, it would be a National Electrical Code violation of Article 250.4(A), among others.

 

[BigChrisDude]

Indeed, respect to all rules and regs.
In 24vDC applications where small low current solenoids are used to switch air valves, if the solenoid ground is left un-connected, does that affect the electrical noise around the circuit? - Meaning if the solenoid is properly connected to ground then the noise is minimised?
Thanks for any input.

 

[Lancie1]

Meaning if the solenoid is properly connected to ground then the noise is minimised?

Yes, but for a 24 VDC solenoid, the noise difference will not be noticable. It will work just as well with the ground disconnected. The ground is for bonding all metallic electrical devices together, so than none can develop a high voltage to ground or between each other - for whatever reason. For example, suppose there is a 400 volt AC cable lying next to the solenoid housing. After a lot of time and vibration, the insulation fails and 400 volts is applied to the metal solenoid case. If it is grounded, a breaker should trip somewhere. If not grounded, the next guy (you) who happens to touch the case will be in for a surprise.

On the other hand, if you have low-voltage shielded signal wires, and you leave the shield wire ungrounded, that can make a big difference in the noise level riding on top of the signal.

 

[BigChrisDude]

Thanks for replies so far.
I like safety so I'll ensure all solenoid ground connections are made to ground.

Any views on the isolation gained from using the PLC outputs to switch relays to then switch the solenoids, and negating the need for catch diodes on the solenoids? - Again all relating to the same 24vDC system, with separate power supplies for the PLC and the solenoid power.
Cheers.

 

[Lancie1]

The diodes are mainly to protect equipment from arcing currents. For example, a PLC output connected to a relay can receive a jolt when it switches off (relay coil can back-feed current). A diode can help protect the PLC output.

Do an Advanced Search on this site, and you will find dozens of previous discussions on this topic.

http://www.plctalk.net/qanda/search.php?searchid=2985899

 

[BigChrisDude]

Thanks for the search results - I shall definitely have a good read through those.
Indeed I always place a diode on the DC relay coils - I was just wondering whether a diode was necessary across the solenoid (on its own 'noisy' power) as the relay provides the isolation to/from the PLC output?

 

[Lancie1]

I was just wondering whether a diode was necessary across the solenoid (on its own 'noisy' power) as the relay provides the isolation to/from the PLC output?

Yes, but do you need to protect the relay contacts from arcing when the solenoid coil backfeeds as the relay contact opens? It comes down to how much insurance you are willing to buy and how much risk you want to assume. "Complete protection" does not exist.