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Voltage free contact
Voltage free contact

How does a voltage free contact work?

1 out of 1 members thought this post was helpful...

This means that the relay contact will be free of voltage. There is no voltage will be there in the relay contacts. So you can switch to any system without any fear.

Best of luck.

1 out of 1 members thought this post was helpful...

What would be possible application of such contact? I know contacts are mainly used to break circuits. Does voltage free mean that no voltage is present on contact when it is open (no voltage at all)?

By Michael Griffin on 20 August, 2007 - 11:37 pm

It's just a relay contact or opto-isolator or transistor. You need to supply your own signal voltage. The voltage that is present is whatever you supply.

Hmm, if I understand you correctly, 90% of all contacts are voltage free. For example, if I have standard ordinary relay that has NO and NC contacts, all of them are voltage free, right? Contact that is "no voltage free" has a built in power supply or similar?

By David Adams on 21 August, 2007 - 5:24 pm

Technically, all contacts are voltage free until you connect them to a voltage source. The term "voltage free" means they do not yet have a voltage connected to them so you may provide your own based upon the needs of the machine and/or system involved. They are typically used to isolate two machines, each having it's own voltage source, from one another. That way, machine builder "A" does not care what voltage machine builder "B" uses (one machine may have 24 VDC control power and the other may have 120 VAC control power).

By Michael Griffin on 22 August, 2007 - 10:39 pm

Yes, you could say that an ordinary relay has "voltage free contacts". You wouldn't normally phrase it that way though. The term is normally used for complex devices that have relay (or opto-isolator or transistor) outputs.

For example, suppose you had a panel meter that turned on an output if a voltage reading were within limits. If it simply provided you with access to a pair of terminals which connected directly to the contacts, then these would be "voltage free". If however, there were a 24VDC power supply inside the panel meter and one of the terminals output 24VDC when "on", then it wouldn't be "voltage free".

I think you should be able to see the difference. A "voltage free output" simply closes a switch inside a device and you have to feed a voltage into it somewhere in order to get a signal back out. Again, this isn't a term that you would apply to a simple device like a relay; it is something you would use for a complex device that has relays inside it which it uses to signal (or handshake) with another device.

There are several reasons for designing devices this way. One is that this way it can be electrically compatible with a wide range of signals since *you* provide the voltage which is output. The other reason is that it is cheaper that way since the device doesn't have to have a 24VDC power supply inside it just to provide a signal output. Instead, they rely on you to provide that.

This is one of those terms which has come into use as a short hand description for a common configuration. Note as mentioned above that a "voltage free contact" isn't necessarily an actual relay. It could also be a transistor or opto-isolator.

If you had a PLC and one of its inputs was connected to a running signal from a pump and the connection was a volt free contact, how would the PLC know that the running signal was high or low? Or would this situation never arise, i.e. would volt free contacts only be used by PLC output cards so that the PLC could generate a signal to maybes to start the pump?

Any help would be appreciated.

By Michael Griffin on 28 March, 2008 - 11:42 pm

In reply to dude1977: A voltage free contact is just a switch. You would need two wires from the PLC to the pump contact.

If we assume the PLC has sinking inputs, then one wire would go to the PLC input, and the other would go to 24VDC (or whatever the voltage is). When the contact closes at the pump, the PLC input is connected to 24VDC and sees this as "on". When the contact at the pump opens, the PLC input is disconnected from 24VDC and sees this as "off".

If the PLC uses sourcing inputs, then the second wire goes to the I/O card common instead of 24VDC.

> Hmm, if I understand you correctly, 90% of all contacts are voltage free. For example, if I have standard ordinary relay that has NO and NC contacts, all of them are voltage free, right? Contact that is "no voltage free" has a built in power supply or similar? <

you have two circuits one for the relay coil and one which is "volt free" the contacts.

For example when the coil is 240v the relay contacts can be another voltage and circuit for example 12 v dc from another source.

By Curt Wuollet on 21 August, 2007 - 2:27 pm

It means that there is no voltage on the contacts,
open or closed, unless you supply it. And the
whole idea is that you can use it for whatever
you need within it's ratings. It's just like a
standalone relay as far as contacts are concerned.
These are often referred to as "dry contacts" as
well, which is a bit confusing as it is not really
related to "dry switching".



It can be used to interface with another machine that has a different voltage to the machine with the relay. For example, you want to give a machine running signal to the other machine, so you energise the relay and use the volt free contacts to tell the other machine that the system is running.

Hope this helps.

Voltage free contact means that the contact is free to carry whichever voltage you choose to feed/supply it with.

One moment you may send 5V through that contact, the next moment you may send 10V through it, the next moment 0V, the next moment 3V, etc, etc, etc. Basically, the choice of voltage to be sent through that contact at any given time is free, i.e., free choice of voltage usage through it.

Imagine you have an actuator that opens o% on oV, 30% on 3V, 50% on 5V, 75% on 7.5V and 100% on 10V.

The moment you need the actuator to open 50%, you send a signal of 5V through that volt free contact, to a PLC controller, which in turn sends (let's say, for the sake of simplicity of explanation) 50V to the actuator. The next moment you need the actuator to open 75%, so you send 7.5V through that voltage free contact, to the same PLC controller, which in turn sends 75V to the actuator. And so on and so forth...

Understand now, J.C.? :)

Joao Monteiro

A voltage free contact provided on a particular contactor does not carry the syatem voltage which basically activates that particular contactor's coil itself. It is free to be used to control another system with different voltage, obviously no contact will be voltage or potential free if it is opearting a control system.