Open Collector vs Dry Contacts

[TwoBlocked]

I have it clear in my mind what the difference is between an open collector and dry contacts, but I heard a "guru" in our outfit with quite a bit of knowledge and experience refer to them as the same thing. Perhaps others view them to be the same also?

It happened when a few of us were determining the I/O point needed for a permissive to another system. At the time I was not familiar with ABB Total Flow systems and asked if the discrete output was a relay output. A "guru" was consulted and he replied without hesitation that is was a "dry contact". I wasn't so sure because the card's I/O points were programmable to be either DIs or DOs. I didn't argue, but looked up the literature and saw that when programmed as a DO, the I/O point was a actually an open collector. Since the other system had it's own power supply, I opted to install an interposing relay, even though the other system's permissive input was sourcing. That is, the circuit would work going to an open collector, but would use a ground path to complete the circuit scince they had separate power supplies.

Anyhoo, Some of you would wonder about the background context, so I thought I would give it up front. But my real question is if it is very common for an open collector to be referred to as a dry contacts?

 

[__dan]

Collector is a part of a transistor. So the term open collector would only indicate to me it is a transistorized output. Dry contact only refers to a mechanical relay contact as I have seen it used. So I would say the guy you consulted with just made a mistake. Happens to everyone.

Not sure if guru is used in a semi derogatory manner. No one knows everything. Guys who pretend to knowledge they don't have are generally, magicians (used in a good way unless they do bad things with it).

Then there are the factory support engineers. Your question, would the transistorized output work with the other transistorized input, I would want to consult with the factory engineer. There are specs and methods of Art.

You cannot use the circuit with two different power supplies connected even if it works.

But it's possible the other card, does not (have to) use the local power supply at the card, and if the transistor arrangements are compatible, it *could be approved by the factory*, possibly with a small ballasting shunt resistor to give the field wiring loop noise, some place to go.

Possibly there is a way to do what you saw possible, but the interposing relay is a very common way to do it.

 

[TwoBlocked]

.....

Not sure if guru is used in a semi derogatory manner. No one knows everything. Guys who pretend to knowledge they don't have are generally, magicians (used in a good way unless they do bad things with it).

Then there are the factory support engineers. Your question, would the transistorized output work with the other transistorized input, I would want to consult with the factory engineer. There are specs and methods of Art.

.....

Thanks for the reply! I used the term guru in a good sense, as someone experienced and knowledgeable that others turn to. Not a factory situation here - gas field service. The input from the other system would not have to be a solid-state device, just be wired to provide DC voltage with a common return. We use these open collector outputs to pull in solenoids by providing DC voltage to the sol at all times, and the open collector output provides the return path when energized by the PLC logic. It throws typical electricians off balance because they see it as "breaking the neutral".

 

[rlundsrud]

I am going to make an assumption but I believe you are referring to a triac output versus a relay output. If that is the case there are differences between them.

A "dry contact" is typically referring to a relay output, either a form A or form C. This is a physical switch operated by a relay coil. It can switch AC or DC loads and is capable of switching larger loads than a triac.

A triac output (triode for Alternating Current) is used to switch AC loads. Early versions required the switching of the common as switching the hot would destroy the output. It is limited to the loads it can handle as compared to a relay contact.

With that said, both types require a power supply so I suppose you could call them both dry contacts. In my experience, only relay contacts are reffered to as dry contacts.

 

[hillbilly1]

Many moons ago. Sam’s Club tried switching dc through Novar IOM’s. Didn’t work out! I had to redesign the control circuit. If I remember correctly it was some refrigeration alarms? That was probably close to almost 40 years ago.

 

[rlundsrud]

Many moons ago. Sam’s Club tried switching dc through Novar IOM’s. Didn’t work out! I had to redesign the control circuit. If I remember correctly it was some refrigeration alarms? That was probably close to almost 40 years ago.

In cases where I need to switch a DC load I just add a relay that is controlled by the triac ouput. I also only switch the common as you never need to worry about damaging a triac ouput when wired that way.

 

[hillbilly1]

In cases where I need to switch a DC load I just add a relay that is controlled by the triac ouput. I also only switch the common as you never need to worry about damaging a triac ouput when wired that way.

I think that was what I did at that time, and ran the dc through the relay. I think it was a dc buzzer. The EMS prints shown the buzzer ran directly through the IOM.

 

[TwoBlocked]

I am going to make an assumption but I believe you are referring to a triac output versus a relay output. If that is the case there are differences between them.

A "dry contact" is typically referring to a relay output, either a form A or form C. This is a physical switch operated by a relay coil. It can switch AC or DC loads and is capable of switching larger loads than a triac.

A triac output (triode for Alternating Current) is used to switch AC loads. Early versions required the switching of the common as switching the hot would destroy the output. It is limited to the loads it can handle as compared to a relay contact.

With that said, both types require a power supply so I suppose you could call them both dry contacts. In my experience, only relay contacts are reffered to as dry contacts.

No, I was referring to an open collector, not a triac, but the same confusion with a dry contact could happen. I am trying to remember of anytime I saw a triac used for sinking, rather than sourcing, and any possible drawbacks.

 

[rlundsrud]

No, I was referring to an open collector, not a triac, but the same confusion with a dry contact could happen. I am trying to remember of anytime I saw a triac used for sinking, rather than sourcing, and any possible drawbacks.

I guess I learned something new on this. I have never used this type of I/O with any of the control systems I work with, I actually had to Google this one. I suppose I see why your in house "guru" called both types dry contacts given the need for a pull-up resistor to ensure proper switching of the output. I personally disagree with this but that is my opinion. To me the important part of this designation is the word "contact" which is not a component in solid state switching.