May 17, · Simply summarized, dry contacts open and close a separate isolated circuit, and the output power is completely removed from the input power. Meanwhile, for wet contacts, the output power is immediately supplied along with the input and is only sent to the output terminal when the switching action has occurred. Dry contacts have a power source going through them that is independent of the control circuit. It’s like a contactor or relay. You have your control voltage that controls when the the contactor or relay coil is energized, and you have the dry contacts that the coil causes to close because of the action of the coil.
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It could be stand-alone, such as a pressure switch mounted on a boiler. Or, if the contact is mounted as part of an electrical unit that has a power source, then the "dry" contact has no pre-wired electrical connection to that power source. For example, a magnetic motor starter may have an extra contact that is not connected to the control circuit that starts and stops the motor. However, when the motor starts, the "dry" contact changes state, because the mechanical action of the starter moves the contact.
Usually some other relay or device has cobtact job of starting or stopping the current. For example, a reed relay matrix switch is normally switched with all contacts dry. After the contacts are all connected, a wire spring relay contacr a supervisory scan point, through which the current flows. Dry contacts are just open or closed contacts, it has no power to it.
Wet contact is a contact closure to supply voltage or ground. A telephone circuit, as well geography what does it mean alarm circuits has some current, often 20mA. The loop usually has a resistor so if the circuit is closed or opened it will trigger the alarm. This way circuit failures are reported. In telephony it is used for signaling and it keeps all the connections from corroding and making aidible noise. Hope this properly answers your question Guru.
A dry contact is a low power contact - so low that it is not self cleaning and therefore a vacuum switch or a gold plated switch so oxidation is not a factor. Dry contacts are just two pieces of a precious metal touching or not touching. A wet contact is a blob of Mercury that wets each contact that makes the connection, or, when the contacts are pulled apart, the blob of Mercury separates and breaks the connection.
The advantage of using the mercury is that any corrosion due to arcing does not cause the connection to fail, the corrosion is just part of the mercury drop, and ix mercury moves to take it's place.
A dry contact is an electro-mechanical contact cntact is not connected to any power source or load within the ia in which it resides. It is a "spare" contact that you may use for your own purposes. Examples of outputs that are not dry contacts are: open collector outputs, CMOS output, voltage output, semiconductor output. Trending News.
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How AB's settlement foreshadows Watson's future. Tom Brady isn't a fan of the NFL's new number rule. Answer Save. It's a contact in a relay or switch that is dry, as what is the fastest growing fruit to Mercury wetted. Violet W Lv 7. Still have questions?
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Transformer Dry Contact Closure Interface Series
Jan 31, · Basically “dry” contacts is a switch that has no shared power source or supply integral to the control. A common example would be the contacts in a compressor contactor. The contactor has a 24v coil (in residential) but the power supply through the contacts doesn't have any connection to the coil. Dry contact means no energy is supplied to the contacts (completely passive and isolated). In order for the DAQ to read it, it needs to have a defined voltage, Either a pull up to the positive supply (logic 1) or pull down resistors to the negative supply (logic 0) or an additional IO. Feb 04, · Dry contacts are just open or closed contacts, it has no power to it. Wet contact is a contact closure to supply voltage or ground. A telephone circuit, .
The boiler manufacturer in this case has low voltage terminals to engage the call for heat, and high voltage terminals to engage system pumps. Dry contacts have a power source going through them that is independent of the control circuit.
You have your control voltage that controls when the the contactor or relay coil is energized, and you have the dry contacts that the coil causes to close because of the action of the coil.
So, when the coil closes the dry contacts a load voltage runs through those dry contacts to energize a motor, valve, compressor, or otherwise. Again, the control voltage that energizes the coil on the relay is independent or separate of the load voltage that actually energizes the equipment.
When the coil is energized from the furnace or air handler as indicated by the control signal in the image it closes the contacts which are wired independent of the 24v that energized the coil.
Now one of the screw terminals on the dry contacts will have the line voltage as indicated by the image , which in this case is v.
And when the coil closes the dry contacts the voltage is carried through to the other terminal which is the load terminal indicated by the image , and energizes the compressor.
Another example of what I am trying to say is found in this drawing of a relay that powers up a blower. Wet just means that the contacts indicated on that equipment have power. Wet contacts are typically powered by the boiler or other unit to energize field installed controls and equipment such as thermostats, motors, pumps.
Most High limit controls, most Fan limit controls, most pressure switches have control voltage running to them and then through them. Thermostats are wet contacts. They get their power from the circuit board and it directly influences the activity of the circuit.
Wet contacts are powered contacts that directly power the equipment. They fear the additional field wiring will be too taxing on their circuit board. So they decided it was a better idea to have the voltage they supply from their circuit board run through field installed dry contacts for both the low voltage for thermostats and such and for their high voltage contacts for pumps and motors and such.
The image above is how I wired the system. On this job there was an existing Fan Center Relay that energized the system pumps and provides the 24 volts for the thermostat circuit. The normally open contact is the dry contact that I used for the v pumps. So, one side of the contact is fed v, and when it closes it travels through the terminals to energize the pumps. I used terminals 2 and 4 on the relay as the dry contacts the manufacturer requires to energize the call for heat.
So the Fan Center Relay becomes my dry contacts to energize the system pump, and the new SPST relay I now installed becomes the dry contacts for the low voltage call for heat circuit. The dry contacts, in essence, become the thermostat for the boiler.
Now remember, wet contacts means the equipment supplies the power. So, a possible application for wet contacts in a boiler might be a set of terminals to run a circ. The boiler control will energize that terminal when the Aquastat calls for heat. If there are 2 terminals in that circuit, one will be the Hot, and the other may be the Neutral for the pump. That is what wet contacts typically do.
Alternatively, some manufactures use dry contacts in their terminal strips. Again, the manufacturer does not provide power to dry contacts, you have to provide the power to the terminals for your load. A boiler example is when there is a low voltage signal for space heat, the circuit board on the boiler might close a set of dry contacts on the terminal strip that will be rated for 3 amps at v. So, you apply v to one terminal and you can wire the other terminal to the pump.
Be sure the neutral side of the pump is wired to the boiler as well. When the call for heat is terminated, the circuit board will open the contacts shutting down the pump.
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