Mechanical Relay Primer - Revision history

Mparadis at 20:45, 6 June 2016

2016-06-06T20:45:24Z

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			==Applications==

			===Using Relays as an H-Bridge for Motor Forward/Reverse===

			[[File:1014_1_Bridge_Diagram.jpg\|right\|450px\|link=]]

			Connect the load (typically a DC Motor) to the COM terminals of the relay. The NormallyOpen (NO) terminals are connected to the power supply (VCC), and the Normally-Closed (NC) terminals are connected to the ground (GND) of the power supply. You can toggle the corresponding output to switch the relays.

			Looking at the diagram, when LeftCtrl is enabled and RightCtrl is disabled, the current will flow from the NO terminal of relay K1 through the motor and into the NC terminal of relay K2. This will cause the motor to rotate in one direction.

			Similarily, if LeftCtrl is disabled and RightCtrl is enabled, the current will flow from the NO terminal of relay K2 through the motor and into the NC terminal of relay K1. This will cause the motor to rotate in the opposite direction.

			When both LeftCtrl and RightCtrl are disabled, both ends of the motor will be shorted to ground and no current will flow. When both leftCtrl and RightCtrl are enabled, both ends of the motor will be shorted to VCC and again, no current will flow.

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	==Conclusion==		==Conclusion==

Mparadis: /* Single Pole, Single Throw (SPST) */

2016-06-06T20:44:00Z

Single Pole, Single Throw (SPST)

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	====Single Pole, Single Throw (SPST)====		====Single Pole, Single Throw (SPST)====
			[[File:1014_2_Relay_Diagram.jpg\|right\|300px\|link=]]
	This is a simple switch with only one path for the current to follow. The relay is either designed to be '''normally open (NO)''' or '''normally closed (NC)'''. If it is normally open, the arm of the switch is held away from the contact with a spring when the relay is off and the electromagnet pulls the arm to make contact and close the switch when the relay is turned on. If it is normally closed, the arm of the switch is held to the contact when the relay is off, and the electromagnet pulls the arm away when the relay is turned on.		This is a simple switch with only one path for the current to follow. The relay is either designed to be '''normally open (NO)''' or '''normally closed (NC)'''. If it is normally open, the arm of the switch is held away from the contact with a spring when the relay is off and the electromagnet pulls the arm to make contact and close the switch when the relay is turned on. If it is normally closed, the arm of the switch is held to the contact when the relay is off, and the electromagnet pulls the arm away when the relay is turned on.

Mparadis: /* Basic Use */

2016-06-06T20:30:55Z

Basic Use

← Older revision		Revision as of 20:30, 6 June 2016
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	Relays are designed to be controlled by a particular voltage applied to the coil. If you buy a relay product from Phidgets with integrated electronics, this is taken care of for you- you'll be able to switch the relays via USB or with any Phidgets device with enough [[Digital Output Primer\|Digital Outputs]] (see table below), depending on which relay board you buy. If you are integrating your own relay, you'll want to keep an eye on the specifications. The relay coil is also specified for a particular current. For example, a 5V relay might require 70mA. If you connect this relay to a very weak source of 5V, like a digital output, the output will not be able to provide 70mA, and there will not be enough current to switch the relay.		Relays are designed to be controlled by a particular voltage applied to the coil. If you buy a relay product from Phidgets with integrated electronics, this is taken care of for you- you'll be able to switch the relays via USB or with any Phidgets device with enough [[Digital Output Primer\|Digital Outputs]] (see table below), depending on which relay board you buy. If you are integrating your own relay, you'll want to keep an eye on the specifications. The relay coil is also specified for a particular current. For example, a 5V relay might require 70mA. If you connect this relay to a very weak source of 5V, like a digital output, the output will not be able to provide 70mA, and there will not be enough current to switch the relay.

	~~(table: +mechanical_relay)~~


	~~{\|border=1~~
	~~\|+'''Mechanical Relays'''~~
	~~! Phidgets Product #~~
	~~! Connects to~~
	~~! etc...~~
	\|-
	~~\| 1014_2 \|\| USB \|\| ...~~
	\|-
	~~\| 1017_1 \|\| USB \|\| ...~~
	\|-
	~~\| 3051_1 \|\| Digital Output \|\| ...~~
	\|}


	The load side of the relay also requires a minimum current. This minimum load current (also called "wetting current") is a requirement because the electricity needs to conduct through an oxide layer that has formed on the contacts of the relay. If the minimum load current requirement of your relay is too high, you won't be able to use it to switch a signal (such as the data line on an analog sensor), you'll only be able to switch power to a circuit. If you're using a relay to switch a signal, [[#Contact Bounce\|contact bounce]] might also be a problem.		The load side of the relay also requires a minimum current. This minimum load current (also called "wetting current") is a requirement because the electricity needs to conduct through an oxide layer that has formed on the contacts of the relay. If the minimum load current requirement of your relay is too high, you won't be able to use it to switch a signal (such as the data line on an analog sensor), you'll only be able to switch power to a circuit. If you're using a relay to switch a signal, [[#Contact Bounce\|contact bounce]] might also be a problem.

Mparadis: /* Single Pole, Single Throw (SPST) */

2013-10-18T19:28:38Z

Single Pole, Single Throw (SPST)

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	====Single Pole, Single Throw (SPST)====		====Single Pole, Single Throw (SPST)====

	This is a simple switch with only one path for the current to follow. The relay is either designed to be '''normally open''' or '''normally closed'''. If it is normally open, the arm of the switch is held away from the contact with a spring when the relay is off and the electromagnet pulls the arm to make contact and close the switch when the relay is turned on. If it is normally closed, the arm of the switch is held to the contact when the relay is off, and the electromagnet pulls the arm away when the relay is turned on.		This is a simple switch with only one path for the current to follow. The relay is either designed to be '''normally open (NO)''' or '''normally closed (NC)'''. If it is normally open, the arm of the switch is held away from the contact with a spring when the relay is off and the electromagnet pulls the arm to make contact and close the switch when the relay is turned on. If it is normally closed, the arm of the switch is held to the contact when the relay is off, and the electromagnet pulls the arm away when the relay is turned on.

	While the decision to use a normally opened or normally closed relay may seem arbitrary at first, it is actually very important for certain applications. If you choose a normally closed relay, it means your load will remain powered as long as the relay coil is '''not''' powered. If some other part of your system fails, causing your relay to lose power, your load will remain powered even though it should not be. For loads that are potentially dangerous when left on (such as a heater, or a high-power load) it is best to use a normally open relay, so that in the event of a system failure, the load will remain off. On the other hand, circuits that are designed to be powered most of the time but temporarily interrupted will use less power if they use a normally closed relay. An example of this type of circuit is the radio in a car: When the engine is started, the radio circuit is momentarily switched off because the engine needs a large amount of battery power to start.		While the decision to use a normally opened or normally closed relay may seem arbitrary at first, it is actually very important for certain applications. If you choose a normally closed relay, it means your load will remain powered as long as the relay coil is '''not''' powered. If some other part of your system fails, causing your relay to lose power, your load will remain powered even though it should not be. For loads that are potentially dangerous when left on (such as a heater, or a high-power load) it is best to use a normally open relay, so that in the event of a system failure, the load will remain off. On the other hand, circuits that are designed to be powered most of the time but temporarily interrupted will use less power if they use a normally closed relay. An example of this type of circuit is the radio in a car: When the engine is started, the radio circuit is momentarily switched off because the engine needs a large amount of battery power to start.

Mparadis: /* Arc Suppression */

2013-02-22T15:13:34Z

Arc Suppression

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	For AC powered applications, you can put a Metal Oxide Varistor (MOV) across the load terminals of the relay in order to protect it from voltage spikes. An MOV will not remove the entire arc, but it will be helpful in circuits up to several hundred volts. You can buy an MOV from any electronics distributor, like this [http://www.digikey.com/scripts/dksearch/dksus.dll?keywords=tmov20rp200e MOV] from Digikey.		For AC powered applications, you can put a Metal Oxide Varistor (MOV) across the load terminals of the relay in order to protect it from voltage spikes. An MOV will not remove the entire arc, but it will be helpful in circuits up to several hundred volts. You can buy an MOV from any electronics distributor, like this [http://www.digikey.com/scripts/dksearch/dksus.dll?keywords=tmov20rp200e MOV] from Digikey.

	For low-voltage AC applications, a bi-directional transient voltage suppression diode ("transil diode") that is rated for higher than the AC voltage of the circuit can be placed across the relay terminals in order to protect it from voltage spikes. You can buy one from any electronics distributor, like this [http://~~parts~~.digikey.com/1/~~parts~~/~~718140-transil-600w-47v-bidir-do-15-~~p6ke47carl~~.html~~ Transil Diode] from Digikey.		For low-voltage AC applications, a bi-directional transient voltage suppression diode ("transil diode") that is rated for higher than the AC voltage of the circuit can be placed across the relay terminals in order to protect it from voltage spikes. You can buy one from any electronics distributor, like this [http://www.digikey.com/scripts/dksearch/dksus.dll?keywords=p6ke47carl Transil Diode] from Digikey.

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Mparadis: /* Arc Suppression */

2013-02-22T15:13:03Z

Arc Suppression

← Older revision		Revision as of 15:13, 22 February 2013
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	For DC powered applications, one simple method of mitigating the effects of contact arcing is to place a feedback diode across the load, as pictured. This will allow some of the residual electricity in the circuit to recirculate through the load instead of contributing to the arc that forms when the switch makes the transition from closed to open. You can buy a suitable diode from any electronics distributor, like this [http://www.digikey.com/product-search/en/discrete-semiconductor-products/diodes-rectifiers-single/1376383?k=10a02 Diode] from Digikey.		For DC powered applications, one simple method of mitigating the effects of contact arcing is to place a feedback diode across the load, as pictured. This will allow some of the residual electricity in the circuit to recirculate through the load instead of contributing to the arc that forms when the switch makes the transition from closed to open. You can buy a suitable diode from any electronics distributor, like this [http://www.digikey.com/product-search/en/discrete-semiconductor-products/diodes-rectifiers-single/1376383?k=10a02 Diode] from Digikey.

	For AC powered applications, you can put a Metal Oxide Varistor (MOV) across the load terminals of the relay in order to protect it from voltage spikes. An MOV will not remove the entire arc, but it will be helpful in circuits up to several hundred volts. You can buy an MOV from any electronics distributor, like this [http://~~parts~~.digikey.com/1/~~parts~~/~~714083-varistor-thermal-200v-10ka-20mm-~~tmov20rp200e~~.html~~ MOV] from Digikey.		For AC powered applications, you can put a Metal Oxide Varistor (MOV) across the load terminals of the relay in order to protect it from voltage spikes. An MOV will not remove the entire arc, but it will be helpful in circuits up to several hundred volts. You can buy an MOV from any electronics distributor, like this [http://www.digikey.com/scripts/dksearch/dksus.dll?keywords=tmov20rp200e MOV] from Digikey.

	For low-voltage AC applications, a bi-directional transient voltage suppression diode ("transil diode") that is rated for higher than the AC voltage of the circuit can be placed across the relay terminals in order to protect it from voltage spikes. You can buy one from any electronics distributor, like this [http://parts.digikey.com/1/parts/718140-transil-600w-47v-bidir-do-15-p6ke47carl.html Transil Diode] from Digikey.		For low-voltage AC applications, a bi-directional transient voltage suppression diode ("transil diode") that is rated for higher than the AC voltage of the circuit can be placed across the relay terminals in order to protect it from voltage spikes. You can buy one from any electronics distributor, like this [http://parts.digikey.com/1/parts/718140-transil-600w-47v-bidir-do-15-p6ke47carl.html Transil Diode] from Digikey.

Mparadis: /* Arc Suppression */

2013-02-22T15:10:41Z

Arc Suppression

← Older revision		Revision as of 15:10, 22 February 2013
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	The main cause of failure for mechanical relays is electricity arcing across the contacts during switching. To lengthen the lifespan of your relay, you can add various circuit elements that mitigate arcing.		The main cause of failure for mechanical relays is electricity arcing across the contacts during switching. To lengthen the lifespan of your relay, you can add various circuit elements that mitigate arcing.

	For DC powered applications, one simple method of mitigating the effects of contact arcing is to place a feedback diode across the load, as pictured. This will allow some of the residual electricity in the circuit to recirculate through the load instead of contributing to the arc that forms when the switch makes the transition from closed to open. You can buy a suitable diode from any electronics distributor, like this [http://~~parts~~.digikey.com/1/~~parts~~/~~1850825~~-~~diode~~-~~std~~-~~rec-10a-100v-r-6~~-10a02~~-t.html~~ Diode] from Digikey.		For DC powered applications, one simple method of mitigating the effects of contact arcing is to place a feedback diode across the load, as pictured. This will allow some of the residual electricity in the circuit to recirculate through the load instead of contributing to the arc that forms when the switch makes the transition from closed to open. You can buy a suitable diode from any electronics distributor, like this [http://www.digikey.com/product-search/en/discrete-semiconductor-products/diodes-rectifiers-single/1376383?k=10a02 Diode] from Digikey.

	For AC powered applications, you can put a Metal Oxide Varistor (MOV) across the load terminals of the relay in order to protect it from voltage spikes. An MOV will not remove the entire arc, but it will be helpful in circuits up to several hundred volts. You can buy an MOV from any electronics distributor, like this [http://parts.digikey.com/1/parts/714083-varistor-thermal-200v-10ka-20mm-tmov20rp200e.html MOV] from Digikey.		For AC powered applications, you can put a Metal Oxide Varistor (MOV) across the load terminals of the relay in order to protect it from voltage spikes. An MOV will not remove the entire arc, but it will be helpful in circuits up to several hundred volts. You can buy an MOV from any electronics distributor, like this [http://parts.digikey.com/1/parts/714083-varistor-thermal-200v-10ka-20mm-tmov20rp200e.html MOV] from Digikey.

Mparadis: /* How it works */

2012-10-16T21:27:51Z

How it works

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	You should be careful when using mechanical relays in applications that require very frequent switching. Typically, mechanical relays can only manage one contact every few seconds- any quicker and they may begin to overheat.		You should be careful when using mechanical relays in applications that require very frequent switching. Typically, mechanical relays can only manage one contact every few seconds- any quicker and they may begin to overheat.

			===Safety===

			[[File:relay_safety.jpg\|thumb\|300px\|link=\|Two circuit diagrams showing the improper and proper ways of switching mains electricity with a relay.<br />[[Media:relay_safety.jpg\|Full-size Image]]]]

			Since relays switch high currents and voltages, standard electricity safety precautions apply. Make sure you never touch the terminals while the relay is powered.

			When placing a relay in a circuit, it is always a good idea to put it between the power supply and the load, especially when using higher voltages. If the relay is instead placed between the load and ground, the circuit will still work the same, but when the relay is open, the load will still be directly connected to the power supply. This could cause safety concerns because someone might touch the terminals on the load, thinking it's safe because the device appears to be off. If the electricity finds a path to ground through their body, they will be electrocuted. If the relay is placed between the power supply and the ground, electrocution would only be a risk if the live terminal on the relay is touched. Again, the relay terminals should always be properly covered to avoid the risk of electrocution.

			When an mechanical relay fails, it often fails permanently closed. This is because the contacts can actually weld to each other. See the section on [[#Arcing, Interference, and Sticking\|Arcing, Interference, and Sticking]] for more information.
			This means that as long as the power supply remains on, the load will be powered, possibly creating a fire or safety hazard.

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	==Choosing a Relay==		==Choosing a Relay==

Burley: /* Products in this Category */

2012-08-10T18:30:40Z

Products in this Category

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	Contact arcing and switch bounce can be an obstacle for some applications, but both are common problems and can be worked around fairly easily.		Contact arcing and switch bounce can be an obstacle for some applications, but both are common problems and can be worked around fairly easily.
	Mechanical relays don't last as long as solid state relays, but if you take care of them, they can be a good cost-effective alternative.		Mechanical relays don't last as long as solid state relays, but if you take care of them, they can be a good cost-effective alternative.

	~~==Products in this Category==~~

	* [http://www.phidgets.com/products.php?product_id=1014_2 1014_2 - PhidgetInterfaceKit 0/0/4]
	* [http://www.phidgets.com/products.php?product_id=1017_1 1017_1 - PhidgetInterfaceKit 0/0/8]
	* [http://www.phidgets.com/products.php?product_id=3051_1 3051_1 - Dual Relay Board]

Mparadis: /* Switch Type */

2012-08-02T20:31:29Z

Switch Type

← Older revision		Revision as of 20:31, 2 August 2012
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	One of the major characteristics of a mechanical relay is the design of the switch inside. A switch can generally be defined by the number of '''poles''' and '''throws''' it has. The number of "poles" refers to the number of individual circuits the switch controls. The number of "throws" refers to the number of positions the switch arm can occupy.		One of the major characteristics of a mechanical relay is the design of the switch inside. A switch can generally be defined by the number of '''poles''' and '''throws''' it has. The number of "poles" refers to the number of individual circuits the switch controls. The number of "throws" refers to the number of positions the switch arm can occupy.

	[[File:Switch_types.jpg\|border\|~~300px~~\|link=]]		[[File:Switch_types.jpg\|border\|400px\|link=]]

	====Single Pole, Single Throw (SPST)====		====Single Pole, Single Throw (SPST)====