Editing Relays

Jump to navigation Jump to search

Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.

The edit can be undone. Please check the comparison below to verify that this is what you want to do, and then save the changes below to finish undoing the edit.

Latest revision Your text
Line 7: Line 7:
  
 
=== Description ===
 
=== Description ===
A PIC can't source more than 25mA, so how do I turn on/off a light bulb? With a relay!
+
A PIC can't source more than 25mA, so how do I turn on/off a light bulb? With a relay!  
 
 
A PIC can't source more than 25mA, so how do I turn on/off a relay? With a transistor!
 
  
 
Relays are those large black boxes that go 'click'. There is a coil inside. When current flows through the coil a magnetic field causes the the internal paddle to move postions. This paddle usually carrys large currents or large voltages.
 
Relays are those large black boxes that go 'click'. There is a coil inside. When current flows through the coil a magnetic field causes the the internal paddle to move postions. This paddle usually carrys large currents or large voltages.
Line 21: Line 19:
 
=== Relay circuit setup ===
 
=== Relay circuit setup ===
  
Many microcontrollers cannot source enough current to feed the control coil. So Q1 is any old BJT (2N3904 is a good one) to turn on/off the relay.
+
When switching large loads, you can get something call ''fly-back current''. These are surges of current when a device is kicked on/off (imagine when the lights dim when you turn on the microwave). Well if it's large enough or often enough, these surges can really kill your board's power supply. The fly
 
 
When switching large loads, you can get something call ''fly-back current''. The fly-back current can cause surges of voltage when a device is kicked on/off (imagine when the lights dim when you turn on the microwave).
 
These surges can arc and destroy nearby components.
 
  
 
[[Image:Relay-Example.jpg|Example Relay Circuit]]
 
[[Image:Relay-Example.jpg|Example Relay Circuit]]
  
The diode 1N4148 (simple, cheap, can handle ~200mA current) is there to direct the fly-back current to a safe path. Without that diode, the flyback current could arc and destroy some nearby component.
+
Many microcontrollers cannot source enough current to feed the control coil. So Q1 is any old BJT (2N3904 is a good one) to turn on/off the relay. The diode 1N4148 (simple, cheap, can handle ~200mA current) is there to supress any fly-back current from entering the main power supply. Be sure to watch how you polarize your diode.
Be sure to watch how you polarize your diode.
 
  
 
=== Documents ===
 
=== Documents ===

Please note that all contributions to OpenCircuits may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here.
You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource (see OpenCircuits:Copyrights for details). Do not submit copyrighted work without permission!

Cancel Editing help (opens in new window)

Template used on this page: