Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Basic information about relays

Status
Not open for further replies.

mince

Member level 5
Joined
Feb 19, 2005
Messages
82
Helped
10
Reputation
20
Reaction score
4
Trophy points
1,288
Location
Texas
Activity points
1,894
Can someone please explain some basic stuff about relays? Like how much voltage you need to turn it on, maximum current, etc.

Thanks.
 

Relays

Depends WILDELY on the relay itself. Logic level relays can be triggered on as little as 5volts and 30ma's. These are very low level signal relays though, not really rated for much current. The stuff you can pull off of common PCB's are generally rated for 9-12 volts and 50ma's or current draw, though latching relays can save power. Also issolation can be a problem if you're using a relay for switching much higher voltages. You also have to watch out for inductive kickback from a relay coil. You might be tempted with a 5volt low current draw relay to drive it directly from TTL logic, and then find out later that your circuit's I/O pin gets fried. Because when the coil is turned off that magnetic field collapses, and considering the rise and fall time of modern TTL logic this can cause a monstrous spike voltage that will fry connected micro controllers or TTL logic device. The BARE minum protection is a diode connected in reverse across the coil so that when the voltage is disconnected the diode shows up as a dead short for the reverse current that the voltage spike generates, and as large a resistor as you can manage between the triggering source that will still trigger the coil so that the current is as low as possible. I've heard people go to some rediculous length to avoid voltage spikes from a relay coil.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top