Electronics: When do I need a relay?

[Jzero]

In short, I want to run a small (5.6") LCD monitor straight from a PC power supply. It is 12VDC-ready and it has its own internal power switch. Can I just wire it to a molex connector and plug it straight in, or do I need a relay?

In general, when is a relay necessary?
Thanks

 

[bmacd]

no need for a relay in your example.

-=bmacd=-

 

[Colt45]

relay is generally when you want to control different voltages.

eg, using a 12v supply to turn on/off a 120v item, or vice versa


or, you want a low current circut to turn on a high current circut.

 

[Heisenberg]

Shouldn't be a problem just wiring it directly. A relay is necessary when you want a small amount of current (or voltage) to control a larger amount. I.E. you want to use a transistor to switch a light bulb - a transistor can't handle that much current (or voltage) so you use a relay.

 

[AlienCraft]

Simply speaking, a relay is a switch that is made (or broken) when a control voltage is applied to another set of terminals. You could use a relay to control the on/off switch. is this what you wanted to do?

 

[etech]

Originally posted by: Heisenberg
Shouldn't be a problem just wiring it directly. A relay is necessary when you want a small amount of current (or voltage) to control a larger amount. I.E. you want to use a transistor to switch a light bulb - a transistor can't handle that much current (or voltage) so you use a relay.

Another reason to use a relay is distance. If you want to switch a large load a long distance from a switch and you don't want to run the large wire or suffer the voltage drop of the long run. You would use a relay close to the load and just run small wires from the switch to the relay.


In this case you don't need a relay.

 

[compudog]

A relay is used when you have a high current device that you want to switch on or off with a low power control circuit. For example, the horn in your car uses a relay. The switch in your steering wheel only powers the coil of the relay, which then closes, and allows the higher power horn to sound. You don't want to run the high power/current through the small contacts of your horn button as they would burn out from the current.

 

[Zenmervolt]

Originally posted by: compudog
A relay is used when you have a high current device that you want to switch on or off with a low power control circuit. For example, the horn in your car uses a relay. The switch in your steering wheel only powers the coil of the relay, which then closes, and allows the higher power horn to sound. You don't want to run the high power/current through the small contacts of your horn button as they would burn out from the current.

Unless, of course, the auto maker in German, in which case they run the entire 25 amp defroster/mirror heaters through a switch without a relay, causing the switch to burn out every couple of years. (Ah well, I'm sure it seemed like a good idea at the time...) :|

ZV

 

[Jzero]

You could use a relay to control the on/off switch. is this what you wanted to do?

That would be a cool feature, but right now I have no qualms about having to push the power button on the display to make it come on.
Thanks for the tips, everyone