by ZerOne » Sun Jan 13, 2013 5:27 pm
A quick 101 on relays for you.
A relay is just like a switch, only it uses power to toggle the switch, instead of someone physically jamming their finger on button to flick the switch.
With relays, these are the specs used to decide what relay is suitable for your purpose.
(Im only listing Automotive uses here, things get a little more complicated when you are talking about high voltages, actuators, and latching (flip flop) relays).
1) Trigger voltage (Volts)
This determines how many volts you need to turn the relay On.
Typically automotive relays have a trigger voltage of "around" 12V (Actually 7-8Volts, but lets not complicate things here).
Low power relays can have triggere voltages as low as 4V, and reed relays can be as low as 1.5V
For Automotive uses, 12V is the standard.
2) Current Carrying Capacity (Amps)
The higher the rating, the more current (and conversly power) the relay can handle before being fried.
For switching a camera, this is not really relevant, as a cameras composite output is around 35ma - 45ma (At 5-6Volts)
You only really need to worry about current capacity when using relays to switch high power devices such as headlights, AC compressors, Starter motors, winches, etc)
A standard Automotive relay can handle 20Amps safely. (Good Bosch relays can handle around 40Amps)
3) Number of "Poles"
A pole is very similar to a physical switch.
If you have 1 pole (Single Pole), then the relay can switch one channel or device.
If you have 2 poles (Double Pole), then the relay can switch two separate channels or devices.
E.g. If you wanted to switch a video signal on or off, you would use a single pole relay
If you wanted to switch stereo audio (either line level, or speaker outputs), then you would use double pole (2 poles),
so as the audio channels are kept separate.
4) Number of "Throws"
A Throw is like how many outputs a relay has.
A single throw (1 throw) relay will either allow the signal to pass, or not.
It has only 1 output, therefore if the relay is on, the voltage, power or signal will pass through the relay.
A double throw (2 throws) relay will either send the power, voltage, signal to one output if the relay is off,
or it will send the power, voltage, signal to the second output if the relay is on.
5) Normally Open / Normally Closed Output.
This term mainly deals with double throw relays, where it specifies if the output terminal is connected when the relay is off, or on.
Normally Closed means that the Normally closed output is connected when the relay is not powered.
Normally Open means that the Normally open output is connected when the relays is powered. (I.E. it is normally not connected (Open Circuit) when the relay is not connected to any power source.
So some examples.
A 12V SPDT relay is a relay with a 12V trigger voltage, which has a single pole (Can handle one device, channel), but has two outputs.
This relay would be suitable for your Video input for your camera.
You would connect camera 1 to one output, and camera 2 to the second output, and connect your Video Input to the relays input.
When the relay is not powered up, Camera 1 will be connected to the Video Input.
When you apply 12V to the relay, Camera 2 will now be connected to the Video Input.
I hope this helps.
(Whats the saying, you can give a man a fish, and he will eat for a day...
Or you can teach a man to fish, and he will eat for a lifetime !)