Why do we need a switch before a relay?

[Hondaboi1729]

So in high current circuits which involve say a starter motor in a car we need a starter relay since a normal switch cannot handle the high current flow.
I think the reason I'm asking this question is because I don't understand what exactly makes the relay be able to handle the current unlike the switch which activates it.
Is it

a) because the actual metal in the original switch will melt or be damaged if high current yet the metal contact in the relay can tolerate much higher heat

b) Or can a relay handle it because the electromagnetism is so strong when the metal contacts in the relay switches it won't budge.

If it's a) then why do we need a switch before the relay or even a relay switch/mechanism that works via magnetism at all but instead only one strong switch with a contact that can handle the current?

 

[256bits]

It's c)
think about heavy duty cable length ( and currents ) and remote location control.

 

[Asymptotic]

If it's a) then why do we need a switch before the relay or even a relay switch/mechanism that works via magnetism at all but instead only one strong switch with a contact that can handle the current?

You could build an ignition switch with an appropriately rated contact set in the 'start' position to switch starter motor current, but:

• it would also be necessary to run two large gauge conductors to it from the battery, and from it to the starter motor.
  
• they'd need to be larger gauge than the single, high current conductor normally employed to prevent excessive voltage drop across these longer runs.
• the ignition switch would need to be physically large enough to fit the larger, high current contact set.
• all of the above pushes up costs far beyond the cost of a relay circuit.

Give it some thought, and you can probably come up with other reasons.

 

[Averagesupernova]

Doesn't everyone want one of these on their dashboard?

 

[Nidum]

In more general applications relays are used not only as power switches but also as logic elements . Not common now but the control and safety systems for many types of equipment used to be based on relay circuits .

Simplest example of this is in the No Volts Release type starting switch for a machine tool . This is basically a combination of a low power push button switch and a power relay . If there is mains power available the relay latches on when the button switch is pushed . If the mains power fails the relay turns off and stays off even when mains power is restored . It won't turn on again until operator presses button switch again . This prevents potentially dangerous equipment from starting up in an uncontrolled way after a stoppage .

In automatic machinery though the complexity of relay systems knew no limit - whole racks of them controlled all the different motions of the machine .

Multiple varieties of relay with all manner of special functions were produced specially for control systems .

The principles continue in use to this day but actual relays have now been largely replaced with all electronic Programmable Logic Control units or full computer control .

Power switching relays are still in quite common use though they are now rapidly being replaced by semiconductor devices for all except the highest power levels .

 

[OCR]

Lol...

Doesn't everyone want one of these on their dashboard?

However, that might be more pleasant... than this . ..

 

[Averagesupernova]

Lol...

However, that might be more pleasant... than this . ..

That used to be common on semi tractors. I believe the mad Max movie The Road Warrior's semi was this way if memory serves me.

 

[OCR]

... I remember them... of course, I was just a kid then. ..

That used to be common on semi tractors.

I believe the mad Max movie The Road Warrior's semi was this way if memory serves me.

Yeah, I think you're right...
Maybe I can find a video...?

 

[OCR]

That particular compressed air starter seems to be on a 4,487 horsepower diesel engine for a standby generator.

Also... see here .
Oh, and back to the original question...

Why do we need a switch before a relay?

Starter solenoid ...

 

[anorlunda]

Is it

a) because the actual metal in the original switch will melt or be damaged if high current yet the metal contact in the relay can tolerate much higher heat

b) Or can a relay handle it because the electromagnetism is so strong when the metal contacts in the relay switches it won't budge.

The answer is (a), except that most damage occurs when you try to open the switch and stop the current.
The picture below shows the type of ignition switch you would need on the dashboard if there was no solenoid.

Before the 1970s, relays had bad reputations in power plants and other process industries. Poor reliability of interposing relays was the problem. Because of that, the conservative engineers and operators preferred to eliminate the relays by directly switching the high power in the control room. SB-1 switches like the one in the picture were typical of what they used. As others in this thread mentioned, the wires to the switches had to be suitably thick.

Similar thinking also led to distrust of boiler pressure gauges that depended on remote sensors. So they piped high pressure steam to the meters in the control room. However, if the meter's glass broke it would cook everyone in the room like lobsters.