Further explanation .............. below is a dialog that I had by PM with a member....
What exactly does a solenoid do...what's its function in a siren wiring project, please?
a solenoid is an electro mechanical switch that has the ability to transfer high amperage power from one side to the other. It is either a grounded switch that you apply power to operate, or an ungrounded switch that you switch either both the power and the ground, or just one side, depending on application. An example of a solenoid on a car is the part that is mounted between the battery cable and the starter motor. Starter motors need high amperage to operate, so there is a solenoid (switch) mounted between the wire that connects the battery to the starter. This solenoid is controlled by the ignition switch. You turn the ignition switch and it sends a low amperage current to the solenoid. The solenoid then closes, and connects the two high amperage wires together.
I'm still in the dark. If it transfers high amperage from one side to the other---why is it necessary? Why not just let the current flow straight to the siren (thru a switch). In other words, what would happen if you wired a siren without a solenoid? Thanks
If you were to run the siren through the switch, the switch would burn up, since it is only rated for about 20 amps, and most mechanical sirens draws 50 - 75 or more amps, depending on model. There are some sirens that draw less amperage, however, even the smallest siren will probably draw more than the average dash switch can handle with few exceptions. You also have to realize that if you were to use the switch, even if it were designed for higher amperage, you would need to be turning it on and off to get the desired results, and usually a switch isn't mounted in a location that is convenient to operate multiple times while driving. This is why the horn ring is used to switch the siren on and off through the solenoid. There are some floor switches that are capable of handling the high amperage requirements of the siren, however, they present other problems, such as routing heavy copper cables to and from the floor switch. Most of the floor switches that I have seen installed switch the solenoid for convenience. Also keep in mind that switches are designed to be used intermittently, and operating a siren through a mechanical switch just isn't practical, or convenient.
An example of a switch that can handle the higher amperage is the Cole Hersee battery transfer switch that is commonly found in fire trucks and ambulances with dual batteries.
My experience with this stuff is from the 1960's funeral home ambulance I worked on. By 'switch' I meant push button (dash, floor). So the solenoid allows only momentary bursts of high amps thru the switch, just enough to spin the siren motor (or starter motor), right? And if you held down the dash mounted siren button too long, the switch would still burn up, right?
What you have experienced with a dash mounted push button switch, or floor mounted foot operated switch, was exactly what I have described. It was connected to the solenoid under the hood. Most people never understand what is actually happening when they push the button, only expect the siren to work when they do so. Solenoids are not new, and have been around since the 1930's. It is like the switch that operates the heater blower in your car. You move the switch, and it connects to a relay (solenoid) and turns on the blower motor. You are not aware of the relay, nor do you care. All you want is to have heat. When it no longer works, you assume that the switch has gone bad, but it actually is the relay that has gone defective. Change the relay and it works the way that it originally did. The switch itself never carries any high amperage, only enough amperage to activate the solenoid.