Auxiliary Solenoids

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« on: December 28, 2007, 11:34:34 AM »
I'm sure most of you have seen some engines/boats use an auxiliary solenoid to run the starter, but what exactly does it do? What's the purpose of it? In this article I will explain exactly what the solenoid does and why we use them.

A solenoid is nothing more than a high current relay. A relay is nothing more than a switch, but instead of being actuated by a mechanical lever like an ordinary switch, it is actuated by an electromagnetic coil. When voltage is applied to the coil, the coil becomes an electromagnet. This magnet pulls the moving contact of the switch (normally referred to as the "common" contact) down to make contact with a second stationary contact, which completes the circuit it is connected to so that the circuit can receive positive voltage, thus turning it on. When the voltage is removed from the coil, it no longer is magnetic, so it cannot pull on the switch. Thus, a spring pushes the switch open, turning off the circuit that it is connected to.

The switch is in no way connected to the internal coil, so in order for the switch to work, a voltage source has to be connected to one of the switch's terminals. It does not matter which terminal of the switch it is connected to, just as long as it is connected to one of the switch's terminals. This terminal of the switch becomes the "line" side. The device to be switched on and off gets connected to the other terminal of the switch, and this side becomes the "load" side of the switch. The other side of the device gets connected to ground.

One terminal of the coil gets connected to ground. It does not matter which terminal of it goes to ground, just as long as one of the coil's terminals gets connected to ground. The other terminal of the coil goes to a mechanical switch, and the other terminal of this switch goes to an external voltage source, being the (+) terminal of the battery.

Switches are not polarized/non-directional...meaning that it doesn't matter which terminal of the switch goes to the "line" and which goes to the "load".

On a solenoid you have a total of 4 terminals: 2 big terminals and 2 smaller terminals. Refer to figure 1 below.

The two bigger terminals are the switch and the two smaller terminals are the coil. When you apply 12 volts to the smaller terminals, there will be a complete circuit (more commonly referred to as "continuity") between the two bigger terminals. You will also hear a 'click' sound as the switch closes.

When you take the 12 volts off of the two smaller terminals, you will have a broken circuit (more commonly referred to as "no continuity") between the two bigger terminals. You will also hear a 'click' sound as the switch opens.

What a solenoid is used for is to turn the starter on and off. The bendix on the starter is also a high current relay and the solenoid is used to switch the bendix/relay coil on the starter on and off. Figure 2 below shows a typical starter circuit with an aux solenoid.

Voltage to the starter is pulled directly from the battery. When voltage is applied to the "S" terminal of the starter, a contact in the bendix closes to a second large terminal on the back of the bendix that the starter motor is connected to, allowing the starter to turn on. The aux solenoid is what turns this "S terminal voltage" on and off. When you turn the key switch to the start position, the "BAT" terminal of the key switch contacts the "SOL" terminal of the key switch, which sends voltage to the coil in the aux solenoid, thus turning it into an electromagnet. The electromagnetic coil pulls the switch in the solenoid closed, which connects the + terminal of the battery to the "S" terminal on the starter bendix.

Now you're probably wondering "Why are we using a switch to control a switch that controls a switch?"'s why.

You could use the key switch to control the starter motor's "S" terminal directly. However, there will be a lot of arcing in the key switch everytime you turn it to Start due to the high current draw of the starter bendix. On top of that, the current would have to travel down 15' - 20' of wire just to get to the key switch, then another 15' - 20' back from the switch to the starter. That's 30' - 40' of wire total that the current must flow through. The longer the wire run, the more resistance the current encounters, which causes a voltage drop to the starter's "S" terminal. An aux solenoid can function with a voltage as low as 9 volts, however a starter bendix cannot. By using the solenoid and locating it close to the engine, the "S terminal current" has a lot less distance to travel from the battery to the "S" terminal (no more than 5' total). On top of this, you usually use a super heavy gauge battery cable to feed the line side of the aux solenoid's switch, which can more than handle the current drawn by the starter bendix coil with almost no voltage drop in between. Since an aux solenoid coil draws a lot less current than the bendix coil, this eliminates the 'arcing' in the key switch, thus making it last longer.

Another benefit to using an aux solenoid is that you can use the line side of the solenoid's internal switch (the larger terminal of the solenoid that the (+) side of the battery connects to) as a master terminal to connect everything that receives power all the time regardless of key switch position. You can also connect the charge wire of the alternator to this terminal as well.
« Last Edit: December 28, 2007, 11:41:01 AM by Jetaholic »
To err is human, to forgive divine...except for running Fords ;D

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