Solenoid Lessons Learned

[Mike and Theresa Vaccaro]

Ron and Stu,

Recently added a solenoid to the glow plug circuit and learned a quite a bit.  Have shamelessly stolen most of this information from the Cole Hersee web site that has a great tech support section, including computer based training.  

'Course I figured this out after the fact (I had no idea that some solenoids are isolated and don't ground through the feet!)...thought some of this information might save some folks some time.  Was thinking that if you sanity check the content, it might be worth adding to your FAQ on the same subject?

Solenoid (soh'-luh-noyd) is an electrical switch operated by energizing an electromagnet.

Solenoids enable a low current control circuit to switch a load circuit that carries a much higher current

INSULATED means, the studs are electrically isolated from the solenoid housing. When referring to solenoids, insulated means the prevention of electrical current to flow from any stud to the housing (ground).  In an INSULATED SOLENOID, none of the studs are electrically connected to the solenoid housing at any point.

A GROUNDED SOLENOID is one in which one of the coil connections is connected internally to the housing.  This allows the housing to serve as a ground.  The advantage of the grounded solenoid is that it requires less wiring for the electrical system, because the coil gets its ground from the housing, when it is bolted to the vehicle chassis.

The following are some general rules to help you determine if an insulated or grounded solenoid is required.

Choose insulated:
1) If it is a highly corrosive area.
2) If the mounting location is not electrically conductive. A fiberglass boat would be a good example.
Choose grounded:
1) If your mounting location is already grounded or will be grounded to the battery.
2) If the mounting location is not exposed to corrosive elements that would contaminate the solenoid case connection to chassis ground.

The round, threaded bolts protruding from the housing of the solenoid are called studs or terminals. Studs and terminals are the same thing and are used to make the connection to whatever you are wiring.

All solenoids fall into one of two broad categories:
• Intermittent (maximum "on" time followed by a minimum "off" time)
• Continuous (unlimited "on" time)

The INTERMITTENT DUTY solenoid is a solenoid that is designed to be "on" for a limited time followed by a minimum "off" period.

A CONTINUOUS DUTY solenoid is a device that can be left "ON" continuously while carrying a specified current.

Generally speaking, the voltage of the battery used by the equipment determines the voltage rating required.  Thus a 12v solenoid is appropriate for most of our boats.  The amperage rating depends on the individual circuit.  Cole Hersee produces one 12v Coast Guard approved model that has both continuous and intermittent limits.  In other words, it can handle high amperage for short periods of time (intermittent duty) or lower amperage for longer periods of time (continuous duty).  To properly size the solenoid, you’ll need to know the maximum amperage the circuit is designed to carry.  In the case of the glow plug circuit, a typical plug will pull about 25 amps during the initial surge, and stabilize at 10-15 amps.  A solenoid rated for 30 Amps continuous should be able to handle this circuit.  Wiring hook up depends on the type of solenoid selected.  The simplest installation on the M-25 type engine is to use a grounded solenoid through bolted using the heat exchanger mounting bolts.  If the solenoid is not engine mounted, it can be of either type, however it probably makes more sense to use an isolated solenoid and run the ground from a stud, vs. the footâ€"keeping all of the electrons in a circuit designed to hold them.  There are a great number of solenoids that would work for this application ranging in price from about 8 bucks to over 40, just keep in mind that they are not all wired the same way.  

Cheers,

Mike

[Stu Jackson]

Nice report, Mike, thanks.

Here's the link to the Projects solenoid article.  

http://c34.org/projects/projects-glowplugs.html

If you haven't done this yet, I highly recommend it.  Simply as a safety feature, you can start your engine sooner (12 seconds vs 30 seconds).  I know we don't travel that fast, but sometimes seconds do count.

Given these two references, it's easy to do.

Stu