Continous Solenoid: Normally ON vs Normally OFF? 1

[daveward]

I'll be installing a heavy duty continuous solenoid as a battery disconnect on my vehicle. Assuming there is an equal chance the solenoid will be ON or OFF (averaging 12 hours per day ON & 12 hours per day OFF over its life) does it matter whether I use a Normally ON or a Normally OFF solenoid?

Also, do I have to find a solenoid that exceeds the max cranking amperage of my battery, like 800 CCA, as an example?

Thanks for your time. Dave

 

[circuit1952]

I'm not sure what you are trying to do, but I would use a normally open (what you are calling "normally off&quot solenoid. If the switch that operates the solenoid fails open or the wire between the swith and the solenoid breaks and you are using a normally closed ("normally on&quot solenoid, how will you shut it off?

The solenoid must be rated for at least the maximum current that will be drawn, which may be less than the maximum cranking amp rating of your battery.

 

[ubrales]

Dave

Try a latching relay/solenoid with a manual override to handle unexpected malfunctions. One short signal will change its state from NC/NO to NO/NC, therefore there is no continuous current draw for the coil. As far as sizing this device - the starter draws up to 300 Amps, however, check the rating on the starter solenoid for the answer. If you are engineering an anti-theft device, look into a simple fuel shut-off solenoid. This can disable the vehicle in 3 minutes or less, in mid traffic where the thieves are unlikely to start any troubleshooting.

Ubrales

 

[daveward]

Re posts from circuit1952 and ubrales: thanks for the ideas! I can see why my terminology was wrong...it does make more sense to use the normally open/normally closed terms. Thanks for the caution on unexpected failures...something important to consider. And I like the fuel shut-off idea, but will hold off on that unless my solenoid idea doesn't pan out.

My number one purpose is to have a remote and easily accessed switch to disable the battery from either 100% or 99% of the car 's electrical functions. This will act as an anti-theft device, but more importantly will allow me to easily disable the very remote, very hard to get at battery (not impossible to get at, just not convenient to unhook the cables at the terminals every time I want to tinker). And I've looked at the big cable sized battery disconnect switches, but would like something tiny/inconspicuous rather that clunky and obvious.

If I were looking for a "latching" relay, is that the name they would be under in catalogs? I haven't seen that term before. Thanks again to both of you. Dave

 

[carnage1]

I agree with the normaly open relay for all those resaons and one more a normally closed relay would require a continues current draw any time the car was shut off, so imagine leaving it for a weekend

as for latching relays you can buy them that way or construct a circuit that makes it latching but either way I definately recommend it, also try to make sure the relay you select won't open and close due to the vibration of the car. (not likely on a high amp relay but something to keep in mind)

p.s. I vote for a spell check im an elecrtrical engeneering tech, I can't spell.

 

[76GMC1500]

I would just splice a switch into your fuel pump's power wire, it's much cheaper than buying a solenoid and will do just as well at preventing your car from being stolen. That is what you are trying to accomplish?

 

[Doug020668]

You can actually buy Isolator solenoids in 12v & 24v to do this job. They draw little current and will handle 800-1000A easily. Isuzu & nissan trucks do it, most jap earthmoving gear does it, and Ambulances in Western Australia use the 12V ones as a means of linking the main and auxillary batteries together for charging & emergency starts.

Otherwise, you could use the continuous solenoid idea, and parallel a Starting type solenoid across it only during cranking. Other ideas come to mind but its too hard to describe them in text.

Hope this helps

 

[daveward]

Doug020668...thanks for the idea. I've just taken a quick look at isolator solenoids.

If I get your idea, I'd connect the isolator solenoid to my new convenient switch, where the purpose of the new isolator solenoid is to interupt the 12V+ heavy 1 gauge wire as it leaves the battery. So when the new convenient switch is OFF (all battery related functions disabled by the inactive isolator solenoid), the car cannot start, and all usually hot terminals will be dead, even though the battery terminals have noit been touched. When the new convenient switch is ON, the isolator solenoid allows all normal electrical activity AND alllows the ignition switch to start the car. Do I have it?

In my search this morning I saw 200 amp units. I'll keep looking for more powerful ones, unless you can recommend a source. Thanks very much.

Dave

 

[daveward]

As a follow-up to my research, I've found a nice latching relay ($39) advertised specifically as a "remote battery disconnect relay". Its specs include: 100 amps continuous, 500 amps for 30 seconds. I called the manufacturer's tech support and after repeating my question several times I'm still unclear whether a battery with 800 cold cranking amps will blow this solenoid.

The solenoid will be spliced into the +12v battery cable very near the battery terminal. The tech rep said that "because I won't be energizing its coil" when I crank the starter, the new relay should be fine. Sorry...I just don't get it.

When the specs say 500 amps for 30 seconds it makes me think they're talking about the time it takes to start the vehicle...yep, it'll be started in less than 30 seconds, so I'm covered there, but...as far as the maximum amperage...can someone tell me (in english) why I shouldn't be concerned that there's a difference between 500 and 800? Doesn't the 800 amps travel through my brand new $39 500 amp relay when the starter cranks? What is it I'm missing?

Is it because its the starter solenoid that has to be able to deal with 800 CCA?
Is it because the starter won't draw 800 amps even though the battery could supply 800?

Thanks.

Dave