AVR-Chat

"Speaking of which, remember these are open contactors.  If you need to 
avoid sparks you may need to enclose them."

Remember, too, that if there are combustible fumes you'll need more than 
just an enclosed contactor.

How about your batteries, Steve?  Are they in an enclosed space?  If so, 
you might want to have some sort of air exchange to ensure you don't 
have a buildup of gas in the enclosure.  An AGM battery, although 
"sealed" will still produce gas at the end of its charge cycle and if 
the internal vapor pressure gets too high due to a high charge voltage 
they'll vent...and that's hydrogen.

If you're reversing, you might also want to make sure you bring your 
motor to a full stop before reversing polarity of drive.  That can be 
accomplished by using a slight delay between forward and reverse and 
some breaking resistors in the circuit, and an auxiliary contact on your 
contactors.  If you try to "plug reverse" it'll probably peg your fun 
meter, but in the wrong direction.


REB


Robert Adsett wrote:

>At 07:49 AM 12/15/2007 -0800, Steven Hodge wrote:
>  
>
>>Robert, thank you for your valuable comments.   "Contactor"!   I had never
>>heard that terminology.   All that is ever used in the marine world is
>>"solenoid", or sometimes "relay".  But you are right, searching on that term
>>does the trick.  I am very rapidly converging on using the SW500.  It's $10
>>cheaper than the SW200 and is higher rated and continuous duty.   I looked
>>at the SD402 but for $25 I can just add a standard marine manual battery
>>disconnect switch (meeting the amp requirements) to the SW500 to accomplish
>>the same.
>>    
>>
>
>Welcome to my world.  It's a bit of an industry separator.  In the EV 
>industry relays are low current devices and solenoids are used to open and 
>close hydraulic valves.
>
>The SW500 is probably overkill for you but that's not a bad thing.
>
>  
>
>>As for the mosfet idea, your post prompted me to estimate the cost of the
>>parts to make it (mosfets, copper bus bars, pcb manufacture, and other) and
>>it came to roughly the same as the SW500.  So that made things a no-brainer.
>>Even though now a dead idea, I do, however, want to comment that my existing
>>single-mosfet board which carries 45 A is operating two motors that are
>>switched on/off externally.   So it presumably gets hit with inductive
>>spikes.  I do have protection diodes on it.   Admittedly currents are much
>>less than 400, but I have never had any problems with it.
>>    
>>
>
>Yep, but here is a big jump in complexity once you start paralleling 
>(larger than is immediately apparent).   With a 12V but you are probably a 
>lot further away from breakdown than I usually am but keep in mind that 
>even uS pulses will short a MOSFET after which it will either fuse due to 
>the very high current or just act as a wire.
>
>  
>
>>I would not use automotive stuff either.   Equipment rated for marine use
>>has very stringent specs, especially electrical.   A fire on a boat is an
>>extremely serious event.
>>    
>>
>
>Speaking of which, remember these are open contactors.  If you need to 
>avoid sparks you may need to enclose them.
>
>  
>


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