AVR-Chat

At 09:53 AM 12/14/2007 -0800, Steven Hodge wrote:
>
>
> >12V? I^2R losses will eat you alive! 48V would be better, but I'll
> >assume you're stuck with it.
>
>You got it.   Boat electrical is moving (very slowly) to higher voltages,
>but I'm stuck with the legacy 12 V.
>
> >Turning off this monster will be interesting.
>
>Well, the actual thruster control circuitry which came with the unit does
>this.  It uses solenoids and handles the actual turning on/off of the motor.
>All I have to do is provide 2 active-low signals, one for each direction
>(port or starboard).   And, as I said, protect those solenoids from
>(literally) melt-down.
>
>However, I separately need to have, for safety reasons, a remote means of
>enabling/disabling the local thruster battery power, ie, a "battery
>disconnect switch".   This is the "other story" I mentioned.    I could use
>a commercial mechanical switch but these cost around $100 and, more
>important, it would be difficult to access.  Hence the "remote" part.
>
>This switch only has to carry the 400 A, not make or break it.   One can buy
>from the thruster company a remote switch to do this, but it costs around
>$800  (yes, that is no typo).   Commercial solenoids that can carry 400 A
>continuous do not exist insofar as I can tell.

Actually they are fairly common.  You are just using the wrong 
keywords.  What you want is a contactor.  For this kind of work I prefer 
Albright.  Unlike many companies they actually provide specifications for 
their contactors

For example

An SW200
         - 250A continuous
         - 450A 30% duty cycle, ~360second on time.  They would likely 
consider this intermittent duty if I recall correctly
         - breaking current 1500A at 48V
         - < $150

An SW500 is rated to 500A, that's a telecom unit so you may have to special 
order to get a 12V coil.

I really like their SD250 which combine a line contactor with an emergency 
disconnect.

I've had good luck with Tecknowledgey

http://www.tecknowledgey.com/catalog/default.php?cPath=57

Knowledgeable and good service.  They asked me a number of questions and 
then let me specify exactly what I needed once they realized I knew what I 
was talking about when I asked for what I asked for.

>The best I have found is 250
>A, so two of these in parallel would work, but the cost would still be $250
>for two of them.

If they are automotive I wouldn't trust them.  They are almost certainly 
less capable than their rating, probably a good deal less.  I've had bad 
experiences with people putting an automotive solenoid where a proper 
contactor should be used in order to save money.  Also they don't parallel 
well under arcing conditions.  I am seriously NOT impressed when it comes 
to automotive ratings on electrical power devices, when you can find 
ratings they are usually seriously close to the edge of the devices 
capability at best.

>Thus I'm making my own using eight IRF2804 mosfets in
>parallel.  I've used one of these quite successfully for a few years now for
>a 45 A load.  These mosfets can carry up to 75 A continuous and have an
>incredibly low on-resistance of only 2 milliohms.   These babies hardly get
>warm at all with 45 A flowing through them.  I put heat sinks on but I'm not
>convinced they are necessary.

Umm, paralleling MOSFETs to carry current is not a trivial 
task.  Especially for turn on/turn off.  They are also a LOT more sensitive 
to inductive kickback than contactors. There are all sorts of interesting 
traps here.

Robert

http://www.aeolusdevelopment.com/

 From the Divided by a Common Language File (Edited to protect the guilty)
ME - "I'd like to get Price and delivery for connector Part # XXXXX"
Dist./Rep - "$X.XX Lead time 37 days"
ME - "Anything we can do about lead time?  37 days seems a bit high."
Dist./Rep - "that is the lead time given because our stock is live.... we 
currently have stock."