AVR-Chat

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.

>As for the battery, well, what can I say?   It was recommended by a person
>who installs about 60 thrusters a year in boats.   The thruster specs only
>call for a minimum of 800 CCA and this is 1100.   There are probably 3
>issues that play into this.  The first is that typical use, as I said, is
>one to a few bursts each lasting 5-15 seconds or so.

  That does make a difference.  It's only a few multiple of automotive 
start times.

>Continuously on for
>periods of one to a few minutes are the exception.

That's the one that was worrying me.  Discharging at (a guess) > 4C is not 
a good thing to do to a battery.  At the short duration though that is what 
automotive batteries are optimized for.

>The second is that the
>thruster does not get used anywhere near as frequently as a normal starting
>battery.   In a year, in my case, maybe a couple of dozen times.   The third
>is that weight in the bow of any boat (where this necessarily has to be,
>because it moves the bow sideways) is to be avoided as much as possible.
>Thus it's better to live with under-rating it for the extreme circumstances
>rather than have to always suffer the negative consequences of the extra
>weight of an additional or larger battery.  (The thruster itself weights 60
>lbs.)

That leads me to think a DC/DC from your main battery bank might be 
worthwhile.  It would keep the thruster battery charged up but the current 
draw would be minimal.


>Once again, thanks for your help.

Not a problem.

A couple of items to keep in mind

To Albright an intermittent duty coil is used on direction contactors and 
is expected to run some thing like 40-60% on with 10's of seconds to 
minutes between direction changes.    Continuous can handle more but an 
intermittent coil may well work for you.

The SW500 is a telecom contactor so a 12V coil may take extra lead time.  I 
expect it will be available but it might be uncommon.

Tek will probably question you on your selection to make sure you are 
making the right choice.  They seem quite knowledgeable from my contact 
with them and they don't do much second guessing once it's clear you know 
what you are doing.  I've had the same issue with a number of contactor 
manufacturing companies, they don't want to provide specifications they 
want the details of your application so they can pick the contactor for 
you.  It's one big reason I like Albright.

Good Fortune.

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."