AVR-Chat

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.     

 

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.

 

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.

 

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

 

Once again, thanks for your help.

 

Steve

 

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Robert Adsett
Sent: Friday, December 14, 2007 4:46 PM
To: AVR-Chat@yahoogroups.com
Subject: RE: [AVR-Chat] Re: ADC Vref

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."

 

__________ NOD32 2724 (20071214) Information __________

This message was checked by NOD32 antivirus system.
http://www.eset.com



[Non-text portions of this message have been removed]