AVR-Chat

My application was for a hydraulic pump that only needed to run for a few
seconds every few minutes.

-----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Steven Hodge
Sent: Friday, December 14, 2007 11:14 AM
To: AVR-Chat@yahoogroups.com
Subject: RE: [AVR-Chat] Re: ADC Vref

Well the gotcha seems to be the continuous part.   Starter relays in cars or
trucks are only designed for a few seconds of continuous use.   The thruster
is limited in total use to something like 5-7 minutes, but it is also
required, and is designed for, being able to be actually on continuously for
a good chunk, if not all, of that 5-7 minutes.  It's like a huge starter
motor that is actually required to be on continuously for periods of up to a
few minutes.   Web searches seem to come up with many in the 200 A ballpark,
but not 400 A.   

 

Besides that the IRF2804's cost less than $4 each.   Steve

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Philippe Habib
Sent: Friday, December 14, 2007 10:34 AM
To: AVR-Chat@yahoogroups.com
Subject: RE: [AVR-Chat] Re: ADC Vref

 

Would a heavy duty starter solenoid from a diesel truck do the job for you?
I had a project a few years ago where the truck part was less than 1/4 the
cost of the commercial part.

-----Original Message-----
From: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com>
[mailto:AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> ] On
Behalf
Of Steven Hodge
Sent: Friday, December 14, 2007 9:54 AM
To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> 
Subject: RE: [AVR-Chat] Re: ADC Vref

>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. 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. 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. I use Micrel 5014/5015 drivers with them.

Thanks for everyone's help. This forum is looking to be very helpful, like
the boat ones I am on. I hope I can reciprocate sometime.

Steve

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