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From: "red rover"
References: <3E2476F6.firstname.lastname@example.org> <7z3V9.3139$%W2.email@example.com> <firstname.lastname@example.org>
Subject: Re: Selecting a heatsink?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Thu, 16 Jan 2003 00:21:40 -0500
NNTP-Posting-Date: Thu, 16 Jan 2003 00:19:44 EST
Organization: Bell Sympatico
"Fritz Schlunder" wrote in message
> Err... MOSFETs are cheap these days and you can get a lot of bank for
> buck with them. Relays on the other hand have limited switching
> expenctancy, subject to strong vibration upsetting performance, often cost
> much more than a comparable MOSFET, are physically very large, make
> noise when switching, have slow switching performance, aren't readily
> available for very high current low voltage use, and consume non-neglible
> drive power...
I only commented that a switch or relay could save power which is a valuable
commodity on a sail boat. The OP said he "will turn it on once a day" so I
see switching life cycle will be a problem. Switches and relays can have
shock/vibration performance but it isn't really needed here. A sailboat
have high enough shock to bother any relay/switch. A switch that can handle
these currents could be very small and a relay would only be required if you
want the switch activated by some electronics or wanted to run small wire to
control switch. You can get cheap automotive relays as well as pcb relays
are sealed and good for 100,000 cycles. Audible noise probably isn't a
in this application nor is switching time. Of course if he could use a
the power consumption is practically zero. The relay could use several 100mW
Still less than his proposed solution and probably smaller when you throw in
heatsink and pcb (you could get relay and switch packages that wouldn't
and additional pcb or housing).
> A much better question in my mind is why doesn't the OP use a lower on
> resistance MOSFET such that no heatsink is necessary?
> A MOSFET such as the IRFZ48V
> (http://www.irf.com/product-info/datasheets/data/irfz48v.pdf) or similar
> MOSFET could very easily handle 6.5amps DC continuously with no heatsink
> other special thermal provisions at all (even with significantly high
> ambient temps, say over 80 celcius).
This does look like a much more suitable part. The rds would probably be
18 milliohms (due to the temperature effect you mentioned). The power
dissipation would be about .8W which is not insignficant without a heatsink.
Tjunction would be about 47C over ambient so in my earlier guess of a 55C
the junction would be a cool 102.2C. However, be aware the case would be
so you wouldn't want to touch it.
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