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Path: utzoo!watmath!kpmartin
From: kpmartin@watmath.UUCP (Kevin Martin)
Newsgroups: net.cycle
Subject: Re: Electric vest current draw
Message-ID: <9887@watmath.UUCP>
Date: Sat, 17-Nov-84 14:11:17 EST
Article-I.D.: watmath.9887
Posted: Sat Nov 17 14:11:17 1984
Date-Received: Sun, 18-Nov-84 03:39:07 EST
References: <584@ccice5.UUCP>
Reply-To: kpmartin@watmath.UUCP (Kevin Martin)
Organization: U of Waterloo, Ontario
Lines: 49

>Question #1.)
>I have (on order) a Eclipse-brand electric vest. Does anyone what these
>vests draw, current-wise? I'm guessing 4-5 amps.
Sounds like a reasonable guess, for the purposes of addressing your
next question.

>Question #2.)
>Is there a convenient circuit to regulate the jacket temperature?
>
>If the device was placed so that it would be cooled by the wind
>(when the bike was moving), I think it would
>"regulate" the jacket temperature quite nicely.
>	Ray Downes


Let's see... If the device you are regulating with is just running as
a regular series variable resistor, it dissipates the most heat when its
resistance is equal to that of the vest. At this point, the total resistance
is twice that of the vest, the current is half of the maximum, and both
the vest and the regulator are dissipating 1/4 of the top heating capacity
of the vest.

For a vest that draws 6 amps (72 watts, R=2 ohms), the total resistance
at this point would be 4 ohms, the circuit would draw 3 amps, and both the
vest and the regulator would be generating I**2*R = 3*3*2 = 18 watts.

I guess with a good power transistor, lots of silicone grease, and a good
solid piece of metal to attach it to, the idea would work.

A somewhat less wasteful regulator would use some form of switching
(alternating between full-on and full-off, with the transistor dissipating
very little power during either state), but the problem lies in
finding a transistor (or other semiconductor device) which:
1) Has a *LOW* "on" voltage drop (transistors usually have 1 or 2 volts,
   at 5 amps, that's 5-10 watts, or 8-16% of your heating capacity).
2) Switches rapidly (since power dissipation by the transistor is highest
   during a switching transition)
3) Switches both on and off (i.e. can't use an SCR easily)

Making a continuously-variable duty-cycle oscillator with a 555 is the
easy part. Finding a device which satisfies the above constraints is tough.
I think I remember finding a Toshiba power RF FET which filled the bill,
but it did so in two ways... I think it cost about $60 for some other
manufacturer's replacement.
I'm not sure what rate I would run the thing at. Maybe 10Hz? The bike
may have instrumentation which goes fubar when there is noise on the DC
supply, and a switching device like this would certainly generate noise!

                         Kevin Martin, UofW Software Development Group