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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Ease up on the current
Message-ID: <17660034@hpfcdj.HP.COM>
Date: 9 Nov 89 20:02:41 GMT
Article-I.D.: hpfcdj.17660034
References: <763@tardis.Tymnet.COM>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 22

>the current on it. Basically, the power (DC) comes in, passes
>through a resistor, a transistor, and drives a light display.
>The resistor and transistor get quite hot. I am using a 5 watt

The resistor is there to limit the current; increasing the resistance
will reduce the current.  When calculating the value of resistor to use
and its power rating, use Ohm's Law (V=IR) to determine the current passed
by the resistor for a given voltage across it (in re-arranged form, Ohm's
Law becomes I = V/R), and then P = I^2 x R for the power dissipated by the
resistor.  Use *at least* the next larger standard power rating (i.e., if
you find that the resistor is dissipating 3W, use no smaller than a 5W
resistor).  One problem is that the voltage across the resistor may not be
a constant for your circuit - it won't be, for example, if the light in
question is an incandescent bulb rather than an LED.

You need to provide a better description of the circuit for more specific
recommendations.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.