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From: davidc@vlsisj.VLSI.COM (David Chapman)
Newsgroups: sci.electronics
Subject: Re: Ease up on the current
Summary: larger resistance
Message-ID: <15385@vlsisj.VLSI.COM>
Date: 10 Nov 89 01:05:47 GMT
References: <763@tardis.Tymnet.COM>
Reply-To: davidc@vlsisj.UUCP (David Chapman)
Distribution: na
Organization: VLSI Technology Inc., San Jose, CA
Lines: 33

In article <763@tardis.Tymnet.COM> jenglish@tardis.Tymnet.COM (Jim English) writes:
>I know this is probably elementary knowledge, but I'll ask
>anyway. What would I need to do to a simple circuit to lighten
>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
>resistor, and it hasn't helped. Do I put a cap across the 
>resistor? 

The normal way to reduce the current is to use a larger resistor (in
ohms, not in watts).  Unfortunately, this also reduces the voltage
to the display.

Hmmmm, maybe not.  If the transistor is doing any voltage regulation
then it might not be so bad.  Try a somewhat higher resistor (increase
in 5-10% steps) and see what happens.  Stick with the 5W size, though.  
The wattage is the amount of energy the part can dissipate without failing.  
It doesn't _necessarily_ mean it would be cooler if it had a higher wattage 
(but you don't really want to know about thermal conductance :-).

The transistor definitely should NOT be hot, though.  You might want
to compute the current through it to make sure you're not exceeding
its limits.  The easiest way is probably to measure the voltage across
the resistor.

If you're still having problems, post the input voltage, the resistance,
and the output voltage required (to the display) and we'll take a look
at it.
-- 
		David Chapman

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