# Which is right size resistor

Discussion in 'The Projects Forum' started by mrel, Oct 27, 2010.

1. ### mrel Thread Starter Senior Member

Jan 20, 2009
135
1
Hello
Have 6 volt battery , device call for 3volts at 350ma .
Could it be a 10ohms resistor at 25 watt or am I wrong.?
mrel

2. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
104
You don't mention what the device is.

If its current varies, there is no way for you to use a fixed resistor to get exactly 3V.

Can you consider the use of a 3.3V low dropout voltage regulator instead?

3. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,227
Perhaps a LM317 could do the job here.

4. ### Wendy Moderator

Mar 24, 2008
21,417
2,948
A regulator is definitely better, however...

3V ÷ .35A = 8.57Ω , R = E/I

3V X .35A = 1.05W , P = E I

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
727

The 350mA is the Maximum current the device will draw, usually the average is less than that.

Trying to use a resistor voltage divider for power wouldn't work out too well.

As others have already stated, an LM317 or other regulator would be the way to go.

6. ### mrel Thread Starter Senior Member

Jan 20, 2009
135
1
thatone guy
Do you have that circuit plan for using the LM317
mrel

7. ### mrel Thread Starter Senior Member

Jan 20, 2009
135
1
I am using 6 volts battery on a dc to dc conventer that need 3 volts at 350 MA so am I doing right thing using resister to drop the voltage from 6 volt to 3 volts and 350 MA
Is there better way to drop voltage from 6 volts dc to 3 dc volts at 350 MA?
mrel

Mar 24, 2008
21,417
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9. ### wayneh Expert

Sep 9, 2010
15,004
5,454
I agree with Bill's picture, worth a thousand words.

You could alternatively drop the 6v thru 4 diodes to get ~3.2v (exact value would depend on the diode choice). Not very elegant, but I suppose it would work.

10. ### mrel Thread Starter Senior Member

Jan 20, 2009
135
1
Bill Mareden
What is valve of R2 looks like adjustable resistor.
mrel

11. ### Wendy Moderator

Mar 24, 2008
21,417
2,948
It is meant to be an adjustable power supply, however, put a fixed resistor in there and it would work very well for your application. You could also build it as is and adjust it for the voltage you need.

For 3V a 180Ω in parallel with a 2.7KΩ (both being R2, 168Ω) would create 3.008 volts, ± the tolerance of the chip and the resistors.

12. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
It's kind of a shame to use a linear regulator to step 6v down to 3v for use as an input to a DC-DC converter. The efficiency will be less than 50% due to the power dissipation in the regulator and R1/R2 divider.

There are some very interesting all-in-one IC packages available nowadays that are switching buck regulators; they just need caps on their inputs and outputs - and they're tiny. However, QFN and similar SMD/SMT packages are somewhat difficult for hobbyists to use; you almost have to make a PCB.

Just a couple of examples:
http://www.mouser.com/ProductDetail...r/FAN5362UC29X/?qs=l8TZ72NkpXmSWLFFmL%2bWeg==
http://www.mouser.com/ProductDetail/Enpirion/EP5352QI-T/?qs=AG1tZYOK7s50PQLZ9ZDk7A==

Happened to run across the VIPER16 family during a search; pretty interesting switching regulator in a 7-pin DIP package.
http://www.mouser.com/ProductDetail...IPER16HN/?qs=sGAEpiMZZMthiYuEY6QoeRtlBkWtHgST
Kind of complex for a beginner to attempt using though.