## Ohms Law application

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simf14
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### Ohms Law application

Kind Sirs:

I would like to drop 3.3V, 3As down to 1V, 3A with a ceramic resistor if possible. Could someone show me the correct way to figure out the proper value of the resistor and its watts?

Thank you.

MrAl
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### Re: Ohms Law application

simf14 wrote:Kind Sirs:

I would like to drop 3.3V, 3As down to 1V, 3A with a ceramic resistor if possible. Could someone show me the correct way to figure out the proper value of the resistor and its watts?

Thank you.
Hi there,

Subtract and divide, R=(3.3-1)/3=0.766666 ohms.
The power is P=I*I*R=6.9 watts, so use a 10 watt or better 15 watt resistor.

You can get pretty close to this required R value by connecting three
resistors in parallel: a 1 ohm, a 5 ohm, and a 10 ohm. The 1 ohm
should have a 10 watt rating, the 5 ohms should have a 2 watt rating,
and the 10 ohm should have a 1 watt rating.

You should realize however that in using a resistor lots of power is wasted.
You should also realize that if the load (at 1v) does not draw the 3 amps
all the time (maybe 1 amp during startup) then the output voltage will
be higher than 1v.

Another thing to think about is that when using 5 percent tolerance units
the range of total parallel resistance can swing from 0.73 to 0.81 ohms, so
you may have to adjust the final value if you are looking for a very
accurate value.

Perhaps you can tell us about what this will be used for so we can give
LEDs vs Bulbs, LEDs are winning.

simf14
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Hello Mr. Al!!!

I am trying to decrease the output of a 120V AC/DC 3-12V variable output, 4A switching power supply down to around a 1 V-8V range.

I tried attaching a 10 Ohm, 10W ceramic block resistor but this did not drop the voltage at all when measured by an Ohm Meter. Could my resistor be bad? Or do I need a load on the circuit when I tested it?

philba
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Are you trying to drop the voltage to supply power to a circuit? What is it?

I think you will find it hard to maintain your target voltage. You will be better off building a variable voltage regulator based system.

The problem comes about because to get your 3V down to 1V you will need a voltage divider but you have to factor in the draw of your circuit. The voltage will change with the circuit's current draw. If it's constant then it could work.

simf14
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Thank you guys, your right. I am going to build something with a LM330 power transitor.

Thanks again.

MrAl
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Hi again,

That chip doesnt sound like it can put out enough current for your
application. Did you check the specs on that chip?
LEDs vs Bulbs, LEDs are winning.

simf14
Posts: 24
Joined: Sun Jul 29, 2007 2:00 pm
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will try a LM338K...5A with 7A peak...should be plenty...thanks MrAl!!!

cp out

MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
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Hi again,

Guess what? One little more gotcha with some regulators...
that is, they only go down to 1.25v without a negative bias on
the lower side of one of the voltage set resistors.
If 1.25v instead of 1.00v is ok then you have no worries.
I think for that connection you connect the ADJ terminal to ground,
but still making sure you have the minimum load (100 ohms on
the output should do it).
LEDs vs Bulbs, LEDs are winning.

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