Ohms Law application

[simf14]

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]

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
some more informative advice and even perhaps a better solution.

[simf14]

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]

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]

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

Thanks again.

[MrAl]

Hi again,

That chip doesnt sound like it can put out enough current for your
application. Did you check the specs on that chip?

[simf14]

will try a LM338K...5A with 7A peak...should be plenty...thanks MrAl!!!

cp out

[MrAl]

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).