Re: MMf-EMf(VMf) two ways to cause a current.


"Xtrchessreal" <markgharrison@xxxxxxxxxxx> wrote in message
news:1138420896.288876.217560@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> When designing a new electronic device you need to know the total
> amount of power dissapation of the device especially if you intend to
> build a DC power supply for the delicate circuitry within. The DC
> power supply needs to be designed for the total dissapation and then
> you need to also know the maximum current that can be supplied by the
> power supply.

You are approaching this in the wrong way. When designing a new electronic
device (as far as the power is concerned), first you see what kind of
voltage and current it needs to work. You then design the power supply to
provide the voltage needed and the current needed. Using ohms law W = E I,
you then know the wattage the power supply will be supplying.

> Specifically, current is not possible without voltage or induction so
> how can you have a voltage be created by a resistor that is designed to
> limit current?

Let's say you have a 9 volt battery. You put a 1000 ohm resistor across it.
Without making any calculations, you know that the resistor will have 9
volts across it. To know how much current is going through it, you need to
use Ohms Law, I = E / R. The resistor is limiting how much current the 9
volt battery can make go through the 1000 ohm resistor. To know how much
power the resistor is dissipating, we use ohms law W = EI. Now put two 500
ohms resistors in series, across the 9 volt battery. Because the two 500
ohms resistors in series still equal 1000 ohms, the current flow will still
be the same as before. If you measure the voltage across each of the
resistors, it will be 4.5 volts. The current through each resistor causes a
voltage drop of E = IR. Sometimes we use resistors to create a given voltage
(voltage divider circuit), sometimes we use a resistor to limit the current
(input current to a transistor).


> I know this is done in many ways and and is done all the time. Maybe
> it would help to know what the power dissapation is for some of the
> devices in an amplifier circuit using an Op amp while it is in its
> linear operation and then its maximum dissapation. Then I can think of
> it like I think of the 120 VAC in my house on a 20 amp breaker - there
> is a limit to the Current and the Voltage and it is easy to figure the
> circuitry.

I think you are getting hung up on power dissapation. Power dissapation is
more of a byproduct. Sure you need to know what the power dissipation is of
the components (but not to be able to understand how a circuit works). The
resistors and capacitors of an Op amp circuit are used to set the gain and
frequency characteristics of that stage.


.

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