queries about amps and volts

[kizzap]

i am a bit confused now because of everyone talking about voltage drops through resistors etc. i previosly thought that resistors lowered the amperage that was going through the resistor. can someone please shed some light on this subject?

if the above is true(that if you put a voltage through a resistor it will lower) then what would be the way to provide a variety of different amperages from power supply that gives out 25volt and 800 milliamps (I think-se below)? would it be possible to have it so that there is one ground and seperate outputs that as they go furthur along slowly reduce in amperage?

I have a multimeter and when i was testing the amperage on the power supply from above it gave out a reading of 0.8 on the 10amp setting. would this be 800 milliamps or 8 amp?

Kieran

 

[Phasor]

The first thing we need to do, is clarify the difference between current (amps) and voltage (volts).

I'll use the water analogy (some people will groan at me :roll: ) - Voltage can be compared to water PRESSURE (psi or kPa). Current can be compared to water FLOW (litres/min). Consider a closed tap. You have a pressure difference (voltage) from the pipe side of the tap to the tap opening. But there is no flow, because the "RESISTANCE" of the tap is very high (in fact, infinite). When you open the tap a bit, you lower the resistance, which allows water to flow (current). Providing that the pressure remains the same, then the more you open the tap, the more water flows. Also, If there is only a little pressure, only a small amount of water flows, but increasing the pressure also increases the flow.

Electrically, this is described by Ohm's Law:
V = IR
where V is volts across a component (or group of components)
I is the current, and
R is the resistance

One of the common misunderstandings that people have, is that power supplies supply a set voltage AND current. NOT TRUE. A supply will (generally) provide a constant voltage, but will only supply as much current as you allow it to. The value of this current will depend on the RESISTANCE of the load. The higher the value of resistance, the less current flows.

(There are also constant current supplies, which do the opposite, but that is another issue altogether.)

voltage drops through resistors

It is better to talk about voltage ACROSS components, not THROUGH them. Conversely, we usually talk about current through components, not across it.

I have a multimeter and when i was testing the amperage on the power supply from above it gave out a reading of 0.8 on the 10amp setting. would this be 800 milliamps or 8 amp?

How were you testing it? It is extremely bad practice to put an ammeter directly across a voltage source. Ammeters generally go in series with a load. On the 10A setting, 0.8 indicates 0.8A or 800mA.

 

[kizzap]

so what you are saying is that there is no easy way(resistors, etc.) that i would be able to lower the amperage from a power supply?

And when you were talking about it being bad practice to measure the load through a circuit while there is a voltage passing through it what would the 'acceptable' method of measuring this. i was measuring it through two contacts that were related to each other but they were not t any time touching. please enlighten me.

Kieran

 

[Klaus]

Kieran, maybe a good book about electricity is in order?

Yes, there are easy ways to limit the current from a power supply, the easiest would be a smaller power supply that can only supply the maximum current you are after.
You have to understand that the current through a given load depends on the load in ohms and the power supply voltage.

Your second paragraph is a bit unclear, you *have* to learn to use the correct phrases like: current is 'through' a load; voltage is 'across' a load. The load being the resistance (lamp, motor, whatever) you connect to the power supply.
Until you grasp the fundamental importance of this others may not be able to understand what you are talking about, hence my suggestion of reading a basic book on electricity.
If you are any good on surfing the web you will almost certainly find a basic electricity text somewhere here, try google.

Klaus

 

[mattg2k4]

**broken link removed** has 6 volumes on electronics. The first two volumes, DC and AC, are well worth a read for some basic info.