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current draw and limitation

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manicool271989:
Hey guys I'm new to this blog (well at least to the forums anyway) I just wanted to discuss something interesting a housemate of mine brought.

   See I use a plug in dc converter to power my breadboard projects at various (6, 12) voltages. One day my housemate asks me if there is a limitation on the current that can run through my circuit projects because of the ac to dc adapter? I told him no there shouldn't be a limit and my reasoning was that since the adapter gives out a particular voltage and with the simple relation that V = IR you can ideally have from 0 to infinite amount of current going through the circuit. That's what I said, but I began to have second thoughts because I kept hearing current being drawn from a power supply during lectures at school. I kinda feel ashamed to be asking such a basic material question, but i really would like to know if my initial reasoning is right or not and clarification on what current draw means? (My guess would be that a load, such as a resistor, when connected in a circuit has some current going through it and that current is the current being drawn?)

Sorry if this is posted in the wrong place, but again this is my first time posting on the forums.

-manicool271989

Feanor:
You are on the right track. The load/resistance that you connect across the terminals of your voltage supply allows current to flow from one terminal to the other. This is the current drawn by the load.

So if you put a very low resistance between the terminals then you will get a very high current, from ohms law. I = V/R.

The current will never be infinate however, infact it will probably not be very high at all for two reasons.

First of all every power supply has what is called an internal impedance, this is a resistance built into the power supply, it comes from the physical way the supply is built, if there are copper wires in the supply then there is resistance in the copper, if it is a battery then there is resistance in the electrolyte, etc.

Secondly most power supplis have overload protection, this means that if too much current is drawn from them then the power supply will stop applying a voltge to the terminals. this can be done in many different ways, fuses, circuit breakers, altering PWM in switch mode power supplies.

Mechatrommer:
in the dc converter there is transformer (coil)... bigger transf = bigger current limit, smaller coil/tranf=smaller max current (limit) but i cannot explain it technically or theoritically, just from experience. the biggest transformer/dc converter that i have built/own only capable of producing 18A max when shorted (DMM Ampere measures) at 12V. i have alot bigger transf, but not assembled yet. for discreet components dc conv. you short it? the chip will burst opened!

edited: i just checked my dear transf psu, its a 18V 8A max, not 18A as slashed above. well, doesnt make any significant difference isnt it ;) :P :D

manicool271989:
Thanks for your answers!

So I did a little more research (mainly wikipedia) to figure out how the internal resistance of the power supply would affect the the voltage at the output of the internal power supply circuit (where in my case I assume I get 12 or 6 volts depending on the setting). It seems that when the load draws current from the power supply the internal resistance (lets assume impedance is only because of resistors) multiplied by the current being drawn (IR) gives a voltage difference that is equal to the difference between the voltage at the output of the power supply when there is no load (which I assume is a short circuit) and the voltage at the output when there is a load connected. Is this a right understanding?

Mechatrommer:
V is constant. only I is changing depending of load (R) you are connecting.

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