# Hi there, i have a question on maximum current inputs of IC

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1. Apr 3, 2015

### BigBen

hi there..i have a question...back at my work, we have a power supply which have a variable current and a variable voltage...my senior said, he have a ic board to check, and then he uses a 5 volt supply with unknown current on it to power up this board,
he then claims that you can supply the board with any high current as long as the voltage is still 5V...is that possible?..he then says an analogy with
a car supply..suppose that the car battery has 12v 40A..and you put your car stereo only..he then says that the car stereo wont eat up the whole 40A..
and then he says that its the same in semiconductors...

2. Apr 3, 2015

### Staff: Mentor

No, but I think that is a communication issue: if you set the voltage at the board to 5 V, the board will have some fixed current (given by its internal resistance*) - no matter how much the power supply is able to deliver.

*the current might vary over time if the board does something (like computing) so it is not as simple as a resistor, but then its current is still independent of the power supply.

3. Apr 3, 2015

### BigBen

thanks for the reply..so it means that the ic in the board would not be affected with the large amount of current entering it...i mean..this power supply that im refering to was not its original supply
.this power supply was mostlikely for a car battery with variable voltage and variable current..so..
if i set this power supply at 5 volts and 10amps..and put it as a supply for this board with no current regulator circuit before this ic board..the board wont be destroyed?

4. Apr 3, 2015

### wud-wurks

The power supply defined as variable voltage and current is in fact a constant voltage WHEN SET to say 5V and the load that is applied to it will draw the required current subject to the rating of the power supply. So let's say you need a 5V rail to test an IC circuit. You set the PS to 5V and on most supplies you can set a current limiter, particularly useful when you are first powering up a new circuit! , you then adjust the current limiter to allow the load to take as much current as it needs, the 5V remains stable at 5V throughout the process.

If it helps an analogy is water from a tap. The pressure (voltage)is always the same in the pipe leading to the tap, but the flow of water (current) is determined by the amount you let through the valve on the tap (current limiter). Taking this analogy to its logical conclusion if the tap is opened fully the max amount of water flows (current) that the pipe can deliver at the same pressure (voltage).

5. Apr 3, 2015

### Staff: Mentor

No, it means there is no large amount of current flowing.
That is not possible. You probably set it to "5 V but at most 10 A".

6. Apr 4, 2015

### wud-wurks

It seems to me that you have a fundamental misunderstanding of the relationship between a voltage source and the current that a load will take.

If you use the very simplistic formula that V=IR where V is the voltage I is the current and R is the resistance. Now allow the load i.e. the resistance to be 10 ohms and the voltage to be 5V from the formula the current will be 5 divided by 10 which is 0.5A. SO even if you set the PS output current to limit at 10 A and connected the load (with 10 ohm resistance) the current from the power supply would only be 0.5A.

As I say this is a very simplistic example and loads are normally complex but the principle above will still apply.

Hope this helps!!

7. Apr 4, 2015

### Staff: Mentor

It does not mean that. You provide the IC with the voltage it is designed for, and the IC will take only the amount of current it needs. Usually its need will change from moment to moment, so the current it takes will not be constant.

If a power supply says on its label: 5V, 10A this means when it is switched on it will give a steady 5V but any amount of current from 0 to 10A max as determined by the "resistance" of whatever it is powering (using the Ohms Law idea).

(The "resistance" of an operating IC is a constantly varying property.)