# Total current drawn by the circuit???

#### abuhafss

Aug 3, 2010
348
Hi

Please see the attached two similar circuits. In the top circuit:

Load R1, 1.2k is connected across 30V supply - Current drawn is 25mA
Load R2, 180 is connected across 12V supply - Current drawn is about 67mA

In the second circuit, R2 is connected to 12V output of the voltage regulator 7812.
How do I work out the total current drawn?

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• Power Consumption.png
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#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
This sounds like a homework question so I've moved it to the homework help forum.

In the second circuit, do you need to work out the total current drawn from the power source at the left side?

The first resistor, marked L1 (should be marked R1), has a known voltage across it, and a known resistance. So you can calculate the current flowing through it using Ohm's Law: I = V / R, where I is the current flowing in the resistor (in amps), V is the voltage across the resistor (in volts), and R is the resistance (in ohms).

Have a look at the 7812 data sheet at http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF. This data sheet is actually for the whole 78xx family; the 7812 is the member of the family that has a 12V output voltage. Although your schematic doesn't have the input, ground, and output terminals marked, the connections are obvious because the supply voltage enters on the left hand pin, and the bottom pin is connected to the negative rail of the circuit.

So you should be able to see what's happening in that circuit, and understand that the second resistor also has a known voltage across it. So there will also be a known current flowing through it. And this current must come from somewhere; this somewhere has to be the power source (via the regulator).

If you want to be very pedantic, you could look at the specification for maximum and minimum output voltage of the regulator, and calculate the current through R2 in each case. But I doubt you would be marked down if you assume the nominal output voltage.

Finally, the regulator actually draws some current itself, for its internal operation. This current is called the regulator's "quiescent" current or "ground" current, because it flows out the ground pin (rather than the output pin). Regulator data sheets always specify the maximum ground current that the regulator will draw; often this is specified at more than one combination of input voltage and output current.

Regulator data sheets often also have graphs in the "typical characteristics" section that show how the ground current varies depending on input voltage, input-output voltage difference, and/or output current as well, so you can get a typical figure as well as a guaranteed maximum.

You should now be able to draw three loops on that diagram, starting from the positive terminal of the power source, and returning to the negative terminal, and you should be able to mark these loops with current figures. The power source is providing the current for all of those loops, so you can add the currents together to get the total load current on the power source.

Aug 3, 2010
348

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
You're welcome You can click "Like" at the bottom of my post to thank me formally.

#### abuhafss

Aug 3, 2010
348
Surely. I should have done it earlier but I missed it.

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