# How can I measure Quiescent current of LDO's?

#### dhlee

Aug 27, 2014
17
I thought one, make sure the load is infinite.
Is this available?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,761
Infinite load = no load at all or short circuit, depends on what you understand when you say infinite (infinite resistance or infinite current)
In you case, quiescent current is measured at the input of the LDO with no load attached.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
The quiescent current in a regulator is the current that flows into the input, but not out the output. Therefore it must flow through the ground connection. It is also called the "ground current". So, measure the current flowing through the ground lead.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,889
The quiescent current in a regulator is the current that flows into the input, but not out the output. Therefore it must flow through the ground connection. It is also called the "ground current". So, measure the current flowing through the ground lead.
Kirchoff's Law in action: What goes into a node must also go out of the node.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,889
For Kirchoff's Law, a node is any bounded region, not just a point where two or more conductors are connected. In this case the entire 3-terminal regulator with input, output, and ground is considered a node. The Law quite simply states that all the current entering a bounded region (a node) must be equal to all the current leaving the bounded region. It doesn't matter what kind of circuitry is inside the region!

Why is this so? Because otherwise there would be an accumulation of charge, either positive or negative, from the excess of current entering or the excess of current leaving the region. You think a Van de Graaf generator is a counter-example to Kirchoff's Law? Think again. All those charges (current) being carried on the belt to the top of the generator eventually come back down when current is drawn from the charged sphere at the top.

Kirchoff's Law is used together with Ohm's Law to analyze circuits. No matter how many parallel and series connected components you have, application of these two laws will allow you determine the voltage drop across and current through each component.

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