# Limit current

1. Mar 13, 2015

### chikou24i

We see always in source meter machines a LED wich indicates the limit current. I want to know what is the limit current and what is the relationship between this later and the resistivity.

2. Mar 13, 2015

### phinds

What is a "source meter machine" ?

3. Mar 13, 2015

### Staff: Mentor

Hi chikou24i. Could you explain what your "source meter machines" do? Can you provide us with a URL to a site showing one, so we can establish what it is.

4. Mar 13, 2015

### soarce

5. Mar 13, 2015

### KenJackson

Regardless of the application, LEDs need a current limiting resistor in series with them. That's because LEDs are diodes, and a diode will attempt to pass almost infinite current if you put anything but a very small voltage across it.

But with the resistor in series, as the current increases, more and more voltage is dropped across the resistor instead of the LED until a kind of equilibrium is reached. Adjusting the value of the resistor or voltage adjusts the current and thus the brightness.

The LED manufacturer specifies a maximum current, so you just need to pick a resistor that won't allow the current to exceed that maximum.

6. Mar 14, 2015

### chikou24i

7. Mar 14, 2015

### davenn

On a power supply like you have shown, many have a current limiting adjustment
This can be set to limit the current from the power supply to a maximum output
This can save the circuit being powered from "burning up"

That is one purpose of a current limiting

Dave

8. Mar 14, 2015

### jim hardy

I think they make it fairly clear in the pdf documentation referenced
http://www.es-france.com/pdf/AX501.pdf?osCsid=552b012b969105dbebe981f664fa19ae
that the supply has active current limiting via the "Current Limit" knob beneath the ammeter.

"Active current limiting" means that the regulator compares output current to a set value, set via that knob, and reduces voltage to whatever value will provide only that much current.. An LED indicates when the current limiter has activated.

I didn't see mention of "resistivity" in the documentation.

Be aware that some supplies have a substantial capacitor right at the output terminals. If the current limiter controls current into that capacitor, then one could momentarily draw more than limit current from the supply while that capacitor discharges into the load.

old jim