LDO current measurement

[poiureza]

Hello,

I try to measure the Iq or ground current of a LM2396-3.3Volt LDO using a DMM

The datasheet here, figure 4 on page 8 shows about 10µA for Iout=0

Whether I measure {DMM probes between 9V battery and LDO in} or {DMM between LDO ground and battery groud} I always get ~1.5mA
Output voltage is ok @3.3V

Where am I wrong ?


FWIW, a 78L05 also reads about 1.4mA

[PRR]

Do you have the capacitor(s) connected? And a 100 μA load?

[Rob Strand]

I try to measure the Iq or ground current of a LM2396-3.3Volt LDO using a DMM

The datasheet here, figure 4 on page 8 shows about 10µA for Iout=0

Whether I measure {DMM probes between 9V battery and LDO in} or {DMM between LDO ground and battery groud} I always get ~1.5mA
Output voltage is ok @3.3V

Where am I wrong ?

The circuit can misbehave when you insert devices and long leads into the circuit.

Can I suggest a different experiment:

- wire the circuit as per the datasheet with the input and output caps present
- no load
- measure the input current on the positive lead of the *input supply* ie.
  before the input cap, not between the input cap and the regulator.

The idea here is the circuit is set-up in an ideal configuration where
the measurement will have minimal impact on the behaviour.  Also that
the ground lead current must equal the input lead current as there is no load.

FYI:
- To match the datasheet you need to make sure the input voltage is a healthy amount above the output voltage; see datasheet for conditions.
- As the regulator approaches dropout it is possible that some regulators will draw more current.
- Another variation is the regulator quiescent current varies with load;  my experiment isn't suitable
for measuring 1mA quiescent with a 1A load.

[poiureza]

Thank you both

I measure the LDO (2936, not 2396 as I wrote earlier ) part alone, outside any circuit.
I put the 2 caps and I still get 1.5mA

THEN ...

I switched the DMM scale from mA to µA, and lo and behold it reads 13.5µA as per datasheet.
Funny, I was expecting to read something like 0.01 on the mA scale (just as you can read Ohms on the kOhms scale) but it seems that it's not working like this for current.

I learnt something new today



Edit ...

Then I measured my 78L05 the same way and it also reads 13.5µA ... while the datasheet says 4.0mA Iq
I'm at a loss ...

[PRR]

Test your meter. Get a 9V battery and a bunch of resistors. Make known currents. 9V in 1Meg is 9uA, in 1k is 9mA (or a bit less because the battery may sag).

If you have been careless with your meter, you may have burned-up the current reading input. They can fail different ways giving different errors. (That's the reason I "never" use a multimeter's current ranges, preferring volts across a known resistor; also my meter current inputs are usually blown-up and on the good FLUKE that is a $35 fuse.)

[Rob Strand]

Then I measured my 78L05 the same way and it also reads 13.5µA ... while the datasheet says 4.0mA Iq
I'm at a loss ...

I'm with PRR, looks suspicious.

- Use your multimeter on DC volts (2V range at first then, 200mV for the measurement).   
- Add a series resistor between the input PSU and the input cap.   Tweak the resistor value to get just under 200mV drop; don't be too fussy getting exactly 200mV.   
- Measure the DC voltage across the resistor. 
- Then calculate I = V_measured / R_tweaked.

[poiureza]

Good call

Known current readings are all over the place and the fuse appears to be broken indeed
Funny how some measurements are still available (the voltage readings look still ok for instance)

[ElectricDruid]

It's always the current readings that get fried, in my experience. I've blown several meter fuses one way or another.

[poiureza]

FWIW I checked the LDO currents with a 1K resistor and voltage difference and Iq appears to be in the datasheet ballpark, finally
78L05 : ~2mA
LM2936 : ~20µA

[PRR]

Funny how some measurements are still available (the voltage readings look still ok for instance)

Sure. An amp meter is normally implemented as a voltmeter and a resistor. If you read the Ohms of a wall outlet, boom, the resistor burns-up but the voltmeter may survive.
Is postimg going to be down all week???
**



And things are different between pedal (or car) grade meters, domestic meters, and utility worker meters. You think you are working on 9V battery but you get careless and read the wall voltage. Small parts make small sparks at short distance. Here inside my house I can't get over 666 Amps for even split second. Out on the street the available energy is much higher. That's kinda why my pro-grade Fluke takes a large rugged and costly fuse.