Electronics > Beginners

Current Draw at Power Supply vs Current Measured across a series shunt

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steamedhams:
I am measuring power consumption of a pcb. The current draw of the power supply reads 0.5 amps.
When I measure across a series shunt on the board (1 mOhm), the voltage across is 2.7 mV which means 2.7 amps are flowing across that resistor.   :wtf:

What is the deal here?? There are a couple of boost/bucks converters on the board, regulators, inductors...etc Is that jacking up the current??

Thanks in advance   

SuzyC:
Either your power supply or  your shunt resistor or your mV measuring device is lying to you. Check them out.

Gyro:
A couple of random thoughts in order of likelyhood...

1. Make sure you are measuring directly across the resistor terminals leads, ie. not sharing solder joints or pads with the current connections, at the 1mOhm level, this can make a huge difference. You need 4 wire aka Kelvin connection. Make separate connections for current and measurement.

2. Your current draw could be very spiky in nature and your measuring device isn't handling it well.

3. Thermocouple effects - check the reading on the meter with the power off and make sure you're getting a zero reading.

My bets are on 1 though.

SuzyC:
My bet is on 2.

Gyro:
Yes, it's one or other. Unless the meter has a serious zero offset, thermocouple effect would probably top out at the 10's of uV level.  :)

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