Yeah, it's confusing to me since I've only been working on high voltage stuff.
The rules are the same no matter what voltages you are working with.
So, all I gotta do to complete the circuit on a circuit board with a multimeter so I can check for voltage is to connect the black/negative lead of the multimeter to the main neutral coming in?
.... Only if you are checking circuitry on the mains side of a power supply.
If you are checking on the other side of an isolated power supply - as is the normal case for transformer and switch mode power supplies - then you don't want to go anywhere near
any connections on the mains side of things. Find your reference point on the low voltage side and keep away from the mains side.
There's a bad diode on a circuit board I"m testing. I removed the bad diode, and I want to test the voltage coming into the solder pad where the diode used to be.
If this diode is on the mains side of the power supply, then the neutral is a reasonable reference point to choose.
If this diode is on the low voltage side of the power supply, pick your reference point on that side - away from the mains.
A DMM checks differential voltage across 2 points, it doesn't care which side is ground.
However, the red probe has less capacitance to the real ground and the black one has less capacitance, so that might be important when probing high frequency stuffs -- you don't want to interfere its operation.
A word of advise: don't work on mains powered SMPS unless you exactly know what are you doing. Otherwise there's quite a chance you will see the Creator himself if you did something stupid.
Thanks. So, multimeters only checks the difference between the two probes. I didn't know that. I thought it actually "completed" the circuit, with current flowing from one probe to the other.
When you are measuring
current - the actual draw in amps - then the meter
does become part of the circuit and the circuit cannot operate if you disconnect one of the leads. When measuring current, the meter should be considered to be a short circuit. When measuring current ALL the current that flows through the path being measured goes through the meter.
When you are measuring
voltage, the meter is only sensing how much the potential is at a given point - and it only needs a "whiff" to be able to do that.
Ideally, a voltage measurement should not affect the circuit being measured.
Ideally a voltage measurement won't take any current from the circuit. In the real world, these ideals are not truly found - but modern DMMs are so very good that for most measurements you are likely to do, you will be able to consider them as 'ideal'.