extreme noob - how to measure voltages

[glesconz]

Hi, just wondering, if I want to measure the volts at certain points on a project, where do I put my DMM probes? Do I need to do anything before measuring volts?

Thanks,
Glenn

[salocin]

Just connect the negative probe to a ground point and the positive probe to the point you want to measure.

[R.G.]

Salocin is correct. You find ground, connect the black probe lead to it, and then touch the red probe lead to the point you want to measure.

If you're completely new to this, you may need to know some more. "Ground" is any point in the circuit you decide is the reference point for all voltages. It *can* be literally anywhere, but if you don't pick a place where the rest of the world would probably pick, all the voltages you measure will be off from what they measure. In general, since one end of the power supply is almost always used by the rest of the world as a designated ground, it makes sense to pick one end of that, and that's what the world does. Since signals into and out of the circuit need to be referenced to a ground, one terminal of the input jack is always ground; and circuit ground is almost always taken to be the side of the power supply that connects to input jack ground.

You need to set up your meter correctly. Meters almost always have a red and a black wire lead. They commonly have several places to plug the leads into the meter for different purposes. In general, one jack will be marked "Com" or "common", maybe even "Gnd". The black lead goes there, to keep polarity straight in your head. The red lead plugs into several different places, but the one you want is marked "V" or "Volts" or "Volts/ohm", or some thing with a V or Volts on it. You can use either lead in either hole, as they are just wires, but keeping Black in "common" and red in "volts" means you don't confuse yourself while making measurements.

Finally you have to pick the right scale and settings. Meters may read volts AC or DC, ohms of resistance, amperes of current, transistor gain, capacitance, inductance, some even frequency or temperature. Until you understand more, stay with volts and ohms. For reading volts, set the scale to the smallest scale that's bigger than the power supply in the circuit you'll be measuring. For a 9V battery powered circuit, that's usually 20V. If you set it too small, many voltages you read will be bigger than the meter can read and off scale. If you set it too big, say at 200V, then the voltages will read fine, but will suffer from being inaccurate, like trying to feel the temperature of something with gloves on. Little variations will be hard to tell.