A device that runs on AC current it's voltage inside is measured by always turning the multimeter's knob to AC voltage:same for DC right?

I've bought a cheap multimeter and it's not automatic. I've to manually rotate the knob.

I live in North America. AC current is used in my home. Does this mean I can always measure voltage by turning the knob to AC voltage part(to appropriate measurement amount/unit) and measure voltage at two parts of the device's circuit that runs on AC current without burning the fuse of multimeter? or is there any exception?

When should I turn the knob to DC voltage? Is it when a device run on DC power source?

I saw electricians right away turn the knob to AC or DC for measuring Voltage and Ampere and figure this out without any problem.

So a device that runs on DC current it's voltage inside the device will always be measured by rotating the knob to DC voltage and that runs on AC current it's voltage will always be measured by rotating the knob to AC voltage. Is this true? Is this the general rule?

You turn the knob to AC voltage when you want to measure a AC voltage, or the AC component of a voltage. You turn the knob to DC voltage when you want to measure DC voltage. "Wall" power will be AC, but inside a device this AC is usually converted to DC, which then actually powers the device.

Most of the time, AC is for measuring wall power and related, and DC is for measuring stuff like batteries or the power actually running circuits inside some device.

However, it seems your real confusion is between AC and DC voltage. Surely there is a lot about that already out there. Once you understand that, it will be obvious what to tell the meter to measure.

• Thanks for helping me find the problem I'm having with the understanding. I guess I've to search wikipedia for that(the difference between AC and DC voltage). I really don't want to burn the fuse of multimeter. Again thanks for help. – mvr950 Aug 3 '12 at 19:50

Your confusion is regarding two things. AC versus DC, and Amps vs. Volts. If you select the wrong setting for AC or DC, the worst you will get is the wrong reading. But if you use the current ranges (Amps) incorrectly, you will blow the fuse. Don't ever put the meter across a power source in the Amps position. And it's best to not use the Amps ranges at all, until you understand how they work.

• Thanks for cautionary advice. I'll will not open any device right now. I'm at learning stage. I hope I'll be an expert. If I have any problem learning I'll definitely post the question hear. Thanks to all for your kind comments. – mvr950 Aug 3 '12 at 20:53

So a device that runs on DC current it's voltage inside the device will always be measured by rotating the knob to DC voltage

Generally it is a workable assumption that inside a device powered by DC you will also find only DC. - BUT this may not always be true. It is possible to generate AC or high-voltages (i.e. dangerous) inside a DC-powered device.

A device that runs on AC current it's voltage will always be measured by rotating the knob to AC voltage. Is this true?

Generally inside an AC-powered device you find a section that has AC and other sections that have DC. (Olin Lathrop has already mentioned this in his answer.) Electrical engineers can generally identify which parts are which by looking at it.

I really don't want to burn the fuse of multimeter.

This really is not an issue when you measure voltages. Burning the multimeter (or its fuse) happens generally only when measuring currents.

Also, for measuring voltages, there is not really any harm in selecting "AC voltage" for measuring DC or vice versa. You just will not get the correct result.

Given your current understanding of electricity however I would strongly caution against opening electrical devices to poke around inside! Getting shocked from accidentally touching a live wire is a really poor way of learning about electricity.

That being said, battery-powered devices will generally be safe to open. You might want to check back here before you do however just to make sure.