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I have used a cheap multimeter to measure voltages in simple DC circuits, but I have seen pictures of them plugged straight into the mains and used to measure various home-built generators.

Why doesn't the higher voltage fry the multimeter, and also in theory could a small cheapo multimeter be safely used to measure very high voltages? If you get the setting wrong on the dial, does this matter?

I'm not planning to plug one in, nor would I recommend anyone who doesn't know what they're doing to do this either, I'm just wondering how it works.

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WARNING: Note that some cheap meters are not suitable for use with 230 VAC AC mains. Some meters may have AC voltage ranges able to conceptually measure to well above AC mains voltage BUT have internal componentry not certified, suitable or safe at eg 230 VAC. Use of such meters to measure such voltages is akin to a safer than usual game of "Russian Roulette" which still may end in death.

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Failures may occur to power dissipation in components or to voltage breakdown even when power dissipation is within bounds. Voltage ranges are usually less stressed than most other ranges when overrated.

Auto-ranging meters start at the highest range and work down until the reading becomes a certain percentage of full scale on that range. Switching can be via eg MOSFETs used to either short resistors in dividers or to pick voltages from appropriate tapping points.

In cheap meters protection is limited.
Current much over 200 mA on low current ranges will usually blow the internal fuse.
Too high current on the high amps range (10A, 20A, will blow the shunt or a fuse if fitted.
High voltage on lower voltage AC or DC volts or on current ranges will sometimes destroy the meter (Ask me how I know :-) ).

All that said: Over ranged inputs will not necessarily stress other than the input circuitry. Higher than minimum necessary input resistor wattages can protect short term. Zener diodes or other clamps can stop high voltage getting into the circuitry proper.
Very keen manufacturers can provide electronic switches. These can be a simple as a high voltage MOSFET in series with the input) which can be turned off when needed. This will add some error due to voltage drop but this can be controlled and designed for.

So eg an overvoltage condition is applied to a current input, the input resistor starts to dissipate excessive power, the inside end of the resistor is clamped by a zener or TVS (transient voltage suppressor) and a fast MOSFET switch is triggered to disconnect the overload. The higher than necessary power dissipation rating of the input circuitry provided enough time for the protection to act.

Real world example:

This Intersil application note -
AN046 Building a Battery Operated Auto Ranging DVM with the ICL7106 provides specific examples of the design of autoranging equipment and how the issues involved are addressed.

Here is how the front end looks conceptually:

enter image description here

and here is how it ends up in practice

enter image description here

http://www.intersil.com/content/dam/Intersil/documents/an04/an046.pdf

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    $\begingroup$ Too high current on the high amps range will set a cheap multimeter on fire. The amp-meter for these ranges is some 5cm of thick copper wire between the two terminals, with voltage between the terminals measured (voltage drop-off from resistance of the thick copper wire), unfused. $\endgroup$
    – SF.
    Feb 17, 2015 at 12:08
  • $\begingroup$ You don't need much to destroy a cheap multimeter. A car battery when using the wrong setting on the meter should do the trick. $\endgroup$
    – Mast
    Dec 25, 2015 at 11:47
  • $\begingroup$ @SF. Why fire? Doesn't it just make a conductor melt? $\endgroup$
    – neverMind9
    Dec 18, 2018 at 12:13
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    $\begingroup$ @neverMind9: The conductor melting temperature is much higher than ignition temperature of the plastic casing, and the copper is thick enough to give out plenty enough of heat to ignite the casing before it melts. $\endgroup$
    – SF.
    Dec 18, 2018 at 12:42
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Simple answer is good multimeters have good protection circuts. When the measuring source outside of the meters capabilities of the meter, the meter will give out of range message.

There are four main multimeters categories. They are

  • Category I: used where equipment is not directly connected to the mains (CAT I)
  • Category II: used on single phase mains final sub-circuits (CAT II)
  • Category III: used on permanently installed loads such as distribution panels, motors, and 3-phase appliance outlets (CAT III)
  • Category IV: used on locations where fault current levels can be very high, such as supply service entrances, main panels, supply meters, and primary over-voltage protection equipment (CAT IV)

It is the user responsibility to us the appropriate multimeter for the appropriate task. Below is a Fluke 87 III multimeter. In the bottom right hand corner it states CAT III

Fluke 87 III

Below is a mulitmeter which is in the el-cheapo classification and there is no visible notice of multimeter category classification. This is a situation where user be aware is very important.

Craftsman Meter

Both type of multimeter play in important role in the area of engineering.

Basic Protection

Most basic form of protection is a fuse. Depending on the price point and manufacture this protection can vary. A high quality multimeter might use a High Rupture Capacity (HRC) fuse where as a low quality meter would use a glass fuse.

High Rupture Capacity (HRC)

HRC fuse

Click on image for a larger version of the image.

Glass Fuse

Glass fuse

Advance Protection

Beyond the basic protection circuit there are many stages of other protection circuitry consisting of diode, Wire Wound Resistors, Zener Diodes, MOV (Metal Oxide Varistors) and thermistors (PTC – Positive Temperature Coefficient). The configuration of these components are depends on manufacture, multimeter category, price point other factors. This links explains the few advance circuits of the Fluke 27 multimeter.

El-cheapo multimeters don't have good protection thus is prone to damge.

Reference:

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    $\begingroup$ Yes please do add more facts - like how do the resistance change to match the voltage without allowing much current? What is used for the over-voltage protection? etc. I'd like to see how it works rather than just "yeah some <magic> makes it all ok" :) $\endgroup$
    – jhabbott
    Feb 9, 2015 at 13:15
  • $\begingroup$ Note that a CAT II meter is not safe for mains measurement use despite the wording in the provided table. Such meters may not protect against mains spikes / surges / ... which MAY result in meter failure and, worst case, can result in the user's death - and have done so. $\endgroup$
    – Russell McMahon
    Dec 24, 2018 at 7:27

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