Multimeter and component testing

DSC_7714

One application for multimeters is to test different types of components, some of the test are fairly easy, while other requires a bit more knowledge and there is always a few pitfalls.
My examples are with fairly large leaded parts, they are the easiest to take pictures off, but the test can also be used on other sizes of parts (Like SMD).
I use a couple of different meters, sometimes to show something, other times just to show that the actual meter is not that important as long as it supports the function.

    Fuses
    Incandescent lamps
    Resistors
    Resistors, low-ohmic values
    Resistors, high-ohmic values
    Capacitors
    Diodes
    Leds
    Transistors
    Transistors again, BJT types
    Transistors again, MOSFET types
    Other
    What to look for in a multimeter
    Alternative ways to test components
    Notes



Fuses

DSC_7696

For fuse testing the best range is continuity, but ohm can also be used. It is best to remove the fuse from the circuit before testing it. To test it just touch the two ends with the probes, it do not matter what end you use the red and black probes on, fuses do not have polarity. The buzzer in the meter will sound if the fuse is working and the displayed value will be a fairly low ohmic value (Fraction of an ohm to a couple of ohms).

DSC_7700

A typical multimeter mA fuse, it is ok. These fuses do have around 1ohm (Here 1.55ohm) and adds significantly to burden voltage in the high mA range.

DSC_7701

A 0.1A fuse that is working.

DSC_7704

This fuse is blown, in continuity the meter will show OL in that case and there will not be any sound.

DSC_7709

But if you are using the ohms range it may show a high value instead, how high depends on how dry your fingers are and how hard you press them against the fuse/probe tips.



Incandescent lamps

Incandescent lamps will probably soon be a thing of the past, but for now they are still used in cars and as indicators (Mostly older, LED's are much better for this task) and for some special applications.
Testing them are best done in ohms mode (Continuity can be used for some bulbs).

DSC_7812

Measuring resistance between the two terminal, there is no polarity. For high power lamps it can be close to zero.
This value cannot be used to calculate the power (watt) of the lamp, the resistance of incandescent lamps will increase dramatically (like 15 times) when the filament changes temperature from around 25įC to 2200įC .. 3200įC

DSC_7819

A signal bulb, this is very low power and has a high resistance, a continuity test would not work for this bulb.

DSC_7815

A blown bulb, if I had touched the probe tips the value would have been lower (Se last image in fuse section).



Resistors

DSC_7664

Resistors are very simple to test, put the meter in ohms mode and connect the probes to a loose resistor, then read the value on the display (This works most of the time).

DSC_7649

A 1kOhm resistor

DSC_7651

A 10kOhm resistor.

DSC_7659

A 100kOhm resistor. This resistor looks to be out of tolerance, it is marked as 5% and I measure considerably lower.

DSC_7654

A 1Mohm resistor. This is 20% below marked value, could I be doing something wrong?

DSC_7653

What about a infinite resistance, i.e. no resistor? I get 3.3Mohm, that is very far from infinite!
The problem is that I am touching with my fingers, the path through the human body is not anywhere near a infinite resistor, if you are sweaty it can easily be as low as 100kOhm, the body is only about 1kOhm, the high resistance is due to bad contact on the skin. Pressing hard will reduce the resistance.

DSC_7663

Measuring the correct way, i.e. without touching, the 10Kohm resistor is slightly closer to 10Kohm.

DSC_7661

The 100kOhm is easily within its tolerances now.

DSC_7662

Same with the 1Mohm resistor.

DSC_7693

There are other ways to get around the body resistance, here I am only touching the resistor on one side, because the other side is isolated it works fine.

DSC_7694

Or I could use alligator clips and not touch the resistor at all.

Measuring a resistor in-circuit with power off, may sometimes work, but often it will not because some other resistors or components will affect the result.



Resistors, low-ohmic values

DSC_7665

Another problem is measuring low ohmic resistors, here I measure a 0.1ohm 1% resistor. It do not look to be anywhere near 1%.

DSC_7666

There is a reasons for that, it is called contact and cable resistance. Here I measure with both probes on the same side of the resistor. It will not show a 0 on this meter with 0.001ohm resolution.

DSC_7667

Pressing the "Null" button, this is also called REL on some meters (delta symbol on display) and measuring again gives a significantly better result, but a normal ohmmeter is not the best tools for measuring low ohmic values.

DSC_7668

If you really want to measure low ohmic value, the solution is 4-terminal equipment. On these jaws the upper and lower jaw is connected to different cables and the two parts of the jaws have no electrical connection.

DSC_7669

This means I have two plugs from the jaws.

DSC_7670

And with a proper 4-terminal meter I can measure the resistance. There will be some variation depending on where I connect the wire on the leads.

There are cheaper ways to make 4-terminal measurements:

DSC_7678

A lab power supply with adjustable current limit can make it fairly easy to check low ohmic resistors.

DSC_7679

First I adjust the current limit to 1A using a multimeter, it is more precise than the meter in the power supply. The voltage is not important, a few volt is more than enough for resistors below 1ohm.
When doing the test I keep an eye on the power supply meter to check that everything looks correct, i.e. it is delivering 1A.

DSC_7673

I connect the power supply to the resistor I want to test. Now the current in the resistor is 1A.

DSC_7674

A check with a voltmeter across the resistor will show the voltage, that is the same as the resistance, i.e. 100mV is also 100mOhm or 0.1ohm (As long as the current is 1A).

DSC_7680

The 4-terminal wire from above can be used with this solution, making it much faster to check many resistors.
I have one wire from each jaw connected to the power supply, the other wire to the meter.

DSC_7681

Here I check a 10mOhm resistor.

The only problem with this solution is how much power the resistor can handle, at 0.1ohm or lower the power is low enough, between 0.1ohm and 1ohm some non-power resistors may be overloaded, above 1ohm the 1A current is for power resistor. Reducing the current to 0.1A will reduce the power and the displayed value must be multiplied with 10.



Resistors, high-ohmic values

What about resistor at 100Mohm or more, how are they measured? There are basically two ways, either make the meter very sensitive or increase the test voltage. When working with these kind of values humidity can easily affect the result, fairly dry surroundings are important.
For testing these resistor I do not use probes, but more fixed connection where I can keep my fingers away, with the first meter due to noise, with the second meter I also have to be careful with the voltage (The meter probes are designed for this, but do not use fingers to press the resistor against the probe tips, this would also spoil the measurement).

DSC_7790

Some meters can measure nS (nano siemens), that is 1/Gohm, i.e. 1nS is the same as 1Gohm and 0.1nS is 10Gohm.
Just because a meter can show nS, do not mean it can measure very high values, but most can. On the above picture nothing is connected to the meter.

DSC_7791

A 100Mohm resistor is easy enough to measure: 1/10.09 -> 0.09911Gohm or 99.11Mohm

DSC_7792

A Gohm also works: 1/1.07 -> 0.9346Gohm or 934.6Mohm.
The 10Gohm resistor looks like a lost cause, but it is not.

DSC_7799

First lets use the REL/NULL function. I had to use a non-conductive stick to press the button, the setup is rather sensitive.
With Siemens the value for parallel resistors can be subtracted and added, like Ohm can for resistor in series.

DSC_7800

It did improve the result for the 1Gohm resistor: 0.98nS -> 1.02Gohm

DSC_7801

The 10Gohm gives 0.11nS -> 9.091Gohm, it is fairly close.

DSC_7796 DSC_7795

Lets look at high voltage instead, this is an insulation meter to check if insulation works correctly. It can test with up to 1000V (Be careful with your fingers). At 100V the maximum is 66Gohm and at 1000V it is 260Gohm. Other insulation meters will have other limits.

DSC_7797

There is absolutely no problem testing the 1Gohm resistor.
When testing resistors it is important not to go above the rated voltage of the resistor, it is fairly easy with this meter.

DSC_7798

The 10Gohm resistor can also easily be measured.



Capacitors

Capacitors exist in values from a few pF up to 1000's of uF or even F for super capacitors. This means the difference between the smallest values and the largest values used is around 10 000 000 000 000 times (10pF to 100F), there can be a few zeros more or less depending on what exactly is included. A multimeter cannot cover that range, usual the low range is covered (more or less), the problems is the higher ranges. The best a multimeter can do is around 100000uF or 100mF or 0.1F (All 3 are same value), they do not handle F and some meters cannot do more than around 100uF.
Multimeters are not very precise when checking capacitors, or maybe it is better to say that some capacitors are not very precise. The value may change with temperature, frequency and voltage and some capacitors leaks current. This do not prevent multimeters from being useful to check if a capacitor is close to expected value or establish the value of an unknown/unmarked capacitor.
Multimeters only check the capacity, but especially with old electrolytic capacitors the parameter called ESR (Equivalent series resistance) is important to check. In some cases, especially with higher voltage, leakage current may also need checking, this requires another type of meter.

DSC_7760

Letís first do a test without any capacitor, this is 0.087nF or 87pF, that is not correct. Specifications for multimeters nearly always says to use the REL/NULL button in the low capacity range, letís try.

DSC_7761

Now it shows 0 (It changes up to 0.003). This is much better.

DSC_7762

First capacitor is marked 27pF, it must be well above that?

DSC_7765 DSC_7781

Letís check with a real LCR meter, capacitor is in the slot just above the input terminals. It says 26.91pF, this means the error was from the multimeter, not the capacitor (In my reviews I only check down to 1nF).
With an LCR meter I can also check at different frequencies, here I measure at 1kHz and 100kHz. This capacitor does not change with frequency.

DSC_4620

There is a way around this, first I measure a 1nF capacitor (The 27pF is NOT connected).

DSC_4622

Then I put the 27pf in parallel with it and measure again. Now I can calculate the values as 1092pF-1069pF -> 23pF, this is a much better result. This method is described in the manuals for some multimeters.

DSC_4630 DSC_4629

When measuring low values, the test probes will also affect the result, if the two test wires are close or not can easily affect the result with up to 50pF! To avoid that use very short test leads, twist them or be sure they do not move after using REL (This was not the problem before).

DSC_7763

A 2.2nF works much better.

DSC_7764

Same with a 10uF film capacitor.

DSC_7758

But this 3900uF capacitor is too large for the meter. It took the meter a long time to discover that.

DSC_7774

Letís try another meter. On this meter there is one digit less in the low capacity range.

DSC_7775

But the reading of the 27pF is fairly good (The meter took a few second to measure showing 0.01, 0.02 and then 0.03).

DSC_7776

It can also measure the 3900uF. the value is showed in mF (This is normal for multimeters) and the 4.2mF is the same as 4200uF. With electrolytic capacitors the leakage current may sometimes prevent the meter from measure them.

DSC_7779

Connecting a electrolytic capacitor with the correct polarity can be important, even though the voltage from the multimeter is low enough not damage the capacitor.

DSC_7783

Not all meters have problems with small values, this meter handles it nicely.

DSC_7784

It can also handle large electrolytic capacitors.

DSC_7785

Very very small capacitor do not really work, this capacitor is marked 2.5pF and the display changed between 1pF and 2pF.

DSC_7788 DSC_7789

My LCR meters do not believe the 2.5pF, but both insist on 3.7pF.



Diodes

Symbol DSC_7746

Diodes have polarity, i.e. it is important to place the red and black probes correctly. The identification on the diode is often a colored ring (Silver and black in the picture). If voltage is applied according to the + and - current will flow in the diode.

DSC_7726

This diode (1N4007) has a 0.536 volt drop when conducting according to this meter.

DSC_7728

And it is blocked the other way around as it is supposed to be.

DSC_7731

This diode (1N4148) is smaller and has a slightly higher voltage drop.

DSC_7730

And also blocks the other way around. If the diode has 0V in any directions it is dead, OL in both directions also means a dead diode.

DSC_7732

It looks very precise when we get the diode voltage with 3 digits, but with another meter I get another value.

DSC_7733

Same with the other diode. What is correct?
The correct answer is both!

Curve

Here I did curves for voltage vs. current for the two diodes. The difference it because the two meters use different test current. For Southwire the current is around 2mA at 0.6V, Fluke is about 0.4mA.
But the precision is not completely wasted, diodes can be used as temperature sensors where the voltage drops about -2mV (-0.002V) for each degree the temperature increases, the meters has enough resolution to make a rough temperature estimate.

DSC_7803

Not all diodes are around 0.6V, a type called Schottky diode has considerable lower voltage in the on direction. These diodes are often used to reduce loses in switch mode power supplies, but they are also used in other places.

CurveSB

The voltage across a Schottky diode is around 0.2V



Leds

Symbol DSC_7745

The LED uses the same symbol as a normal diode, except a few arrows are added to symbolize that it radiates something. For 3mm and 5mm leds it is usual the long pin that must be connected to plus.

DSC_7735

Leds are a type of diodes, but have higher voltage drop. The red one is about 1.8V. There is light in it, but due to the light used for taking photo, it cannot be seen.
Like diodes it is important to put the probes on the right way around, it is often the long pin that need the red probe, but not always.

DSC_7737

Yellow has slightly higher forward voltage drop.

DSC_7736

And green a tiny bit higher again.

DSC_7738

The blue led uses another technology and has much higher voltage drop.

DSC_7739

The white led is similar to the blue led. It is a blue led, but with phosphor to convert the blue light to white light. There are also some red, green and other colors that uses phosphor leds, they will all have about this voltage.

DSC_7740

Leds blocks the other way around. If the led has 0V in any directions it is dead. The OL can be either due to the meter not supporting the led type or because the led is dead. On some meters the led might light up, even when the meter shows OL (This, of course, means the led works).

DSC_7742

This meter uses a slightly different test current and the voltage is different (See curve below).

DSC_7744

But what is this, is my white led defective? The answer is no, but the meter cannot check white and blue leds. Multimeters come in 3 types for diode testing: In led bulbs for mains there are often used more leds inside each chip, i.e. they cannot be tested with a normal multimeter.

Curve

The forward voltage depends on color and current, this means different meters will show different forward voltage.
This voltage depends mostly on the led color, not as much on package like SMD, 3mm, 5mm or power (There will be a small variation depending on size of led).

Testing in circuit with power off is sometimes possible. The result must not show 0 volt or a voltage above the expected forward voltage. If the forward voltage is lower than expected, it may be the circuit that is doing it.



Transistors

DSC_7619 DSC_7620

Many cheap multimeters includes a build in transistor tester, it is marked "hFE" on the range switch, that is the letters used for gain in a transistor. This tester is fairly useless for a couple of reasons: It do also compromise the safety of the meter (Maybe not as much on a cheap meter, they are often low safety anyway). Safety distance from metal part inside meter to where you can put your finger is fairly low on a transistor tester socket and if you have left a transistor in the socket it is non-existent (The inside of a multimeter can be at high voltage when measuring high voltage).

DSC_7608 DSC_7610

Here are 3 of the transistor sizes: SMD small signal (SOT23), leaded small signal (TO92) and power (TO220) and the two common transistor types: BJT and MOSFET, as I said above only one of these are supported.

DSC_7611

A transistor of the right type in the socket and the range switch in the hFE position, then I can see the gain (at a unspecified current). This transistor obvious work or do it?

DSC_7612

What about a resistor? This resistor has a gain of 300, i.e. it is better than the transistor to amplify current? This is, of course, wrong. The problem is that the tester only measures current, the base current is defined with a resistor. Anything that passes current from C to E connection will be shown as a working transistor with some gain, even a resistor or a damaged transistor.



Transistors again, BJT types

When not using a transistor tester, how can a transistor then be tested?

NPN PNP

It is not that difficult, the above schematic shows how a transistor will measure. It is basically two diodes and can be tested with any multimeter that has a diode test mode. There is only one problem and that is knowing what pin is what.

TO92Pinout1 TO92Pinout2 SOT23Pinout1 TO220Pinout1

Here is some of the most common pin-outs. In SMD a package very similar to TO220 is often used, it has the same pin-out.

DSC_7639
DSC_7640

For demonstration I use a transistor in TO220 package, it is large and easier to take pictures of.
The centre pin is the same as the tab on the TO220 package, this is often used in circuits and can also make it easier to measure.

DSC_7642
DSC_7641

First test is the B to C path, with the red pin on B the diode has a forward voltage and will conduct, the other way around the diode is blocked. This is a NPN transistor, a PNP would be the opposite way as can be seen on the diode symbols at the start.

DSC_7643
DSC_7644

The E to B path is tested the same way and gives the same result.

DSC_7645
DSC_7646

E to C path is blocked both ways.

DSC_7647

But things are not always that easy, this transistor shows a diode from E to C, is something wrong?

DSC_7656

There is also a resistor (Note meter is in ohm) from B to E, what is going on?

TIP120

Here is the schematic of the transistor, it is a Darlington transistor (Two transistor connected in this way) with some resistors and a diode. The resistor is not exactly the same as the schematic shows, but close enough.
With some meters you have to be careful when measuring the resistor, it is in parallel with a diode and if the ohm measure voltage is high enough to turn the diode on, the value will be wrong. Solution: Swap pins and measure the other way around or manually select a higher range (It will usual mean lower test voltage for the same resistor)


The diode tests will usual also work in-circuit with power off, except the blocked paths may not show OL, but instead a value. If they are close to 0, something is wrong and you probably need a new transistor.

TO92Pinout1D TO92Pinout2D SOT23Pinout1D TO220Pinout1D

Cheat sheet for NPN transistors, place the red probe at the back of the arrow and black at the point of the arrow to measure a diode drop. For PNP it is the opposite way around.



Transistors again, MOSFET types

symbolnmos NFET         symbolpmos PFET

Here is the symbol for a N-channel and P-channel MOSFET and how it looks when measuring on it. What can usual be checked is that the diode is there and the switch is open. The switch will only be open if the G voltage is 0, with enough positive voltage (For N-channel) on G the switch will close.
This MOSFET is called a enhancement mode MOSFET transistor, it will turn on when a voltage is applied to the gate, how much voltage depends on actual type. There exist another FET type called depletion mode, that is on until negative voltage is applied. This type is very rare as MOSFET, but is sometimes used in JFET versions (A JFET replaces the capacitor with a diode in the non-conductive direction), these are also fairly rare.

SOT23Pinout3 SO8 SO8dual

Some common SMD versions. The first SO8 is usual easy to see on the circuit board, because most pins are connected. The second SO8 is two transistors in one package.

TO92Pinout3 TO220Pinout3

The most common leaded versions.

DSC_4655

The centre pin and the tab is connected as usual for TO220 packages.

DSC_4652
DSC_4661

The gate is not connected to anything, I could connect test probes either way and the result would be the same.

DSC_4662
DSC_4663

G is a capacitor to S and D.

DSC_4660

Before testing, be sure the gate is discharged, this is done by shorting G to either S or D.
Correctly G must be shorted to S, but because there will be a short between S and D if there is voltage on the gate capacitor, shorting to D will also work.

DSC_4651

The diode is working.

DSC_4650

And the MOSFET is blocked the other way around.

DSC_4661

A little trick, if it works depend on the diode test voltage (3V or more is best) and the actual MOSFET.
I am measuring between S and G with the positive pin on G (For a N-Channel MOSFET), this means I am charging the G capacitor and turning the MOSFET on (If I have enough voltage in diode mode).

DSC_4653
DSC_4654

The mosfet is turned on.

MOSFET can often be measured in-circuit with power off, there the gate will usual be discharged, i.e. the diode can be checked and there must not be around zero ohm between S and D.



Other

The multimeter can be used to test many other components and parts, here a few examples.



Bridge rectifier

Bridge

A bridge rectifier is 4 diodes and each diode can be tested.

DSC_7807

Here I am testing one of the diodes.

DSC_7804

Testing from + to - is also possible, here with the red probe on +.

DSC_7805

Moving the red probe to - show two diode voltage drops (Beware, not all multimeters can handle that).



Relay

Relay

A relay is a coil (Measures as a resistor) and some contacts (The pin numbering on the relay do not match the one I test below).

DSC_7810

I can check that the coil have a non-zero resistance.

DSC_7811

And I can check the normally closed (NC) contacts. The normally open (NO) contacts will read OL or the resistance of my fingers. Continuity can be used for this test.
The relay test is not complete, because the NO contacts is not tested in closed condition, this would require some external power to activate the relay.



Passive buzzer

DSC_4637

A buzzer is a capacitor and will measure as such, it may also be possible to hear a tone from it, but that depends on the meter.

DSC_4638

Another buzzer.



Active buzzer

DSC_4639

A active buzzer marked with + and -, here I use diode mode and the buzzer will make a weak sound.

DSC_4641

Even this 12V buzzer will sound with the diode voltage.



Switch

With a switch it can be useful to check if it is momentary or toggle, connections of the pins, especially if it has a indicator light and for old switches also if it even works.

DSC_4642

The indicator light, it is a red led. A incandescent bulb will show a value higher than a direct short, in ohms it may be from a few ohms to a few hundred ohms.

DSC_4646

I can also test the rest of the switch in diode mode, 0V means a connection.

DSC_4645

Continuity is faster, because I do not need to look at the display, I can just listen for the beep.



What to look for in a multimeter

For component testing you have to look for these parameters:


Alternative ways to test components

There are alternative devices to test components, some more precise, some more convenient, some more universal, some cheaper. Common for all dedicated component testers is very basic input protection, they are not supposed to be plugged into mains voltage like a multimeter and do simply not need much protection (Be careful with charged capacitors around component testers, they cannot handle them).

The cheapest type is a circuit board with a microprocessor and a display and not much else, it can test many component types and value.
I have tested a few of them:

Component tester Multifunction Tester T1
Component tester MK-168
Component tester Fish8840


One brand have made their own series of component testers, they are not as cheap or universal as the above, but higher quality:

Semiconductor tester Peak DCA75
LCR tester Peak LCR45


And then there is LCR meters, they use AC with selectable frequency to test resistors, capacitors and inductors:

DSC_7767 DSC_7771

Bench meter versions of these meters have better precision and may also be able to apply DC voltage/current to measure at the correct operating parameters.



Notes

I show a different way to do low ohmic measurement here Measurement on Flashlight This method can be used with simpler power supplies, but requires a power resistor and more calculations.