Checking voltage on a board with a multimeter

[Pcmaker]

Super easy question. Probably laughable to you guys.

On high voltage circuits, such as residential plugs, you put the red multimeter lead on the hot and the black lead into the neutral or ground to check the receptacle. However, I have little knowledge on how to do it on circuit boards.

On electronics with a 2 prong plug, without a ground on the plug:

If you have the board plugged in in front of you and you use a multimeter to check voltage, you put the red meter lead on the pin and the black on the neutral or ground. Is there a "ground" on these boards? Can you check AC and DC voltage by placing the red lead on a pin you want to check and black lead on the neutral mains coming in to the board? Or is there a specific spot on the board you put the black lead on?

[Brumby]

Firstly, there will be people concerned that you might be probing around mains power without this fundamental knowledge.  I hope you don't - but if you do, I hope you are using a meter that is properly rated.  That is what the Cat ratings are all about.

Now ... on to the question.

The first thing you need to do is separate the idea of "ground" from your measurement thinking.  There is no fundamental need for a "ground" point anywhere in a circuit.  What you are calling a "ground" is actually a reference point.  The fact that it is connected to the soil under your feet is neither here nor there as far as voltage measurements on the circuit are concerned.

In its purest sense, a reference point can be chosen quite arbitrarily - but there are some conventions which tend to be followed, especially when there is a DC voltage circuit behind a power supply.  In these circuits, there will tend to be two power rails - the one with the higher potential will be called the positive and the other will be called the negative ... but even these labels are misleading when it comes to measurement.

The convention is that the "negative" power rail is assigned as the reference point - and all voltages are measured with respect to that.  In the case of a polarity sensitive analogue meter, this means the "negative" lead (usually black) must be connected to this point, so that all voltage measurements taken will be from points that are higher in potential.  Digital meters usually don't have this restriction, since they cater for polarity in their circuitry and will show a minus sign if need be.

Things get more interesting with split rail power supply circuitry - or with multiple power rails - but the principle is the same.  One point will be the reference for any given measurement.  Whether it is connected to an actual ground is a separate consideration.

[Pcmaker]

Firstly, there will be people concerned that you might be probing around mains power without this fundamental knowledge.  I hope you don't - but if you do, I hope you are using a meter that is properly rated.  That is what the Cat ratings are all about.

Now ... on to the question.

The first thing you need to do is separate the idea of "ground" from your measurement thinking.  There is no fundamental need for a "ground" point anywhere in a circuit.  What you are calling a "ground" is actually a reference point.  The fact that it is connected to the soil under your feet is neither here nor there as far as voltage measurements on the circuit are concerned.

In its purest sense, a reference point can be chosen quite arbitrarily - but there are some conventions which tend to be followed, especially when there is a DC voltage circuit behind a power supply.  In these circuits, there will tend to be two power rails - the one with the higher potential will be called the positive and the other will be called the negative ... but even these labels are misleading when it comes to measurement.

The convention is that the "negative" power rail is assigned as the reference point - and all voltages are measured with respect to that.  In the case of a polarity sensitive analogue meter, this means the "negative" lead (usually black) must be connected to this point, so that all voltage measurements taken will be from points that are higher in potential.  Digital meters usually don't have this restriction, since they cater for polarity in their circuitry and will show a minus sign if need be.

Things get more interesting with split rail power supply circuitry - or with multiple power rails - but the principle is the same.  One point will be the reference for any given measurement.  Whether it is connected to an actual ground is a separate consideration.

Yeah, it's confusing to me since I've only been working on high voltage stuff. 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?

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.

[Mr. Scram]

Yeah, it's confusing to me since I've only been working on high voltage stuff. 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?

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.

All a voltmeter does is measure the voltage difference between two points. On a PCB, ground is considered the reference point and therefore "zero". It's not actually zero in any empirical sense, but arbitrarily decided to be zero. However, all other voltages are referenced to it. So, if something is indicated to be a 9 volt power supply, we simply mean to say say it's 9 volts above ground. The difference between the positive of the power supply and the ground is 9 volt.

If you want to measure the voltage coming in, use one lead for the incoming voltage, the pad, and one lead for the zero reference, ground.

Note that you can also use other parts of the circuit as a reference, but remember that you're always measuring the voltage difference between the two points you are probing. Also note that you normally shouldn't probe both the AC and DC side of a circuit at the same time, which your question seems to suggest.

[Mr. Scram]

Is it a device with an AC to DC converter in it, or is it a device that works on AC, and in what area is the part you're trying to probe?

[Pcmaker]

Is it a device with an AC to DC converter in it, or is it a device that works on AC, and in what area is the part you're trying to probe?

It's a DeWalt battery charger. It's the missing diode to the left of the transformer. When I put my multimeter on DC and put the leads on the pads of where the diode used to be, on the back of the board, I'm getting 0 voltage.

The diode was bad. I got 0.000v either way on the diode with the tester.

It's labeled D3 and it's to the left of the transformer on this picture.

[Pcmaker]

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.

[Awesome14]

AC mains power, i.e. residential electrical power, in the USA, is 240VAC. It's divided in half, so most of it is 120VAC. This is NOT high voltage. Hi voltage in an electronics sense is 1,000 volts or more.

More to the point, I question how you checked your diode. Did you measure the continuity both ways? Were you working on a live circuit? Do you have a diode-check function on your meter? Working on devices plugged into the wall is a good way to do harm!

Thirdly, mains power has a ground and a neutral. The neutral is actually the negative. The ground and the neutral join at the breaker panel. Electronic circuits have nothing called neutral.

And last, when you measure voltage, you are measuring potential, not current (amps). The meter does not complete the circuit when you measure potential, because the input impedance of the meter is quite high. Otherwise, when you measured the voltage present at a wall outlet, you would complete the circuit and trip the breaker.

If you want to check the voltage at the solder pad of the diode, solder a wire to it, connect the wire to a meter, and then power up the device.

[Pcmaker]

More to the point, I question how you checked your diode. Did you measure the continuity both ways? Were you working on a live circuit? Do you have a diode-check function on your meter? Working on devices plugged into the wall is a good way to do harm!

The charger wasn't plugged in. I measured the diode with the diode function of the multimeter. I measured both ends. I usually get .400 to .900 volts one way and OL the other way.

First, I measured the diode while it was still on the circuit board. Then I desoldered the diode and measured it again with the diode testing function of my multimeter.

I get 0.000 both ways.

If you want to check the voltage at the solder pad of the diode, solder a wire to it, connect the wire to a meter, and then power up the device.

Wouldn't you get the same reading if you put your leads on the pads as soldering a wire to the pads then putting the leads to the wires?

[Awesome14]

The idea is that you don't need to manipulate the charger and probe around. You'll get the same reading, but you won't slip and short something out. But everyone learns in his own way. I hope you get it fixed.

[David Hess]

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.

A current does flow between the probes but when making voltage measurements, it is very small.  On a good multimeter it will be 0.1 microamps per volt of DC and about twice that per volt of 60 Hz AC and on a cheap multimeter it might be 5 to 10 times higher which is still very small.

For ease of use, I replace the negative probe with an alligator clip so I can leave one hand free.

[Brumby]

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'.

[Mr. Scram]

A current does flow between the probes but when making voltage measurements, it is very small.  On a good multimeter it will be 0.1 microamps per volt of DC and about twice that per volt of 60 Hz AC and on a cheap multimeter it might be 5 to 10 times higher which is still very small.

For ease of use, I replace the negative probe with an alligator clip so I can leave one hand free.

To be absolutely clear, for almost all practical intents and purposes, no current flows. If you assume that nothing flows through the meter while measuring voltages, you'll be fine and right in pretty much all of the cases. I understand that people try to be correct, but it doesn't really make things much clearer.

The rule of thumb is that when you're measuring voltage, no current flows through the meter and the meter is no part of the circuit. If you're measuring current with a regular style multimeter, it does become part of the circuit, and as much current flows through it as through the rest of the circuit.

[3db]

Bollocks !!

Volts across a 10M or 100M resistor ?
AC or DC ?
It's important to consider the effect of the instrument when connected to the circuit.
This advice is good for all,even beginners.

[Brumby]

Nobody is saying that you shouldn't consider the effect of a meter connected to a circuit.  It is always something you must keep in mind.

What is being said is that most of the time, that effect is so minor that you do not need to worry about it.

For circuit impedances over 100K I might start worrying about a DMM - but there is a lot of measuring that happens well under that.  In fact, I can't remember when I actually measured a voltage on a circuit with a high enough impedance for it to be an issue.

[David Hess]

When dealing with AC power circuits, sometimes a lower multimeter input resistance is better to prevent errors from leakage currents and capacitive coupling.  Some multimeters intended for power line applications have a special low input resistance voltage measurement mode.  This can crop up with dealing with line side bridge rectifiers where the relatively high diode capacitance and leakage causes false readings.

When I did a lot of work in this area, I kept a 3/4" banana plug with me to insert between my 10M multimeter and test leads with a 120k shunt resistance installed between the leads.

[Brumby]

That's the flip side ..... the effect of the circuit on the meter!

[Pcmaker]

If the DMM measures the difference between the 2 leads, what happens when you're measuring something with 2 hots like a 220-250v outlet? Does it detect that there's 2 hots and switches to another setting?

On the low voltage side of an SMPS, as previously advised using an alligator clip for the neutral side of the multimeter, can I just leave the clip on a single reference point, then use the red lead of the meter to check for voltage on different points of the low voltage side of the circuit? The problem for me is finding a reference point. Can I just clip it to the outgoing side of any diode or negative side of a capacitor?

[David Hess]

If the DMM measures the difference between the 2 leads, what happens when you're measuring something with 2 hots like a 220-250v outlet? Does it detect that there's 2 hots and switches to another setting?

If it is 240 volts AC split phase with two hots and one neutral which is common in the US, then the hot-to-hot measurement will be 240 volts AC and either hot-to-neutral (or ground) measurement will be 120 volts AC.  The measurement between neutral and ground should be zero unless there is currently a load on the neutral.

If dealing with three-phase, then things can get interesting.

On the low voltage side of an SMPS, as previously advised using an alligator clip for the neutral side of the multimeter, can I just leave the clip on a single reference point, then use the red lead of the meter to check for voltage on different points of the low voltage side of the circuit? The problem for me is finding a reference point. Can I just clip it to the outgoing side of any diode or negative side of a capacitor?

The reference point or "common" for the off-line switching regulator is usually the negative side of the output from the bridge rectifier.  This point is separate from chassis ground and the output side of an isolated off-line converter.

[Brumby]

I was typing up an answer - but David beat me to it.

I do have this to add, though...

Does it detect that there's 2 hots and switches to another setting?

No meter will detect anything of the sort.  It will just measure the voltage between two points.  That's it.  It doesn't care which side is hot and whether there is an earth point of not - it just measures voltage between two points.

The problem for me is finding a reference point.

Bingo!  Now you are thinking the right way.