Posted on August 17, 2015 by - Tech Talk

Do you know the difference between a loading meter and a non-loading digital multimeter (DMM)? Both are critical troubleshooting tools for appliance repair and both belong in your tool bag. In this post, I'll explain the difference between these two types of meters and the situations when you would want to use each type of meter. Read, watch, and be illumined!

The Importance of Input Impedance

Most digital multimeters sold today are for testing electrical and electronic systems, such as those commonly found in appliance repair, and have high impedance input circuits, typically greater than one megohm (1 million ohms). Impedance is the term used to describe total circuit resistance which includes regular resistance, as well as resistance from reactive components such as capacitors and inductors.

Fun math fact you can use to impress the ladies: The symbol used for impedance in mathematical equations is Z. Total input impedance, Z, is the sum of resistance, R, plus capacitive reactance, Xc, plus inductive reactance, Xl.

Z = R + Xc + Xl

Once you know the total impedance, Z, you can treat it just like you would total resistance, R, in any of the Ohm's law equations.

The takeaway point here is that impedance is the term for all types of resistance in electric circuits.

A DMM’s high input impedance means that when it is placed across a circuit, it will have little impact on that circuit because it will draw hardly any current, not even measurable using common equipment. You want this for most voltage measurement applications, and it is especially important for sensitive electronics or control circuits. On these types of circuits, if you draw any measurable current with your meter, you could affect the circuit by inducing a failure mode known as "loading down,” and your measurements would be meaningless.

In contrast, other troubleshooting tools such as solenoid testers generally have low impedance input circuitry of around 10 K-ohms (10,000 ohms) or less, which means that they will draw some current when placed in a circuit. These are called loading meters because in drawing significant current they are, by definition, placing a load on the circuit being tested. While these meters aren't fooled by ghost voltages, they should only be used for testing power circuits or other circuits where the low impedance will not load down the circuit voltage or alter circuit performance. Their great strength, however, lies in the very fact that they aren't fooled by ghost voltages or open neutrals.

Ghost voltages and open neutrals are two of the major landmines waiting to trip you up while you're troubleshooting a tricky circuit on a service call. For this very reason, I always keep a loading meter in my tool bag.

I’ve encountered techs who don’t see the need for loading meters, but they learn real fast after they've wasted a lot of time or gotten their asses kicked on a service call chasing ghosts or open neutrals. If it hasn't happened to you yet, then this just means you have some fun learning experiences to look forward to!

Open neutrals are pretty self-explanatory – where the neutral side of a circuit is open either due to a break in the wire or high resistance (burnt, corroded, loose, etc.) connection. But let's talk more about those scary-sounding ghost voltages.

What are Ghost Voltages and Where are they Encountered?

Ghost voltages occur from having energized circuits and non-energized wiring (such as a “dead” wire that should be energized but has an open connection to either Line or Neutral) close together, such as in those wire harness bundles commonly found in all major appliances today. This can result in a buildup of static charge that a high-impedance meter (a DMM) will read as voltage if you place its leads between the open circuit and the neutral conductor. A low-impedance loading meter, on the other hand, will not be fooled by this ghost voltage because its high current draw will immediately discharge the static buildup.

Ghost voltages can sometimes be 80% of the actual Line voltage. Spooky! So in a 120 vac power circuit, ghost voltages will often be in the 75 to 95 vac range. In some of my videos, you'll hear me refer to these as "junk voltage." Same thing. If you don't recognize ghost or junk voltage when you see it, you'll end up wasting additional time on service calls chasing your tail and going down rabbit holes. Or worse, you'll get completely faked out and confused, unable to solve the problem.

Examples of common places you'll encounter ghost voltages in appliance repair situations are a wire with an open thermal fuse that’s near a live wire or an open neutral wire in a wire harness.

So hopefully you can see from what we’ve talked about that one of the desirable characteristics of a loading meter (also called a LoZ meter, by the way) is that it have as low an input impedance as possible or practical and, as a result, a high current draw.

With a DMM, on the other hand, you want as high an input impedance as you can afford so it draws hardly any current and thus doesn’t affect, or load down, the circuit being tested.

Now that you have a background on loading versus non-loading meters and low input versus high input impedances, let's watch a video showing a practical comparison of a couple of different types of meters. It gets really crazy as I use one meter to measure other meters. It’s Meter Mania!

### 7 Responses to “The Low Down on Loading Meters and DMMs”

1. matt nei on

I’m trying to wrap my head around this. The DDM has high overall resistance, or said another way the circuit of the meter has high overall resistance. Which means less current moving through it’s circuit, and thus less of a load. Is that correct?

Tim

• Scott Brown on

Exactly right!

Here again, Ohms Law keeps your thinking straight on the relationship between voltage, current, and resistance. In fact, it’s impossible to understand electricity without knowing Ohms Law.

Ohms Law says E=I*R or I=E/R.

From this simple equation, we see that as R increase, I decreases. As the input resistance increases, the current drawn by the meter decreases.

Z is a term for resistance which includes resistance from capacitors and inductors which is called reactance, X. In other words, Z=R+X. We can substitute Z for R in the Ohms Law equation:

I=E/Z

Here we see that a low Z meter will draw more current than a meter with a high input Z.

Ees seemple, da? Anyone can understand electricity using 7th grade math.

2. Keinokuorma on

One thing, if you can get your hands on one… an old style analogue meter. While they have their certain shortcomings they are fast to read, they mostly load circuitry enough not to be fooled by ghost voltages, and they visually show fluctuations. Also they mostly don’t require batteries to measure voltage or current.

• Scott Brown on

This is a great point and I’m glad you raised it. I almost included this in the post but decided against it because I wanted to stay focused on the concepts of loading vs. non-loading meters, which is totally new information for most appliance techs.

I also believe that the modern DMMs are superior for most troubleshooting situations on modern appliances where we’re testing inputs and outputs of digital control boards.

For AC power supply measurements, it’s more economical to carry either a separate wiggy (rugged, low cost) or use the LoZ function built into some DMMs. Just makes for less stuff to carry into a house on a service call and frees up some room in the cramped tool backpack.

But analog meters do have the advantages you mention and are generally not fooled by ghost voltages. But because of their low input impedances, they are not recommended for testing computer control boards DC power supply outputs because they can load down these voltages and cause misleading results.

• Keinokuorma on

Modern DMM’s can carry analytically important features, like DC quality (ripple amplitude and RMS effect etc), logical high/low indicator and the like, doing all this at the same time as measuring the main variable. Analog meters lack the speed required for ripple and seldom have a logic state indicator. They can, if carelessly used, load a logical signal line low and cause anything from an erratic interpretation of the result to malfunction of the measured system.

For analyzing slow fluctuations on a supply rail of a board, etc, the AMM may be easier to read when comparing with other machine events. Say, if everything goes haywire when triggering a certain relay, and the only other event associated with that is a drop of supply voltage to the board, a bad supply may be the culprit. Yet the DMM reading might not update often enough to register the momentary change.