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Your loading meter will not pickup junk (or ghost) voltages because it places too much of a load on the circuit being tested and so bleeds them off. Refer to the Voltage Measurements and Meters Webinar. Only the DMM can pick these up because of its much higher input impedance.
My loading meter (the Wiggy– again refer to the webinar) can accept specialty probes that let me get into tight places for back probing.
My Fluke 117 has a LoZ function (low input impedance– loading) which also accepts specialty probes. Like the Wiggy, it is not faked out by ghost voltages, either.
Make sense?
Glad you could attend the webinar, Keith!
Sounds like you’re referring to the GE dryer example we looked at in the webinar. In that example, the RC timer board intermittently supplies Neutral to the timer motor during auto dry. Line is always present at the timer motor whenever the main drive motor is running. So we’re looking for a positive, unambiguous, UNMISTAKABLE indication that Neutral is present at the timer motor and because this 1) completes the circuit for the timer motor and 2) proves that the RC timer board is working correctly.
A DMM can (and does) pickup stray, capacitively coupled voltage which could read 30 vac, 66 vac… some unpredictable voltage we discussed in the Voltage Measurements and Meters Webinar. This leaves you scratching your head and wondering, “Is this because the RC timer board isn’t making a good connection?” In other words, the DMM does not give you a clear unambiguous measurement because of the possibility of ghost voltage.
Loading meters, on the other hand, are not fooled by ghost voltages because they place too much of a load on the circuit and bleed off such voltages so they never have the opportunity to head fake you.
The other benefit with using a Wiggy loading meter in this case is that it tells you if a good Neutral is present without even having to look at the meter because it vibrates in your hand– a really nice feature when working on live circuits because you can keep watching your hands and probes the entire time.
Make sense?
One thing to keep in mind about ohms testing: it is merely preliminary and not diagnostically conclusive. This is because something can test good on ohms but still be bad (failing under load).
The mantra is this: If something tests bad on ohms, it’s bad; but if it checks good on ohms, it could still be bad.
For this test, a savvy tech would check voltage across the timer pins by back-probbing the molex connector with it still connected. If the contacts are good, the voltage across the contacts should be 0 VAC.
But if the contacts are bad, you’ll read some voltage significantly more than 0 VAC. You would read line voltage if the contacts were completely open. You could read something less than line voltage if the contacts are damaged and dropping significant voltage across them as current flows through them. This voltage drop could be enough to keep the motor from getting the full operating voltage it needs to operate.
Make sense?
just a bit nervous about taking apart the appliances. I don’t want to be embarrassed should I stumble.
Don’t worry about disassembly. There are so many online resources available today, especially Youtube, that you can review disassembly procedures before you go out. This would be part of your pre-diagnosis and why you always get the model number before you go out on the call. With the model number, you can look up technical literature at Appliantology and watch Youtube videos that show disassembly.
Compared to the diagnosis, disassembly really is the trivial part of the repair. This is not to say that they’re all easy to do– some require a fair amount of manual dexterity and skill with hand tools– but it’s very straight-forward to understand and see what you need to do. Sweat thou not, my fine young apprentice– you’ll do great!
This video explains the pre-diagnosis process and using Appliantology as an information tool:
https://www.youtube.com/embed/Wz638D6unes
If you’re lucky, you’ll read a valid line voltage that’s actually capable of delivering current. But if you have an open neutral, you may get head-faked into thinking you have a valid power supply and then incorrectly conclude that the load is bad because you were actually reading a ghost voltage. Read this article for more on ghost voltages: https://mastersamuraitech.com/the-low-down-on-loading-meters-and-dmms/
(That article link was posted in the unit and you should have already read it.)
Also, this podcast episode talks more about ghost voltages and loading meters: https://mastersamuraitech.com/master-samurai-tech-radio-episode-3/
– When measuring AC power voltages used to drive a load, use a loading meter only. NEVER use a DMM on these types of voltages unless your DMM has the LoZ function.
Okay like motors, compressors, pumps?
Exactly so. You should use your loading meter (or loading function on your DMM) anytime you want to see if an AC load is getting a valid power supply. Examples of AC loads are as you have listed. There are many others: heating elements, wax motors, lights, etc.
– When measuring a digital logic voltage, such as a data line, use a DMM only.
Like voltage from a pressure switch back to control board to make water level decisions?
Probably yes, depends on the particular circuit.
The data lines I’m referring to are those that operate on low DC voltage (usually 0 to 5 VDC) and consist of pulses which, in turn, correspond to data, 1’s and 0’s, that the computer interprets as information.
An example is the data line between two boards talking to each other, eg., between the computer board and motor inverter board in a variable frequency drive system. You’ll get into this in more detail in the Motors module and in the Advanced Schematics course.
At first I thought maybe when current actually flows the voltage goes away and becomes amp flow.
This is a common “tech myth.” It goes along with the mistaken notion that current flows in magical and mysterious ways, having a mind of its own.
The reality is quite different and is fully explained by Ohm’s Law (in the simple circuits that we deal with as appliance techs):
Electrical current is composed of tiny atomic entities called electrons. They are negatively charged. Their whole mission in “life” is to seek out electrical neutrality. Voltage is a difference in electrical charge or potential between two points. When presented with a voltage difference, electrons in a conductor will always seek to avoid the more negatively charged voltage and seek out the relatively more positive voltage.
Voltage drives current.
A voltage difference between two points creates the potential for electrons (current) to flow IF there is a complete, unbroken path in which the electrons can flow. If the electrons have an unbroken path, then the electrons will flow, seeking to neutralize that potential voltage difference.
You have to use the Ohm’s Law equations to keep your thinking straight about electricity. Never rely on tech myths and stories to understand electricity– math and measurements are the only two means we can understand electricity. And you need both. The math explains how it works and the measurements make it real to us.
But you helped by explaining (I think, tell me if I’m wrong) that when we measure voltage we are actually measuring potential difference between two points?
So with switch open there’s 120vac at L1 and 0vac at N so it measures the difference of 120vac.
With it closed we get 120vac at both L1 and N so difference is zero.
I’m I thinking about it right now?
By proving no voltage difference with switch closed you verify 120 is also present at motor. Correct?
All 110% correct! You have snatched the pebble from my hand!
you force the board to do a defrost diagnostic test and it works during the test, then can we assume its the board?
If by “works” you mean that either 1) I measure a current draw that accords with the expected reading (estimated by Ohm’s Law) or 2) the heater gets “hot” (a less exact assessment but is usually sufficient), then yes.
Power for the defrost system circuit is supplied completely by the computer board. If defrosting is not occurring automatically, the most likely problem be an algorithm. The other possibility is a compressor relay stuck closed, keeping the compressor running during defrost, which would cause the evaporator to not defrost completely. But since this relay is also located on the computer board, the solution is the same: replace the board.
As you hopefully learned in the Temperature Sensors webinar recording, bimetals are notoriously flaky. Knowing this, I would replace the bimetal as well even though is was not proven to be a proximate cause of the defrosting problem in this case due to my functional understanding of bimetals. Also, 1) I would already most likely have the freezer section torn down anyway to clear the frost from the evaporator so the additional labor to replace the bimetal would be minimal and 2) the cost of the bimetal as a component of the complete job rate for the repair is minimal and 3) it results in a more reliable repair because a part with a know-common failure history has been replaced, preventing a follow-on service call at some indeterminate period of time after replacing the computer board.
Make sense?
When you tested the switch at the control board connection, could not the open been in the wire?
It is possible that the open could have been in the wire (or at the connection point with the door switch) but not probable.
Keep in mind the point of this video (and all videos on that unit, titled, “Electrical Measurements in Appliance Repair”)– to illustrate basic electrical measurements in appliance repair. In this case, I was illustrating the use of continuity measurements in appliance repair.
Module 3 is still Basic Electricity where we are teaching foundational electrical concepts. We’ll apply these later on in Module 5, Troubleshooting.
Basic switch function: A closed switch should act like a piece of wire (if the switch is good).
Every uninterrupted point on a wire looks identical to electrons. An ideal conductor (wire) should have zero resistance to current flow (in reality, there is some measurable resistance, especially with long runs of wire, but we ignore that when troubleshooting circuits). If resistance is zero, then Ohm’s Law tells us that the voltage drop will be zero:
E = I * R
In other words, if you take your meter, set it measure AC volts, and measure the voltage across any two uninterrupted points along that wire, you will measure zero. Every point along that wire in continuity with each other will have a voltage potential difference of zero. These are the two criteria for Electrically Equivalent Points (EEPs).
The motor was determined to be bad because I proved that it was getting a valid power supply yet not running. This is basic troubleshooting: testing inputs and outputs for a component. If you have an input that meets specification (power supply, in this case) but the output does not meet specification (mechanical rotor rotation in the motor, in this case), then the component being tested is deemed bad by definition because it’s output does not meet specifications.
Make sense?
This is also true! And it illustrates an important point: Electric current (electrons) are driven by a voltage difference between two points. The key word is difference.
If you were to measure the voltage in the conductor under the bird’s feet, you would measure 0vac. But there really is voltage on that line– about 14,000 v for most grid lines– but that’s with reference to the earth, or ground.
If you put one of your meter probes under one of the bird’s feet and then put your other meter probe on the grounding conductor, you would measure a large voltage, whatever the grid voltage is at that location.
So whenever you talk about voltage at a particular location, you also have to talk about what the reference is for that voltage.
I’ll give you the simple, one-word answer then elaborate more if you need it: shunt
The bird’s body would be a load to the current. The conductor is shunting that load.
Shunts are special cases. They look like parallel circuits but they are not because the resistance in one branch is effectively zero so the equation for equivalent resistance in parallel circuits is undefined (divide by zero error). Meaning that shunts are not parallel circuits at all. They are just shunts.
Also an important followup for students– These forums are for questions pertaining to your course work.
Specific appliance repair problems should be posted at Appliantology and include the complete model number so we know what we’re dealing with.
Before troubleshooting any appliance problem, you always get the model number so you can research the technical literature. This is SOP for any troubleshooting situation.
If the line seems fuzzy between posting a question here or at Appliantology, ask yourself this, “Do I need a model number to answer my question?”
If the answer is ‘yes,’ then the question belongs at Appliantology.
If the answer is ‘no,’ and the question is more conceptual in nature, then these forums are fine.
Hi Sal,
Post this as a new topic in the Kitchen forum at Appliantology. Be sure to include your model number in your post title so we can pull the tech sheet and take a look.
Thanks!
Scott
Hi John,
First off, you should only ever make AC load voltage measurements with a loading meter, such as a Wiggy, or the LoZ function on your DMM. This completely avoids the issue of getting faked out by junk voltages, aka, ghost voltages. More on that in this blog post:
https://mastersamuraitech.com/the-low-down-on-loading-meters-and-dmms/
Whenever possible, you want to make these measurements with the load connected. This is because you want to observe the circuit while preserving its actual operating conditions. Also, if the Neutral is gated by a triac, it requires a certain minimum current flow before it will “switch on” and close the Neutral, completing the circuit.
You can measure the supply voltage to the load with it still connected by “back probing” the molex connector. These can usually be done at the control board for convenience. Sometimes, it’s a tight fit so you may need to use specialty probes to facilitate this measurement. Here’s a nice set from Pomona: http://amzn.to/1OlFj6V
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