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What's the difference between voltage and voltage drop?


Samurai Appliance Repair Man

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You're measuring voltage at the board between two points in a circuit for a specific load. You measure rated voltage but the load isn't functioning. What one additional electrical measurement will tell if you're measuring voltage or voltage drop?  

19 members have voted

  1. 1. What distinguishes voltage from voltage drop?

    • Drop
    • Dunno
    • Phase
      0
    • Amps
    • Volts
      0
    • Nuttin'
      0
    • Ohms
    • Temperature
      0

Bonus question: What are the troubleshooting implications either way, voltage or voltage drop? 

13 Comments


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Voltage across a load means that there is an open somewhere in it.

Voltage drop is the normal measurement across a load if current is flowing through it.

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  • Team Samurai
Samurai Appliance Repair Man

Posted

2 hours ago, LouisM said:

Voltage across a load means that there is an open somewhere in it.

You mean: NO voltage across a load means that there is an open somewhere in it. [By "it" I assume you mean the power supply circuit for that load.]

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If the load has an open, such as an open motor winding.

Troubleshooting implications:

If the heater element has voltage present, but no heat, you can check for resistance, or use the clamp meter to measure current.  A voltage drop would indicate that the element is working properly, so the problem must lie elsewhere.  In-spec current measurement says that the LOI is good, so you need to move over to the second load of interest.

 

 

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  • Team Samurai
Samurai Appliance Repair Man

Posted

21 hours ago, LouisM said:

If the load has an open, such as an open motor winding.

If the open is in the load only, there will still be voltage across it. But is this voltage or voltage drop? 

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ARARichmondCA

Posted (edited)

Looks like I am the 1 out 5 who answered ohms. I thought voltage drop across a load depends on the ohms of that load. Maybe I better go back to studying! I understand that by voltage, we are talking about source voltage or the voltage of the power supply and by voltage drop we are referring to the voltage across a specific load which is determined by the resistance of that load. So, if we measure across the load and get source voltage, the voltage drop will equal the source voltage if it is the only load the circuit. 4 people said one would measure amps next to determine if we were reading voltage or voltage drop. I'm not sure how measuring amps would lead us to a determination. But I would love if someone could explain it to me because I am obviously missing something basic.

Edit: We could read 120 volts across the load and if it were not functioning, we would just be reading L1 to N (voltage) rather than voltage drop. But, if there is amperage present, then it should be in correct proportion to the Resistance of the load 120 vac= 3 amps x 40 ohms or 2 amps x 60 ohms, etc). In such a case the component would be getting current and NOT doing work, so it would have to be some kind of mechanical failure or defect in that load, correct? If there were no current flowing, we would just be getting a reading of the L1 side and neutral side. Am I on the right track? 

Edited by ARARichmondCA
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  • Team Samurai
Samurai Appliance Repair Man

Posted

16 hours ago, ARARichmondCA said:

I thought voltage drop across a load depends on the ohms of that load.

Although that is true, that's not the question here.

To expand on the original question, the scenario is that you're making a voltage measurement between two points in a circuit. How do you know if you're looking at voltage or voltage drop on you meter? 

 

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ARARichmondCA

Posted

Thank you. If there is current, you are measuring voltage drop. If there is no current, you are measuring voltage. Hence, the next test to make is amps. Then replace the component either way, right? I'm playing parts changing monkey's advocate here because I see one in the lirro and I am trying to evolve! As a green tech who blindly gropes for solutions, it's the only thing I know to do next. But I have a feeling you are going to tell me something different. 

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In a series circuit with more than one load or a bad switch (higher resistance than 0), you will get a voltage drop across each load. if a switch is bad feeding a load it could have enough resistance to drop enough voltage to keep the load from working and could even damage the load?

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I should have also added you can drop voltage across bad connections at plugs and connections of all kinds throughout a circuit keeping the correct voltage from getting to the load.  

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Son of Samurai

Posted

3 hours ago, suampman said:

In a series circuit with more than one load or a bad switch (higher resistance than 0), you will get a voltage drop across each load. if a switch is bad feeding a load it could have enough resistance to drop enough voltage to keep the load from working and could even damage the load?

Yes, a switch that has gone high-resistance (but not completely open) will drop voltage and effectively act as a load in series with any other loads in that circuit. Current will still flow through the circuit, and there will still be voltage drop across the loads, but it will be less than the spec.

Unlikely that this would cause damage to the loads, but it does mean that they might not get enough voltage to function properly. The bigger risk is that the high-resistance switch will start producing enough heat to melt things.

A circuit with a switch that has gone completely open, on the other hand, will have no voltage drop or current anywhere. But you will measure voltage across that open switch.

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Yes, I agree. 

Getting back to the original question voltage verses voltage drop: total voltage should be equal the total of the voltage drops in the circuit. If you are talking electrical circuits It would be junctions switches loads etc.. If electronic circuits, resistors etc. (I no longer work on circuit boards. Just troubleshoot to them and change them.) Many different things can cause drop voltage. Sometimes they are not supposed to drop voltage, sometimes designed to work that way.

If a motor should be started at 120 volts but something in the circuit is dropping that voltage down to 80 or 90volts it will heat up and burn a winding pretty quick cause it can't get started. Of course that depends on the type of motor also. 

I do appreciate these questions and answers because I have to get back to thinking about the basics. Thanks.

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