Weird voltage from switch single pole switch

[Jpflex]

Fluke 117 dual impedance meter is best of both worlds. Great trouble shooting meter that won’t break the bank.

What is a dual impedance motor? Most DVOMs I know employ 10 mega ohms of impedance

 

[ptonsparky]

What is a dual impedance motor? Most DVOMs I know employ 10 mega ohms of impedance

It has an additional low impedance setting to weed out those capacitive voltage readings that mean little most of the time.

 

[gar]

230617-1000 EDT

Jpflex:

Most digital meters will have an input impedance of about 10,000,000 ohms on all ranges. This constant impedance is in contrast to a Simpson 260 that has a fixed sensitivity of 20,000 ohms per volt, and an input impedance that varies with meter range setting..

Back to the digital meter. Since its nominal input impedance is constant, and high for different ranges, then because of this high input impedance it presents a small load on the source being measured, and it only takes a small leakage current to obtain a meter reading. By adding shunt resistance in parallel with the meter to lower the meter resistance you can greatly reduce this stray leakage reading on the meter.

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[don_resqcapt19]

Fluke 117 dual impedance meter is best of both worlds. Great trouble shooting meter that won’t break the bank.

If you have an older Fluke meter, or other brand with the same lead connection spacing, that you like, but is only high impedance, you can get their "stray voltage adapter" to use with your meter.Fluke

SV225 Stray Voltage Adapter

• On energized wires, the meter will indicate the real voltage
• On non-energized circuits the meter will read close to zero (even if there are stray voltages)
• It can be used with all modern meters with standard input spacing
• Allows for discharge of residual voltage from capacitors up to 1000 V
• Rated CAT III 1000V, CAT IV 600 V

 

[Jpflex]

W

230617-1000 EDT

Jpflex:

Most digital meters will have an input impedance of about 10,000,000 ohms on all ranges. This constant impedance is in contrast to a Simpson 260 that has a fixed sensitivity of 20,000 ohms per volt, and an input impedance that varies with meter range setting..

Back to the digital meter. Since its nominal input impedance is constant, and high for different ranges, then because of this high input impedance it presents a small load on the source being measured, and it only takes a small leakage current to obtain a meter reading. By adding shunt resistance in parallel with the meter to lower the meter resistance you can greatly reduce this stray leakage reading on the meter.

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What do you mean by adding a shunt resistance in parallel with the meter ? I assume you’re talking about the load being measured by the DVOM leads?

Adding a parallel load to the meter would lower resistance

 

[Joethemechanic]

Adding a parallel load to the meter would lower resistance

That is exactly what you want it to do

 

[Jpflex]

That is exactly what you want it to do

So meter calculated mili amperes draw to determine voltage between points?

 

[don_resqcapt19]

So meter calculated mili amperes draw to determine voltage between points?

Just a load that shunts out the ghost or phantom voltage.

 

[Jpflex]

I’m not sure what this means, do you mean this switch leg voltage disappears? Isn’t voltage only consumed after a final load or resistance?

 

[Joethemechanic]

Think of it this way. Have you ever built a crystal radio? They work best with a really high impedance earpiece. If you hook a standard 8 ohm impedance speaker up to them you hear nothing.

Your digital meter is like the high impedance earpiece. Where as a Simpson 260 is more like that 8 ohm speaker

 

[gar]

230617-1800 EDT

Jpflex:

You need to study electrical circuit theory.

.

 

[hbiss]

I’m not sure what this means, do you mean this switch leg voltage disappears?

As soon as you try to draw some current from it, that's exactly what will happen.

That's also the reason non-contact voltage testers are not to be relied upon.

-Hal

 

[kwired]

I’m not sure what this means, do you mean this switch leg voltage disappears? Isn’t voltage only consumed after a final load or resistance?

Voltage drops across a resistance.

With capacitive coupling you get on an otherwise isolated conductor the source is the "capacitor" but it is a weak source and it doesn't take much current to "short out" the source.

 

[Joethemechanic]

230617-1800 EDT

Jpflex:

You need to study electrical circuit theory.

When I was in high school electronics class in nineteen hundred and seventy six. We had to draw a schematic for a superheterodyne radio receiving apparatus with a pencil and paper from memory in order to pass the course.


Unfortunately younger generations are not taught stuff like that anymore. It's really a shame.

BTW I doubt I could do it now

 

[dkarst]

As others have said, capacitance is in the wires in the wall.

Fluke adapter is $100. You can make your own (less safe, I'm sure) from a Pamona MDP-02 and a 3 killohm resistor (3,000 Ohms, 1/2 watt). Use heat shrink to cover the leads and only use the adapter after you have determined there is no 120 or higher voltage on the wires to be tested.

The low-Z meters such as Fluke 117 and the Fluke stray voltage eliminator both use PTC devices which rapidly increase in resistance if there is current flow. Otherwise they present the ~ 3kohms which drains the coupled voltage and allows the meter to read correctly.
Just be aware if there really is 115 volts present and you put a 3kohm resistor you will quickly dissipate 4W which will overload the 1/2 W resistor.

 

[Jpflex]

As soon as you try to draw some current from it, that's exactly what will happen.

That's also the reason non-contact voltage testers are not to be relied upon.

-Hal

You mean like a “voltage pull-down circuit?” Such as how some automotive vehicles would use a computer module to send out an open circuit voltage from a resistor in series to a switch for a door window.

When the switch is pressed the circuit completes and voltage drops to near ground low potential but draws a current

Wave shows as a square DC wave signal to be used by computer to send power to window motor from another circuit

 

[Jpflex]

When I was in high school electronics class in nineteen hundred and seventy six. We had to draw a schematic for a superheterodyne radio receiving apparatus with a pencil and paper from memory in order to pass the course.

View attachment 2565870
Unfortunately younger generations are not taught stuff like that anymore. It's really a shame.

BTW I doubt I could do it now

I do understand electrical theory. I passed the FCC central electronics technician license back in my 20s as well as advanced level electronics L1 Through ASE

HOWEVER I went into working on automotive computers and DC systems right after so I did not get much exposure to AC, inductance, power factor etc, until 20 years later as of now

 

[Jpflex]

When I was in high school electronics class in nineteen hundred and seventy six. We had to draw a schematic for a superheterodyne radio receiving apparatus with a pencil and paper from memory in order to pass the course.

View attachment 2565870
Unfortunately younger generations are not taught stuff like that anymore. It's really a shame.

BTW I doubt I could do it now

Having to memorize a schematic and reproduce as you described in high school seems extreme but I’m aware how things continue to become “dumbed down” over the years

 

[Joethemechanic]

Having to memorize a schematic and reproduce as you described in high school seems extreme but I’m aware how things continue to become “dumbed down” over the years

Believe me at the time I thought it was extreme. I'm trying to remember if we had to show the values for the components or not. The instructor was an Extra Class Armature radio operator back when you had to demonstrate code proficiency and know a lot of electronic theory. One of those guys that could hear code and it turned into words in his head. Same thing on the key. It was like his thoughts turned right into dots and dashes

 

[Jpflex]

Believe me at the time I thought it was extreme. I'm trying to remember if we had to show the values for the components or not. The instructor was an Extra Class Armature radio operator back when you had to demonstrate code proficiency and know a lot of electronic theory. One of those guys that could hear code and it turned into words in his head. Same thing on the key. It was like his thoughts turned right into dots and dashes

What do you mean by code?