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Author Topic: Accurate Measurements on pulsed system's harder than you think.  (Read 83343 times)

tinman

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Accurate Measurements on pulsed system's harder than you think.
« on: December 09, 2015, 01:59:10 PM »
Today i made a some what confusing discovery that could have large ramifications.
Yes-another look at the humble pulse motor.

For all those that think you can measure some pulsed systems accurately-think again,as it aint just cut and dried.

In this thread,i will show you that using a CVR and scope will not always lead to accurate measurements. I mentioned some time back,that using resistors(CVR's) dose not always give accurate result's,and in some system's your scope could be leading you up the garden path.

The simple pulse motor can be a real bitch to measure P/in and P/out accurately-unlike some of the EE guys may tell you.

In my video series to come,i will show you (using an incandescent bulb) how our scope shows us one thing,and our incandescent bulb shows us the opposite.

Simply by placing a cap within the pulse circuit,we can throw accurate power measurements out the window-->even though our CVR remains on the input side. With pulsed systems,we can increase the current flowing through an incandescent bulb,raise the voltage across that incandescent bulb,but have the bulb dissipate less power-as in the way of heat and light. This is the opposite as to what should be seen across a resistor,as an incandescent bulb is just that-->a resistor. The only way to increase the heat and light output of an incandescent bulb is to increase the current flowing through it,and this in turn will show a higher voltage across it-as the resistance rises as the heat (and light) increases.

This is very important to know,and i will show you that it is wrong. This also means that what you may see on your scope across your CVR may not be actually what is really happening-->the incandescent bulb shows this to be true.

Or is there really some truth to the claim of radiant energy,or some form of cold electricity within the humble pulse motor?.
!! Dose ohm's law always apply with pulsed system's?.

I will post the first video here as soon as it is uploaded.


Brad.

lasersaber

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #1 on: December 09, 2015, 02:44:41 PM »
Sounds interesting.  I am looking forward to watching your video.

gyulasun

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #2 on: December 09, 2015, 02:52:36 PM »

....
With pulsed systems,we can increase the current flowing through an incandescent bulb,raise the voltage across that incandescent bulb,but have the bulb dissipate less power-as in the way of heat and light. This is the opposite as to what should be seen across a resistor,as an incandescent bulb is just that-->a resistor. The only way to increase the heat and light output of an incandescent bulb is to increase the current flowing through it,and this in turn will show a higher voltage across it-as the resistance rises as the heat (and light) increases.
....

Hi Brad,

This is surely going to be a good topic. 
Why I put in bold above your statement is that I think an incandescent bulb is a nonlinear resistor, unfortunately and this fact can indeed lead a tinkerer up the garden path, especially when the duty cycle of the pulse motor current also changes.  ;)

EDIT I added the voltage-current curve of an incandescent lamp, taken from this video at random:
https://www.youtube.com/watch?v=rNsykkSR3wg     Ohm's law is not valid of course.

Gyula

fritz

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #3 on: December 09, 2015, 03:06:17 PM »
.... even changing (+) and (-) tips of your multimeter may completely change the scene. (-) is connected with the shield and has higher capacitive load to ground as the (+) tip.


tinman

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #4 on: December 09, 2015, 03:13:11 PM »
Hi Brad,

This is surely going to be a good topic. 
Why I put in bold above your statement is that I think an incandescent bulb is a nonlinear resistor, unfortunately and this fact can indeed lead a tinkerer up the garden path, especially when the duty cycle of the pulse motor current also changes.  ;)

Gyula

Hi Gyula

Both voltage and current will rise together with an incandescent bulb,in that the more current flowing through it,the higher the voltage across it.. This makes them linear. The incandescent bulb is just a resistor,and ohms law should tell us how much power that resistor is disipating when we know the voltage across it,and the current flowing through it--which we do !! apparently !! :o
With an LED,voltage will hit the rated voltage,and then the current will continue to rise,while the voltage remains close to the same. This makes the LED a non linear device.


Brad

tinman

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #5 on: December 09, 2015, 03:18:15 PM »
.... even changing (+) and (-) tips of your multimeter may completely change the scene. (-) is connected with the shield and has higher capacitive load to ground as the (+) tip.

In the first video,the pulse motor will be running from a battery,and we will be using the scope to measure the voltage across a CVR,and the voltage across the incandescent bulb.
If the avreage voltage across the CVR rises,then that tells us the average current has risen. If you raise the avarage current flowing through an incandescent bulb,then you also raise the avaerage voltage across that incandescent bulb-which means you should be now dissipating more power from that bulb-->in the way of heat and light. The more power you disipate from an incandescent bulb,the brighter it shine's-the more light it puts out.


Brad.

gyulasun

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #6 on: December 09, 2015, 03:18:45 PM »
Hi Gyula

Both voltage and current will rise together with an incandescent bulb. This makes them linear.
...

Just uploaded the current-voltage characteristic of an incandescent lamp in my previous post.  You are right that both voltage and current will rise but NOT linearly with each other like with a normal resistor.
LEDs are also nonlinear loads, and I agree with what you wrote on them.

EDIT  Yes, when we know the average current via a light bulb and we know the average voltage across it, we can use Ohm's law in that case.   8)

Gyula

tinman

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #7 on: December 09, 2015, 03:29:13 PM »
Just uploaded the current-voltage characteristic of an incandescent lamp in my previous post.  You are right that both voltage and current will rise but NOT linearly with each other like with a normal resistor.
LEDs are also nonlinear loads, and I agree with what you wrote on them.

Gyula

Gyula

That is a lumens output curve per V/A,not a voltage/current curve.

Anyway,regardless of that,we know ohms law still applies,in that we can safely say that the dissipated power from the bulb should increase if the current flowing through it increases,and the voltage across it increases. This should result in more light output-should it not?,as this is how incandescent bulbs work.

Brad

tinman

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #8 on: December 09, 2015, 03:30:56 PM »
OK,here is the first video,and i am off to make the second.

https://www.youtube.com/watch?v=QfqvIjABXZ8

gyulasun

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #9 on: December 09, 2015, 03:36:59 PM »
Dear Brad,

In the meantime I added a sentence to my previous post and the use of Ohm's law is ok for the average current and voltage for an incandescent bulb.  8)

The VI curve I took from the video link was done from the current and voltage measurements, they changed the voltage across the lamp from zero to about 6 V and measured the current with an Ammeter in series with the lamp.  Of course the brightness of the lamp changes as the voltage across the lamp changes.

Gyula
 

seychelles

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #10 on: December 09, 2015, 03:59:20 PM »
Works out to be the same a joule thief, because of dc pulsing visually more lumens
  when you connect the cap the pulse is filtered dc voltage  and current is higher but
the lamp show less light because there is no pulsing..light illusion . BUT a circuit diagram
will get me out of the bush..

seychelles

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #11 on: December 09, 2015, 04:01:11 PM »
AGAIN A LITTLE BIT OF THAT WHITE'S MAN MAGIC STUFF..

tinman

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #12 on: December 09, 2015, 04:15:54 PM »
Works out to be the same a joule thief, because of dc pulsing visually more lumens
  when you connect the cap the pulse is filtered dc voltage  and current is higher but
 . BUT a circuit diagram
will get me out of the bush..

First ,we are using an incandescent bulb-not an LED.

Quote
the lamp show less light because there is no pulsing..light illusion

yes,your eyes can be fooled with LED's,but incandescent bulbs do not work that way.
What if we use a solar panel to measure light output--can a solar panel be fooled like your eyes can?
So we shall see.

centraflow

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #13 on: December 09, 2015, 06:10:32 PM »
The resistance R of the bulb will change as it heats up. Raising V will cause I to increase, creating heat which will change R untill it reaches it's stable temperature


My 2p worth


Regards


Mike 8)

verpies

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Re: Accurate Measurements on pulsed system's harder than you think.
« Reply #14 on: December 09, 2015, 06:49:37 PM »
In this thread,i will show you that using a CVR and scope will not always lead to accurate measurements. I mentioned some time back,that using resistors(CVR's) dose not always give accurate result's,and in some system's your scope could be leading you up the garden path.
Yes, an inductive CVR can lead you up the garden path.
Non-inductive CSRs (CVRs) do not lead up the garden path.

OK,here is the first video,and i am off to make the second.
https://www.youtube.com/watch?v=QfqvIjABXZ8
In this video you seem to be using a wirewound CVR that is inductive.

Also, the light bulb has a coiled filament which makes it inductive - maybe that amount of inductance does not matter at these frequencies but it will at higher ones.
I recommend automotive dome light bulbs with straight filament, which are not inductive and are much better candidates for power measurement via optical coupling to the photovoltaic cell.

P.S.
Could you draw a schematic of your setup?  It can be on a sheet of paper or a mouse-scrach in MS-Paint.
It is hard to follow even that amount of wires from the video ...especially at high zoom.
« Last Edit: December 09, 2015, 11:00:29 PM by verpies »