Doing Without an Oscilloscope!

[resonanceman]

The idea is to be able to do without an expensive scope and derive accurate assumptions about the waveform using equipment which can be purchased for under 120USD.

I think it would behoove us all to have such an ability to extrapolate this type of information and may help relieve hobbyists on shoestring budgets.

I have observed several multimeters which can asses the following:
Duty cycle,
Frequency,
Voltage DC and AC,

From these: voltage AC, voltage DC, frequency, and duty cycle; I think a reasonable assumption of the waveform can be derived.

I aim to find a method for doing so, and request help with the subject matter.

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One simple way to verify your AC is to test the AC voltage, ..then switch the multimeter to DC.  If there is a DC voltage, then the AC has a DC bias offset in the direction of the polarity of the voltage reading proportional to the voltage reading.

This is a[n unprofessional] way to do things but with limited resources . . .  I want to figure out a way to determine the characteristics of the waveform without having a scope or any professional ( expensive ) equipment.

As I do not have an oscilloscope, it would be nice if, when we do attempt to test the ideas, someone having one could verify the outcomes.

As it stands, the numerousness of circuits which create pulses, wobbling waves, unbalanced AC, etc creates a situation in which methodology takes precedence over a single circuit design.  Therefore, I think it unwise to attempt to match circuits diagrams and presume success without taking these things into account.

I hope to use math and perhaps a come up with some simple solutions.  Perhaps even create a Java web applet that can show an approximation of the waveform.

I know that only so much can be derived from basic measurements, however, we may also come up with a means or apparatus, which when employed, yields more data which can be taken into account.

One thing we may incorporate is the properties of the multimeter being used ( RMS ), etc, internal resistances, the list goes on.

Any additional information or ideas would greatly be appreciated, and I hope to collaborate a solution to the problem of not having the budget to afford ( say, ) a five thousand dollar shelf piece.

Jadaro

Interesting research

Do you  have any  recommendations yet?

Perhaps  this  would  be a good place for  people to put in a good word about  the equipment  that  has worked well for them,.........and  ways that  they  have been using it that might be unique

I  have been thinking  of looking  for a new DMM
Right now I don't  have anything  that will read current .
I also don't have anything that  reads over 600 V

One of the things that is keeping me from looking for  a good DMM is time
At this  point  I have to many ideas I want to try  to spend time  looking for  a DMM


gary

[jadaro2600]

Thank you all for your recommendations; even so, for the advice as well.

IST is right, the device is a mainstay, however, for those budding hobbyists with a need...

I seem to have some ideas, but they are generally lacking coherency.  I will most likely wind up buying a scope so that I can test the ideas myself.  This may be a long time in the process though; it may even become a personal endeavor, seeing as how scopes were made for this purpose..   Additional recommendations are always welcome!

Here's what I have so far, and not having a scope make things difficult to adjust or test:

Information is subject to the scrutiny of other forum dwellers! please be advised.

The duty cycle on most inexpensive DMM's is given as a percent, this is usually the same setting on the DMM associated with testing the frequency of the wave in question.

If a waveform has a 13% duty cycle, then it's 'on' for 13% of the of the time...  the closer it is to 50% the closer it is to being true alternating current ..granted, this could be sinusoidal OR square .. the measures without an oscilloscope, in this case, become unclear.

I recently took measures from a hobby transformer for a train set: on it's AC side...

AC voltage was 21.66
DC voltage was -0.039 ( -0.04 )
Duty cycle was 50.3%
Frequency was 59.9

In this instance, I can see that there is a DC component to the waveform as well as it being on more than it is off. Using these types of measure, I was hoping to derive a means to model the waveform.  I can see that the measure are somewhat unrelated though, after testing a few equations I had come up with.

(0.003*21.66 ) = .06498
Which is close to the DC voltage of the AC..but not exactly.

The problem is, most multimeters are RMS devices, which can bungle measuring DC on an AC source ...  which is where a bridge rectifier may be used ...of course, we would have to measure the properties and voltage drops across the diode and take this into account.

I will post more in the future about this, as I still believe it has some relevance.

[jadaro2600]

Jadaro

Interesting research

Do you  have any  recommendations yet?

Perhaps  this  would  be a good place for  people to put in a good word about  the equipment  that  has worked well for them,.........and  ways that  they  have been using it that might be unique

Sears sells both Craftsman and Fluke multimeters, though a good fluke will set you back several hundred dollars, a craftsman will not, ..

I have two Crafsman DMM's, one which has a diode testing setting, measures current, AC/DC voltage, temperature with a thermojunction probe..measure frequency up to 10mHz ...  the other does all of these things, and has a transistor testing plug ( which I have not figured out how to use, and would greatly appreciate someone telling me what the readouts are supposed to mean ) Both measure up to 1000v dc, 650V ac., capacitance, continuity, resistance.  Very handy tools they've made.

The diode test setting tells me the voltage drop across the diode, which I find useful when I can't read the part number...   I believe the model for this DMM ( number ) is 82400, but craftsman doesn't seem to have any more for sale.  It was about 60USD... and for it's functionality, was well worth the money VS a Fluke which could do the same thing.  The other is the 82139.  Both can take relative readings, hold the screen so you can remove the probe..etc.

[ATT]

I've never used one, but pc scopes are available, even ones that don't require proprietary DAQ cards or USB interfacing.

The 'soundcard' ones look like they only read AC, but I imagine you could exercise a little creativity and come up with a linear VCO that was DC-drive to get the data past your AD converter.

Without access to the source code, you couldn't tell the scope you were reading DC levels, so you'd still be getting an AC waveform on the display from your VCO output.

Of course, it wouldn't be that big of a deal to cobble up a display just for DC, but this would preclude visualizing DC with an AC component or vice versa but, hey, it's a free scope, use it for waveforms and save them out right to your drive.

Think about buffering the input, limit the signal to 1.7v so you don't blow your sc.


good free example sc scope:
http://www.zeitnitz.de/Christian/scope_en

general info:
win:
http://www.ehow.com/how_2278973_use-sound-card-as-oscilloscope.html
linux
http://www.micahcarrick.com/06-06-2006/pc-sound-card-oscilloscope-linux.html
any:
http://www.allaboutcircuits.com/vol_6/chpt_4/13.html

Tony

[wings]

I've never used one, but pc scopes are available, even ones that don't require proprietary DAQ cards or USB interfacing.

The 'soundcard' ones look like they only read AC, but I imagine you could exercise a little creativity and come up with a linear VCO that was DC-drive to get the data past your AD converter.

Without access to the source code, you couldn't tell the scope you were reading DC levels, so you'd still be getting an AC waveform on the display from your VCO output.

Of course, it wouldn't be that big of a deal to cobble up a display just for DC, but this would preclude visualizing DC with an AC component or vice versa but, hey, it's a free scope, use it for waveforms and save them out right to your drive.

Think about buffering the input, limit the signal to 1.7v so you don't blow your sc.


good free example sc scope:
http://www.zeitnitz.de/Christian/scope_en

general info:
win:
http://www.ehow.com/how_2278973_use-sound-card-as-oscilloscope.html
linux
http://www.micahcarrick.com/06-06-2006/pc-sound-card-oscilloscope-linux.html
any:
http://www.allaboutcircuits.com/vol_6/chpt_4/13.html

Tony

try this free software site :
oscilloscope
wave generator
filter
toroid coil
...........

http://www.tech-systems-labs.com/freesoftware.htm