STEORN DEMO LIVE & STREAM in Dublin, December 15th, 10 AM

[Omnibus]

Omega_0,

I'd like to tell you that I value very much your input as well as the input of several other researchers. I'm reading your posts regularly although I don't respond every time.

Now back to I and V measurements. How are you solving the problems with the differential and current probes? Probes are more expensive than many of the scopes themselves. Can you think of any way to use usual passive voltage probes for measuring current and would you trust voltage measurements with a passive probe (say, ground loop problem) in what we're studying?

[Omega_0]

Omni,

Thank you for your kind comments. All kinds of feedback is welcome.

Well, I've not reached that stage yet to begin worrying about accurate measurements. Right now trying to get the mechanical stuff right. I'm using the scope and passive probes only to take crude measurements.

Surely, you can't trust these readings as far as the absolute values are concerned, but they are fine for measuring trends and finding optimal spots, like how the RPM varies with current, the absolute value of current is not very important here.

When it comes to measuring power so accurately that the difference in transients shows up and the integrations make any sense, a better scope and probe will be required. Investing in a $2000 probe is out of question so I may try some op-amp based home made circuit to enhance accuracy.

I'm hopeful that with a 300% OU the chances of measurement error will be slim. So lets see.

[FatChance!!!]

I may try some op-amp based home made circuit to enhance accuracy.

Whom besides yourself will determine the accuracy of your home made circuit?
The basis of accuracy and evaluation is having more than one skilled people doing the measurements.
The risk of an induced error by a "home made" circuitry is probably a lot higher than using better equipment.
Try to borrow or rent the equipment when your design is ready for final measurements.

BTW, a good digital oscilloscope from eBay won't cost you any $2000.
And you don't need extreme bandwith for simple motor current measurements.
Here's some links for cheap investments, shipping included in the price.

Used Tektronix 468 100MHz Dual Digital for $197
http://cgi.ebay.com/Tektronix-468-100MHz-Dual-Digital-Storage-Oscilloscope-/120519021000?cmd=ViewItem&pt=BI_Oscilloscopes&hash=item1c0f7e51c8

Used Hitachi VC-6015 Digital Storage Oscilloscope for $249
http://cgi.ebay.com/Hitachi-VC-6015-Digital-Storage-Oscilloscope-/170182642807?cmd=ViewItem&pt=BI_Oscilloscopes&hash=item279fad0c77

New ADS1022 Digital Storage Oscilloscope 25MHz for $289
http://cgi.ebay.com/ATTEN-ADS1022-DIGITAL-STORAGE-OSCILLOSCOPE-25MHz-/320517094572?cmd=ViewItem&pt=BI_Oscilloscopes&hash=item4aa04ebcac

Used EZ Digital OS 5060A Oscilloscope 60MHz for $313
http://cgi.ebay.com/EZ-Digital-OS-5060A-Oscilloscope-TESTED-GOOD-/290431517816?cmd=ViewItem&pt=BI_Oscilloscopes&hash=item439f114478

[Omega_0]

Whom besides yourself will determine the accuracy of your home made circuit?

Calibrations and baseline measurements. Although I'm not an expert in that field.
Thanks a lot for the links.

I have a cheap but decent USB scope but we were talking about probes. If all else fails and the measurement is not definitive, (and the claim shows some promise), I'll surely invest more in measuring stuff. Time is not ripe yet.

[Omnibus]

Calibrations and baseline measurements. Although I'm not an expert in that field.
Thanks a lot for the links.

I have a cheap but decent USB scope but we were talking about probes. If all else fails and the measurement is not definitive, (and the claim shows some promise), I'll surely invest more in measuring stuff. Time is not ripe yet.

That's a very prudent approach and I second it. Here's what I found as a substitute (for Tektronix or Agilent) probes: http://www.caltestelectronics.com/www/ItemDetails.asp?cat=Cat1&ItemCode=CT2593-2&ID=1.c.1&ID1=1510&PN=General%20Purpose%20Differential%20Probes&SubCat=Differential%20Probes. I'll try to use it with a precision resistor in series to do the current measurements. This way it may be even better than using a current probe, especially for our purposes.