Electronics > Beginners
Dirt cheap and simple scope-based component tester - curve tracer
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w2aew:

--- Quote from: jackasspenguin on June 11, 2012, 01:32:16 pm ---Hmm, while I'm considering buying a filament power supply I'm trying to think of a way to use something I either already have or buy something more financially appropriate.  I'd love to build this. Although I would rather not have an 8lb iron core step down transformer to live with it. I mean in reality, if I had a filament transformer I'd rather build a tube amp to put it in. So my considerations would be something like this perhaps... This is a 220V input 12V AC output transformer, but wouldn't the output simply be half of whats rated? 110V ac input you'd get 6V AC out right? I Googled octopus component tester and what I found asks for 3 volts off the center tap of a 6V ac transformer.  I'm just not sure what values or component I'd need to tweak in order to conform the circuit to suit the power supply.  I'm not dead set on this power supply.  I just thought there may be something out there over the shelf that might be a better choice.  Johnny W :)

http://www.ebay.com/itm/20W-Max-220V-to-12V-AC-0-086A-Home-LED-Light-Power-Supply-Electronic-Transformer-/400303095053?pt=LH_DefaultDomain_0&hash=item5d33ec7d0d

--- End quote ---

There's really nothing set in stone with this.  Using a 220VAC - 12VAC transformer on 110VAC will indeed give you 6VAC out.  It's really up to you to decide how much AC voltage you want to use as your excitation voltage.  You can always use a voltage divider at the output to cut it down further, as most of the Octopus schematics show.  You can even use an audio isolation transformer on the output of a function generator if you want.  You are also free to choose what current limit/sense resistor you want to use.  My simplified example was used to simply show the concept.
mianchen:
Alan,

Thanks for the very informative / educational videos -  You've got a new Youtube subscriber.

I've tried your curve tracer with my Iwatsu ss-5705 today, it didn't work as expected. Sorry for my n00b question - is it because CH1 and CH2 are completely out of phase? I try to locate an 'invert' switch on the scope, but couldn't see anything. Does it mean that I have to build an inversion circuit to use my scope as a curve tracer?

Thank you in advance.

I've attached some pics:

Wave forms of CH1 and CH2 when connected to the 'Octupus'


X-Y Mode is on:



Measuring a capacitor



Measuring a diode

w2aew:
Really no need to build an inverting circuit.  Just recognize the fact that +current through your DUT is deflecting in the downward direction instead of up.  You're waveforms look OK, there's some phase shift beyond the 180 degrees due to the non-inverted ch2, but that could be inherent to the scope, and I wouldn't sweat it.  Looks like your scope doesn't have an Invert function on CH2 because it includes a Ch1-Ch2 feature.  Many other analog scopes have only a Ch1+Ch2 (ADD) feature, so the Ch2-INV was added so that you could also do the subtraction. 

tekfan:

--- Quote from: mianchen on June 11, 2012, 04:07:53 pm ---I've tried your curve tracer with my Iwatsu ss-5705 today, it didn't work as expected. Sorry for my n00b question - is it because CH1 and CH2 are completely out of phase? I try to locate an 'invert' switch on the scope, but couldn't see anything. Does it mean that I have to build an inversion circuit to use my scope as a curve tracer?

--- End quote ---

Make sure that you have both inputs DC coupled. That should take care of the problem. You can invert the Y axis by pulling the channel B position control out (it's labeled PULL INV).

w2aew:

--- Quote from: tekfan on June 11, 2012, 05:10:35 pm ---
--- Quote from: mianchen on June 11, 2012, 04:07:53 pm ---I've tried your curve tracer with my Iwatsu ss-5705 today, it didn't work as expected. Sorry for my n00b question - is it because CH1 and CH2 are completely out of phase? I try to locate an 'invert' switch on the scope, but couldn't see anything. Does it mean that I have to build an inversion circuit to use my scope as a curve tracer?

--- End quote ---

Make sure that you have both inputs DC coupled. That should take care of the problem. You can invert the Y axis by pulling the channel B position control out (it's labeled PULL INV).

--- End quote ---

Excellent point about DC coupling - and nice catch on the pull-inv function - I missed that in the photos.  Sounds like you've got him all sorted now.
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