# help with octopus curve tracer

Discussion in 'The Projects Forum' started by nonelectron, Apr 3, 2014.

1. ### nonelectron Thread Starter New Member

Apr 3, 2014
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Hi there guys and gals!!

This is my first time posting on this forum so don't bash me to hard.
I recently bought an oscilloscope and want to make an octopus curve tracer. There are many schematics online that I have seen. Most of them are using a 120v ac in to 6.3v ac out. Then they're only using one secondary wire and the center tap to get 3.15v ac. also the device under test(DUT) is seeing a max 1v 1ma? am i understanding this correctly?

Basically what i want to do is use a transformer that i already have. Problem is its output is 9v 800ma? I don't know if it has a center tap or not as i haven't tore the wall wart apart yet. i have learned OHM's law and am getting familiar with Kirchoffs law thinking i can figure out the different resister values to get me down to 1v 1ma for the DUT. Am i on the right track . please help me with this so i can learn in the process

2. ### wayneh Expert

Sep 9, 2010
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I think your wall wart is unlikely to have a center tap. You might consider just using a pair of AAs.

What is your DUT? Note that you will supply a voltage and it will draw whatever current it wants. Or, you can supply a fixed current by varying the voltage. You cannot usually specify both at the same time.

Somebody here probably knows all about octopus curves, but not me.

3. ### nonelectron Thread Starter New Member

Apr 3, 2014
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I dunno about the AA's. Seems as though it needs to be an ac voltage.

4. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Why do you think you need a center tapped transformer?

Why do you think you cannot use 9VAC?

If 9VAC is too high, you can string a voltage divider (adjustable?) across the transformer. You just need to use that Kirchhoff's* law you are getting familiar with to select the proper resistor values...

*Please note proper spelling of Kirchhoff

5. ### nonelectron Thread Starter New Member

Apr 3, 2014
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Well that is what I was saying about using the 9v power supply. I know it's usable. And didn't think I needed a center tap. I knew that I needed to drop the voltage. I was just unsure if I was on the right track or not by using Kirchhoff's law. Am I right by thinking I need 1v at the DUT probes? Oh and thanks for the correct spelling of Kirchhoff

6. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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You haven't said what the DUT is -- and my crystal ball is in the shop right now.

7. ### nonelectron Thread Starter New Member

Apr 3, 2014
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Well the DUT will be many different things. Not just 1 specific device. Resisters,diodes,capacitors,Zener diodes. If you google octopus curve tracer you'll see what I'm talking about. I'm just learning and thought this would be a good starter project and help me learn and understand what things do

8. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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No need. I know what you want to do, I am just wondering if *you* do. When you do *your* google search, note that they usually make the voltage (and current) adjustable in some fashion. This is because each device has different operating points and limitations.

A 1V curve tracer will not adequately test a LED or zener, though it will work fine for a small-signal diode, a resistor or a cap.

A 9VAC signal may exceed the maximum voltage rating of some parts (think "reverse breakdown voltage", but will be require to test, say, an 8V zener.

The current limiting "sense" resistor must also be sized appropriately for the DUT. For example, there is a huge difference in the rise time between a 10pf cap and a 100uF cap for a given current.

If you *really* want a good education, first try to predict the voltages and currents you need for a particular part, and the scope pattern you expect to see. Then, build the tracer and compare your prediction against the actual result. If you see any differences, try to understand why.

9. ### Alec_t AAC Fanatic!

Sep 17, 2013
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For testing that range of DUTs you will need a voltage >>1V. 9VAC would be fine (unless you need to test, say, high voltage zener diodes) providing suitable resistors are used in conjunction with it.

10. ### wayneh Expert

Sep 9, 2010
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If you think you only need 1mA thru your DUT, choose resistors for your voltage divider so that the total divider current is at least about 10mA, or at least ten times the amount that you want to siphon off to the DUT. This ensures that adding or removing the DUT has minimal effect on the voltage provided by your divider.

11. ### nonelectron Thread Starter New Member

Apr 3, 2014
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Thanks for the input. I guess it boils down to me needing to do some more homework

12. ### BR-549 Well-Known Member

Sep 22, 2013
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Hello nonectron. You have just discovered the most valuable tool for in circuit testing. I used to do a lot of bench work. I used to get all the dogs. With this gadget, you can repair circuits without knowing the circuit specs or function. A most useful tool. Search youtube for oscilloscope octopus. Enjoy.