Author Topic: Curve tracer designs?  (Read 4806 times)

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Online 0culus

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Curve tracer designs?
« on: November 18, 2018, 02:01:24 am »
So I've been thinking that it would be handy to have a curve tracer in my lab, but even more, it would be fun to make it myself. I've looked at a variety of designs, some requiring mains voltage, some not. I tried soldering this one up on a protoboard today because I have all the parts on hand and no transformer required (except I omitted the switch for a prototype): http://www.electronics-lab.com/diy-generic-curve-tracer/

But, it seems to not be working at all, mainly the 3904 never starts oscillating so there's no signal being put through the DUT.

Are there better designs for a curve tracer that are similarly parts-efficient? Thanks!
 

Offline JoeO

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Re: Curve tracer designs?
« Reply #1 on: November 18, 2018, 03:00:56 am »
The schematic shows a 3M resistor while the parts list calls out a 2M resistor.

Have you tried them both?
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Online 0culus

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Re: Curve tracer designs?
« Reply #2 on: November 18, 2018, 05:04:07 am »
The schematic shows a 3M resistor while the parts list calls out a 2M resistor.

Have you tried them both?

Yeah, I tried 3Mohm too and it still doesn't work. There's 0.584 V on the base of the transistor and 6.160 V on the collector in that arrangement, and there is nothing at the emitter. Fairly sure the transistor isn't backwards.
 

Offline SeanB

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Re: Curve tracer designs?
« Reply #3 on: November 18, 2018, 09:24:20 am »
Try another 2N3904, and what capacitor are you using for C1, C2 and C3, they need to be 63V or higher mylar foil types, as ceramic or electrolytic capacitors leak way too much. Bias is turning the transistor on, but either there is leakage or a bad connection.
 

Online 0culus

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Re: Curve tracer designs?
« Reply #4 on: November 18, 2018, 08:38:25 pm »
Try another 2N3904, and what capacitor are you using for C1, C2 and C3, they need to be 63V or higher mylar foil types, as ceramic or electrolytic capacitors leak way too much. Bias is turning the transistor on, but either there is leakage or a bad connection.

OK. I only have a few of those kind of caps unfortunately, and not in the right spec. Guess it's time to order some. The 4.7nF ceramics I have don't seem to be leaking DC, but the 1uF electrolytic I used is definitely leaking.
 

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Re: Curve tracer designs?
« Reply #5 on: November 18, 2018, 09:47:05 pm »
So there's an 890 Hz mostly clean oscillation happening on the collector of the 3904. I think I'm going to order some better quality caps and some other parts and build another prototype on breadboard.
 
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Offline m3vuv

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Re: Curve tracer designs?
« Reply #6 on: November 19, 2018, 12:24:27 am »
i built the same jobby a while ago,never did get it working properly!
 

Online 0culus

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Re: Curve tracer designs?
« Reply #7 on: November 19, 2018, 12:55:22 am »
Hmm, maybe it's time to find a better circuit then.
 
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Online Wolfgang

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Re: Curve tracer designs?
« Reply #8 on: November 19, 2018, 02:56:17 am »
Hi,

have you seen the curve tracers of ChangPuak from the ETH in Zurich, Switzerland ?

https://www.changpuak.ch/electronics/Curve_Tracer_advanced.php
 
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Offline johnwa

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Re: Curve tracer designs?
« Reply #9 on: November 21, 2018, 10:23:32 am »
Hi,

Here is an overview of a curve tracer I built recently, don't know if this is of any help:

It all started when I was wondering what to do with this old oscilloscope that a friend gave me some years ago. I already had an analogue scope, and I was thinking of giving this one away to some deserving newbie, but it was in pretty poor shape: noisy switches, timebase not working, and I had already needed to make a repair to the EHT supply using the tripler out of an old TV set. So I decided, rather than burden someone else with it, I would convert it into a dedicated curve tracer.

To provide the drive signal, I used an amplifier out of some old PA equipment. This circuit used a couple of quasi-complementary 2N3055s, and was good for about 40W into 8 ohms. Conveniently, I managed to find some suitable +/- 35V supply rails inside the scope to power it. Maximum output swing is about +/-33V.

For the oscillator, I got lazy, and ordered a sine/square/triangle signal generator module from ebay, for the grand sum of $3.50. This feeds a front panel attenuator (I repurposed some of the controls in the timebase module), and some switchable level shifting to provide positive, negative, or AC output. I also added an adjustable current limiting circuit to the amplifier. (Let me know if you would like details).

The output of the amplifier is connected to the CH1/X input, which then becomes the drive output. The CH2 input has a 1 ohm resistor connected across it, to act as a current shunt. The input attenuators work as normal, with the CH2 one being recalibrated in amps instead of volts.

I also added a base drive staircase generator, based on the Elektor December 1989 circuit. (I think someone may have posted this on the forum somewhere - search around)

In the picture, the original X deflection board is at the back, then comes the amplifier module. The function generator is at the top, with the staircase generator below it. The current limiter is partially hidden behind the heatsink.

Edit 21/11/2018: spelling
« Last Edit: November 21, 2018, 10:26:27 am by johnwa »
 
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Online Wolfgang

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Re: Curve tracer designs?
« Reply #10 on: November 21, 2018, 12:59:33 pm »
Love it. POOGE at its best ! :)
 

Offline rhb

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Re: Curve tracer designs?
« Reply #11 on: November 23, 2018, 03:27:13 pm »
I wanted to build one of these many years ago, but never got around to it.

In 2018  I have to ask a simple question.

Why would you not use a fast ARM MCU with stereo ADC/DAC feeding suitable op amps?  One channel for the base drive and one for the collector with the ADC taking readings.

I have a Peak Technologies DCA75 which does a decent job, but it only reports Vbe_sat and Vce_sat.  It will do curve tracing with the PC software.   My BSIDE ESR02 Pro reports Vbe_turn_on, but not Vbe_sat, etc.

I've not done any in depth analysis of what a good transistor tester should do, but it seems to me that there is room for considerable improvement at low cost.
 

Online 0culus

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Re: Curve tracer designs?
« Reply #12 on: November 24, 2018, 04:08:06 am »
Hi,

Here is an overview of a curve tracer I built recently, don't know if this is of any help:

It all started when I was wondering what to do with this old oscilloscope that a friend gave me some years ago. I already had an analogue scope, and I was thinking of giving this one away to some deserving newbie, but it was in pretty poor shape: noisy switches, timebase not working, and I had already needed to make a repair to the EHT supply using the tripler out of an old TV set. So I decided, rather than burden someone else with it, I would convert it into a dedicated curve tracer.

To provide the drive signal, I used an amplifier out of some old PA equipment. This circuit used a couple of quasi-complementary 2N3055s, and was good for about 40W into 8 ohms. Conveniently, I managed to find some suitable +/- 35V supply rails inside the scope to power it. Maximum output swing is about +/-33V.

For the oscillator, I got lazy, and ordered a sine/square/triangle signal generator module from ebay, for the grand sum of $3.50. This feeds a front panel attenuator (I repurposed some of the controls in the timebase module), and some switchable level shifting to provide positive, negative, or AC output. I also added an adjustable current limiting circuit to the amplifier. (Let me know if you would like details).

The output of the amplifier is connected to the CH1/X input, which then becomes the drive output. The CH2 input has a 1 ohm resistor connected across it, to act as a current shunt. The input attenuators work as normal, with the CH2 one being recalibrated in amps instead of volts.

I also added a base drive staircase generator, based on the Elektor December 1989 circuit. (I think someone may have posted this on the forum somewhere - search around)

In the picture, the original X deflection board is at the back, then comes the amplifier module. The function generator is at the top, with the staircase generator below it. The current limiter is partially hidden behind the heatsink.

Edit 21/11/2018: spelling

Oh that's really neat! Thanks for sharing. That would be a totally awesome use for an old scope in the future.


I wanted to build one of these many years ago, but never got around to it.

In 2018  I have to ask a simple question.

Why would you not use a fast ARM MCU with stereo ADC/DAC feeding suitable op amps?  One channel for the base drive and one for the collector with the ADC taking readings.

I have a Peak Technologies DCA75 which does a decent job, but it only reports Vbe_sat and Vce_sat.  It will do curve tracing with the PC software.   My BSIDE ESR02 Pro reports Vbe_turn_on, but not Vbe_sat, etc.

I've not done any in depth analysis of what a good transistor tester should do, but it seems to me that there is room for considerable improvement at low cost.

While that would likely work, I personally prefer analog test equipment. :) My main scope is a Tek 2465B.
 

Offline rhb

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Re: Curve tracer designs?
« Reply #13 on: November 24, 2018, 05:13:03 pm »
Certainly a valid reason.  I bought a refurbished Tek 485 from Reed Dickinson via eBay precisely to have an analog scope to double check strange stuff from DSOs.
 

Online 0culus

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Re: Curve tracer designs?
« Reply #14 on: November 24, 2018, 09:12:49 pm »
I will probably try to get a decent digital scope at some point to have access to the nice features they have, but I certainly wouldn't want it to be my only scope!

On another note, I tried building another prototype of this circuit with some better caps and I am pretty sure the design originally mentioned in the OP doesn't work.
 

Offline rhb

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Re: Curve tracer designs?
« Reply #15 on: November 25, 2018, 12:33:52 am »
 
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Offline glue_ru

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Re: Curve tracer designs?
« Reply #16 on: November 25, 2018, 02:03:16 am »
The schematic shows a 3M resistor while the parts list calls out a 2M resistor.

Have you tried them both?

Yeah, I tried 3Mohm too and it still doesn't work. There's 0.584 V on the base of the transistor and 6.160 V on the collector in that arrangement, and there is nothing at the emitter. Fairly sure the transistor isn't backwards.

would it help to add a schottky diode  to get better switching
 

Offline rhb

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Re: Curve tracer designs?
« Reply #17 on: November 25, 2018, 02:30:05 am »
The schematic shows a 3M resistor while the parts list calls out a 2M resistor.

Have you tried them both?

Yeah, I tried 3Mohm too and it still doesn't work. There's 0.584 V on the base of the transistor and 6.160 V on the collector in that arrangement, and there is nothing at the emitter. Fairly sure the transistor isn't backwards.

The voltage drop across the BC is lower than across the BE junction.  Forty years ago I built a bunch of very simple things using 2N2222s from Radio Shack, but none of them worked.  I went around asking everyone I knew how to identify the C & E leads.  No one knew.

As a break from looking through a microscope all day for my MS I attended an Electronics 101 class in the next building taught by a solid state physicist who had built a computer out of TTL logic just before the 8080, 6502 and such appeared.

One day he said, "The forward resistance across the base collector junction is lower than the forward resistance across the base emitter junction because the greater area of the collector exceeds the effect of the greater doping of the emitter."  All I had was an analog VOM, but sure enough, he was right and the reason nothing worked was the pinouts on the back of the package were wrong!

With modern DMMs you need to use the diode test function as the resistance ranges don't put out enough voltage to forward bias the junctions.
 

Online 0culus

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Re: Curve tracer designs?
« Reply #18 on: November 25, 2018, 03:26:06 am »
No worries, mate.

https://www.tinaja.com/ebooks/TTLCB1.pdf

page 219

Wow, that is very cool. Thanks! Saving that!

The schematic shows a 3M resistor while the parts list calls out a 2M resistor.

Have you tried them both?

Yeah, I tried 3Mohm too and it still doesn't work. There's 0.584 V on the base of the transistor and 6.160 V on the collector in that arrangement, and there is nothing at the emitter. Fairly sure the transistor isn't backwards.

The voltage drop across the BC is lower than across the BE junction.  Forty years ago I built a bunch of very simple things using 2N2222s from Radio Shack, but none of them worked.  I went around asking everyone I knew how to identify the C & E leads.  No one knew.

As a break from looking through a microscope all day for my MS I attended an Electronics 101 class in the next building taught by a solid state physicist who had built a computer out of TTL logic just before the 8080, 6502 and such appeared.

One day he said, "The forward resistance across the base collector junction is lower than the forward resistance across the base emitter junction because the greater area of the collector exceeds the effect of the greater doping of the emitter."  All I had was an analog VOM, but sure enough, he was right and the reason nothing worked was the pinouts on the back of the package were wrong!

With modern DMMs you need to use the diode test function as the resistance ranges don't put out enough voltage to forward bias the junctions.

My Fluke 87V seems to have no trouble driving it in resistance mode; it would appear that I don't have the 2N3904 backwards by that heuristic. The base-collector is ~475k and the base-emitter is ~486k. In diode test mode, the base-collector is 0.707 V and the base-emitter is 0.712 V.
 

Offline rhb

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Re: Curve tracer designs?
« Reply #19 on: November 25, 2018, 04:02:54 am »
Don wrote wonderful books.  There's an entire generation that acquired much of their knowledge of electronics from Don and a handful of other writers.  If you can explain it to a teenager, you *really* do know what you're talking about.

It took about a minute for me to find the circuit even though it is not indexed as a "staircase generator".  I knew it was in there, so I just had to think of synonyms.  I have been surprised at how often that's the book I grab off the shelf.  I read it so much I know what's there and I'm just checking that my memory is correct.

If you want to really be old school you can build it all from discretes.  Might have to.

BTW the "which is C and which is E" was my favorite question when I ran across someone who was *really* good at electronics.  No one was able to answer it until I ran across someone online a few years ago.  Fundamentally the difference between a scientist and an engineer.  Scientists rely on first principle and engineers on rote.  I feel pretty sure everyone I asked enjoyed the answer.  I *only* asked people who were "walks on water" good.  People like that enjoy such  little subtleties.  It is the pleasure derived from such things which made them good.
 

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Re: Curve tracer designs?
« Reply #20 on: November 25, 2018, 06:13:43 am »
Which circuit are you talking about?

This is a book I might have to find a hard copy of.

That is a neat little subtlety, thanks for sharing!
 

Offline rhb

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Re: Curve tracer designs?
« Reply #21 on: November 25, 2018, 01:15:19 pm »
Which circuit are you talking about?

There are only two circuits in this discussion that I know of. The one you've been trying to get to work and the one on page 219.  If you're asking about the reference to discretes, that was the counter chips.

This is what is commonly referred to as an R2R ladder DAC.  A relevant concept from the same book is using a counter to index a memory and using the output of the memory to feed the R2R.  That will let you correct for resistor tolerances by just modifying the contents of the memory.  The JDS 6600 arbitrary waveform generator uses an R2R ladder driven by an FPGA.

Quote
This is a book I might have to find a hard copy of.

As it happens I have a spare I got when someone was cleaning out there office.  How about $4 plus media mail?

Quote
That is a neat little subtlety, thanks for sharing!

Don's books are full of neat little circuits.  I'd bet you would find a lot of them in Tek and HP gear of the era.  The Tek 577 probably uses the circuit Don showed.
 
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Re: Curve tracer designs?
« Reply #22 on: November 26, 2018, 04:04:23 am »
Which circuit are you talking about?

There are only two circuits in this discussion that I know of. The one you've been trying to get to work and the one on page 219.  If you're asking about the reference to discretes, that was the counter chips.

This is what is commonly referred to as an R2R ladder DAC.  A relevant concept from the same book is using a counter to index a memory and using the output of the memory to feed the R2R.  That will let you correct for resistor tolerances by just modifying the contents of the memory.  The JDS 6600 arbitrary waveform generator uses an R2R ladder driven by an FPGA.

Quote
This is a book I might have to find a hard copy of.

As it happens I have a spare I got when someone was cleaning out there office.  How about $4 plus media mail?

Quote
That is a neat little subtlety, thanks for sharing!

Don's books are full of neat little circuits.  I'd bet you would find a lot of them in Tek and HP gear of the era.  The Tek 577 probably uses the circuit Don showed.

Turns out amazon has a number of books by him.  :-+
 

Offline rhb

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Re: Curve tracer designs?
« Reply #23 on: November 26, 2018, 02:22:24 pm »
I asked Don.  His best guess is that with foreign press runs the TTL CB sold about 1.4 million copies.  That means that pretty much anyone with a strong interest in electronics in the 70's bought a copy.

Pretty heady stuff for a 30 something tech writer. It went through 3 printings in the first year!

The other books which are still very relevant are the CMOS CB and the Active Filter CB.  The RTL CB might be useful if you need to build basic logic.  Unfortunately, lots of the 7400 series devices are no longer available except at steep prices.

The schematics and service manuals for the gold standard, the Tek 577, are available.  I suggest studying those.  The Tek service manuals of that era include a detailed explanation of the theory of operation of *every* circuit in the instrument.
 

Online 0culus

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Re: Curve tracer designs?
« Reply #24 on: November 26, 2018, 05:11:46 pm »
I asked Don.  His best guess is that with foreign press runs the TTL CB sold about 1.4 million copies.  That means that pretty much anyone with a strong interest in electronics in the 70's bought a copy.

Pretty heady stuff for a 30 something tech writer. It went through 3 printings in the first year!

The other books which are still very relevant are the CMOS CB and the Active Filter CB.  The RTL CB might be useful if you need to build basic logic.  Unfortunately, lots of the 7400 series devices are no longer available except at steep prices.

The schematics and service manuals for the gold standard, the Tek 577, are available.  I suggest studying those.  The Tek service manuals of that era include a detailed explanation of the theory of operation of *every* circuit in the instrument.

Very cool. I've got the ones Amazon still has in my cart. I will grab those manuals too. I've always wanted a 577 but sadly I haven't the space in my current lab!
 


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