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Old 8th December 2006, 09:40 PM   #1
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Default Curve Tracers

Let's open a discussion about Curver Tracers.

There are a lot of older used Curver tracers on the market. many with spec sufficient for DIY use.

When shopping for such a unit. what specs are important to the average DIY guy???

If you wanted to match small signal devices i would imagine most any would be ok for audio use. But what about for drivers or output devices. What sort of specs should one look for? a high current rating? a high bandwidth?? etc.


Discussions please.
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Old 23rd December 2006, 01:00 PM   #2
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So i got to thinking. and that usually gets me into trouble....I have been looking for some sort of Gear to help me match large batches of transistors. Of course nothing exist's and even trying to find any sort of transistor test gear has been hard.

Everyone seems to like to just use a power supply and a some resistors to do there matching. Much like Rod Elliot has show how to do here:
http://sound.westhost.com/transistor-matching.htm

This is a pretty good article for anyone looking to gain an understanding on how to do this. People also keep telling me that the B-E voltage is important as well as the Hfe.

Then i came a cross this. well, not actually this unit, but one just like it. cant seem to find the original unit i saw but this device is interesting!

http://www.measurementcomputing.com/...1TWMQTMLF6EHJ3

This is a USB powered multi channel 12 bit DAC. Now this got me thinking. I have seen several units like this. the one shown has 8 single ended or 4 differential inputs with a 10 volt range.

Now why couldnt i build Rod Elliots transistor matcher as shown but set up for the range i need. then use one of these DAC's connected to a PC and wired in place of the multi meters to record the results?

I would put a transistor in line with the Base drive and trigger it with the PC for a 300Ms pulse. have the analog inputs wired across the base resistor and the emitter resistor and write a simple piece of software to record the values during the 300ms pulse.

Now this unit i could only do 2 devices at a time with 4 differential channels. But they sell similar units with many channels. with a 32 channel DAC i could test 16 devices at a time. greatly speeding up the process and the results would be much closer i would imagine.

Seems logical anyway. I mean if all we are trying to do is measure the voltage across the resistors then this should work.

With a 12 bit DAC in differential mode, this gives us 2048 bits. that should get us down to about .005v per bit. If i need to measure 75ma then i should get a reading of 150bits back from the DAC. if everything is set up right and if my math and my understanding of these devices is correct.....I think...



Zc
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Old 23rd December 2006, 02:50 PM   #3
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AHHH i found it again! here is the USB DAC i found the first time that got me thinking about this....


http://www.hytekautomation.com/Products/IUSBDAQ.html



Zc
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Old 23rd December 2006, 04:49 PM   #4
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I tooo have been lately been giving thot to a USB (or Firewire) I/O box to act as a basis for a curve tracer (i'm interested in tubes thou).

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Old 23rd December 2006, 06:31 PM   #5
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Measurement Computing used to give away "Softwire" for free, -- then the company was acquired by National Instruments and, fearing that Softwire would undermine "Labview" they obsoleted the program -- will no longer support SW after June of 2007. Should be a case-study at the Harvard B-School on how large American companies kill innovation. National Instruments is very litigious...

I spent a couple weeks working on an article only to find that they had essentially put a bullet into this fine program so had to let the mag know that it was no-go.

Still, if you have modest competency programming VB or C you can use their utilities to program the USB-DAC.

Quite frankly, for these applications RS-232 is OK if you still have a serial port -- and you can purchase a BasicX BX-24 with on board ADC and program and debug with no hassle. You can also use the FTDI chips for USB communications -- just remember USB is a BUS, not a PORT. USB hasn't received the DIY popularity of serial or parallel port because of the licensing issues.
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Old 23rd December 2006, 07:20 PM   #6
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I thought of using a basic stamp but that seemed like a whole lot more parts to aquire and assemble and for less then 100 bucks either of these seemed like a reasonable solution.

The first one i listed is actually a bit nicer and i like the software better. the second one, is a bit cheaper however.....

I like the fact that the measurment computing unit has a software package(albiet at extra cost) that is a plug in to MS excel, you could set up a spread sheet and have it plop the numbers right in!

Heck i bet you could set up excel to even do a graph right from those numbers! and i bet it wouldnt be that hard to come up with a GO/NOGO status indicator that would tell you right away if the DUT meets what ever criteria you set up. Oh that would be nice.


So there seems to be some interest in this. lets start designing something.


Zc
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Old 23rd December 2006, 07:47 PM   #7
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From QST magazine July 2006 A Low Cost Automatic Curve Tracer
And the softwares available here.

Regards
James
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Old 24th December 2006, 03:31 AM   #8
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Quote:
Originally posted by tvi
From QST magazine July 2006 A Low Cost Automatic Curve Tracer
And the softwares available here.

Regards
James
There are a number of problems with this system as described. Limited drive current, Only able to test 2 junctions at a time, use of a PC sound card as an accurate device, only able to test one device at a time etc etc.

It is an interesting approach and one that could have possibilities if it could be expanded.

Using a PC/Soundcard to drive the Base/Gate of a device and then reading a value across a base and emitter resistors to plot both curves would be useful.

Again we are relying on a PC soundcard to do the work. and as described in the article, requires special calibration etc as a straight DC value cannot be read.

This leads me back to my original idea of the USB DAC.

a Basic stamp or PIC and a ADC and a RS-232 device and various parts could be used to build the equivilant circuit, but the USB DAC seems to have everything needed built in except a power supply and some resistors.



With the USB DAC i have shown, this same sort of thing could be done using one of the analog outputs to drive the DUT and then use two pairs of analog inputs to read the voltage drop across the B and E resistors. seems to be a much simple solution and more accurate to boot i would think. and if extreme accuracy is required, 16 bit USB DAC's are available.


Is my thinking incorrect here? maybe i am over simplifiying how this works?


How does a commerical Curve Tracer test devices at high currents? I think it does this in short bursts from what information i can find. How much drive curent would be required to drive a BJT to say? 10 amps?


Zc
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Old 24th December 2006, 01:30 PM   #9
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Quote:
Originally posted by Zero Cool


There are a number of problems with this system as described. Limited drive current, Only able to test 2 junctions at a time, use of a PC sound card as an accurate device, only able to test one device at a time etc etc.

It is an interesting approach and one that could have possibilities if it could be expanded.

Using a PC/Soundcard to drive the Base/Gate of a device and then reading a value across a base and emitter resistors to plot both curves would be useful.

Again we are relying on a PC soundcard to do the work. and as described in the article, requires special calibration etc as a straight DC value cannot be read.

This leads me back to my original idea of the USB DAC.

a Basic stamp or PIC and a ADC and a RS-232 device and various parts could be used to build the equivilant circuit, but the USB DAC seems to have everything needed built in except a power supply and some resistors.



With the USB DAC i have shown, this same sort of thing could be done using one of the analog outputs to drive the DUT and then use two pairs of analog inputs to read the voltage drop across the B and E resistors. seems to be a much simple solution and more accurate to boot i would think. and if extreme accuracy is required, 16 bit USB DAC's are available.


Is my thinking incorrect here? maybe i am over simplifiying how this works?


How does a commerical Curve Tracer test devices at high currents? I think it does this in short bursts from what information i can find. How much drive curent would be required to drive a BJT to say? 10 amps?
Zc
I use the USB1208FS and the PMD1208LS from MCC so have some experience with the devices.

1) DACs is DACs -- they can only source a few ma per side
2) no isolation of the DAC or ADC
3) relatively slow sample rate
4) no access to the innards

Thus, you have to us the DAC to drive an opamp like one of the heftier Burr-Brown varieties or a discrete amplifier.

If you are testing tubes or high power MOSFETs I would highly recommend isolating the DAC and ADC from the power supply. You can test small signal transistors, zeners, avalanche diodes with the USB DAC as it exists with no problems.

The software which you get with the MCC devices is pretty limited. As I said earlier, when they packaged Softwire with the devices you could write modules into the program which dumped directly into Excel, allowed you to to FFT's etc. Now they want near a kilobuck for the software.

Dr. Steber's program for the sound card input is not difficult to implement. This is about the third iteration (it first appeared in Circuit Cellar about 5 years ago.) A little imagination will tell you how to overcome the hurdles you cite.
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Old 24th December 2006, 02:30 PM   #10
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Hmmm, there has to be a better way....


the DAS-Wizardô 3.0 for Excel software is the Excel module software i was reading about. it is $149.00 but then we start getting to a price point where i think other options become available.

Some oversea's company is surley missing the boat on this one. we need a plug in USB or Serial PC curve tracer with at least a 10 amp capability.



Is the voltage through the device important. IE if we test at a specific current level, does the voltage matter? If we test a device at 1 amp with a 10 volt power supply, will the results be similar if we were to test at 1 amp with a 100 volt supply?, granted this would be pulse testing to avoid device heating etc.


Zc
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