DIY Curve Tracer
I will try to provide some links to some of the posts within the thread.
Alfredo's AS-BUILT Schematics
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For some time i have been using my oscilloscope XY, camera and photoshop to create the curve family for some tubes. The process was time consuming as I had to layer all single curves into one.
I was playing around with some 555 ics i had on my bin and i was able to make a simple curve tracer driver.
For the plate sweep i use a variac into a HV transformer to I can limit the voltage excursion. I take the HV ac and put it to a full wave rectifier to get humps above ground.
See attached block diagram for the basic design. It works fairly well, see picture of 12ax7.
For now it only works with small signal tubes, I need to add a stair amplifier to test power tubes or anything that needs over 12V swing on the grid.
I also used a series resistor after the rectifier to limit the current for the 12ax7. I will need to build a series resistor box to handle different current rated tubes. I will also wire a permanent one on a Radio Shack project board.
Also a challenge is taking the current reading of a pentode's plate as my oscilloscope does not have differential probes. I may need to build a simple probe.
I still have not got around to drawing a detailed schematic. I will post when it is complete. I want to run it for some time to make sure it prevails.
But btw, i use my other bench tools to complete the setup.. I use external power supplies for Vcc, tube bias, heater supply and HV ac.
You've got the curve tracer bug! (Been there!)
For a simple staircase waveform generator, I used half of a four-bit binary counter IC (HC393?) to get up to sixteen steps, with a passive resistor ladder on the outputs (but into an opamp buffer amplifier) to do the digital-to-analog conversion. I put that through a step attenuator and then a small power amplifier. A chipamp (e.g. LM1875, LM3875 or larger) might work well for the power amplifier, although I used an opamp with a BC139/BC140 push-pull booster amp inside the feedback loop, since I didn't need too much power. I configured the small power amplifier as a Howland-type voltage-controlled current source, but could switch in a power-resistor load to make it into a voltage source. That way I could get either calibrated current steps or calibrated voltage steps. I also had an inverting opamp amplifier that could be switched in, before the power amp. That way I could get either polarity for the staircase.
The clock pin signal for the four-bit binary counter IC was a pulse that was derived from the sweep signal. That way everything stayed synchronized very nicely. You want to switch to the next stairstep for each new sweep cycle.
I used "current sense" resistors in each of the three DUT (device under test) connections, with an opamp-based differential instrumentation amplifier's inputs across each one. I had switches that enabled selecting any of the three DUT currents for the vertical axis display and any of the three DUT voltages (i.e. across any two of the DUT connections) for the horizontal axis. And I had final inverting opamp amplifiers that could be switched in, for horizontal and for vertical, so I could flip the polarity of the display independently for each axis.
You might need switchable voltage dividers for everything, or something like that. I was only doing semiconductor DUTs, and only had a maximum of +/- 15V for the sweep signal amplitude, and only up to about 1.5A available for the sweep.
And yes, I had a whole series of switchable current-limiter resistances. I even had a z-axis output, for blanking the "retrace" when I used a sawtooth sweep waveform. (I also had selectable triangle and sine sweeps, and selectable sweep frequencies from 30 Hz to 22 kHz.)
Your biggest problem might be getting everything to stay calibrated when its temperature changes.
You can see an "older" (2nd-last) version of my curve tracer, and the specs, at Gootee Transistor Curve Tracer and Electronic Component and Device Tester . (Everyone please note that I do NOT sell these, any more.)
Let me know if there's any information I can provide that might be helpful.
Impressive! Thats a serious curve tracer!
I am pretty shure that using 555 is not the best. Using an astable and the reset trigger is not perfect, another type of frequency multiplier has to be more accurate. But it does work.
Can you share a diagram of your differentail instrmentation opamp?
Here's a version that doesn't use an actual instrumentation amplifier topology but is one that I liked:
Just what I need, ......
Basic Testing of Semiconductor Devices
[QUOTE=gootee;2297440]Here's a version that doesn't use an actual instrumentation amplifier topology but is one that I liked:[/MQUOTE]
A different approach
Still playing around with the circuit.
I changed the design to include a PLL locked a line frequency.
With the previous version, the # of steps per family jumped from 4 to 8 to 12 due to the way the 555 monostable works.
Now I put a Comparator IC referenced to the voltage of the largest step wanted for each family. So in theory it goes from 1 the the max number of steps the staircase gen can produce (limited by Vcc). The problem is that once you go past 10 the oscilloscope has trouble keeping up with the refresh rate.
I like this version better. Once I build it on a permanent board, I will post the final schematic.
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