Curve tracer further checkout but offset function still doesn't work

CONTINUING CHECKOUT

I set up the 1K resistor and scope probes in order to verify that the offset circuits work in both polarities and for both N and P type tested devices now that I have repaired the bad solder joint I discovered. 

Generating base voltage steps

Injecting offset to step voltage

I also worried about a jumper that I had put on a header because its purpose was not documented anywhere and there were no diagrams to show which jumpers were to be inserted. 

The assembly instructions stated that a jumper goes on the pulse polarity header on the left two of three pins and that a jumper goes on the P1en header. However, there is also a P2en header and no mention at all of this. I had put on a jumper but after looking at the schematics, it appears there are two ways to produce pulses for the triangle generator, one uses a second schmitt trigger (P1 route) and the other bypasses the second schmitt trigger (P2 route).

In reading the detailed blogs about the design process the role of both paths became clear. The triangle wave is applied to the emitter-collector (source-drain) of the device at each step of the base (gate) level. The design steps the base/gate at the end of each direction of the triangle, thus the upward slope of the triangle feeds the device on one step, the downward slope feeds the device on the next step, and so forth.

An alternate mode is to apply the entire triangle, both up and down, to the device during each step of the base/gate supply. The path P1 triggers the step generator at the start of an upward portion of the triangle wave, while path P2 triggers at the apex as it starts downward. If P2en is not jumpered we get the full triangle per step, whereas with both we see the alternating use of up and down slopes that is desired.  



The offset circuit had no effect at all on the signal coming from the resistor in the test socket. I began verifying connectivity of all the components in the circuit using a DMM. 

COSMETIC UPGRADE COMING

Two of the control knobs, 3/4" diameter, were designed to use with 1/4" round shaft controls. The switches used for these positions have 1/8" flatted shafts instead. I have used a workaround of a plastic insert with a bit of bare copper wire to match the flat section, but that isn't a high quality solution and is susceptible to slippage. 

I received two adapters which convert 1/8" shaft to 1/4" knobs and can accommodate flat sections as well. I will use these to improve the existing knobs for selecting the number of steps and selecting the voltage/current levels to be used with a particular device under test. 

I still have an issue in that the knobs I am using are not only 1/4" shaft diameter, but the two setscrews are placed 120 degrees apart and neither is in position to fasten the knob to the flat spot with the indicator line pointing at the correct spot. I need to add a half-moon shaped piece into the adapter in order to lock it to the flat spot. Alternatively I need to epoxy a flat plate inside the adapter to fit to the flat spot. 

Comments

Post a Comment

Popular posts from this blog

Bizarre behavior detected in a Raspberry Pi Pico based design and partially understood

ORIENTATION OF THE PRODUCT CAN CAUSE THE PI TO FREEZE AT STARTUP This particular product with a Raspberry Pi Pico would not boot up when sitting flat on the table in its normal orientation, but if tilted or turned it would come right up. When I hooked up a USB serial connection to the debug header, it worked fine even flat on the table.  INVESTIGATION BY DESIGNER AS WELL AS ME I first assumed that I had introduced this problem in the product, but happened to describe it to the maker who was able to reproduce it reliably. I continued to try variations to determine whether it was pressure on a board or component that caused it. I also searched the web for any reports that might cause Pi Picos to fail to boot based on orientation, but found nothing.  The designer found signals introduced on a serial connection trace when sitting in the normal (failing) orientation but not injected when rotated or otherwise turned. Traces were routed from the UART0 on the Pico to a header, for use...
Read more

Incompetent coders - the frustration of using Vivado to build FPGA designs

IDIOT SOFTWARE MUST BE RESTARTED CONSTANTLY TO SEE INPUT FILE CHANGES When I update a source file, often Vivado is somehow working from a cached copy and never notices the changes. It is highly frustrating to fix an issue, rerun a Linter or Synthesis and get the same old errors. The only solution is to shut Vivado down and restart. I remember hitting this many times in past years as I worked with the toolchain, but it has changed from sporadic to nearly constant failure to see the changed timestamp. I can't be the only person running the toolchain under Windows, yet this kind of bumbling misperformance has been a feature for years. 
Read more

Found correct mod for the Arduino Mega 2560 to avoid powering it from USB

Image
USB HANDLING CIRCUITS IN THE MICROPROCESSOR REQUIRE USB VCC TO RUN The reason my first attempt failed, simply removing the poly (resettable) fuse, is that the fuse is between the USB connector and the microchip which needs the USB VCC connected. I had to reinstall the fuse.  BETTER CHANGE IS TO REMOVE A MOSFET THAT ROUTES POWER The Arduino uses a power mosfet and a comparator to decide when the external power is adequate to run the board, disconnecting the USB VCC from the board. It does leave that wire connected to the USB circuitry in the chip, however. With the transistor removed, the USB power never gets to the board so it is always powered by the external (1130) power. This means the board powers up and down with the main system, whether or not a USB cable is connected to anything else. When the board is powered up, the cable is active as a serial connection, exactly what I wanted to happen.  In the picture below, the green circle shows the reinstalled polyfuse. The red c...
Read more