Recent Posts

41

General Technical Chat / Re: Do you think an LED is a resistor?

« Last post by Kim Christensen on Yesterday at 11:46:44 pm »

From: A.C. Fischer-Cripps, in Newnes Interfacing Companion, 2002
3.3.7 Log amplifier
A non-linear resistor is connected into the feedback circuit. In practice, this can be a diode, but a transistor connected as a diode is used since the forward biased transfer function is more accurately exponential. The exponential nature of the forward biased diode leads to a logarithmic decrease in gain of the circuit as the input signal is increased.

Notice that they didn't write this:

A non-linear resistor is connected into the feedback circuit. In practice, this can be a resistor, but a resistor connected as a resistor is used since the forward biased transfer function is more accurately exponential. The exponential nature of the forward biased resistor leads to a logarithmic decrease in gain of the circuit as the input signal is increased.

 

42

Repair / Re: TDS744A: 1M input impedance measures: Ch3 = 160Kohm, Ch4 = 706KOhm

« Last post by bdunham7 on Yesterday at 11:44:30 pm »

I'm not sure how to actually use the 5mV/div, 2mV/div and 1mV/div setting because the scope will not allow me to select those fine vertical resolutions when I'm plugging passive probes in (min vertical resolution then becomes 10mV/div).

If you connect 10X probes w/ readout or select 10X manually for non-readout probes, the vertical scales are all multiplied by 10.  So with a 10X probe, the apparent 10mV/div setting is actually the 1mV/div setting to the scope.

43

Projects, Designs, and Technical Stuff / Re: custom LCD design

« Last post by doanerocks on Yesterday at 11:40:10 pm »

@Kim Christensen yes that would definitely make sense for the first couple to just take a off the shelf parts.  I was hoping to skip a step but I should probably do a 6 digit seven segment for the top and put a separate 16 or 20 x1 character display on the bottom.   

44

Projects, Designs, and Technical Stuff / MCP3008 and MCP23017 experiments - first project verification

« Last post by novice_electronic on Yesterday at 11:39:43 pm »

Hello there! I'm Karol, and I've embarked on the journey of automating my gardening beyond mere experimentation. I've drafted a PCB design, although I must confess, it's a tad messy. Before investing in components and PCB fabrication, I'm seeking validation that I'm on the right path.

The PCB integrates two MCP3008 and MCP23017 chips. The MCP3008 manages temperature sensors, while the MCP23017 controls servos/relays. Both chips are powered by an external 5V supply. I've included TVS diodes on both the external 5V line and the internal Raspberry Pi line, although I suspect the latter may be redundant. Additionally, I've added resistors to drop the supplied voltage to the chips to 3.3V, even though they can function at 5V, which might be unnecessary. To stabilize the 5V external line, I've inserted a few 100nF capacitors, albeit without extensive research. Zener diodes have also been added for extra stability.

Does this layout and design make sense? I'm open to any feedback or suggestions before I proceed further.

I have included ZIP file containing KiCad project

45

Repair / Keithley 2400 locks up when buttons are pressed

« Last post by tomn on Yesterday at 11:39:14 pm »

I have a Keithley 2400 C10 with the following issue:

• The unit powers up, the display works, and it can make measurements when commanded over RS-232, so the ROM self-test passes and the analogue section works.
• After pressing some buttons on the front-panel, it becomes unresponsive – buttons do nothing, the display does not change, and it no longer responds to commands over RS-232.
• Some buttons cause it to crash instantly (e.g. on/off and front/back), while others take a few presses (e.g. edit crashes on the third press), and some don’t crash at all (e.g. digits, menu). The behaviour is always the same.

I’ve been working on this for a few days and haven’t really got anywhere.

Here’s what i’ve tested so far:

• Power supplies seem fine. I’ve checked the incoming 12V and 5V supplies, and all analogue supplies.
• Reset for the display and CPU seems fine – held low for ~250ms after 5V is stable.
• It behaves exactly the same with the analogue board removed.
• Communication between the display and main board seems OK. I can see communication in both directions, and when it’s crashed I can see that the display board still sends key-presses. I can’t see the “Pulse train every 1 ms.” from the main board to the display board listed in the service manual, but this seems unlikely on a 9600 baud serial line? I’ve attached a trace of the display rx, tx and reset lines during startup with two button presses before it crashes. Does this look right?
• Looking at the activity on the CPU external bus, it is definitely still running while crashed, but is probably in a tight loop (repeating patterns on address lines).
• Exactly the same issue occurs when simulating key-presses by sending SYST:KEY commands over RS-232.

Anyone got any bright ideas, or things to test? I’d like to avoid having to disassemble the code and work out where it’s crashed with a logic analyser, but that feels like where this is heading…

If I get it working, I’d be interested in a source for a new button pad – this one has always needed a lot of force and cleaning didn’t seem to help.

Cheers!

46

General Technical Chat / Re: Do you think an LED is a resistor?

« Last post by Sredni on Yesterday at 11:38:36 pm »

Oh new words and definitions to play with. How about we take the IEEE dictionary as the authoritative reference:

static resistance (semiconductor rectifier device) (forward or reverse) The quotient of the voltage by the current at a stated point on the static characteristic curve.

Yes, this is the only one I have used. Did you not realize it?

small-signal resistance The resistive part of the quotient of incremental voltage by incremental current under stated operating conditions.

small-signal A signal which when doubled in magnitude does not produce a change in the parameter being measured that is greater than the required accuracy of the measurement.

As I said in my opening statement, you're just playing with small signal analysis a well known and entirely un-novel method. There is nothing interesting here as most anything can be described as a small signal resistor (with bounds on some other dimension). So you're still wrong and trying to twist definitions to your liking while ignoring the consensus and the established science.

No, I have not used small signal analysis. Where did you study small signal analysis? You are mistaken. Please seek tutoring from someone you trust to straighten this out.

47

Repair / Re: Unknown IC

« Last post by fzabkar on Yesterday at 11:36:15 pm »

https://ipv4.google.com/search?q=filetype%3Adoc+OR+filetype%3Apdf+%22absolute+maximum%22+1446i

http://markingcodes.com/search/c/1446i

48

RF, Microwave, Ham Radio / Re: WSJT-X on Pi

« Last post by ZigmundRat on Yesterday at 11:33:56 pm »

You’re in dependency hell due to severely mismatched package dependencies. If you’re trying to do this the simple way, Bullseye (the version you have installed) seems to have a WSJT-X 2.30 package available. If you want to run 2.6.1, then it looks like installing Debian ‘bookworm’ is the way to go as WSJT-X 2.6.1 is available there.

The Bullseye package is wsjtx_2.3.0+repack-2_armhf.deb
The Bookworm package is wsjtx_2.6.1+repack-1_armhf.deb

Both should be available in the standard archive and installable via apt.

Hope that helps.

49

Projects, Designs, and Technical Stuff / Re: Nichicon UPJ for switching power supplies - what's so special about these caps?

« Last post by wraper on Yesterday at 11:33:13 pm »

LM317 is not an LDO and therefore is stable with low ESR capacitors such as MLCC. UPJ series have low ESR and high ripple current rating. Therefore they can survive way more load/stress than general purpose capacitors.

50

General Technical Chat / Re: What did you buy today? Post your latest purchase!

« Last post by cosmicray on Yesterday at 11:32:24 pm »

An assortment of enameled wire sizes (22-32 AWG) from Remington Industries.