Help Proofing Breadboard Connections - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: September 16, 2018, 06:33:34 am
content: I have been working on Jay’s Dual Dynamic Electronic Load. The circuit has been hacked to remove the original oscillator portion of the circuit and to add back an auxiliary input, for an independent function generator. This is, now, on the breadboard.I do not have a LT1013 op amp, in stock, as is specified, by the schematic. I am using a LM741, just for testing the circuit concept. I have corrected the pin connections, to the correct pinout.I have limited experience, with my test equipment. I would appreciate your help, in confirming that I have made the connections correctly.The PSU is a BK Precision, 1735, Linear, 30v/3a. It has the common Negative, Earth and Positive banana jack connectors.Negative and Earth are not tied,at this point.The Frequency Generator is part of a Tenma Universal Test Center, 72-1005, 0 – 20MHz. It settings are 500KHz, at 1v.The oscilloscope is a Tektronix 2215, Analog, 60MHz. It settings are x10 probe - 1V/D - 10mS/D. I am confident enough to have looked (directly/independently) at the FG signal, on the oscilloscope, and it appears correctly.Things get a little sketchy, for me, working with earth ground and the negative voltage supply. The drawing shows how I have things connected. But, before powering everything on, together, I wanted to verify the setup.Of particular interest is the probe ground connection. Should it go to earth ground, or the negative supply? My guess is that it goes to earth, because, when I had it attached this way, while testing only the FG signal, the negative component, of the sign wave, appeared below the zero volt base line. The op amp output is connected to the oscilloscope probe input.Thoughts and suggestions, please and thank you.

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2018, 07:29:30 am
content: I doubt very much that your circuit's GND is actually GND, rather it is the 0V output of a isolated PSU.The probe's shield is, loosely speaking, the "reference potential" used when making the measurement, and should be connected to the circuit's reference potential. In your circuit the PSU's 0V output will be that circuit's reference voltage, so it looks like you have connected the scope probe correctly.A word to the wise: the probe shield's is directly connected to the scope's case and hence the mains protective earth. Never attempt to connect the probe's shield to anything that cannot be at that potential; if you do then large currents will flow along the probe's shield! If that is a problem when trying to make a measurement, never ever disconnect the scope's protective mains earth since that can be extremely dangerous to operators and the scope. Instead you should use an appropriate probe.In this case, even if the circuit's GND really is main protective earth, no damage will be done and the measurements will be correct. (Caveat: provided the two mains protective earths are at the same potential, which is probably the case - but cannot be guaranteed in all installations!)

author: t1d
author_rank: Super Contributor
timestamp: September 16, 2018, 08:03:42 am
content: Thank you, tggzzz.Excellentinformation! I look forward to powering up the circuit.I am aware of some of the issues, and care needed, with using an oscilloscope. Thanks for the reminder.I have been working on a Differential Probe circuit. I will order it, with my next larger PCB panel. Here is the link.http://www.analog.com/en/design-center/reference-designs/circuit-collections/ltc6268-10-oscilloscope-differential-probe.html#cc-overviewThere are some odd tricks, in the notes, to minimize RF problems, so be sure to look at those... Guard rings and intentionally exposed traces...

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2018, 08:22:33 am
content: Quote from: t1d on September 16, 2018, 08:03:42 amI haven't examined that circuit in detail, but any 4GHz opamp will require very careful attention to layout; at those frequencies "earth" and "0V" are merely fictitiousAlso be aware of the common mode and differential mode voltage limits, and that high voltages often jump "insulating" gaps.FFI, have a look at some of the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: t1d
author_rank: Super Contributor
timestamp: September 16, 2018, 11:32:20 am
content: Good stuff, tggzzz. Thanks.I did a power-on test, with the circuit. Using the LM741 op amp, I was able to maintain a very acceptable square wave, to 5000KHz. I don't know the correct term, but there was "echoing." However, the shape was not greatly deformed.Interestingly, there was a prominent spike, at the top of the rising edge, at lower frequencies. It disappeared, the faster I tuned it.

author: Old Printer
author_rank: Frequent Contributor
timestamp: September 16, 2018, 01:30:11 pm
content: I am not the one to proof your breadboard, but in case you have not seen this YT video.https://youtu.be/mJL11UF1arQIt is on power supply's, and in particular, joining two isolated supplies in various configurations. Alan uses discrete units, but the basics hold true for two isolated supplies in one like your BK 1735. On the chance you have not seen his channel, W2AEW, Alan has made videos on many basic and more advanced electronic topics. He works for Tektronix and is particularly knowledgable on T&M gear like scopes.

author: t1d
author_rank: Super Contributor
timestamp: September 18, 2018, 10:31:14 am
content: Yes, Old Printer, I know of Alan's channel... I'm a big fan. I think I did watch that particular video. I would have watch several. Those videos are where I got enough information to know that care needs to be taken, when working with this type of powering scheme. Thanks for your reply.