Is the common ground between probes logic free? - Page 1
Forum section: electronics / beginners

author: MathiasSven
author_rank: Newbie
timestamp: June 30, 2023, 03:53:20 am
content: Hello everyone, first post!I understand this sort of question has been asked before, but I haven't been able to find an answer to this specifically. Essentially, I am just starting out with Electronics and was working on a power supply for a DIY waveform generator, while making the power circuit I seem to have killed a voltage regulator, I desoldered it to test is separately.With no load on the circuit, no current should be going through the GND pin of the regulator, so in my test I didn't bother doing a proper connection to the circuit's ground and just used the GND of the probe since the other probe was already at the correct reference, however it seems the voltage regulator when dead became a resistor between Vin and GND thus current went through my probes. I didn't realize it at first (I didn't have the Ammeter initially) and there were no sparks, so my question is, while this is bad and should serve as a lucky warning since this wasn't high voltage, do low currents going through the probes damage the scope/its calibration? Can I treat the common ground between the probes as a 26-28 AWG wire without any logic, and as long as the current that would go through them isn't too high there shouldn't be any long term damage?

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2023, 04:08:35 am
content: Your scope and probes are almost certainly fine, you can check the ground side of the probes with a DMM if you like. The lessons you should learn are 1) start by only using 1 ground clip at a time, remove the others 2) if you must use more than one ground clip, only connect them all at a single point unless 3) if you must connect the ground clips at different points (mostly for HF signals) then those points must be guaranteed to be at very nearly the same potential and you must think the whole thing through very carefully. Don't proceed to the third scenario unless you really understand things and really need to. Your case here is a 1) scenario--one ground clip only is needed.

author: MathiasSven
author_rank: Newbie
timestamp: June 30, 2023, 04:51:35 am
content: Understood!My main concern is that it could have somehow messed with the calibration, I am not doing any HF stuff yet so I will remember to always use one ground probe. Thank you

author: srb1954
author_rank: Super Contributor
timestamp: June 30, 2023, 05:19:39 am
content: Quote from: MathiasSven on June 30, 2023, 03:53:20 amThere is still current flowing out of the ground pin of the L7812 regulator even if there is no load on the regulator. The internal circuitry of the regulator has its own operating current, typically around 5mA, which flows out of the ground pin. The ground pin of the regulator must always be connected if the regulator is to function correctly as loss of the ground connection may damage the regulator itself or other circuitry run from the regulator output if the regulator output rises to the maximum incoming supply voltage.QuoteIt never pays to rely on a scope probe for providing the circuit ground connection. In the event of a fault condition you might get excessive current flowing through the probes and that could damage the probes or the input of the scope. Saving a piece of wire for a proper ground connection is false economy if you risk damaging your probes or scope.Your circuit doesn't show a smoothing capacitor on the output of the bridge rectifier. You will need one of these if you want to get a smooth DC out of your supply. The smoothing capacitor has appreciable ripple current flowing though it so a proper ground connection to this is also necessary.

author: MathiasSven
author_rank: Newbie
timestamp: June 30, 2023, 06:08:15 am
content: QuoteAh right, although I wouldn't have worried over 5 mA, when it was dead it had over 600 mA going through it! That is what scared me about possible damage to the probesQuoteI invested in a 60 euros kit of different kind of test leads yesterday because of this, it mainly happened because the rectifier was in a perfboard with the transformer output and I didn't want to solder the regulator down. Trying to connect a wire to the free regulator was a bit painful, so I just skipped it, thinking "they will connect through the oscilloscope anyway". I will not make this mistake again.QuoteThis was just the setup I used to test the regulator. I spent about a day breaking my head on why the voltages and currents were all over the place. I think I killed both regulators when I soldered the L7912 incorrectly, desoldering and testing it separately was just to confirm that they were dead.I included a diagram of what I was making with the voltage readings I got from my multimeter that I drew up when I was talking about it in a discord channel with some people. This is besides the question of thread but yeah, right now I am just waiting on a new 7912 since I killed the only one I had.

author: srb1954
author_rank: Super Contributor
timestamp: June 30, 2023, 02:37:11 pm
content: Quote from: MathiasSven on June 30, 2023, 06:08:15 amYes, these voltage readings don't look good.I assume that the -16.8V shown on the input to the 7812 regulator and the +16.8V being shown on the input is simply the multimeter probes being inadvertently reversed but you should double check the wiring of the bridge rectifier to make sure it is putting out the right polarity voltages for the respective regulators. Applying reverse voltage to most ICs is almost a guarantee of instant destruction. Also, triple check the wiring to the 7812 and 7912 chips; these have reversed positions of the INPUT and GND pins on each type.The 16.64V output of the 7812 indicates that the chip is possibly damaged and there is an internal short or low resistance path between Vin and Vout. Put a load resistor, say 1k, on the output to see if the voltage drops significantly. Little or no voltage drop from the 16.64V level with the extra load indicates an internal short.The -9.75V output of the 7912 might indicate that the regulator output is oscillating. Have you monitored the O/P with a scope? The 7912 requires a good, low impedance, capacitor on the output to make it stable. You might try a 1uF tantalum capacitor across the O/P to see if that improves the O/P voltage stability.Another circuit improvement that is desirable where you have both positive and negative regulators is to have reverse biased diodes across each output. This is usually shown in the respective datasheets and application notes for these regulators. These diodes prevent damage to the regulators if its output is pulled below or above their respective ground pins by current from the other polarity regulator flowing back through the load circuitry. This is particularly applicable to the 7812 which may latch up and draw lots of current if its output is pulled below -0.6V.

author: MathiasSven
author_rank: Newbie
timestamp: June 30, 2023, 06:22:23 pm
content: QuoteYes, apologies, that was a mistake I did when making up the diagram.QuoteSadly, I believe that is exactly how I killed the regulators, I soldered the 7912 thinking it had the same pinout as the 78 series and then when I plugged in the transformer to check the voltages I noticed they were all wrong and a bunch of current was running through the circuit with no load. The readings in the diagram were after I had desoldered and resoldered with the correct pinout.QuoteI did do some tests on the circuit before I desoldered the regulators. On the output that was reading +16.64, when a load was placed (I think I used something in the low kohm range) between it and GND the voltage would drop to +12.7, while on the -9.75 if under the same load it would drop to -1.5vAfter I had the regulators out of the perfboard I did do a scope reading, which going full circle is what led me to this question. You can see my suboptimal test setup on the first picture, and the output of the scope from the 7912 that was in the board.The third and fourth images are the readings from one of my working 7812 and a reading from a different 7812 than the one that was in the board, but one that I had also somehow killed. The voltage drop on the rectifier output was from the current that was going through the scope.QuoteQuoteThank you for the advice! I don't have any tantalum capacitors on hand but since I am waiting on the 7912s I will purchase some as well. This circuit is meant to power a DDS for a DIY waveform generator so if that rail was ever pulled that low (lower then 3 volts) the circuit would turned off I think, but I will the diodes just in case, replacing these is no fun.

author: Damianos
author_rank: Frequent Contributor
timestamp: July 01, 2023, 09:01:01 am
content: Some thinking about the diagram shown in replay #4here- What is the role of F2? If it blows, then the circuit will be floating and the situation becomes as described in the initial post! Note that these regulators have internal current limiting.- The values of C3 and C4 are "huge"! Use these capacitors in parallel with C1 and C2, to "keep" the input voltage while the circuit is loaded. In place of C3 and C4 use 1μF tantalum or 10μF electrolytic.- The input voltage is at the low side for proper operation.- Use external protection diodes, as described in data sheet.Study the data sheets and the relative application notes to understand how they operate and find ideas on how to design and implement your power supply. Always analyse the circuits, because even these official documents may contain errors (that unfortunately are never corrected!)!

author: EPAIII
author_rank: Super Contributor
timestamp: July 01, 2023, 11:16:11 am
content: Definitely not how to test a Voltage regulator.Moving on, if you are still uncertain about the condition of your scope probes or the scope itself, most scopes have a calibrator output that is used to test things. It is usually a 1 Vp-p square wave, often with a known frequency, like 1 kHz. It can serve as a quick test of your probes and the scope input circuit. Just clip the probes, one at at time, on that calibrator signal and observe the waveform. If it is a good square wave and of the correct level, then you can be 99.99% certain that both probe and scope input are OK.If you haven't noticed, scope probes that attenuate (10X, 100X, etc.) the signal also utilize this calibrator signal to properly compensate them to match the scope's input. These probes have a small screwdriver adjustment which is used to set the square wave to the best possible square shape. This provides the proper frequency response for the probe-scope combination.

author: Infraviolet
author_rank: Super Contributor
timestamp: July 01, 2023, 02:59:28 pm
content: Do scope manufacturers actually give afigure for what amount of current scope ground clips are rated for in the event of a "connected two grounds to different places" accident. I know that where mains voltages (which also share actual ground with most scopes) are concerned putting a ground clip anywhere but true mains ground can fry a scope nearly instantly, but when working with low voltages and current limited supplies and situations where the accidental extra grounding happens between real ground and somewhere which already has a substantial resistance between voltage sources and the point where an accident happens (so the accidental contact happens as if to a voltage source of high internal thevenin resistance) are there published figures for what currents can be tolerated by typical scopes and probes?

author: MathiasSven
author_rank: Newbie
timestamp: July 02, 2023, 04:28:27 pm
content: Quote from: Damianos on July 01, 2023, 09:01:01 amI didn't post the whole schematics, I think it is only relevant when the rest is included.Quote from: Damianos on July 01, 2023, 09:01:01 amI see, I will look into the effects of such changes, I didn't make these schematics, they are from this instructables pagehttps://www.instructables.com/DIY-FunctionWaveform-Generator/for a DIY Waveform Generator, and while I understand why the capacitors are there/what their role is in the circuit here I didn't think much about the values.Quote from: Damianos on July 01, 2023, 09:01:01 amUnderstoodI did have a look and the datasheets now and I see now that on the recommended application it mentions such a diode.Quote from: EPAIII on July 01, 2023, 11:16:11 amIgnoring the incorrect use of the ground probe, does the 7812 image not serve as a way to confirm the regulator is still performing its basic task? It might not be under load but the ones that were broken would no longer regulate the voltage at all. Is there something more wrong about it?Quote from: EPAIII on July 01, 2023, 11:16:11 amAh right, this was mentioned as one of the first things to do on the manual, and while I did do this calibration on all my probes when I got it I didn't think of using it again for testing if they were okay after this incident. I tested them now and it looks just like after I calibrated the probes, so everything should be good

author: Damianos
author_rank: Frequent Contributor
timestamp: July 02, 2023, 10:33:32 pm
content: Quote from: MathiasSven on July 02, 2023, 04:28:27 pmIt is a bad copy ofthisNote also that most (if not all) small transformers are self protected, so using any fuses just complicate things.