I do a lot of test equipment repairs and find that most manufacturers align their resistors. I do too and even rotate axial lead components so the values can be seen after installation - must have OCD too! To set the constant current source, why not just connect your meter in series with the output? Also, as the current source is critical as well as the balance of the op amp for accurate readings I would use the ten turn pots specified. Great project and looking forward to seeing it in operation. Regards, David
It's good manufacturing procedure since many non electronically skilled people can be used for assembly and it makes for greater accuracy in assembly as they can simply take the right colour combination from a pick bin with the components all oriented the same way. In addition it makes for faster and more accurate QA for similar reasons. However, for your own domestic project use, if you are used to reading 4-colour code resistors in any orientation then it may be of little benefit.
Credit where credit is due - this project was originally designed by Electrolab and uploaded to PCBWay shared projects. If you are interested subscribing to the Electrolab RU-vid channel you can find it here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-FBlCEkQsdwM.html
IIRC - I had two hot air recovered soic-8 TL431 chips which failed to work properly - I was using a simple test circuit where an LED lit up at 5.00v. It worked fine for the usual 3-pin TL431C
Hi Richard. In LER #47 talking about hot ground and not to connect oscilloscopes to it , you say you will demonstrate how to get around it. I assume you use an isolated supply but you never actually demonstrate the method. It would be good if you made a video of this method and maybe other good working practices . Your videos are excellent for the beginner and i feel like an apprentice again while watching them. All too often other videos show more theory than practical troubleshooting. Keep up the excellent standard and chiao for now.
If both equipment is use the same AC power adapter either one else device must be possible use isolated transformers power supply for preventing them from short circuit .
### Understanding Hot Ground in Power Supplies and How to Safely Check for Isolation **Hot ground** in power supplies can cause serious problems when working with sensitive equipment like oscilloscopes. This issue occurs when the ground of your power supply isn't at the same potential as the earth ground. Here's a breakdown of the concept and how to check for isolation to avoid any mishaps. #### What is a Hot Ground? In a typical power supply, the ground (or negative terminal) should be at the same potential as earth ground. However, with a "hot ground," the power supply's ground can float or even have a voltage difference from earth ground, potentially leading to: - **Short circuits**: If the oscilloscope's ground and power supply's ground differ, connecting them can cause a short. - **Electric shock hazards**: Touching both the hot ground and earth ground can create a dangerous shock path. - **Damage to equipment**: The short circuit might damage your oscilloscope, power supply, or circuit. #### Oscilloscope and Hot Ground Problems Oscilloscopes are usually grounded to earth through their power plugs. When you connect an oscilloscope's ground clip to your circuit, you assume both grounds are the same. However, if your power supply has a hot ground, this can create a direct short and cause sparks, shocks, or equipment damage. #### How to Safely Mitigate Hot Ground Issues 1. **Use an Isolation Transformer**: This device isolates your power supply from the mains, eliminating the risk of creating a short when connecting grounded equipment like an oscilloscope. 2. **Use Differential Probes**: These probes measure voltage between two points without referencing ground, avoiding ground loops or short circuits. 3. **Battery-powered Oscilloscopes**: Since these are not connected to earth ground, they can safely measure circuits with floating or hot grounds. 4. **Check Power Supply Isolation**: Before connecting equipment, ensure your power supply is properly isolated (more on this below). #### How to Check for Power Supply Isolation To avoid problems caused by hot grounds, you can test whether your power supply is isolated from the mains (AC input). Here's how: 1. **Unplug the Power Supply**: Ensure it’s disconnected from mains power to prevent electrical hazards. 2. **Set Your Multimeter to Continuity or Resistance Mode**: Most multimeters have a continuity setting (often shown as a soundwave symbol). If not, use the lowest resistance range (e.g., 200Ω). 3. **Identify the Input and Output**: Find the AC mains input (marked "L" for Line, "N" for Neutral) and the DC output (marked + and -). 4. **Check for Continuity Between AC and DC**: - Place one probe on the **AC Line or Neutral** pin and the other on the **DC ground (negative terminal)**. - There should be **no continuity** (multimeter shows "OL" or a high resistance). If there is continuity, the power supply is not isolated, which can cause hot ground issues. 5. **Test Both Poles**: Repeat the check for both Line and Neutral to the DC positive terminal as well. 6. **Test Under Power**: After confirming no continuity, power the supply on. Use your multimeter in AC voltage mode to check between the DC ground and a known earth ground (e.g., metal chassis or outlet earth pin). There should be no significant AC voltage. #### Conclusion To safely connect oscilloscopes and avoid hot ground problems, always ensure that your power supply is isolated from the mains. Use the steps outlined above to check for isolation with a multimeter. Additionally, consider using isolation transformers or differential probes for sensitive equipment. Stay safe and avoid unnecessary damage to your gear!
I'm not sure that I'm ocd, however the thing that really grates to me, is that you have two different background coloured resistors! I find that to be much more jarring than which way round they are. Though, I could see, that should they be all of the same colour, I'd then get upset by the orientation, lol.
When doing projects I print the schematic and mark off each component when installed. I check how each part is used in the circuit - and mark off the circuit trace - makes it easy to insure the part is correct, the pcb is. correct and helps me understand how the circuit is supposed to work.
As your going to be useing it as short finder rather than a lab grade micro ohm meter is that level of accurate reading.as long as it can be zeroed it can be used to measure relative values which should be fine for short finding. boards look great.
Those pins on the heat sink. They are split pin and can be tapped out using a flat screwdriver and a small hammer. I did this on the heat sink I used for this project. I don't understand why they are there. They could be added if required.
The gain of a differential opamp amplifier, like in this case, is set by R10, R6, R11 and R7, not just by R11 and R7. In case of a normal, non-differential amplifier it would R11 and R7. (but this is most likely not the source of your problem).
Hi Rich, why not just measure the constant current at the CC terminals and set that to the desired 100mA? or whatever was desired. I thought that was the point to have the constant current applied to the circuit under test. Plus If you look at the 741 datasheet again, it showed the offset null pins were there, but on different pins, 1 and 5. It looks like if you were to modify the board and link pin 5 to 8 it would work with either version of op-amp? For the accurate 10k resistors why not add multi-turn 1k variable to 9.1k fixed resistors? and a similar solution for the 100k's !
It's better to add a 100k multi turn pot in parallel to a value of 12k to get to the value you want as it doesn't involve connecting items in an odd up ended way to fit in the space available. Other people have also said this!
Hahaha, it's definitely OCD, and I do like my resistors the right way round usually, but other times when it for work and I'm in a rush - they work both ways round don't they?!
I like this project and I've convinced myself to building it. But my question is this, this is a milli-ohm tester and I understand is to check as close as possible to a short which is Zero Ohms, but now were getting in micro-ohm testers and soon nano-ohm testers..? but with these testers at what resistance if not "Zero-Ohm" can it be considered a short? I mean at such low resistance milli or micro ohms, can they not all be considered "shorts"
You twigged very early on actualy well done, yeah, I new the 741 had no 'trim' and the circuit called for it and this was a great video ! Do you dislike the 40 ohm setting on the Fluke ? for low value resistors you can calibrate out your leads, it just that you tried several meters and didn't try the Fluke and I'm wondering if there is a problem with the low ohm setting ? once again great video !....cheers.
I'm building this project more for the entertainment and educational value rather than having a need for it - I think so far it has done very well on both counts. For newbies I believe they will learn a thing or three, and for the more experienced it has made for a great conversation, and even some of those are learning a few things (me included). So it's win win really regardless what I actually do with it. Having said that I definitely will compare it head to head with the 'amazing $1 short finder' in part 2.
@@LearnElectronicsRepair Yes ! great video I learnt stacks and I think maybe some more builds like this. I was just curious about the Fluke on low ohm purely out of interest....you cant have to much test equipment !....cheers.
I just finished building mine. For an open circuit it shows 11,...V, just like yours, but for any low resistance it shows exactly 0, and in fact the OP07 output is negative (-1,2V)... The trim pot doesn't seem to have any effect. Any idea what might be wrong? I'll get back to troubleshooting it tomorrow.
I was thinking you had a link to the shared project at PCB Way, but I can't seem to find it on any of your milliohm meter videos. Where should I look to find it? Thanks Richard!
When trying to get a highly accurate resistance it is usually better to use a parallel combination, especially if you use a multi-turn pot as the parallel element. NOTE: in order for this to work it is essential that the existing resistance is ABOVE that required since another resistance in parallel can only LOWER the overall value clearly. Example: you want exactly 10k but only have 10k3 (measured). Using standard parallel resistor calculation we know that R1R2/(R1+R2)= R3 (equivalent parallel resistance) So R1R2=R3R1+R3R2 R1R2-R3R2=R3R1 R2(R1-R3)=R3R1 R2= R3R1/(R1-R3) R2 is what we need to know to put in parallel. Substituting real values: R2=10k x 10k3/(10k3-10k) Working that out gives ... R2=103 x 10⁶/300 or 103/3 x 10⁴ R2= 34.33' x 10⁴ or 343,300 ohms Hmm not an easy single resistor value to obtain but using a 390k pot it's a piece of cake! Using a single value resistor in parallel, even from a 5% or 2% can still dramatically improve the accuracy of the final value needed. In the example used, check to see the result just by using a standard 5% 330k in parallel you can do the calcs and see how much it improves! It's a technique I've used many times in the past to great success. The best thing is that it is SO much easier to add a resistor in parallel to a TTH component than to put one in series ... No need to remove one leg, sometimes awkward to do or risking PCB damage. It is also possible (sometimes) to move an SMD resistor sideways and mount two side by side on the same pads! Hope this helps.
maybe try OP07D op amps, would probably be more accurate .. but that op07 part is pretty old and eol. good luck finding some other than aliexpress copies...
DIPs are becoming harder to find, but the OP07 is still in mass production. A year after your post I checked one authorized distributor which had well over 100 000 pieces in various versions on its shelves for immediate delivery.
DC-coupling opamps is always risky. If you want to use cheap amplifiers in stead of instrumentation amplifiers you probably want to use only AC-coupling and use a low frequency signal to and measure the AC voltage drop. I was in unbelieve when that sheet told you you could replace an instrumentation amplifier with a cheap 741....
Yeah, but google is always right apparently, and it made for an interesting video that I think a lot of newbies will find very educational and a lot of experienced will find very entertaining 😁
@@LearnElectronicsRepair They are not even pin compatible. Pin 5 and 8 are swapped. Normally people leave the offset pins of a 741 not-connected. I don't know what the effect is of connecting just one of them, like you did. You could try disconnecting pin 1 of the uA741 by bending it up.
There is nothing whatever "risky" about DC coupling op amps if you know what you are doing. I've designed op amps into countless circuits and all but a very few have been DC coupled. Many have been in circuits demanding very high performance. It *is* generally more sensible to use an integrated instrumentation amp than a differential amp these days if you need good common mode rejection They are moderately expensive, but so are the 0.1% or better resistors you need to make a good differential amplifier. In this circuit an integrated instrumentation amp with high input impedance could be well protected against over-voltage at the input with just series resistors to limit the current through the internal input protection diodes of the amp. Ordinary 1% resistors would be adequate. They do contribute additional noise but it is of no consequence in this sort of circuit.
I read in an electronics magazine (probably in the 70s) that you can tune resistors by filing them. They filed some types of resistors lengthways and others across. So if you can't find two resistors that are well matched to the required tolerance, you can adjust one to match the other.
That would depend on the resistor construction, carbon resistors maybe but then you would allow moisture into the component and that would create issues. Not suitable for modern resistor construction.
Performance of this circuit is highly dependent on the matching of ratios of resistors. 1% metal film resistors with a temperature coefficient of not greater than ±100 ppm/k should be used.
Having a quick look at the current circuit I can't see any feed back on the current settings, are they relying on the small range of mohms measurement that the current won't charge much?
@d614gakadoug9 well bugger me you right on both counts. well spotted there. 1: there is a feedback D1, D2, R2, and R15 2: it's a real abomination of a schematic. With only a quick glance it's bloody difficult not to see connections when there isn't one. its a 2/10 in schematic drawing from me 🙂 i like the way they put R3 in 🙂 and what's going on by C4!
@@TheEmbeddedHobbyist R3 is pretty much useless. In a situation where there was a large capacitor across the voltage reference R3 might prevent excessive current into the op amp input if the supply voltage dropped very rapidly while there was still charge on the cap across the reference. There is no cap across the ref so that isn't going to happen. C4 is just a power supply decoupling cap. Or are you referring to parts near it?
Those tweezer probes are crap my LCR meter won't calibrate with them but with Kelvin clips it does. There is sufficient length in the unshielded tweezers for them to have stray capacitance and inductance, where as Kelvin clips only expose very short clips.
@@marka1986 They only pop up, so to speak, in the first 30 mins of publishing a video (and they do it on every video) so generally I just delete them - but this video had still not processed when i went to bed last night so I just left it for the morning, I always publish unlisted until YT completes any copyright checks and move to public once YT is happy Anyway last night was a another hot one, it's been like this for the last couple of weeks now, around 40C (104F) in the day and 30C (86F) at night. As we don't have aircon in the house I woke up about 2:30am rather moist LOL and went to get a glass of water so I checked my video and it was ready to publish which i did then went back to bed. So that's what the inevitable porn links didn't get deleted. I would have thought YT could find and remove these no problem, as it manages to remove innocent subscriber comments more than often enough without asking me!
@@KB1UIF Yeah that is not actually correct. YT does not give an option to automatically publish when copyright checks are completed. That would be great (and no doubt easy to implement, but NO, you have to publish manually which could be the next day if you were sleeping. It also has no progress bar or indicator of any kind on the copyright check progress - which in all honesty is ridiculous. I find copyright checks take around 6hrs+ though if you google it, a lot of people say it takes about 10-20 mins. The craziest one of late was the 1500W PA system repair. It took about 9 hours, said there was a copyright claim for the song 'Dance Freak' which I was actually playing in short segments from... wait for it... a YT video (not an official record label one either) and mostly talking over it. I then had a option to mute the song but retain the voice over. I chose that, it said it would 'take a while'. It took over 12 hours and then said there was no copyright issue, without even muting the song!!! Go figure that one out 🙄
Ok, it's the wrong amplifier and you can't fine-tune the DC offset. But the result should not be as bad as this. I am not sure where exactly you are measuring, but it all doesn't make sense. You can't have a differential input of 1.2 V before the input resistors of your amplifier when you short the probes. Are you sure you've not swapped wires in the connection to your probes? Or maybe you problem arises from having 1 offset pin connected (instead of 2). The OP07 and uA741 are not pin-compatible. Maybe you should try to bend up that one connected offset pin (pin1 of your uA741).
I definitely measure 0.002V across the probes when they are shorted together and 1.2V across pins 2 and 3 of the op amp. Once i remove the ua741 the offset goes away. You can clearly see that in the video.
This means the 741 is not working as it should. You connected only one of the offset pins of the 741, because the OP07 and 741 are not pin compatible.... You try to disconnected the other offset pin too, by just bending up pin1 of the 741. You should either connect the offset pins correctly or not connect them. Just connecting one will cause problems.
Before I say a shitty thing, I'd like to know what op-amps did you use? I ask because OP07 is a low offset & trim pinout device, so not much will match this ability for proper functionality. ...and now to be a shit here, but the circuit design is very power inefficient, and a little bit crap in other ways as well, not to mention the shitty need for +/- power that could be fixed in one of two common ways.
The whole schematic doesn't make much sense. The input resistance of the amplifier circuit connected to the sense pins is low, which influences your measurement significantly (because part of the current from the current source will flow into the amplifier), but at the same time creator is stressing you need to use accurate resistors within the circuit.... Why would one need R9 and R8? The zeners already prevent too high input voltages. These resistor just create extra load.
26:10 - Does RU-vid also delete parts of your description?! I couldn’t find the link to your tl431 tester video. [Edit) I found it through a painful search: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-LvGehyWLS80.html here it is to save others the trouble.
