Milliohm tester/continuity tester project - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 10:52:54 am
content: I was thinking of making my own milliohm meter. When thinking about the project, I thought that it might be a good idea to start with a "better" continuity tester.Two things bother me about most continuity testers:First: The buzz - It's this horrible high-pitched shriek. The reason why is simple: Piezo buzzers are easy and power efficient. Since I want my final project to be in a bench-top format anyways, I thought about using a more pleasant sounding pure sine into a speaker. Even with some distortion from the speakers and such it will still not drive you crazy. Might include a volume knob while I'm at it.Second: Thresholds. I've worked with meters that considered anything under 75 ohms good enough to buzz.Veryannoying when you are trying to see where something goes wrong in a 50Ohm system.Now I thought, if I go away from the piezo anyways, why limit myself to one frequency? And thus I have come up with the following idea: Have your lower and upper thresholds settable (either as a range or some other way - still in the concept phase). The system will produce a tone whenever the resistance is less than the upper threshold. The lower the resistance, the higher (or lower) the frequency of the tone. I think this would make it a lot easier to find shorts on a board - it allows you to quickly gauge if you are moving away or getting closer by listening to the change in frequency.Now, I still have to have a think about this and set out some specs. Also need to see if I want to integrate this into an actual m?/mV meter I wanted to make, or if I'm going to do this as a seperate project.What do you guys think about this? Any "gotchas" or issues I might be overlooking here that stops the project right in it's tracks?

author: MosherIV
author_rank: Super Contributor
timestamp: June 27, 2017, 11:09:48 am
content: HiNice idea for a project.Gotchas - do not try to do this with microcontrollers eg Arduino, they are just not fast enough.Do it with good old fashioned analogue parts eg Comparators.Make sure you have a very accurate and steady current source, eg Voltage ref + opamp + bjt in emitter followerGood luck and have fun.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 12:17:19 pm
content: Quote from: MosherIV on June 27, 2017, 11:09:48 amThe fronted will all be analog. Still have to decide what parts I'm going to use, since it depends on what specs I decide to settle for (sense current(s) I'm going to use, how many ranges, what absolute accuracy do I want). I was thinking of then going to a 14 or 16 bit DAC, and then out to a display.Tone generation can be done either with a microcontroller or a VCO. Have to see what options there are for this. VCO seems interesting, a nice project to learn about different topologies and so on.Will have to figure out how I do the turn on the VCO then - I don't want it to buzz at a very low frequency when I'm measuring an open (because the probes are not touching anything). Some stuff I have to play with.

author: HKJ
author_rank: Super Contributor
timestamp: June 27, 2017, 12:37:38 pm
content: Quote from: MosherIV on June 27, 2017, 11:09:48 amThe Arduino is more than fast enough to sample the input value and drive a buzzer.

author: MosherIV
author_rank: Super Contributor
timestamp: June 27, 2017, 12:45:27 pm
content: Something else to consider:microcontrollers vs analogue componnent power usage - Analogue circuits can be down in the nanoWatts, microcontrollers will be up in the 10s miliWatts.So if this is battery powered, micro based one will use up batteries far quicker.QuoteI suggested you use comparators (level sensing opamps) should allow you to detect different resistance levels very easily.The comparator output is pretty much binary (on/high or off/low) so they can be used to drive different frequencies on your VCOs.QuoteDo not forget that DACs actually have a linearity curve, which you will have to calibrate out. At such low currents, the non-linearity may be a big problem.If you use the "Voltage ref + opamp + bjt in emitter follower" I suggested, you can use switches to switch different resistors on the op-amp to give different ranges (adjust the gain of the op-amp to select different constant current ranges).

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 12:54:11 pm
content: Quote from: MosherIV on June 27, 2017, 12:45:27 pmThe thing I'm worried about when using gain switching is noise. If I'm down at the lowest levels, I will be right in the noise. Therefore I was also looking at using different current ranges (I got hold of a box of Coto reed relays, might be able to use those to switch in various current feedback shunts?)Since I'm really looking at high accuracy (sub-0.1 percent), I was hoping of using one low-offset opamp or differential amplifier. I have some AD opamps with sub-15uV offset laying around, might be a nice place to use those. Will have to run the numbers on that. Would also allow me to drive the VCO directly, without having levels. I'm gonna take a good look at realistic specs and what I'm willing to spend, so I have somewhere to start. Power is not an issue, I think I'm going to make this a bench device anyways, so it'll be plugged into mains.Thing that will limit me in terms of accuracy is the out-of-the-box performance of my ADC. I don't have the equipment to calibrate. Perhaps I can go to uni at some point in order to calibrate (I can just use a precision voltage source into the sense terminals I think?)

author: MosherIV
author_rank: Super Contributor
timestamp: June 27, 2017, 01:10:48 pm
content: QuoteWow, you are aiming high. That is going to take some serious engineering to get to 0.1% accuracy!QuoteQuoteWe might be talking cross purposes.I am talking about using opamp to set up the constant current that will be used to measure the resistance (ohms law).Set the current to a know value eg 100mA and the resistance is proportional to the voltage you measureThe current MUST be very accurately set, hence suggesting a Vref and op-ampI am think about something similar to the circuit used for electronics loads :https://coolcapengineer.wordpress.com/2014/09/15/tutorials-simple-electronic-load-circuit/Where R1 is the resistance you are measuring, R2 is replaced by the Vref and you would place different resistors in the feedback to change the range.Since this circuit will be looking at low ohms, you may want to consider 4 wire measurement.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 01:18:44 pm
content: Quote from: MosherIV on June 27, 2017, 01:10:48 pmI see. I thought you were talking about changing the gain of the opamp that is doing the voltage measurement over the DUT.Indeed, that current must be very accurately set.I was hoping to use something like the ADR45xx series of references for both the ADC reference and the current reference. They have 0.02% error, but low drift. If I can be certain that I can calibrate, I can also use a different part that has even lower drift. I want to keep the reference voltage down as low as possible (ideally I drive the reference of of the same supply as everything else analog, and I would like to avoid having to deal with 15V analog supplies...)I'll look at drawing a rough block diagram of what I have in my head already.

author: MosherIV
author_rank: Super Contributor
timestamp: June 27, 2017, 01:32:05 pm
content: QuoteFor this kind of instrument, I would not be that concerned about drift. If the constant current is in the order of 100mA, you should be able to keep the error/drift in the 100s uA region.Another reason to avoid microcontrollers is noise. Any micro is VERY noise when you are measuring things down in the mV or even uV, you may find the noise the micro introduces makes uV measurement impossible.You would have to heavily shield and physically keep the analogue stuff well away from the digital stuff.QuoteA lot of opamps can run single rail. Many now a days are designed to run on 5V, 3V3 or even 3V single rail.

author: bktemp
author_rank: Super Contributor
timestamp: June 27, 2017, 01:35:59 pm
content: Actually, the current must not be accurate. It must only match the reference voltage of the ADC. If you are using a voltage driven current source and use the same reference voltage also for the ADC, the accuracy is then given by the shunt resistor used in the current source.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 02:10:47 pm
content: Quote from: bktemp on June 27, 2017, 01:35:59 pmI was originally considering the same, but then I remembered a appnote from I think Linear, that said that you mustneveruse the same voltage reference for multiple devices, as some other effects (noise injection) would negate any gain due to the correlation of the error. Example in this case: You couldn't try and calibrate for offsets (as the ratio of the offset to the reference is not fixed). There is more to it, and I'm not saying I won't - Just pointing out why I stepped off of the idea. Might get back on it tho, if I can verify that it will provide better performance.That said, it could be very interesting here - a very poor reference could be used - all accuracy would come from the accuracy of the shunt resistor (you basically start measuring the ratio between the shunt and the DUT).I have to think about this.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 03:07:59 pm
content: Here is what I was thinking for the current source. One issue I have with this is that when when no DUT is connected, the voltage will be zero, so the opamp will be turning the MOSFET on hard - leading to a sudden rush of current if the DUT is then connected, and the opamp is turning the transistor off. Perhaps I'll see about adding a resistor in series with the drain of the transistor, acting as some kind of current limit.

author: MosherIV
author_rank: Super Contributor
timestamp: June 27, 2017, 03:22:08 pm
content: QuoteYou should not have much trouble with in rush current, the opamp closed loop sensing should control that OK.You might want to put a capacitor in the feedback loop to reduce the loop bandwidth, to stop any 'ringing'(put a filter on the feeback loop)

author: bktemp
author_rank: Super Contributor
timestamp: June 27, 2017, 03:22:49 pm
content: I had the same problem when building an adjustable current source.I ended up sensing the voltage drop across the mosfet and reducing the set current when the voltage was getting below 0.5V. This worked quite well and gave a nice soft start behaviour because the current basically starts at 0A when a load is connected.It would be interesting to see if there are better ways of doing this.

author: Microcheap
author_rank: Frequent Contributor
timestamp: June 27, 2017, 03:33:29 pm
content: Hi,Have you already checked the project of this guy?https://youtu.be/anE0jDeBuxo?list=PLUMG8JNssPPxGPivQllTex7AEVIIeHFd0

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 27, 2017, 03:36:00 pm
content: Very interesting project. I like the idea of different tones for different resistance levels.Also as someone mentioned: when the tester is initially open circuit, then your opamp will be fully pegged to the rail in an attempt to get some current out of the transistor! When the probes are finally touching the DUT (device under test), it will send a very large current spike!What you should do is to put in parallel with the DUT output a (or a series pair) 1N4001 diode. If no DUT is connected, the constant current will flow harmlessly thru the diode (or diodes). The open circuit voltage will also be limited preventing any possible damage to your DUT.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 27, 2017, 04:43:12 pm
content: Quote from: schmitt trigger on June 27, 2017, 03:36:00 pmThat seems elegant in it's simplicity. Will have to look at the leakage of those diodes tho... If I'm measuring to 4 digits precision using small currents (don't want to damage anything), a tiny bit of leakage could already be cause for inaccuracies. (however, they should scale and be repeatable, so perhaps I can calibrate them out).I will have to look at what other options are there. Will throw this into spice either later today or tomorrow. Also need to look at what I can use for resistors. Since I'm dealing with such accuracy, I might need to go for 4-wire measurement on my shunt resistors in the current source as well. Then again, those reed relays are also bothering me. I should probably move them to the top, so I don't drop voltage over them, but then I need to also switch my sense line... But since I'm not drawing any current a MOSFET on there should probably work.Hard part will be a good VCO, but that is the part I'm looking forwards to.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 28, 2017, 12:51:33 pm
content: I've decided to take a different approach for the current source. Based on a document from Analog, I am going to try the following:I have to see if I use multiple set resistors, copy the entire system for each range, or use a DAC/set of resistor deviders to switch the reference voltage (instead of the current shunt resistor).I don't think the opamp buffering the voltage reference is necessary, but I've added it in case I want to put multiple (3 or 4) of these sources in parallel driven by the same reference.Stuff I'm going to now look at: Either figuring out how I can turn the source off if the current can't flow (to prevent a inrush when it is connected again) or put a diode, or set of diodes in parallel with the DUT as suggested by schmitt trigger. In order to do so I will have to do some maths, so I know what realistic ranges are in terms of voltage drops and currents. I would like to keep the voltage over the DUT as low as possible, but of course the limit here will be the offset of the measuring differential amplifier. Once I know this, I can look at the leakage current at that voltage, and see how much it will throw off the final measurement. It can be that it could be calibrated out, but I would rather not do to much calibration.Quote from: Microcheap on June 27, 2017, 03:33:29 pmI've looked at it before. The thing I don't like is that he has no ranges and that he seems to be too confident that getting the accuracy he claims is easy. Just because I'm using 0.1% parts does not mean I get 0.1% accuracy. (All parts I'm using are at least close to an order of magnitude more accurate than my target, and I am starting to think even then I wont pull it off.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 28, 2017, 01:40:25 pm
content: Quote from: TheUnnamedNewbie on June 28, 2017, 12:51:33 pmJust use a low leakage diode, for example the BAV199. Up to about 100mV forward voltage the leakage current at 25C would be below the bias current for a good FET input opamp, and if you would use 10mA current source even at 500mV the leakage will be below 0.1%.CheersAlex

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 28, 2017, 03:14:35 pm
content: Quote from: TheUnnamedNewbie on June 27, 2017, 04:43:12 pm

author: tigr
author_rank: Regular Contributor
timestamp: June 28, 2017, 04:07:26 pm
content: Milliohm tester.Reverse the polarity of electrolytic capacitors!