ADC Protection Circuit, requesting peer review - Page 1
Forum section: electronics / projects

author: Chris C
author_rank: Frequent Contributor
timestamp: February 14, 2016, 11:35:35 am
content: My employer has asked me to design a portable datalogger for in-house use. Officially, it will be measuring from 0-50VDC, low sample rate. That much is simple.Unofficially though, I expect it will be used past the specs I was given, or downright abused. They'll reverse leads. Try to measure AC volts. Try to simultaneously charge it, and measure voltage on something without a common ground. Use it to test if a 240VAC socket is working. And so on. Seriously, one of the users is someone who in the past tried to see how many amps an AC socket could supply, by putting a multimeter on 10A and shoving the leads into the socket.This is the input conditioning circuit I've designed and simulated (but not yet tested in the real world). It scales inputs of up to 240VAC RMS, down to the acceptable 0-2.44VDC range of the differential 12-bit ADC, which is a LabJack U3-LV (used for expedience's sake). Loss of resolution for the intended 0-50VDC range is acceptable. It will hopefully prevent damage, under all the above conditions - both official and unofficial. And if pushed further, should blow only a few cheap sacrificial components.I haven't really designed any conditioning circuit this defensively before, so I'd appreciate a peer review. All suggestions are welcome.

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2016, 12:01:54 pm
content: Presumably V2 is the input voltage from the device under test.R5 and R6 will need to be special high voltage resistors, or two or three ordinary ones in series to avoid the risk of flashover.You are clamping the differential input using Zeners, however I see no provision to keep the inputs within the rails during large common mode transients (e.g. ESD). Consider adding some reverse biassed Schottky diodes in parallel with the 1K and 3K input divider resistors + a low ESR ceramic cap decoupling the 5V rail right next to them.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2016, 01:59:11 pm
content: If your 5V rail cannot sink current then a 5V6 1W zener would be good across the rail, with a series resistor of 100R 0.5W to the main 5V rail. that will handle any current into the unit common mode way past the limits of the input resistors smoking off the board. 2 cheap components that will add extra protection.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2016, 02:19:48 pm
content: Why R1/R2 and R3/R4 unequal? Does the ADC have a peculiar common mode voltage requirement?You'll want a zener, or pair of clamp diodes, for each input node (I mean, at R1/R2 and likewise -- past the input dropping resistor). As mentioned, if using clamp diodes, add a TVS to the supply as well.Make sure all inputs can tolerate the worst case voltages. During a fault condition, the TVS will drop much more than the nominal value (e.g., a SMAJ5.0A will drop 8-9V at a few amperes). Make sure all devices supplied from that rail can handle the excess!(Note that the ADC inputs will see a similar overvoltage, under those conditions. But as long as the pins are supplied by resistors, they should be okay. You might take a close look at the LabJack circuit or documentation, to see if it's rated for any excess input voltage or current, or if it can tolerate it. Normally, plain CMOS ICs have input clamp diodes, which are okay for up to 10mA input current. A surge might develop 9V at the input-clamping node in your circuit, which goes through a 1k resistor into the ADC clamp diodes, which clamp at 6V maybe, so the 1k resistor drops 3V and delivers 3mA, a perfectly safe condition!)The input dropping resistors should be either large metal oxide/film types (2W or more?), or >= 2 in series (1W or more). That'll keep them cool even for very high voltages, and keep them safe from arc-over even under high category conditions (surge/overvoltage).The combination of arc-over and failed resistors would be the likely failure mechanism for a TVS needing to handle a lot of current. The reason for doubling up on the input resistors is, you avoid any single-fault mechanism where direct arcover can occur. Therefore, even under single-fault conditions, your overvoltage condition will be very modest indeed (only slightly worse than the design expectation). With diodes or zeners in there, perhaps only 6 or 7V.In all cases, with these precautions, it should be quite good.Now, on another important topic:- Do you have any plans for filtering this input?- Is the LabJack filtered?- Is the LabJack grounded or isolated?- Do you have anything else attached to your circuit, or is this snippet truly all of it?If you have GND/+5V connecting to anything else, mind that you can get mains voltage appearing on those as well! So use this very carefully, only with isolation or double insulation between the probe circuit and anything that's grounded or user-touchable.Tim

author: Chris C
author_rank: Frequent Contributor
timestamp: February 14, 2016, 03:15:57 pm
content: Requested info:V2: Represents the input, and with the values shown represents the worst-case scenario I'm designing for.R1/R2 and R3/R4: Are unequal to approximately center possible AC inputs within the Labjack's nominal 0-2.44V range.POWER/ISOLATION: The datalogger will be a tablet computer (likely a Winbook TW700). It will power the Labjack and my custom circuitry via 5V USB. It will normally run on battery, which I wouldstronglyprefer be the only option. But I'm sure I'll get complaints if it can't be simultaneously plugged into to a switching charger (the TW700 has dual USB ports which allow this).PURPOSE: It will be used to monitor the voltage and current from a welding machine, to calculate approximate heat input. Estimated travel speed will be manually provided. (Current is a non-issue, handled via an isolated hall-effect sensor, which is why I'm not showing that circuit. Initial tests will be with a Fluke i410.) I'm not a welder, and know almost nothing about the machines, so I don't know what to expect. I was told it should measure up to 50VDC and 100A, but the welders aren't electricians, so I'm adding a large safety margin. I did at least know to rule out any possibility of TIG or other high-frequency welders.ADC CLAMPS: The Labjack does appear to have some sort of clamping/protection on the analog inputs, though I'm not sure how to interpret the specs, found here:https://labjack.com/support/datasheets/u3/appendix-aThank you everyone for your help on this. I feel foolish for not thinking of ESD, I should know better by now! Here's revision two, with all suggestions so far added, and highlighted in blue. (For clarity, I'm still representing R5/R6 as single resistors, they'll actually be several metal film resistors in series).

author: Chris C
author_rank: Frequent Contributor
timestamp: February 14, 2016, 03:26:10 pm
content: Oh, I forgot this:Quote from: T3sl4co1l on February 14, 2016, 02:19:48 pmI'm going to be taking ADC samples at the Labjack's limit, and averaging/filtering in software down to a lower sample rate. I'll add hardware filtering only if necessary, but will have to do real-world tests to determine that.

author: Kalvin
author_rank: Super Contributor
timestamp: February 14, 2016, 03:35:27 pm
content: About the isolation: The user may be able to touch the tablet computer connected to your measurement instrument, so you need to consider the isolation and safety grounding very carefully. When I look at the schematics there is galvanic path from the 240V to the tablet computer.

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2016, 03:38:21 pm
content: Are you certain the welder is scratch start? If its got a HF start circuit, you could easily get 3KV pulses while its starting. Also if the arc quenches, even without HF start, it could easily see a similar impulse due to the back EMF from the leakage inductance of the transformer + welding lead inductance.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2016, 03:42:42 pm
content: Upgrade the spec with some 100-470pf NPO ceramic capacitors across the R2 R4 parts, this will at some point be connected to a welder with HF start, which will apply several kV of RF energy to the open circuit output. Even a plain iron core transformer can have HF strike assist, so design for handling this RF energy. You will need the high voltage withstand resistors to make a low pass filter to dissipate that energy safely, so really overspec them for both voltage and power dissipation, I would recommend 3 33k 2W high voltage resistors in series, with a 1k 0.25W to make up the resistance, and generous creepage distance around them, and even with some slots to keep leakage down when in a dirty humid environment. You might even want to selectively conformal coat that section to keep it clean and dry.Voltage open circuit of a welder can easily be over 50V, up to 100VAC on some welders, so do not believe the 50V part, same for the current, you will want to have a full scale of 200A to handle most common small welder work, where you can have a 150A current in a 5mm welding rod on thick steel. Been shocked a good few times when welding, even on a plain buzzbox, that one has an open circuit voltage of 72VAC depending on the setting of the core shunt.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 14, 2016, 05:46:07 pm
content: Quote from: Kalvin on February 14, 2016, 03:35:27 pmHmm, you're right.Assuming the datalogger is being operated from battery and is therefore ungrounded, then my exaggerated worst case scenario is 240VAC RMS = 340VAC P-P. If I'm figuring this right, the R5/R6 path will divide that voltage in half; so the datalogger's ground will float at 170VAC P-P. But through a resistance of 100K, so a grounded human touching it will receive no more than 1.7mA. 1mA is the threshold of sensation, <5mA is generally considered harmless.A more realistic use case is 100V. 100 / 2 / 100000 = 0.5mA. The only direct contact point on the datalogger, as the design stands, is the outer shell of a BNC connector. Grabbing exposed connections while using a welding machine isn't a good idea anyway.The tablet will be mounted inside a plastic case, with only the capacitive touchscreen exposed. I assume if it can withstand ESD, it can withstand one or two hundred volts (at 60hz).Note I'm not selling these, just making three for internal use only. Given that, do you think this is good enough? And if not, have any simple suggestions? I have a deadline of 9 days from now.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 14, 2016, 05:58:15 pm
content: Quote from: SeanB on February 14, 2016, 03:42:42 pmI had no idea such a feature even existed on welding machines. I guess it makes sense though, now that you mention it. (Thanks also to Ian.M for bringing this up.)Just to be sure, do you mean a capacitor from R1/R2 junction to ground, and another from R3/R4 to ground?Would also passing the leads through a ferrite be of any use to reduce this RF?

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2016, 06:22:41 pm
content: Be aware that, while you might be able to make something foolproof, you can't make it damnfoolproof - because damn fools are so ingenious.I one made an in-house test set that was protected against reversed input jinputs, overvoltage inputs, open output, shorted output. One was sent back from the test department after the technician (called Bigfoot by absolutely everybody) had set the PCB on fire.What had happened was that a UUT was being correctly detected as slightly out of spec; the tester was doing its job. But Bigfoot decided that it was the tester at fault, that many marginal problems are caused by PSU ripple, and therefore he would put an effing big capacitor across the output.You can't protect against that.Oh yes, why is the bottom section of military 19" racks traditionally left empty? So that squaddies can kick it and not do too much damage.

author: Kalvin
author_rank: Super Contributor
timestamp: February 14, 2016, 06:31:04 pm
content: Quote from: Chris C on February 14, 2016, 05:46:07 pmYou could possibly use a small, isolated DC/DC voltage converter (+5V to +5V) and a linear optoisolator (For example IL300http://www.vishay.com/docs/83622/il300.pdf) to provide the galvanic isolation. Maybe you would also like to add a fuse to the input if someone connects the probes to excessive high voltage, so the fuse will blow instead of the whole measurement device. These suggestion will add some complexity as you need two op amps for the IL300, but hopefully not too much. Given the isolated DC/DC-converter has sufficient isolation voltage of 2500V or even better, your employees would see their children growing up, which is typically highly appreciated by their family. That is also considered as a good thing for the company's public image.Edit: Although you make the device to be a battery operated, you still need the galvanic isolation to get the measured signal safely to the Labjack. The IL300 or similar plus two op amps should be quite simple solution. The op amp on the hot side will be battery operated and the op amp at the Labjack side is powered by USB.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2016, 06:52:33 pm
content: Quote from: Chris C on February 14, 2016, 05:58:15 pmNo difference for the caps, you are just looking at some low pass filtering, so either common mode or to ground is fine. Ferrite beads will do very little here, though a few turns of the output leads through a nice toroid might reduce noise induced into the PC slightly.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 14, 2016, 07:30:13 pm
content: Quote from: Kalvin on February 14, 2016, 06:31:04 pmI was getting ready to write this off as good in theory, excessively cautious in reality according to my previous analysis.But that was before the possibility of a HF starter. 3kV, apparently at >1Mhz. That's brutal. The more I think about this, the more I find I can't convince myself that I'll get this right on the first try. And I don't want to risk people, or the more expensive parts of the project.So your optocoupler approach is starting to look very attractive, even though it's not something I'm particularly comfortable with either, at least any mistakes will be cheap and non-lethal.Working this out in my head, I'll put something similar to my existing circuit on the isolated side. But with higher input impedance, 1Mohm or more, which will ease dealing with high voltage. The two outputs will go into a differential op-amp (with very high impedance), converting the two outputs into one, that goes to the optocoupler driver op-amp. I'll see if I can work out a schematic for that, and post it later.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: February 14, 2016, 08:09:49 pm
content: Don't forget to consider the voltage rating of your input voltage dropping resistors.Common metal film resistors are rated at 250 V DC. So as not to exceed this rating when connecting an AC voltage of 230 V with its peak-to peak voltage of 650 V you will require here resistors rated at 1000 V DC or even more to be on the safe side. I myself have settled for a safety margin of 2.5 resulting in a voltage rating requirement of around 1500 V DC. Based on this then calculate the worst case resistor wattage rating. And then the necessary creepage distance in consideration of the pcb. material. These waters are much deeper than you initially expect.The voltage rating aspect is very often neglected in many designs. Guess why? Cost vs. safety. Who cares.Yours - Messtechniker

author: Messtechniker
author_rank: Frequent Contributor
timestamp: February 14, 2016, 08:22:17 pm
content: And now to testing your setup.This is simple. Take a rod made of glass or plastic.Rub the rod well in a piece of wool. Wool is important here.Hold the charged rod close to the input. Let the spark flash over and see what happens after the discharge.If you circuit survives this test say 20 to 50 times you are probably on the safe side.If not, back to the drawing board.By the way. Insulation resistance of air is 2 to 3.3 kV per mm.Visually assess the gap upon occurrence of the discharge and you will get the approximate discharge voltage.All this is great fun - believe me. But also very important.Yours - Messtechniker

author: Kalvin
author_rank: Super Contributor
timestamp: February 14, 2016, 09:11:56 pm
content: I have something like this in my mind (see the attachment). It will provide a safe, galvanic isolation barrier between the high voltage side and the measurement instrument. The input resistor RC is composed of three or more identical resistors connected in series in order not to exceed the voltage rating of each resistor. The resistors RC, RA and RB provide the attenuation in order to be able measure high voltages (nominal +/- 50VDC). The resistors RA and RB provide 2.5V bias point in order to be able to measure negative or AC voltages, too. The zener diodes provide the clamping function should the input voltage exceed the measurement range. The power rating of the zener diodes and the input resistors should be selected so that they will survive the highest input voltage. The zeners can be replaced by a unipolar transient absorber. The fuse at the input will protect the device from excessive abuse. The op amp will be driving the IL300 linear optocoupler as shown in the Vishay's application note.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2016, 09:17:20 pm
content: Regarding filtering:Ferrite beads are essentially useless, because very few inductors, of any sort, have an impedance greater than 100kohms at any frequency!However, the attenuation can be much greater than the RC filtering shown. So it can still be worthwhile, done at a suitable point.It might be nice to put such a filter on the input terminals, because that will reflect HF energy away from the resistors, rather than letting them burn up (100k is probably a pretty good match to an HF start, so they would get hot, or else just arc over completely..). But now you need a filter that's designed to operate at full fault voltage (which isn't as bad as it sounds; capacitors for that aren't expensive), and has to be terminated. (If the source impedance is unknown, then you have to add some damping as a load, otherwise the filter... kind of, won't. The 100k resistors won't do well enough, because that has the same problem as the ferrite beads: the whole filter has to be designed around a 100kohm impedance, which means preposterously large, high performance inductors, and capacitance near stray-wiring levels. The termination resistor can be cap-coupled in, but it will inevitably dissipate a fair amount of power under unlucky conditions, so it potentially needs to be large anyway.)It's probably better just to use larger (higher value and wattage) resistors, changing the divider ratio. Once the impedance is down a bit (~10kohms), it'll be easier to filter. I'd add a CLC filter, meaning, at each junction (i.e., across R2 and R4), put a cap to ground, then a series inductor, then another cap (the inductors would be inline with where R7/R8 are now). Ending with a series R (the R7/R8 as shown) and C to GND (i.e., split C1 in two parts, each from ADCIN to GND) will filter just that little bit more, plus terminate the filter, so R7/R8 are just peachy to leave in.Also, don't worry about D1/D3, the first diodes take care of that. FYI: you may want to use junction diodes (1N4148, BAV99?) instead of schottky diodes, because of leakage. Schottky diodes are usually too leaky for precision analog inputs. Ditto the low voltage zeners -- zeners only get good above 5V, (which is why 3.3V or lower TVS devices are basically nonexistent).It would be a further benefit to use 1M or larger resistors, because these can be connected between line and ground, safely. I still don't suggest licking the circuit ground while it's probing mains, but it'll help, at least. (Typical limits, from mains, to anything a person can touch, is a couple mA.)The biggest worry to accuracy is, if you're in a noisy industrial environment, what will noise do? If the LabJack ADC does simultaneous sampling of the two inputs, it may be tolerant of a modest amount of RF noise at the input. (That is, the "differential" measurement actually isn't a bad difference.) If it samples the two inputs separately and subtracts the digital results, then any change in noise voltage between those independent sampling events will completely screw it up. (Averaging will still help, but now you have to average out a massive amount of aliased common mode noise, in addition to the differential noise you already have to filter.)So it's always best to filter down to only the bandwidth required, to get the cleanest digital signal possible. And filter that some more, to clean up what aliasing remains, and get a nice silky smooth, higher precision (if not accuracy) result.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: February 14, 2016, 09:24:18 pm
content: Quote from: Chris C on February 14, 2016, 07:30:13 pmNowadays there are other isolation mechanisms available, which may or may not be suitable for your purposes. Start withhttp://www.analog.com/en/products/interface-isolation/isolation/standard-digital-isolators.htmlhttp://www.analog.com/en/products/interface-isolation/isolation/isolated-ad-converters.htmland there are others.

author: Kalvin
author_rank: Super Contributor
timestamp: February 14, 2016, 09:34:08 pm
content: There are also isolated amplifiers with 200kHz BW for around $6:http://www.digikey.com/en/product-highlight/a/avago-tech/precision-mini-isolation-amplifiersEdit: Note the input impedance and the input voltage range.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 15, 2016, 05:18:37 am
content: Wow, thank you all again. I think I've integrated all suggestions in my new design.With the exception of the CLC filter. , it sounds like a great idea. But I admit I'm analog challenged, especially when it comes to inductors (my thinking ferrites might help is proof of that). I'd be guessing at component values, and probably not build a worthwhile CLC, so best for me to omit it unless you feel it's absolutely necessary. I did understand and implement your other suggestions.This is the bulk of it, a protected differential amp. It will be on the HV side, with an isolated power supply. The output will likely go to one of those Avago isolated amplifiers that  linked.(Blue components don't represent anything special here, just forgot to reset them.)V1: Isolated 5VDC power supply. (EDIT: Using XP Power IE0505S-H, 5V-5V 200mA isolated DC-DC converter, 3kV isolation.)XFG1: Input signal.R5 & R6: Each will be 5x 220K metal film resistors in series. Or something similar, dependent on real part prices and voltage ratings. Total power rating no less than 2W. Layout important to reduce possibility of flashover or capacitive coupling! (EDIT: Found a single resistor that will work, Vishay ROX1001M00FKEL, 1M 4W 7.5kV 1%.)C2 & C3: Low-pass filtering. Will adjust if needed based on real-world tests.R1/R2 & R3/R4: Along with R5 & R6, form a voltage divider, and a 2.5V virtual ground for each differential leg.D1/D2/D4/D5/R7/R9: Clamp the differential legs to the supply rails, +/- 0.6V. Should protect against both differential excursions, and differential/common mode ESD. (EDIT: Using ultrafast rectifiers, Multicomp UF4001.)D3/C1: Absorbs any overvoltage in V1 caused by current fed back through the clamps. Shown here in the schematic to represent close proximity to clamping diodes.U2: R2R I/O op-amp. Probably not my final selection, just what's handy in SPICE. But it will be a similar Microchip amp. (EDIT: Using Microchip MCP6283.)R8: Feedback. Not a final value, will adjust as per requirements of the Avago isolated amp selected. (EDIT: Using Avago ACPL-C79B-000E.)

author: DrGeoff
author_rank: Frequent Contributor
timestamp: February 15, 2016, 05:53:38 am
content: Zap the inputs with a 1.2/50us 5kV impulse to test the protection. Both common mode and differential mode.When I used to design this kind of stuff for power station monitoring we used quite a lot of inductors and transient suppressors (MOVs etc), to be able to handle large transients that occur on power lines, either from fault load interruption or lightning.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 15, 2016, 06:33:58 am
content: Quote from: DrGeoff on February 15, 2016, 05:53:38 amI lack proper ESD testing equipment, but I have ionizer modules, like found in air filters. They produce 4.5kV if I recall. Can touch the ionizer output with one hand, then touch the inputs with the other. I'm doing part selection and building my preliminary order sheet now, and it includes several spares for everything on the HV side, in case I succeed in destroying it.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: February 15, 2016, 07:19:54 am
content: Quote from: Chris C on February 15, 2016, 06:33:58 amNo, that's a completely different beast. Different energy profile altogether.Suggest as a minimum to add common mode MOV pair from the inputs to chassis earth. Rate it to clamp above your peak input voltage. If an impulse higher than the MOV rating comes along, most of the energy will be clamped by the MOV. The next stages of protection circuitry can deal with the clamped waveform.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 15, 2016, 04:03:53 pm
content: Quote from: DrGeoff on February 15, 2016, 07:19:54 amI'm not sure how feasible that would be. My datalogger, being (normally) battery-operated and in a plastic enclosure, may not have a "chassis earth" in the way you mean it. But assuming that it did, or that I'm misunderstanding your meaning...I've been reading about the HF start feature on welding machines. It seems some (all?) of them generate the HV through an user-adjustable spark gap. With the peak voltage determined by the gap spacing, and that spacing might be set too large. So assuming this happens, and the peak exceeds my chosen MOV clamping voltage, the MOVs if present will effectively absorb all the excess voltage - at regular intervals, each time the spark gap fires.When the MOVs are conducting, more power is being dissipated than would be through the 2M resistance alone. How much power can a HF start produce? Enough to overheat my datalogger, in an almost sealed enclosure? I couldn't find any references. But one thing is certain - I don't want those MOVs conducting unless absolutely necessary. Only for genuine ESD events, and extreme voltages that the other components truly can't handle. So the MOV threshold voltage needs to be high. No less than 3kV, maybe as high as 6kV.I looked up some high voltage MOVs, finding they're very expensive - costing far more than everything they'd protect on the isolated HV side of my datalogger! To implement what you suggest and keep the cost reasonable, it looks like I'd need a dozen or more smaller ones, in series. With adequate spacing between them to prevent flashover, that's a lot of area taken up by MOVs, which may or may not actually be needed. I still don't know for sure whetherSo I'm going to cross my fingers and hope they're not needed. Only 8 days left until deadline. It's Monday morning here, and time to place my part orders so I'll have something to physically start working with!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2016, 04:13:30 pm
content: You're missing something. HF start is high impedance, probably less than ~amperes. Shunting it to a low voltage with an MOV dissipates extremely less power than trying to shunt nearly all of it.Tim

author: Chris C
author_rank: Frequent Contributor
timestamp: February 15, 2016, 05:28:14 pm
content: Quote from: T3sl4co1l on February 15, 2016, 04:13:30 pmGood to know HF start is high impedance, but I don't understand the other part of your statement. How is passing the HF start through a total of 2Mohm resistance "trying to shunt nearly all of it"? I'm not trying to shunt anything, especially not all (or even a significant portion) of the HF; that would prevent the HF start from functioning properly to start an arc.Only 0.5mA will flow per kV through the resistance, 1.5ma @ 3kV, 3mA @ 6kV, etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2016, 11:33:05 pm
content: Well, I assumed you meant putting the MOVs at the hot ends of the resistors, not the low voltage ends. The low voltage end is already clamped by diodes and capacitors, so a MOV would just be more capacitance.As it is, you only need to worry about burning up the resistors under particularly unfortunate conditions. If they're big enough (in value, wattage and voltage rating) to handle that kind of voltage, then no worries.Tim

author: dom0
author_rank: Super Contributor
timestamp: February 15, 2016, 11:52:53 pm
content: Re. the IL300 approachThe IL300 is a nice part, but I think it's not really in the right spot here. It will be easier to quantize the signal before passing it through an isolation barrier; either place the complete ADC in front of it, or if that is not possible (e.g. you have some ADC card and you absolutely need to use the ADC in the card because $software-driver-restrictions) you can use a modulator.E.g. a SDM or PWM to sample & quantize the signal on the isolated side, pass it through 1 channel digital isolation (hence simple and cheap), and filter it back to analog, feeding it into the ADC. Even a "discrete" design is probably cheaper than a single IL300.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 16, 2016, 04:04:27 am
content: : Yes, I was thinking hot end too, first thing the HV sees. But the HV is always passing a single 1Mohm 8W 7.5kV resistor, and twice that for the complete differential current path (since HV side is now isolated). If the input exceeds those ratings, that's so far above the intended design spec that I consider damage an acceptable outcome.: That would be a better approach. But I'm building only three dataloggers, which would cost my employer $6,000 each commercially, and was given two weeks to do it. Also, I'm programmer by profession, electronics is just a hobby. So I'm taking every shortcut and time-saver possible! Still coming in at about $800 per complete DIY datalogger. I'm sure I could halve that if given more time, but it's a luxury I don't have. As long as I get them built by the deadline, and they perform adequately, me and my boss are happy.

author: Kalvin
author_rank: Super Contributor
timestamp: February 16, 2016, 08:17:34 am
content: Quote from: dom0 on February 15, 2016, 11:52:53 pmJust wanted to mention the optical, linear analog isolation solution as an alternative because it wouldn't require software development. The IL300 is quite popular device for that. Of course there may be some other similar parts which can be used in a similar fashion. Another analogish approach would be to use a voltage-to-frequency converter to drive the optocoupler, and the frequency to-voltage or a Labjack's pulse counter to convert the frequency back to the measurement value. Even a 555 and a comparator could be used as simple PWM-generator which is easy for optical isolation and easy to convert back to an analog voltage with a simple (active or passive) RC-filter.An ADC with a SPI-bus would have been also one possibility as the SPI-bus is optocoupler-friendly interface. With some bit-banging one can read the ADC quite easily.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 21, 2016, 04:12:59 am
content: An update for the curious. It's working well, with some changes:1) Increased the input resistors to 2M. Just seemed prudent given the potentially harsh input.2) The XP Power IE0505S-H, 5V-5V isolated DC converter I ordered is unregulated. Just how unregulated was a surprise to me, having never used a component like this before. Open circuit, the output voltage is almost 6V, which pushes the Vmax of my amps. And there's lots of noise too. So I added a Pi filter, consisting of two 10uF low ESR electrolytics, and a ferrite bead. And after that, a linear regulator. I wasn't sure if any I have on hand had sufficiently low dropout voltage, so I just built one instead, from a TL431 and a 2N2222A. Output voltage is 3.6V, chosen to allow a small optimization (described later). All this omitted from the below schematic to keep it reasonable in complexity.3) After addition of the Avago ACPL-C79B-000E isolation amplifier, I realized I had three voltage dividers doing the same thing - providing a reference voltage at 1/2 Vcc. This caused offset errors, due to part tolerances (even at 1%), and enough current possibly being sourced/sunk from two of the dividers to change their voltage. One of the three dividers was found to be unnecessary, and eliminated. The remaining two were combined into a single divider, followed by an op-amp voltage follower, that distributes the reference to the two points needed. The reference need not be exactly 1/2 Vcc, it just needs to be free of any fast fluctuations. So the divider is a 1K resistor, and a red LED with Vf=~1.8V; allowing me to get rid of a previously separate power LED and resistor. (That's why I chose Vcc=3.6V.)Not shown: The Avago isolation amplifier. And another voltage divider, 3.9K and 1K. Output from U1 goes to 1K and the amp's Vin-. Output from U2 goes to 3.9K. The center point of the divider goes to the amp's Vin+. This keeps normal signals within the specified +/- 200mV differential input range of the Avago. The Avago has a differential output, with fixed 8.2x gain, so up to +/- 2.4V out. The Labjack can read that directly with a single sample. And it conveniently has a range of +/- 2.45V, so no voltage translation necessary for full resolution.Not much electronic work left to do but solve a USB OTG problem. I'll start a separate thread for that.

author: Ian.M
author_rank: Super Contributor
timestamp: February 21, 2016, 08:08:47 am
content: You should probably reduce your Zener D3 to 3.9V or 4.3V (depending) on the tolerance of your 3.6V supply, or remove the Zener and switch to shunt regulation + a series resistor to limit the IE0505S-H output current.

author: Chris C
author_rank: Frequent Contributor
timestamp: February 21, 2016, 10:00:12 am
content: Quote from: Ian.M on February 21, 2016, 08:08:47 amZener D3's 5.1V was chosen when I was planning on having a 5V supply. That value seems out of place now, and if I were to do this all again, I'd choose 3.9V. But since everything will work fine up to 6V, it still functions as a protective device; so I'm using the zeners I already had shipped, rather than raiding my parts bin.

author: Kalvin
author_rank: Super Contributor
timestamp: February 21, 2016, 11:18:16 am
content: Quote from: Chris C on February 21, 2016, 04:12:59 amGood progress!