Kizzap's attempt at a Lab power supply.  - Page 1
Forum section: electronics / projects

author: kizzap
author_rank: Unknown
timestamp: October 24, 2013, 01:17:18 am
content: Hey guys, I'm trying to design a Lab supply for my bench (0-30V, 0-2A although the design could be pushed to 3A with a bigger transformer), based around the LM1085, and using a LM2576 tracking pre-regulator. I've bread boarded a design however I am having a bit of trouble with the CC section, that is actually getting it to work at all.I've attached a sort of flow chart of what I am working with, and how I am hoping it will work. So far I have everything working as wanted with exception of the current control circuit. It takes a few ideas from Dave's power supply video series, in particular the pre-regulated current sense, and put in a few of my own design choices.It should be fairly simple as to how it works:Current flows from left to right on the top wire, this is the main current path. Current is measured pre-regulation, with an instrumentation amplifier. The output of the current sense is compared to the current limit, and if it is higher, the output is fed into the Voltage Control circuit, which is simply a differential amplifier, with the current limiting being fed to the inverting(subtracting) input of the op-amp to reduce the output voltage. The comparison of the set voltage and current limiting is then amplified and sent to the adjust pin of the LM1085 to let it do its thing.Here is where my problem starts:I have tried simulating the design in LTspice, and I have also attempted bread-boarding the circuit, and have proven that the Voltage control circuit to drop the voltage when the inverting input is brought above ground as expected.The problem lies in the comparison of the measured current to the current limit, and having the resultant voltage drop the voltage supplied. My initial design consideration was to use another differential amplifier to compare the current flowing to the limit, however this is not optimal, due to a number of issues. It will work for the initial condition, however once the drop is enacted, the differential voltage will drop, causing the voltage to increase. If the thing does settle, it will settle at a point where there is still too high a voltage and too high a current flowing through the circuit.My next thought was to use a op-amp as a pure comparator, however this is also bound with issues. If my voltage set signal goes from 0-3 volts, and the op-amp rails go +/- 5V, then the rail swing will be larger then my maximum voltage,which will drive the adj pin down to the negative rail, which is not ideal. I could use a diode to stop the output going negative, but then I would have to worry about the voltage drop across the diode, and account for the tempco. of said diode.I am aware I could probably take care of this issue in software, (I am using an atmega as the controller), however this feels a little bit like cheating, so I would rather not go about that route, and have a proper analogue method for the current regulation.So where does that leave me now? To be truthful I don't know. Hopefully someone here might be able to give me a push in the right direction. I was wondering if some form of transistor circuit might be useful, but as to how I would get that to work I'm not sure. I am getting stuck with the feedback part of the system. Help?TL;DR: I am having trouble with a Current sense feedback in a Lab power supply design, and was after some pointers to how to get around my issue.-kizzap

author: nickm
author_rank: Regular Contributor
timestamp: October 24, 2013, 01:52:38 am
content: Post more detailed schematics and waveforms because it's hard to understand whats going on. It sounds like you're worried about stability of the current sense loop which should be easy to deal with.

author: kizzap
author_rank: Unknown
timestamp: October 24, 2013, 05:08:23 am
content: Quote from: nickm on October 24, 2013, 01:52:38 amStability is a concern, but what I am more worried about is accuracy.I've attached a copy of the circuit I am trying to work with, and have some basic notes as to the function of the circuit. Where I have the box is the section of the circuit I am having issues with.Basically what I want to happen is this: If the sensed current is greater then the current set, the op-amp outputs a positive voltage on its output to feed the differential amp thus lowering the output voltage and reducing the current. If the sensed current is lower, then have the output of the op-amp go to a negative voltage, which I then proceed to ignore/take out of the equation, possibly with a diode.I had a thought but I am not too sure if it will work. If I understand op-amps correctly (wouldn't surprise me if I didn't), op-amps work by putting a voltage out towards the rail in the direction of the input which is at the higher potential, so if the + input is higher, it will try drive the output to the +ive rail, and if the - input is higher, it drives the output to the -ive rail. What I was considering, is if it was worth hacking together some form of pseudo "summing" amplifier as shown in the second picture.The idea being that if the voltage out out the current sense minus the voltage of the current limit is greater then ground, then the output will go high, and if it goes the other way, the output falls to the negative rail, however the diode on the output will stop the output affecting the voltage control portion.I'll have a play with LTspice of this idea later, unfortunately I have to go pick up some family from the airport (4 hour round trip.)-kizzap

author: nickm
author_rank: Regular Contributor
timestamp: October 24, 2013, 01:57:20 pm
content: You're correct about op-amp operation. The block diagram and basic operation of the circuit is sound too. The only issue I see is the lack of negative feedback on a few opamps. Without feedback the parasistics of opamps take over and they don't behave like opamps. The high side current sense op-amp marked IC3A should be configured as a diff amp like IC2A and IC2B. In the second picture IC4B will need a resistor from pin 5 to 7. This is where you can control the stability too. By adding a capacitor in parallel with the resistor you can slow down how fast the overcurrent protection reacts to prevent oscillations or improve response ringing.

author: kizzap
author_rank: Unknown
timestamp: October 24, 2013, 02:44:32 pm
content: Quote from: nickm on October 24, 2013, 01:57:20 pmYeah I know that the current sense amp isn't connected all that great, it was just there to show that it is being connected. I'm currently tossing up whether to use an instrumentation amp or just a differential amp to get the current measurement (I'm leaning towards the instrumentation amp atm).As for the feedback resistor between pins 5 and 7 on IC4B, won't that ruin the use of the op-amp? The idea is that the output of the op-amp will go high if the current is too high, which will feed into the inverting input of the differential amplifier, thus reducing the output voltage, reducing the output voltage from the current sense op-amp, which will bring the voltage back into IC4Bs +ive input back towards ground. If I place in a resistor there, the circuit will latch high once current control kicks in, by feeding another +ive input to the additive junction. Even if the feedback was enacted, wouldn't it just lock the output voltage to a certain responce, instead of allowing the system to feed as much voltage to allow the current sensing system to work? Can I not simply utilise the control loop that will occur through the loop that is made?I've attached how I would envision the circuit to be connected.-kizzap

author: nickm
author_rank: Regular Contributor
timestamp: October 25, 2013, 01:37:43 am
content: Adding the feedback will make the response very linear. Without it as soon as the output current exceeds the set current the opamp will swing to the positive rail. By adding the feedback resistor the opamp will reduce the output voltage in a much more linear manner. This way if the load becomes too high the output voltage will be reduced until the output current is equal to the max current with the output voltage less than the set voltage. Running that opamp open loop will almost certainly cause it to oscillate during overcurrent.You're right, configured as is adding the feedback resistor won't work like I said. Instead you can simplify your circuit by changing IC2B to be configured like IC2A and getting rid of IC3A. The current sense will go into the resistor that connects to thenoninverting side and the current set will go into the resistor that connects to the inverting side. This will give you the linear reduction in output voltage.

author: kizzap
author_rank: Unknown
timestamp: October 25, 2013, 02:50:20 am
content: Quote from: nickm on October 25, 2013, 01:37:43 amIt seems that We have come full circle back to the differential amplifier, which I stated didn't work in all occurrences.For example, if I have the voltage set to output 30 volts, with a 10 ohm load attached, I expect 3 Amps to flow. If I have the current limit set to 20mA, I should expect the output voltage to be 0.2V maximum.With the differential amplifier, I would be seeing 3 volts at the non-inverting input, and 0.02 volts at the inverting input, with unity gain, the resultant output will be 2.98Volts. Subtract that from the voltage set, gives the 0.02 volts, which will drive the output down to 0.2Volts as expected. However when this has resolved, we go back to the current comparing differential op-amp, whose output will now be zero, thus bringing the output voltage up to over what it should be.things get even worse when we go to a 15 ohm load. At maximum voltage, there would be 2A of current flowing. Again if I set the current limit to 20mA, I should expect the regulator voltage to drop to 0.3V. I will have 2 Volts at the non-inverting input of the differential amp, and 0.02 Volts at the inverting input, which will result in 1.98 volt output of the current comparing diff. op-amp, which subtracting from the 3V voltage setting, will only bring the Regulator output down to 10.2 Volts! It doesn't even get close to what should happen. Using Ohms law: the current still flowing will be 680mA! Thus the reason I cannot use a differential amplifier for this specific application.I'm not too worried about the linearity of the response of IC2B in the last image, and it does have a feedback loop, but it is not directly tied to the input, rather it has to go around through controlling the regulator to give the response I need. Effectively it is a summing amplifier that is inverting the output to bring the positive input back down to ground if the sensed current is higher then the current limit.-kizzap

author: KerryW
author_rank: Regular Contributor
timestamp: October 25, 2013, 10:51:18 pm
content: You could use a circuit like this one, instead of a diff amp. Resistor values are far less critical.

author: kizzap
author_rank: Unknown
timestamp: February 13, 2014, 03:08:24 pm
content: Wow, been a long time since I updated this thread. I have done quite a lot of work with a lot of assistance from c4757p, and others on the eevblog IRC channel. (check it out if you haven't)Where do I start? I guess with a schematic:http://users.tpg.com.au/luce90/PowerSupply/OutputVoltageControl.jpg(warning: largish file ~1MB)Theory is as follows:A 30V AC voltage is rectified, filtered, and fed into the switching regulator IC1. Following the datasheet for a step-down regulator, a suitable diode, inductor and output filtering capacitors were chosen. The output then passes through R3, which is used to measure the current. The voltage is then fed to IC5, a LM1085, which is a LM317 variant, capable of higher currents.The adjust pin is driven by a series of Op-amps from the combined inputs of Vset, Iset and Imeas. All of these values are in the range of 0-3V. Vset sets themaximum voltagethat the power supply will reach. Iset sets themaximum currentthat the output is allowed to draw from the supply. Imeas is the output voltage from the MAX4080, which is both measured by an ADC (I will get to that later) and compared to the value set by Iset.In gate IC3A, Imeas and Iset are compared, If Iset is higher then Imeas, the output of the op-amp is driven high, which will cause diode D6 to become reversed biased. In this state, all current then flows through to IC4B. If Iset is lower the Imeas, the output of IC3A is driven low, forward biasing D6, which in turn causes the Voltage at the input of IC4B to fall, lowering the overall output voltage.IC4B amplifies the voltage at the input 10x. Diode D7 is used to ensure that the output voltage can only go between 0 and a positive voltage. Taking the feedback voltage from after D7 will negate the voltage drop caused by D7. IC4A is used in driving the adjust pin of the regulator. The output of IC4A will be driven high, until such time that the negative feedback, taken from the voltage regulator output is equal to the input. R8 is placed between the output and adjust pins of the regulator, as, being a 1.25V differential, will cause a ~10mA draw from the output of the regulator, which covers the minimum load the linear regulator requires.The output of IC4B is also fed through to Q1, to act as a tracking pre-regulator (as seen in eevblog video #329) which will set the output voltage of IC1 to be 1.7 Volts above the desired Voltage output. This saves the need for large heatsinks, as the maximum power dissipated will be roughly 5.1W, compared to the 120W that would be required with a purely linear regulator. I also checked the power dissipation of the switching regulator and it is limited to a maximum of 4W. The two regulators will be mounted to a heatsink with a 4.8°C/W thermal resistance.I actually went and took the time to graph out the heat outputs on IC1, IC5 and the rectification diodes: (again, be warned, these are kinda large files, I suck at compressing images.)IC1 power dissipation over voltage and current.IC1 temperature range over voltage and current.IC5 power dissipation over voltage and current.IC5 temperature range over voltage and current.and for shits and giggles:Diode power dissipation over current range.The thing that I find most amazing about this is that the maximum Temperature is 65°C, and that is only if I am drawing 4A through the diodes. So I'm happyThe voltages Vset and Iset are sourced from a DAC (MCP4922), while Imeas, and Vmeas are buffered, and then read by an ADC (MCP3204). Both these devices have a voltage reference of 4.096V, which over 12bits provides a 0.001V/bit voltage output. Both the ADC and DAC are SPI driven by a atmega, which controls these devices, and the output display.As far as logic level voltages apply, they are sourced through MC34063 based regulators.As it stands, I am now onto PCB layout. I have most of the design laid out, however I still have a little more to do, which I plan to finish in the next couple of days. As a teaser I will show you this:I have to say thankyou again to c4757p, and the folks on the eevblog IRC channel, as they have been a great help to me in designing this so far, and I am sure they will be willing to help again in the future-kizzap

author: sync
author_rank: Frequent Contributor
timestamp: February 13, 2014, 03:29:43 pm
content: Where does the +5V and -5V come from?

author: c4757p
author_rank: Super Contributor
timestamp: February 13, 2014, 03:49:46 pm
content: Quote from: sync on February 13, 2014, 03:29:43 pmHe cut off the 34063 section from the schematic picture.

author: sync
author_rank: Frequent Contributor
timestamp: February 13, 2014, 04:17:20 pm
content: Ok, thanks. It must be guarantied that the +/-5V is always present or there maybe turn on/off spikes at the output. Think of the big caps at the main power supply when switching off. They will slowly discharge. When the -5V collapse before then the LM1085 adjustment pin maybe can't driven low enough anymore and the output overshoot. And I think there could be also phase reversal problems with the TL072 without -5V.

author: kizzap
author_rank: Unknown
timestamp: February 14, 2014, 04:19:02 am
content: Quote from: sync on February 13, 2014, 04:17:20 pmc4757p nailed it on the head there. The ±5V rails come from the MC34063 regulators which are powered from the centre tap of the transformer. I elected to exclude the schematic for them, and for the digital logic, as they are simple schematics, based purely off the datasheet.One thing I just checked on, is the use of a TL072 in the driving stage. Not only will it fall short in the voltage rail range needed(voltage range will vary between -5V and up to 31.7V), but the differential input voltage is 30V, so if I'm not careful I might blow something up. So I went searching for an alternate. First was theLT1013 (warning .pdf), but I didn't end up using it, because it has an alternate layout then your regular dual op-amp. The ironic thing is that the alternate layout was a change brought in via a revision, so I have no idea what is up with that. There were others as well but the one I ended up choosing was theLT1490.44V Voltage range, with rail to rail output, 44V voltage differential on the inputs. To top it off it follows a standard pinout for a dual opamp.Back to PCB layout.-kizzap

author: kizzap
author_rank: Unknown
timestamp: February 16, 2014, 01:45:06 pm
content: I have made a couple of changes to the design, nothing fundamentally major, but something that might cause issues if I don't rectify the situation.As I stated, I have changed the value of IC4 to a LT1490(A - if that means anything to you). As shown in the schematic, the +ive rail is being fed by the output of the SMPS pre-regulator, which saves on space, by not needing to supply a +32V rail. As the pre-regulator voltage is always 1.7V above the output voltage, and because the LT1490A is a Rail-to-Rail op-amp, there will be enough headroom for the op-amp to output the required voltage.AHowever where I was concerned is when I want the output voltage low, down to 0 volts even. At zero volts output, the tracking pre-regulator is at 1.7V. I was concerned that it might be an issue if Vcc of the LT1490 was so low, with -5V on Vss. I was chatting to c4757p about this, and he felt that it would be ok, however I have still decided to play it safe, and am going to take the simple precaution of adding in a diode OR gate to the positive supply, with one side feeding from the pre-regulator, and the other side feeding from the local +5V rail for the other logic. I will only be loosing ~330mV which I am confident will not cause an issue for either rail source. Simple solution I think.The only other thing I have changed is replacing the MAX4080T with a MAX4081T. The chips are nearly identical, and will operate identically in this project, However the MAX4081 (as per the Maxim website) is preferred for new designs, so who am I to argueAs it stands, I believe I have finished the PCB layout, with only a few minor tweaks around the place, and it is good to go. I will be endeavouring to try get the PCB out to a board house as soon as financially possible. I can post up the complete PCB layout if anyone is interested.-kizzap

author: IanJ
author_rank: Unknown
timestamp: February 16, 2014, 03:50:19 pm
content: Hi,Just wondered if you had come across a similar issue to my own testing with LM317 variants and the use of pre-regulators.What I found was that when the PSU went into current limiting, lets say a really low current was set then effectively the final output would drive down to almost 0vdc, the pre-regulator would also follow down to about 2vdc (I used a 2vdc differential amongst others during testing).Following that, the problem I found was that when the CC mode was turned off, or the load was removed then the LM317 variant would not always recover, very intermittent. The symptoms being that the final output would stick at 0vdc or near. It had latched up. Only removing power completely would reset it.Seems to be some issue, but not limited to, the set pin being at a higher potential than the supply pin, possibly also the supply pin suddenly going from 2vdc to 30vdc. There may be more to it than that as it's been a while and I don't remember fully the specifics!Anyways, I changed to using a more discrete design using BJT's etc, but I'm still using the LM2576ADJ.Will be nice to here also what sort of noise issues, if any, you get with the LM2576. In my case I ran the 2nd optional inductor. Low ESR caps are a must.I had also played with a simple mosfet pre-regulator, but that had issues all of it's own. The LM2576ADJ is the best so far.Ian.

author: c4757p
author_rank: Super Contributor
timestamp: February 16, 2014, 10:57:47 pm
content: IanJ, do you have a schematic of how you connected the LM317? I'm having a hard time imagining how that could happen in this circuit.

author: kizzap
author_rank: Unknown
timestamp: February 16, 2014, 11:38:46 pm
content: Hi IanJ,As c4757p asked, do you have a schematic for the circuit you were using?I am wondering, if you were driving the adjust pin with an op-amp, and you were somehow pushing an op-amp into phase reversal.-kizzap

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 01:36:02 am
content: Your logic should have two pieces: one, if the current limit is hit, reduce the output voltage until the current limit isn't hit. Two, if the current limit isn't hit, maintain the desired voltage output.the two goals can be contradictory unfortunately. Handling it in software isn't easy.

author: c4757p
author_rank: Super Contributor
timestamp: February 17, 2014, 01:43:56 am
content: Looks like that's what he has implemented to me

author: IanJ
author_rank: Unknown
timestamp: February 17, 2014, 06:44:01 am
content: Quote from: c4757p on February 16, 2014, 10:57:47 pmNo schematic to hand right now unfortunately, but it was basically very similar to Dave's LT3080 psu except using a simple LT1083 and using the same ADJ pin control as Dave's (op-amp driven with transistor to pull CC) and a pre-regulator which was just monitoring the final output and maintaining a 2-3v differential across the LT.When it latched up, no matter what you did even with manual intervention on the ADJ pin or by-passing the pre-reg would the output recover. The only way the LT is by removing power to the it's input and ADJ pins, i.e. reset it.I played with a few things like a much larger pre-reg differential etc but at the end of the day it wasn't 100% reliable when trying to recover from having just existed CC mode. It would still latch up sometimes.I had figured it was the fact my pre-reg was driven from the final output, but after a lockup I would manually bypass the pre-reg thus giving the LT full input voltage, but no matter what I did with the ADJ pin (with no load) the LT was still latched up.So, I figured it was the transient of the input voltage & adj pin to the LT that was somehow was locking it up.I haven't studied your schematic too closely, but am hoping the fact you are controlling the pre-reg via your adj. pin circuit and not via feedback from the LT's final output you will be luckier.UPDATE - Out of interest here's what I finally went for, albeit still being tweaked but fully working c/w the LM2576ADJ pre-reg:-http://www.ianjohnston.com/images/stories/IanJ/PowerSupply/BenchPsuSchematic.pdfIan.

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 12:02:12 pm
content: I would suggest that you split the design into two phases, a CC design followed a CV stage. After you got that working, you may integrated the CV stage into the CC stage.The difficult task here isn't to get it working. The difficult task is to design it in a way so that when the mcu fails, the power supply doesn't go catastrophic.

author: dannyf
author_rank: Super Contributor
timestamp: February 17, 2014, 01:01:57 pm
content: Here is something for you to consider:It only implemented the CC side (but the CV side is almost identical).Imax comes from your mcu to set the current maximum. D1 indicates overcurrent.At this point, the design in inherently unstable - you will need a lot of carefully placed capacitors for that.Hope it helps.

author: kizzap
author_rank: Unknown
timestamp: February 17, 2014, 01:24:19 pm
content: dannyf, if you looked at the schematic in reply #8, you would see how the thing works. IC3B is driven from an ADC. in general operation, it will output it's full voltage into IC4B, which amplifies the signal 10x to drive the LT1085. So the CV mode will work in all circumstances. The CC module is in IC3A, which uses op-amp action to drive the output to a point that gets the inputs to be equal. The output of the CC op-amp is connected to a diode, so it is only able to drag the output of the voltage set pin lower, till the CC condition is met. I fail to see where you think there is a problem?IanJ, I think I get what you are saying. As to why your design was locking up, I have no idea. Rather strange. I wouldn't have thought that taking the feedback voltage for the tracking pre-regulator from the output voltage would have caused an issue. I might go have a look at the 1085 datasheet and see if I can see anything about if the set pin is at a higher voltage then the input, Maybe it is still something to with possible phase reversal on the control circuitry in the Vreg.-kizzap

author: kizzap
author_rank: Unknown
timestamp: March 21, 2014, 12:02:46 pm
content: Time for an update. Where to start?I guess I can start with the good news: I have the PCBs!For those who want to see the bottom:http://users.tpg.com.au/luce90/PowerSupply/boardbot.jpgIn the interest of saving money, and to try out a different PCB manufacturer, I had the boards made by Elecrow. roughly $50 AUD for 5 boards 100x120mm in size (compared to the $100 or so at OSHpark for 3 boards). As an added bonus, I managed to get an extra 2 boards (for a total of 7 PCBs)Inspecting the boards, they look good. There aresomeerrors in the layer aligning and drill hits, however there are no issues with paths being cut. As you can see in the photo however there are some issues with the Silkscreen layer, with a lot of the component designators for a lot of the SMD components being illegible. This however seems to be an issue on my end, not having the thickness of the text high enough to come out correct.Also top left you can see the manufacturers markings.As far as the design goes, there have been a few major changes. Firstly I was using the same Microchip DAC that Dave was using in his uSupply, which if you watched those videos had a number of issues with the DAC in regards to the INL and DNL of the outputs. I was also using going to use the Microchip ADC (and still might-but i'll get to that in a second). I decided to do some digging to try find a better DAC at least. I ended up selecting aTLV5614, feeding two outputs into a differential amplifier for both voltage and current limits. In retrospect, I could have stuck with the original DAC and have done the same thing.I was trying to keep the board size down, where possible, so I decided to ditch the ADC on this board, and leave provision for measuring the voltages I want on a daughter board. I have ample space above the logic, so why not abuse itI do get that this is not ideal to have the ADC off the main board, however in this project it isn't critical to have them on board.I also changed some of the capacitors filtering the output of the Pre-regulator. Using 4 smaller values gives me a little bit more space, and I was able to move the logic voltage supplies to a more convenient position.So at the moment it is now sitting on my desk, waiting for me to buy some parts to get it into working order. When that happens will be anyone's guess...-kizzapFinal rendering:http://users.tpg.com.au/luce90/PowerSupply/final3D.jpg

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 21, 2014, 07:01:54 pm
content: Hey man great work,.Have you considered these.16-/14-Bit DACshttp://www.analog.com/static/imported-files/data_sheets/AD5543_5553.pdfAD7708 16-Bit Resolution ADChttp://www.analog.com/static/imported-files/data_sheets/AD7708_7718.pdfHIGH RESOLUTION, ZERO-DRIFT CURRENT SHUNT MONITORhttp://www.analog.com/en/specialty-amplifiers/current-sense-amplifiers/ad8217/products/product.html

author: kizzap
author_rank: Unknown
timestamp: March 22, 2014, 09:47:45 am
content: Quote from: diyaudio on March 21, 2014, 07:01:54 pmHey, thanks for lookingSadly I am past the point of looking at DACs, although the ones you have linked are nice (albiet a the same price for half the channels...) I should have looked at higher resolution DACs to get around my issue but hey, dems the breaks.One thing I forgot to make note of in my last post is that I have already identified an issue with the MAX4081 which I am using to measure the current flowing through my current sense resistor. I want to be able to measure the current down to 0 volts. What I didn't notice is that the MAX4081 requires aminimumcommon-mode voltage of greater then 4.5V above GND. Thankfully, I can get around this issue, by simply bringing the GND pin on the IC down to -5V. Luckily I had swapped from the MAX4080 to the MAX4081, as the MAX4081 provides two reference pins, which allow the IC to have a DC offset on the voltage. I managed to flukishly connect both of the reference pins to ground, so my output voltage will be ground referenced. Thankfully this solution keeps the MAX4081 within it's operating conditions-kizzap

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 22, 2014, 11:39:14 am
content: Quote from: kizzap on March 22, 2014, 09:47:45 amHiI had the same issue!, after I ordered the two AD8217 current shunt monitor(s) I realised its the incorrect version bah!. The High common-mode voltage ranges from 4.5 V to 80 V. trap for young players.