ICL7107 Volt Meter Hacked To An Ammeter With 2.5V Ref And In Lo Offset - Page 1
Forum section: electronics / beginners

author: Trurl
author_rank: Regular Contributor
timestamp: June 05, 2024, 07:51:07 pm
content: First, a BIG THANKS to:- macboy & Kleinstein(for their tips in the thread "ICL7107 - Is VRef Greater than 1V Possible?" -https://www.eevblog.com/forum/projects/icl7107-is-vref-greater-than-1v-possible/)- David Hess & Terry Bites(for their tips in the thread "How To Make Non-Zero Initial Current Sense Voltage Zero?" -https://www.eevblog.com/forum/beginners/how-to-make-non-zero-initial-current-sense-voltage-zero/)also kevin.gibbs for making me LOL and reminding me to keep it simple.[ Summary ]Solution found to convert a common 20V panel volt meter (based on ICL7107) into an "ammeter"* with a 2.5V reference voltage(instead of the typical 1V reference), and non-zero/positive voltage on the 'IN LO' pin("- input"). This setup is not obvious on any datasheets or tips related documentation - trial and error was key.*Note:This meter is not a true ammeter in the usual sense as it is not in series with the main power circuit nor is it using a shunt resistor in series with the power circuit. It is simply a volt meter tuned to process a current sense voltage output from the device being monitored.[ Why A 2.5V Reference? ]The server PSU I intend to use with the ammeter(HP HSTNS-PD43, from here on "PD43"), has a 25mV/A current sense output(1st far left slender pin on underside of outputs). So in this case, the 2.5V reference voltage eliminates the need for the input voltage divider*(on the panel volt meter), and provides better signal to noise ratio(compared with a typical 1V reference setup).[ Why An IN LO Offset? ]The PD43 PSU's CS(current sense) pin outputs about 26mV even with no load(0 amp) on the outputs, so this has to be "subtracted" from the IN HI signal.The volt meter's TL431 2.5V reference with a voltage divider and potentiometer was originally used for setting the typical 1V reference for the 7107's REF HI. But the TL431's 2.5V output can bypass the voltage divider and be "directly" connected to the 7107's REF HI with a jumper wire, then the freed up voltage divider & pot(which still gets 2.5V from the TL431 through the intact PCB traces) can be used for setting up the IN LO's offset voltage of 26mV (the resistors had to be replaced for a 100 to 1 conversion ratio and the pot allowed fine tuning).[VRef Jump Wire (White) From TL431's Output Direct To 7107's REF HI ]Photo-1Note:The potentiometer was desoldered, its middle(wiper) pin bent 90 degrees outward, soldered with the yellow Offset Wire(for IN LO), then the joint covered with white shrink tube(didn't have yellow). Therefore, the VRef Jump Wire(White) only makes use of the pin-less solder tab that connects directly to REF HI, and the Offset Jump Wire & potentiometer's middle(wiper) pin(which no longer goes through the PCB) is "isolated" from REF HI.[Offset(26mV) Jump Wire(Yellow) Directly Into 7107's IN LO Socket]*Photo-2The voltage divider(between the blue pot and TL431 on upper left side of PCB) previously used for the REF HI input's Vref adjustment(but no longer linked to it) still gets fed 2.5V from the TL431, but the divider's output is now redirected with different resistors to supply 26mV offset voltage for IN LO (this 26mV is eventually subtracted from the IN HI/+input voltage for correct current display).* As mentioned above, the advantage of a 2.5V reference in this case is that the input's 10:1 voltage divider(on my 20.00V range meter there were 3 resistors, now gone) on the right side of the PCB, can be removed completely.Note:on the far right of PCB in Photo-2 above, the resistors have been taken out and replaced with just a bare copper lead , so the CS(current sense) output of the PSU can connect directly(without degradation or added noise) to the 1M resistor just before the 7107's IN HI input.[ How To Setup ICL7107 For Offset Voltage On 'IN LO'(- Input) ]1.The main datasheet ("3.5 Digit, LCD LED Display, AD Converters") does NOT go into details to specify CLEARLY(i.e. "fool-proof" schematic with descriptions etc.) on how COMMON, REF LO, and IN LO should be connected in relation to each other for this type of case(requiring the use of the 7107's differential input capabilities to "subtract" the IN LO's offset voltage), BUT...the 'Application Note' "AN052 (Tips for Using Single Chip 3.5 Digit AD Converters)" DOES suggest an essential step towards the solutionas below (albeit it is more a reference describing setup with an external voltage reference):2.On page 2 of AN052, "FIGURE 2. OPERATION FROM +5V SUPPLY WITH EXTERNAL VOLTAGE REFERENCE" shows a diagram withCOM linked to +5V supply via a 27k resistor, but NOT linked to either REF LO(directly) nor IN LO.Whereas, in most other examples, REF LO and COM are linked directly(some including a direct link with IN LO as well).Photo-3 ("Figure 2")3.On page 4 (of AN052), lower right 3rd paragraph from bottom, it mentions that"if external voltage reference is used, the COMMON pin should have a pullup resistor of 27kΩ connected between it and the positive power supply, as shown in Figure 2."In the photo below, the red shrink tubed link is a series of resistors equaling roughly 27kΩ between COM and positive supply.Photo-4[ PCB / Parts Mods ](Actual traces will likely differ by manufacturer, but the key thing to note are the 7107's pins and how they link or not, and how they are grounded)1. The link between Supply Ground and IN LO must be cut if linked on the PCB.*Otherwise, any offset voltage on IN LO just gets pulled down to nil.Photo-5Note:In Photo-5 above, the lower mid section of the PCB, the trace that linked IN LO(left) directly to ground(right) has been cut (in contrast to IN LO being grounded as in 'Figure 2' above). In my setup, IN LO is only indirectly connected to ground via the voltage divider from which it gets its offset voltage.2. If IN LO and COM are linked on the PCB the link must be cut.In my case, the IN LO is NOT connected to COM on the PCB, so no trace cutting was needed, but if they are linked on the PCB, the link must be cut.3. The link between COM and REF LO must be cut if linked on the PCB.Photo-6*Note:During the trial and error phase, both trace cuts(right cut : breaks link to ground for both COM & REF LO, left cut: breaks link between COM & REF LO) were done with all LEDs desoldered. Only after resoldering the LEDs did I find the potential solution above, at which point I realized REF LO needed to be regrounded, but as the LEDs are so close to the trace cut, a jump wire(black) was used to link the REF LO pin to the nearest ground point(2 o'clock position from REF LO). I could have desoldered the LED to resolder the cut trace, but 7-segments are tedious to desolder, even with a handy brass brazing wire stub to heat 5 pins at once.3. The integrating resistor(470kΩ for 2V full scale) was replaced with 1M+180k* in series.*For a 2.5V Vref the related formula** suggests 1.25M:**"Rint = Full scale input voltage / Iint ...For ICL7106, ICL7107 IINT = 4uA" - Renesas AN052 page 7 "Component Formulae"4. The LED display's decimal point grounding resistor may need to be reoriented/resoldered to be grounded to supply ground.Cutting the link between IN LO and supply ground also broke the link between the decimal point resistor and ground(refer to Photo-5 above).*In Photo-2 above, lower mid section of the PCB, there is a blue resistor next to the marking "R9", and one pin is connected to this marking, but the other pin has been resoldered to a via in 7-o'clock position relative to the other pin. This via(7 o'clock) connects to supply ground on the backside of the PCB, whereas the original 3 o'clock location only links to IN LO after the link between IN LO and supply ground got cut.5. The LED display's decimal point selecting "solder blob" may need to be relocated depending on the original setting vs that intended.I started with a 20.00V setting and had to change it to a 200.0V setting.* In Photo-2 above, lower mid section of PCB, there are markings of "200", "20", and "2" - the decimal displays to the right of the selected(blobbed) number.[ Performance Test Run ]A 12V laptop PSU(with AC grounding, plugged in the same power strip as the PD43 server PSU) and LM7805 5V regulator were used with resistors & potentiometer to emulate the PD43's CS(current sense) output at various outputs of interest.1. First, shorting IN HI & IN LO "zeros out" the meter (hew... it's working!).Photo-72. The PD43's half of full load @ 14V output setting would be 50amps, so the bread board was set to provide IN HI of 1.276V (1.250V +0.026), as 1.250V / 0.025(V/A) = 50A. The meter was within 1 LSB.Photo-83. The PD43's full load @ 14V output setting would be 100amps, so the bread board was set to provide IN HI of 2.526V (2.500V +0.026), as 2.500V / 0.025(V/A) = 100A. The meter's error was within 1%, which is good enough for my purposes, and any decent readings above 100.0 are just a bonus (for me).Photo-94. The PD43's max output is 100amps @14V(1400W PSU), but I tested the meter at near fullscale 5V(4.959V after subtraction of offset) even though I would never use it at this extreme end, to note its performance, and the meter's error was 2.8% of expected reading - refer to post-it in the photo below for details.* I wasn't sure if the meter would even work at this near fullscale input(i.e. I was expecting an "out of range" error or other strange display) as the datasheet states "ICL7107 with ±5V supplies can accept input signals up to ±4V"(p.10 'Reference Voltage'), without mention of what happens beyond 4V nor what happens with a VRef set higher than 1V. So I'm impressed that it even works at all at this extreme!Photo-10Cheers~

author: Trurl
author_rank: Regular Contributor
timestamp: June 10, 2024, 04:02:22 am
content: Well, after the initial post above, I connected the meter to the actual PSU of interest (PD43), and noticed issues not seen with the mock-up tests (using a laptop PSU and simulated CS output). Hence the update below:*UPDATE 1:When the meter was powered by the PD43 via a 5V regulator, and the PD43's CS output is connected to the meter's IN HI, the meter started gettin' "stoopid". Powering the meter with an independent power source didn't help even with a DC-DC converter. But when tested again in the "mock-up" setting using the 12V laptop PSU with a 5V regulator and resistors to simulate a CS output, the meter would work fine. I was stumped. I didn't go so far as doing thisbut you get mydrift(which the meter did BTWas if it were some counter.At one time it wasdisplaying with a negative** reading and disregarding the offset voltagethat was set). Back to scouring the datasheet...** 'Application Note' AN052 p.4 - Accuracy Problems :"Problem- For a constant input voltage, there is a difference in the absolute value of the reading when onlythe polarity is reversed.Action-This problem is called “rollover error”and is usually eliminated by proper selection of the integrating capacitor connected to pin 27. A capacitor with good dielectric absorption characteristics is required; polypropylene or polystyrene are the best types of capacitors to use here.Another possible source is that CREF is too small, or that there is excessive stray capacitance to ground from its pins"With that in mind, I decided to try some more fiddlin' for the last time, and if it didn't work I was considering just getting a clamp type hall sensor or get an in-line hall sensor like those by Allegro (which I've used before). But I didn't have to go there fortunately.Good Results From The Last Fiddlin'1. Replaced the original cheap 0.1uF Reference Capacitor with a decent 1uF film cap(KEMET R82DC4100AA60J) recalling the datasheet mentioning with regard to the Reference Capacitor (p.10) "A 0.1µF capacitor gives good results in most applications. However, where a large common mode voltage exists (i.e., the REF LO pin is not at analog COMMON) and a 200mV scale is used, a larger value is required to prevent rollover error. Generally 1µF will hold the rollover error to 0.5 count in this instance."The white rectangular block marked "1J63" is the installed 1uF film cap which replaced a 0.1uF orange cap similar to the one still on the PCB.Photo-1/82. When an independent power source and a 5V regulator is used to power the meter, and the voltage between the meter's input ground pin and the PD43's CS output pin was checked with a multi-meter, there was almost no offset voltage(about 0.002V at most) unlike when the CS output voltage is checked in relation to the PD43's ground, in which case there was an offset voltage (which I noticed varies between 25~32mV - reason not yet determined).In Photo-2/8 below, the 7107 meter's ground is referenced showing pretty much no offset voltage, whereas Photo-3/8 below with the PD43's ground referenced shows an offset voltage of about 27mV.Photo-2/8 Photo-3/83.So keeping in mind according to No.2 above, thatif the meter is powered by a separate power source(not PD43, the unit it is monitoring) + 5V regulator,the meter does not need to consider an offset voltage, and so I disconnected the yellow offset voltage wire from IN LO, and re-linked IN LO to supply groundvia a wire from IN LO on the input pin section to the meter's supply side ground pin(as in Photo-1/4 above),and voila(!), the (7107) meter would display zero with no load, and displayed pretty much the same as the (Fluke) clamp meterI had on the positive wire between the PD43 and the resistive load. DANG!In the end, "Figure 2" above(from p.2 of AN052, with IN LO grounded as in the diagram) is what worked for the final setupwith the actual PD43(PSU to be monitored) and an independent power source for the ammeter (whereas the volt meter could be connected to AND be powered by the PD43 via a 5V regulator BTW without issues, it's just the ammeter that connects with the PD43's CS output that hadissues - likely due to need for isolation and a larger Reference Capacitor).Having the volt meter share the ammeter's power source also caused ("stoopid") issues in the ammeter, so best to keep their sources separate as well.Photo-4/8 below shows the ammeter(left) powered by a separate 12V laptop PSU & 5V regulator, whereas the volt meter(right) is powered by the PD43 via another 5V regulator.4. Covered up the exposed end of the yellow offset wire with some shrink tube and reoriented it to stay under the 7107 IC for now. I left the wire and divider resistors intact as it may be necessary to reuse it with the separate power source for the ammeter.Photo-5/85. Resoldered the cut link between IN LO and supply ground with a short resistor lead stub, and removed the temporary link wire from Photo1/8.Photo-6/8 Photo-7/8 Photo-8/8*** End of Update ***Cheers~

author: Trurl
author_rank: Regular Contributor
timestamp: June 14, 2024, 11:52:39 pm
content: * UPDATE 2:This second update is sort of a note for UPDATE 1 in the previous post.After realizing that other more compact supply sources for the meter could be used as long as their negative leads (or better yet any regulator's negative leads*) are "grounded" to the same "earth ground" as the server PSU(the unit being monitored), I decided to use a 5V charger "wall wart" hacked to 7.3V with a 5V regulator(LM7805).* I noticed that at least on a breadboard, a 5V regulator's "ground potential" can be offset from "earth ground" by several mV upto about 10mV in some cases**, so to get real serious about zeroing out ground potential differences between supplies(which has a direct affect on accuracy at the meter), it is better to "tap to ground" from either the regulator's negative lead or closer to the meter's negative pin.** The 10mV or so offset case appeared when I tried to power the meter from the server PSU via a 5V regulator and a Murata 5V-5V DC-DC converter. I could not get the ground potential of the regulator down to match that of the server PSU. Not sure what is causing the DC-DC converter's ground potential to be pulled up. Also surprisingly the isolation of the DC-DC converter caused drifting(though not any wild fluctuations as with non-grounded supplies) in the meter which I could not resolve even by grounding the regulator's negative lead to "earth ground". A bit bummed, as the DC-DC converter being so small would have been ideal for my eventual purposes.******UPDATE:Before I tried "grounding" the regulator's negative pin without success, but I tried "grounding" the "isolated" output's negative the the server PSU's ground(not power strip ground) and the meter came to life(no fluctuations)! Hence, I think this could well be the final method for powering the "ammeter" as it is the most compact and simple to set up.In this case(using a different supply source for the meter than the "good" supply* with grounded input), the ground potentials between the server PSU and the meter's supply were manually "equalized"/"zeroed" so the the server PSU's CS(Current Sense) "quiescent output voltage"(about 25~27mV output even with no load) was being processed by the meter and showing a "false" 1.0A reading with no current output.*In hindsight, the "good" supply though "grounded" at the input, likely has decoupling caps and resistors along its internal "grounding" and not a direct short to ground as in the case of manually grounding the negative lead to "earth ground" and so this likely caused a slight positive offset in ground potential to cancel out(mask) the quiescent CS voltage output of the server PSU from the meter, whereas with a manually grounded supply, the quiescent voltage is not cancelled or masked(as the ground potentials are equal) so it must be offset in the differential inputs of the ICL7107.Therefore,I needed to reactivate the offset voltage feed*(yellow wire connected to the divider & TL431) for the ICL7107's IN LO,and recut the link between the PCB's IN LO pin and the supply negative pin.***As in Photo-2 in post#1 above.** As in Photo-6/8 in post#2 above.With the source of the meter's fluctuations resolved* the meter could be fine tuned to match the Fluke clamp meter right down to 0.1A - rock solid!*For more details of the process taken to figure this out refer to these threads:https://www.eevblog.com/forum/beginners/current-sense-output-disrupting-ammeter-powered-with-most-smps-but-not-all()/msg5539861/#msg5539861&https://www.eevblog.com/forum/beginners/safe-simple-way-to-convert-floating-wall-wart-to-grounded/msg5541505/#msg5541505Therefore, the link between IN LO and supply negative will depend on the need for use of the ICL7107's differential input feature(whether there is need for an offset voltage on IN LO). If an offset voltage needs to be used, the link should be cut. If there is no need for an offset, there is no need for the cut.Thanks to everyone that commented and advised along the way!Cheers!