µSupply Rev C Current shunt question - Page 1
Forum section: electronics / projects

author: Unknown
author_rank: Unknown
timestamp: June 05, 2017, 02:55:50 am
content: Hiwe are trying to build/adapt the µSupplyhttps://www.eevblog.com/projects/usupply/(schematics:http://www.eevblog.com/files/uSupplyBenchRevC.pdf) and came across the current shunt which is a total of 1 Ohm.This will result in a voltage drop of 1V per Amp and also the INA219 Bidirectional Current/Power Monitor which is connected to the shunt doesn't seem to be able to monitor the flowing current correctly because its limit according to the datasheet is at 320mV.Adafruit has released a INA219 breakout board where a 0.1 Ohm 1W shunt resistor is mounted which makes much more sense in my opinion because of the much lower voltage drop.Now I'm curious if and how changing the shunt resistor to 0.1 Ohm 1W in the µSupply schematics will influence the differential amplifier and how I can mitigate this.I was thinking of replacing R18 and R24 by 180K resistors to get a gain of 10.What are you thinking about this?Thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2017, 08:25:26 am
content: A 1 Ohms shunt may be OK for current up to about 300-500 mA. This makes sense for the LT3081, as the power dissipation is limited. It also matches the 3 W power rating, as using about 1/10 the power rating is a good size for a precision shunt.I would avoid the µSupply circuit all together. The whole concept of making a lab supply from a voltage regulator chip by bodging in some kind of current limiting is flawed. Thinking a voltage regulator chip would bring you half way to a lab power supply is one of those traps for the young players - not sure why Dave got caught by this.

author: Unknown
author_rank: Unknown
timestamp: June 05, 2017, 09:48:23 am
content: I see, thanks. However I'm trying to improve a LM317T shitty beginners PSU which kind of doesn't contain more than like 5 parts and ended up in this µSupply Circuit. So I'm aware of the fact that this isn't going to get even close to some professional Lab PSU. It would however be much better than the current LM317T circuit I have here. In the end this whole story started by wanting to control the LM317T's ADJ by software just for fun and Daves circuit looks like exactly what I want + even more cool features.I'd really like to build this just for the sake of fun and learning and eventually getting something cooler and better than the L317T 5 parts PSU.So when dropping in the LT3083, amps could go up to 2 or more, thats why I asked about the shunt.Also I don't want to power this by a battery.So any idea how to solve my initial request considering the facts I just posted?Any help would be highly appreciated.Thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2017, 11:31:34 am
content: A smaller shunt is definitely a good idea. Already the high power consumption makes a 1 Ohms shunt in a 1 A range a bad idea.Having more gain in the differential amplification stage is good, as this means less division of the already small voltage at the shunt. However one still has to watch the common mode range for the OP. So a gain of 10 might be slightly too high (depending on the supply and OP) as the OPs common mode voltage would than be at around 90 % of the supply.Having 10 small resistors in parallel for a shunt includes an other trap for the young players: if close together, the power ratings of the resistors do not directly add up, as they heat each other. Especially SMD type resistors are rated under rather optimistic conditions and SMD parts close together have much less heat loss.Daves µsupply circuit uses quite a number of parts. A simple lab supply circuit from OPs and darlingtion power stage could get away with less parts and still get better performance and more to learn from.

author: VEGETA
author_rank: Super Contributor
timestamp: June 05, 2017, 11:53:27 am
content: Why voltage regulator chips do not work for lab power supply? I see their internal circuit and it is exactly like what is expected from a darlington or n-mosfet (like mine)... so it is the power transistor + other features. I wonder why it is wrong.However, one fact to support your point is that professional power supplies never use any of these regulators, but they use npn or mosfets.

author: Unknown
author_rank: Unknown
timestamp: June 05, 2017, 11:55:31 am
content: Quote from: Kleinstein on June 05, 2017, 11:31:34 amI was planning to use one single SMD resistor instead of the 10 pieces. Getting those isn't really a problem at digikey or rs-components.Any idea why Dave changed in Rev A to Rev C from the MAX4080T which has a gain of 20 and would allow a measuring range up to 4A with R=1R = 0.1R = 0.5V/A OUT = 4.096A range = 1mA / bit to the INA219 which even seems to have the wrong or at least not so clever Shunt resistor?edit:after studying the schematics again, I guess his point was to get rid of the ADC which was necessary to read the values from the MAX4080T and to get a device which is accessible via I2C. So I guess he was just simplifying. But that's not an explanation for the weird shunt resistor in Rev C...Rev A:http://www.eevblog.com/files/uSupplyBenchRevA.pdfRev C:http://www.eevblog.com/files/uSupplyBenchRevC.pdfRegarding the opamp I thought of changing the gain to 10 to match the shunt resistor which now would also be a 10th (1/10, say 0.1Ohm instead of 1Ohm) of the value it had before.Isn't this necessary/what would be a better solution?Regarding the number of parts: Yes, it quite has some parts in it but one of my goals was to control it via a microcontroller and all the cool stuff like current limitation/measurement and output voltage sensing should also be included. Ethernet I also want it to have, coding that isn't a problem for me. I don't think I'll be using Daves software much. If it is Open Source, I'll maybe take a look at some genius formulas inside, he might be using but I think that'll be all.What I definitely won't need is the I2C I/O expander or the whole DC-DC/EPOT and battery charging thing.I'm not sure if I'll end up in much less parts in the end, regardless of the fact that I'll have to start completely from scratch. Sure I might learn more but at some point I'd also love to get to an end and start using this thing - this already got way bigger than just controlling the LM317T ADJ Pin using a microcontroller, which was my initial intentionThanks again so much for your help!

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 05, 2017, 03:21:33 pm
content: Quote from: VEGETA on June 05, 2017, 11:53:27 amLook at this thread:https://www.eevblog.com/forum/chat/the-state-of-regulators-for-linear-power-supplies-a-rant-(!)/

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2017, 03:49:29 pm
content: Quote from: VEGETA on June 05, 2017, 11:53:27 amThere are several reasons for not using an integrated regulator:The frequency compensation is usually internal and often made for well behaved loads, not the variable load of a lab supply. There often is no access to the correct point to add current limiting - thus making the added current limit stable under all conditions is really hard and might very well rely an properties not specified in the data-sheets. With the power device in the same package, self heating will be a big problem and thus precision is usually poor. Often there is not even a separate voltage sensing pin. It gets much more precise if the voltage reference and OPs are at ambient temperature. Also the power handling capability of the LM317 and similar is rather limited. Powerful chips like the LT3086 or L200 are relatively expensive. So about the only positive thing about those integrated regulators is there internal temperature protection.Not sure why Dave dropped the max4080. It might be due to wanting less special parts. The 4080 is not very fast, but the LM358 used instead is not faster, especially not with a gain of 10. It could be using the INA219 as ADC - so no more need for a precision current sense amplifier. The point is that this did not solve the problems Dave's circuit has.

author: Unknown
author_rank: Unknown
timestamp: June 05, 2017, 04:05:16 pm
content: In his last video about rev c he is only talking about 1A max load so I guess the note about the LT3083 is just a relict from older designs, which he doesn't pay much attention to.He also said that the major goal of rev c was to consolidate and save money and to make the PSU just good enough for what it is.The INA219 is only used to measure up to 320mA, all above he is measuring with the AVR ADC.He is also talking about the shunt resistor which doesn't drop too much Voltage at 1A and that changing its value would be nasty and would require changing several other things regarding MOSFETs and so on(??). Didn't really understand what he meant there...

author: VEGETA
author_rank: Super Contributor
timestamp: June 05, 2017, 05:20:40 pm
content: He said he dropped LT3080 because of a bug in it. I don't know what is that but looks real since it is well known now.He actually said he wants an off-the-shelf solution for this psu, not doing it by himself (ie, mosfets and npns).

author: Unknown
author_rank: Unknown
timestamp: June 05, 2017, 05:21:48 pm
content: I guess I'm gonna ditch the INA219 as it is ok for me to only be able to measure in 1mA precision with the Atmega builtin ADC.In return, I'll change the shunt resistor to 0.1 Ohm as mentioned (1 single resistor) and change the gain of the differential amplifier from 1 to 2.Also I'll put in a DC-DC step down converter to change the input voltage of the LT308x to Vset + Dropout + little margin to make it more efficient.This should allow me to also use the LT3083 to get more power out of it.As a DC-DC stepdown converter I was thinking about this one:http://www.ti.com/lit/ds/symlink/tps54332.pdfIt is similar to the one Dave used, but allows higher input voltages as I want to power this by 18V (25V rectified).What do you think, might this work? Especially the shunt resistor 0.1 Ohm instead of 1 Ohm and the gain of 2 at the differential amplifier? I'm really not sure if this will break the whole measuring and limiting circuity or if I'll have to change anything else too...Thanks!

author: Unknown
author_rank: Unknown
timestamp: June 05, 2017, 05:29:45 pm
content: Quote from: VEGETA on June 05, 2017, 05:20:40 pmwhere do you know this from and what bug?thanks

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2017, 03:09:01 am
content: Quote from: nemail on June 05, 2017, 05:29:45 pmIf it is the bug I am thinking of then it is not really a bug and the behavior is fully specified in the datasheet. The LT3080 has a pretty limited safe operating area which prevents high output currents at a high input to output voltage difference. The maximum output current drops above about 6 volts of difference. On older regulators like the 317, this is more like 12 volts.

author: Unknown
author_rank: Unknown
timestamp: June 08, 2017, 06:57:02 am
content: Quote from: David Hess on June 06, 2017, 03:09:01 amOk, that wouldn't be a problem, we will be going to use a step-down converter to dynamically adjust the LT3083 input voltage according to output voltage demands.Similar to like David did it with the step-down/boost converter.Another update: we will be going to use the INA219 to measure the whole current range because our smaller 0.1Ohm shunt will allow us to use the INA219 for this.On the other hand, we will ditch the whole opamp + PWM DAC setup completely which is used in Daves schematics for current measuring and limitation.Thats easier and higher res. than using the PWM DAC or a real DAC if we want to go up to 3A of load with the LT3083.