uSupply Problems - Page 1
Forum section: electronics / beginners

author: redder558
author_rank: Newbie
timestamp: April 30, 2018, 12:04:12 am
content: For a while now, I have been wanting to build my own bench PSU. Recently, I have come across Dave's μSupply, but I have seen posts that say that there are some major flaws with it. I want to know what these problems are and if they are present in the pot-controlled design shown in Part 3 (image is attached).Also, what does Dave mean when he puts '4x10k matching' on the schematic?If the problems are major ones, can you also recommend me a better discrete (because they are better according tohttps://www.eevblog.com/forum/chat/power-supply-using-discrete-components-vs-regulator-ic/) PSU schematic?

author: daubmaso
author_rank: Contributor
timestamp: April 30, 2018, 01:26:00 am
content: Hi, I believe most of the issues with this iteration lie with the current limit function of the μSupply. It can cause oscillations that don't exactly create a very clean output when the current limit is triggered. What fixed it for me is changing the value of R2 on the schematic (near pin 2 of the lt3080) from 1k to 100R.By 4x10k matched he means that the 10k resistors should be hand picked and measured to make sure each value is identical across all resistors. This is so that the differential amplifier for the current shunt resistor has as close to a 1:1 gain as possible to ensure an accurate current limit.If you haven't made any power supplies before, I would probably recommend to just use an lm317 without any current limit. You can make a nice little supply with it and a cheap panel volt/ammeter from ebay, for a very small amount of effort. If you do choose this route I would recommend at least a fuse on the input in lieu of any current limiting

author: redder558
author_rank: Newbie
timestamp: April 30, 2018, 02:28:20 am
content: Thanks for the very simple explanation! Also, on your suggestion about the LM317, I was thinking about building something similar (with a few additions) but I kept reading that current limiting was invaluable to many people.I just have two small questions, can I simply replace the lt3080 with the lt3083 without any additions? Also, if I increase the resistance of the voltage / current pot, then will the max theoretical output also be higher?

author: daubmaso
author_rank: Contributor
timestamp: April 30, 2018, 03:06:41 am
content: Current limit would certainly be a very nice feature to have, but when it comes down to it just having a variable bench supply at all is incredibly handy.If you still want the current limit function to work there are 2 methods for doing this. You can either add an additional 3v reference and use that as the input to the current limit pot, or you can replace the 2.5v reference with a 3v one, feed the current limit pot directly from that and modify the gain of the voltage control op amp (for example changing the 12k resistor to 8k would give roughly the same output range as the original schematic).But before you modify anything in this circuit I strongly recommend you watch the μSupply videos very carefully, Dave does a great job explaining every aspect of the supply and how the control works.Here is my version of an lt3083 supply, it was meant to be controlled by a 3.3V PWM signal, but can easily be replaced by a potentiometer.

author: daubmaso
author_rank: Contributor
timestamp: April 30, 2018, 03:13:39 am
content: Sorry I should mention that the file I attached is a LTSPICE simulation in case you're not familiar with it's file extension. So here is the schematic all by itself. Please excuse how messy it is, I threw it together sometime last summer and completely forgot about it until now.V1 is the set voltage for the current limit, and V2 is to set the output voltage of the regulator.If you have any questions about the functionality of this schematic just let me know, I'd be happy to help.

author: redder558
author_rank: Newbie
timestamp: April 30, 2018, 03:46:17 am
content: Wow, thanks for the suggestion to watch the series more carefully, you can really learn a lot from Dave when really paying attention. Anyways, just realized that the maximum output voltage is the gain of the voltage controller op-amp multiplied by the voltage reference. So, in theory, if I change the gain of the op-amp, then I can change the maximum output voltage, right?

author: daubmaso
author_rank: Contributor
timestamp: April 30, 2018, 04:35:15 am
content: Yeah no problem, glad to help! Dave is an excellent teacher that's for sure.Yes that is exactly right, just make sure you change the input voltage to the current adjust op amp if you're wanting the current limit to allow currents greater than 1A. In the schematic I sent you the voltage control amplifier had a gain of 7.2, to allow for a maximum output voltage of 3.3*7.2=23.76v.If you look in the lt3083 datasheet, the block diagram of the output can be found. Essentially it uses a 50uA constant current source and a resistor on the set pin to create a voltage withing the chip less than or equal to the supply voltage. An op amp then compares the constant current source output voltage to the output voltage of the lt3083, and drives the base of the output transistor. By applying our own external voltage to the set pin, the output of the regulator is compared directly to the voltage on the set pin. So the voltage output is roughly equal to the set pin voltage. If we want a higher max output, we simply need to increase the voltage at the set pin. To do this we can either increase the voltage reference going to the pot, or increase the gain of the amplifier.The way the current limit works is by decreasing the voltage of the set pin whenever the current limit is active and essentially acts like a separate voltage regulator. When the voltage drop across the shunt resistor, after it's been amplified by the differential amplifier, exceeds the current set value it turns on the base of the npn transistor, pulling the set pin low. This steeply drops the output voltage until the voltage drop is smaller than the current set input, allowing the voltage to build up again before it gets too high and the npn transistor turns back on. For example if I have the voltage set to 10v, the current limit set to 3 amps and there is a 2 ohm load across the output, the voltage will begin to oscillate above and below 6v because that is the threshold when the comparator is triggered.I hope this clarifies more about the way this circuit works, and I'll include some pictures of how the current limit oscillates. Hope this helps

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2018, 08:53:42 am
content: The main problem is with the current limiting. It can oscillate under some conditions, especially a inductive load.The other weakness is that there is some series resistance, so the output voltage will not be as stable as with good lab supply circuits. The LT308x chips are also rather limited in power handling capability and cooling can be difficult. So the circuit is limited to low power.The root cause of the problem is starting with a voltage regulator and adding current limiting as a kind of afterthought - this is rather difficult to get stable under all conditions. It is much easier to not use the voltage regulator and have full control over the control loops yourself. The only good about those regulators is that they provide an internal current limit and thermal protection. So in some cases one might use LM317 as a kind of protected transistor.

author: Jwillis
author_rank: Super Contributor
timestamp: May 01, 2018, 02:57:16 am
content: Did you consider the L200.It has an output adjustable current up to 2 amps .Can be pulled to 0 Volts with a negative rail .http://www.st.com/resource/en/datasheet/l200.pdf

author: redder558
author_rank: Newbie
timestamp: May 04, 2018, 02:25:23 am
content: Thanks for the input Kleinstein, can you recommend me a good discrete component design that just uses pots? (nothing too fancy needed)

author: Dave
author_rank: Super Contributor
timestamp: May 04, 2018, 07:46:24 pm
content: Find service manuals of old HP power supplies. E3610A, for example - simple and elegant design.

author: hugo
author_rank: Regular Contributor
timestamp: May 06, 2018, 02:49:29 am
content: Not a micro but a better supply:DC2132A 24V 3A Constant Voltage, Constant Current Bench Supply Featuring LT3081, LT8612, LT3092

author: VEGETA
author_rank: Super Contributor
timestamp: May 07, 2018, 12:37:23 pm
content: Quote from: Jwillis on May 01, 2018, 02:57:16 amI like that IC so much, it could be the best to use for such projects.I plan to use one to create my lab power supply, there is a schematic for using it as so. It uses 10k and 100k pots to adjust voltage and current respectively.I want to never use a regular pot, but a digital potentiometer! this way I can set everything via software very easily. There is MCP41100 and MCP41010 on aliexpress for very cheap. This is my idea after thinking about it so much.This way you are following the datasheet and most importantly without any op-amp or stability issues!What do you think?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 07, 2018, 05:47:35 pm
content: When using Digital pot to adjust voltage and current limit, one has to observe that those digital pots are limited to voltage within the supply range of the chip and thus with many chips to a 0-5 V range. Especially the higher value digital pots can be effected from parasitic capacitance and this might cause stability problem - it depends on the circuit: in some circuits it is not a problem, other circuits can suffer.The L200 is nice for a beginners project. However the accuracy of the current limit is still limited and the range for adjustment is not very large. So for a larger current range one would need something like a switchable shunt. Also AFAIK the voltage range of the generic circuit does not go down to 0 V - so it needs some additions to make is work down to 0 V. Also self heating limits the stability / precision as the whole circuit gets hot, not just the power stage.

author: VEGETA
author_rank: Super Contributor
timestamp: May 07, 2018, 07:57:13 pm
content: You would need a negative rail to get it to 0v, which is doable. I saw TL431 or so used to achieve this... make it -5v and it will be enough.I don't understand why digital pots are a problem of stability. They are simply resistors.Current limit will have a drop voltage of around 0.43v between pin 5 and 2, so it won't be a big problem. However, the bench psu circuit shows more details and more resistances with an op-amp... but it won't get worse.getting 10k for voltage control and 100k for current will guarantee 30v 2A, but I don't fully understand the current limit control loop. I would like a nice explanation for it (the bench psu config.)

author: JS
author_rank: Frequent Contributor
timestamp: June 03, 2018, 02:16:14 am
content: Quote from: VEGETA on May 07, 2018, 07:57:13 pmCurrent limit will have a small voltage across it but will be floating high, dpots can't do that, you'd need an external power source and isolation, bit messy.I'm finishing a PCB design for the µCurrent and I used just a diode drop to get 0V in case my results are different than David's but it's just a bridge away of taking it out, I also added differential remote voltage sensing. I forgot to add OCP and OVP but I wanted to, pity the PCB is nice and ready, self etch friendly single sided, for those who want it, I'd probably be posting it at any time now. The board I designed doesn't contain the µC but a pin-header to connect to a control board. PWM filters are included in the PCB and two different voltage sensing outputs, one taken directly from the output and other with a differential amp for remote sensing.I'd like to build it, maybe tomorrow, and fully test it on the week when my new scope arrives, then I start a topic for troubleshooting or sharing (depends on the results)JS

author: mikerj
author_rank: Super Contributor
timestamp: June 03, 2018, 01:23:24 pm
content: Quote from: daubmaso on April 30, 2018, 01:26:00 amFWIW the resistor matching is to ensure the common mode rejection ratio (CMRR) is as high as possible. With poorly matched resistors, changes in the input supply voltage would cause the output of the op-amp to vary, which would be seen as changes in load current.

author: JS
author_rank: Frequent Contributor
timestamp: June 03, 2018, 06:16:28 pm
content: Quote from: mikerj on June 03, 2018, 01:23:24 pmLuckily the source impedance difference is low (the 1Ω resistor, at most, and pretty much fixed) which makes not to need the two sides to be matched, just ratio. You could hand trim a resistor with a file to get as high as you can measure if you want so.Effects in CMRR are quite hard in this case (with the original pre regulation) as it can change 14V in the original design and 2mV per bit, the error introduce by those 14V will make about 80dB needed to be less than 1LSB, or 2mA. I'm not counting the INA current sensing as it has 120dB typ CMRR. The other caveat, aiming for precision and accuracy is that about 2% of the current to the load is going to the Vcontrol pin of the LT3080 which is not being sensed, so there's a gain error in that as well.JS

author: VEGETA
author_rank: Super Contributor
timestamp: June 03, 2018, 07:09:56 pm
content: Quote from: JS on June 03, 2018, 02:16:14 amEPOT is a resistor network eventually so it replaces the resistor... what is the problem? the IC's power source is independent of the resistor array in it so the resistors are somehow isolated like having a separate resistor. Dave used EPOTs in his uSupply so I guess they are suitable somehow, you just have to find the way.Also, if 100K epot is 256 taps that is equal to 8-bit DAC or similar... So if we put 10k epot + 10k pot with each 256 taps in series (instead of the 100k in l200 circuit, but how? how to modify the values) would that be like 16 bit resolution?About the negative rail... Put ICL7660 IC which gives you -5v from your +5v or if you have something like +12v it will output -12v (or maybe limited to -10v) so put TL431 after it (with resistor feedback network) to get your desired negative rail, this will be stable enough.

author: JS
author_rank: Frequent Contributor
timestamp: June 03, 2018, 08:04:49 pm
content: Quote from: VEGETA on June 03, 2018, 07:09:56 pmNo, the potential at resistor terminals must be within digital rails, take a look at the datasheet.It won't behave as 16 bits. Many things there, first you load the first one with the second one so the first one won't behave linearly. This might be desirable as you get more definition in the lower range. Using a 10k pot first ans a 100k pot later will diminish this effect. If you just need more definition ibln the lower range (or upper range) and you are fine with 256 steps you can load the pot with a passive resistor and have a non linear response, this can be tweaked to a steep or shallow curve depending on the ratio of the pot and resistor, widely used, at least in audio were non linear tapers are the usual prefered and not alwaya available.Still, when both pots are on the top, a step in anyone of them would attenuate the same ammount.You could use 3 pots, the first two in feeding the extremes of the last one and track them a step or two appart, behaving like a KVD. Even then, you do have a ton of steps to output now, maybe more than 16 bits, but linearity would be horrible as the first two are ment to be 8 bits anyway and build to a cost with that in mind, even if they error is a fraction of a LSB you can only get that much bettrr out of them for 3 times the cost.JSEnviado desde mi LG-M250 mediante Tapatalk