minimum load circuit for lab psu - Page 1
Forum section: electronics / projects

author: nemail2
author_rank: Regular Contributor
timestamp: May 26, 2019, 10:17:27 pm
content: Hiin this:https://www.eevblog.com/forum/projects/how-to-wiregnd-everything-correctly-to-get-accurate-adc-readings-in-my-project/msg2439009/#msg2439009project I'm looking for a solution to constantly draw about 6mA or so to have a minimum load which helps stabilize everything.LM334 doesn't work out cause it only works down to about 1V when set to about 6mAMy currently used circuit (T8, D11, R25, R46) doesn't work quite well either and the zener diode gets really hot (above 75 °C).The circuit in the attachment works, but it needs -5V which I don't have. I wanted to avoid adding additional switching noise by a, say TC7660 or something like that to generate the negative voltage.Does anyone know of an off the shelf solution for this? The circuit in the screenshot works really nice even down to 0V.Thanks!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 26, 2019, 10:53:39 pm
content: Hi,You can try a current mirror circuit like this:This works well if the V2 is greater 100mV, which is pretty good.The circuit will not work as well in practice because the two transistors are not at the same temperature. The temperature of Q1 is a function of the output voltage of the power supply.I have attached the LTspice model.Regards,Jay_Diddy_B

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 26, 2019, 11:23:35 pm
content: The classic electronic load circuit with an opamp and MOSFET would work nicely down to very close to 0V.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 26, 2019, 11:58:25 pm
content: For the current mirror that Jay_didy suggests, you may find low cost, dual matched pairs from both Nexperia and OnSemi.Do a parametric search on their websites.They should be more than enough for your requirements.Of course, if you want to splurge and money is not an issue, then the MAT01 from Analog Devices is for you.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 27, 2019, 12:21:11 am
content: Hi here is the model for the scheme proposed by NiHaoMike:This depends on the op-amp working with the inputs near the -ve rail. Some op-amps work well like this others do not. Check the common mode range of the inputs and input bias currents etc.I chose 60mV across the shunt to get away from input offset and input offset drift.I have attached the model.Regards,Jay_Diddy_B

author: xavier60
author_rank: Super Contributor
timestamp: May 27, 2019, 12:45:12 am
content: This shows what I did for a Preload circuit.https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2388873/#msg2388873I really don't think accuracy of the current is important, so long as the current path avoids the shunt.I used a MOSFET in another design with no noticeable advantage.EDIT: I should point out that below about 1V, the load becomes resistive.The linked bench supply project does adjust down to 9mV output.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 27, 2019, 12:56:06 am
content: Hi this xavier60's circuit:I have attached the modelJay_Diddy_B

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 27, 2019, 03:20:01 am
content: Quote from: Jay_Diddy_B on May 27, 2019, 12:21:11 amPretty much any general purpose opamp designed for single rail use, including the common LM358, works just fine with its inputs down to and even slightly below 0V. Getting close to the positive rail is where they typically start to misbehave. (Except for rail to rail opamps, of course.)For simplicity, you could use a NPN transistor and a pullup resistor instead of the opamp, although that would make the load degenerate to a resistor below about 0.7V rather than much lower for the opamp solution.Then there's PTC elements, of which the ones commonly found in old CRT TVs tend to work well with a cold resistance of just a few ohms. (That's probably a bit too much of a load for your use, however.)

author: nemail2
author_rank: Regular Contributor
timestamp: May 27, 2019, 07:24:44 pm
content: awesome, thanks for all your help! I'll take a closer look at the various solutions and will then let you know which I implemented in my Lab PSU!!

author: nemail2
author_rank: Regular Contributor
timestamp: May 27, 2019, 07:36:03 pm
content: another thing: what happens if the two NPN transistors in Jay_Diddy_B's circuit actually aren't a matched pair?instead of xavier60's circuit I could just use a LM334 as well, as that works down to 1V as well.I also do like the opamp solution, however if I could get away cheaper with just a resistor and two NPNs, that'd be niceedit: what about this one?https://www.mouser.at/ProductDetail/Diodes-Incorporated/DMMT5551-7-F?qs=sGAEpiMZZMshyDBzk1%2FWi3NKRur4siPQ87U2jNAVT5E%3Dlooks good

author: nemail2
author_rank: Regular Contributor
timestamp: May 27, 2019, 10:45:46 pm
content: Ok so i have kind of made a decision to go with the matched-pair npn solution.That DMMT5551-7-F should be able to dissipate those 6-7mA @ 12.288V max output voltage at V_ADJ.Any objections?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 27, 2019, 10:53:29 pm
content: Hi,We can look at the impact of temperature with this model:The temperature of Q1 is stepped from 35C to 100C.I have added the 47 Ohm emitter resistors to reduce the sensitivity.The downside to the emitter resistors is that the circuit works above 300mVRegards,Jay_Diddy_B

author: xavier60
author_rank: Super Contributor
timestamp: May 28, 2019, 01:48:35 am
content: With the added Emitter resistors, might cause some residual voltage at the output when CV is set to zero volts. It wouldn't be much.Mine causes 9mV.

author: nemail2
author_rank: Regular Contributor
timestamp: May 28, 2019, 06:16:45 am
content: that does look good, doesn't it? 300mV is certainly better than what I'm having now. also more stable. and still cheaper/less parts than the opamp solution.residual voltage at the output of the PSU itself? because of the drain? or because of the lack of a drain when set to 0mV where the load circuit isn't working?sorry for the stupid question...

author: xavier60
author_rank: Super Contributor
timestamp: May 28, 2019, 06:53:24 am
content: Quote from: nemail2 on May 28, 2019, 06:16:45 amWith mine, the small Base bias current causes a small voltage drop across the Emitter resistor while the regulator is off. This small voltage is transferred to the output via the transistor.

author: nemail2
author_rank: Regular Contributor
timestamp: May 28, 2019, 07:18:12 am
content: Quote from: xavier60 on May 28, 2019, 06:53:24 amlet's say I have a short on the output, could that blow the ass out of the transistor? what about a diode to prevent current to flow in the wrong direction?

author: xavier60
author_rank: Super Contributor
timestamp: May 28, 2019, 08:38:37 am
content: Quote from: nemail2 on May 28, 2019, 07:18:12 amA short on the output shouldn't cause a problem.Because I sometimes charge batteries, the last two bench supplies that I built are designed to briefly tolerate applied reverse polarity.I use a diode and a polyswitch just to protect the preload circuit.Not all regulator topologies can be made to tolerate applied reverse polarity such as those with voltage follower output stages.

author: nemail2
author_rank: Regular Contributor
timestamp: June 03, 2019, 09:25:02 pm
content: what concerns me a bit after thinking a while about it (sorry for the late response, didn't have time to hobby around recently) is the negative current when the emitter resistors are in place which is shown in your diagram at voltages below 0.1V and also my spice simulation says the same thing.also, I was thinking about using the BC847ALT1G for this, instead of the DMMT5551-7-F to reduce my BOM (and I have already bought the BC847ALT1G in high quantity (at least in regards of hobbyists use)). they should work as well, right? maximumoutput voltage is 20V. what about backfeed voltage of let's say motors or magnetic coils (relais) would they be an issue for the Vceo=45V of the BC847ALT1G? the DMMT5551-7-F has a much higher Vceo of 160V. not sure if that would even help though, when dealing with backfed voltage like that. i guess I'd need some other protection to mitigate this issue but that's for another topic...thanks!

author: nemail2
author_rank: Regular Contributor
timestamp: July 04, 2019, 06:27:39 am
content: ok so i have built this up with the DMMT5551-7-F matched pair and without the emitter resistors. what should i say... the magic smoke escaped from the DMMT5551-7-F at about 7-8V output set voltage. so i bodged in the emitter resistors (48 Ohms) and replaced the DMMT5551-7-F. The magic smoke did not escape again but now i can't get my outout below 120mV. before those emitter ristors my output went down to 3mV.Why is it so? LTSpice shows some negative current at the emittors, is it something to do with that?As soon as I disconnect the current mirror, the voltage drops to normal (3-5mV).What is going on there?Thanks!

author: duak
author_rank: Super Contributor
timestamp: July 04, 2019, 07:09:50 pm
content: I think the output voltage no longer goes to zero because the current mirror is injecting current into +V_ADJ when the output voltage is << 600 mV. The diode connected transistor T4a will develop a certain collector/base voltage as well as the voltage developed across its emitter resistor. This will forward bias T4b's base emitter junction. When +V_ADJ is lower than T4b's base voltage by a few hundred mV, T4b's base-collector junction will also start to conduct and allow current to flow into +V_ADJ.Did you say that T4 was damaged without the emitter resistors? What I think may have happened was that the thermal dissipation of T4b was exceeded due to thermal runaway. Even though these devices are matched fairly well (the spec sheet says they are adjacent dice) T4b will be hotter than T4a when +V_ADJ is increased and when this happens its collector current will be greater than the reference current. This increases its temperature which increases its collector current which increases its dissipation and so on... Do you have the recomended heat sink area on the PCB?I suggest reducing the value of the emitter resistor for T4a and measure T4b's emitter voltage to determine its emitter current and calculating the sinking current and its power dissipation at various output voltages. You may also find that by reducing the emitter resistor value the current mirror will not inject current into the output.

author: xavier60
author_rank: Super Contributor
timestamp: July 04, 2019, 08:13:37 pm
content: R1 needs to be 56K to reduce the residual voltage drop across R3 to about 10mV when the output is set to 0V.Try 1.5K for R2.A lower value for R3 would further reduce the residual voltage.

author: nemail2
author_rank: Regular Contributor
timestamp: July 06, 2019, 12:20:11 pm
content: Quote from: duak on July 04, 2019, 07:09:50 pmsounds reasonable. yes T4 was damaged without the emitter resistors, the magic smoke did escape.There is quite some area on the PCB but I didn't check it with the datasheet. a friend however told me that he never saw a current mirror without the emitter resistors so i guess I'll just put them in.I'll try to reduce the values and measure things. Thanks!

author: nemail2
author_rank: Regular Contributor
timestamp: July 06, 2019, 12:21:38 pm
content: Quote from: xavier60 on July 04, 2019, 08:13:37 pmI'm sorry, to which schematic are you referring with the part names?Sounds very promising, though! Thanks!

author: xavier60
author_rank: Super Contributor
timestamp: July 06, 2019, 03:35:53 pm
content: Quote from: nemail2 on July 06, 2019, 12:21:38 pmI was using the references from Reply #11.

author: nemail2
author_rank: Regular Contributor
timestamp: July 06, 2019, 04:24:58 pm
content: Quote from: xavier60 on July 06, 2019, 03:35:53 pmOK thanks, will try it out!

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 01:10:54 am
content: OK so first I tried 56k for R1 and 1.5k for R2 and R3. There was still voltage fed into the PSU and I was seeing about 60mV instead of 9-10mV. Certainly better but still not where I'd like it to be.So I went up to 100k with R1 and down to 47 Ohm for R3 (keeping R2 at 1.5k). Output voltage fell to 20mV instead of 9-10mV and according to the 121GW meter no current was flowing via R1 into the PSU (before there was like a few dozens of µA flowing into the PSU). So I'm not sure whether the PSU isn't going down to 10mV because it is virtually unloaded or if the current mirror is still upping the voltage on the PSU output. Also, the current was increasing up to 2.5mA as I was increasing the output voltage of the PSU up to 16V. With the original values, the current stayed stable (at some 0.6mA).I'm not sure how to overcome this. Is it even possible to make a circuit which works down to like 10 mV and stays stable all the way up to 16 V? I mean it would be OK if the circuit does not sink much current at very low voltages, it seems that my PSU is happy with very little amounts of current to stabilize in low output voltage regions.The circuit which I was using in the previous revision (see attachement) with the zener diode basically did the business (after I added a series resistor to the zener it stopped heating up like crazy) but it is quite unstable so I'm having a hard time calibrating that additional current out in software. That thing varies in input voltage and with temperature but at least it brings my PSU down to the desired 9-10mV with ease...I'd also be willing to buy some fancy chippy which can do the business for me, even for quite a price if it does a reasonable job. But I'm afraid, as often, this is nothing you can solve by simply throwing money on it...Thanks for any advice!

author: duak
author_rank: Super Contributor
timestamp: July 10, 2019, 03:01:13 am
content: Three things to think about:- a MOSFET current mirror,- add a negative supply - it just needs a volt or two and a few milliamps.- an incandescant light bulb - consider them to be PTC thermistors and have a wide range of resistance.. Try something like 24 V at 20 mA or even a small line operated one.

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 03:18:20 am
content: Quote from: duak on July 10, 2019, 03:01:13 amregarding the MOSFET current mirror i found this:http://tera.yonsei.ac.kr/class/2013_2_1/lecture/Lect_22%20MOSFET%20Current%20Mirror%20and%20Active%20Load.pdfunfortunately this is way beyond my knowledge. I'll churn through it but I highly doubt that I'll get something useful out of it...negative supply: where would i put that? are we talking about the npn current mirror?incandescant light bulb: sounds like a 50's solution :-D and I can't imagine how that would work (again because of lacking knowledge) :-D

author: xavier60
author_rank: Super Contributor
timestamp: July 10, 2019, 03:30:51 am
content: The residual voltage should simply be due to R1's current flowing through R3 and it's voltage also appearing at the Collector.First check what the real minimum regulated voltage is by checking what the CV op-amp is doing at minimum voltage setting.My bench supply wants to regulate to 0V so the CV op-amp reacts by swinging its output hard low.My other bench supply( 20A ) uses a MOSFET so there is no residual voltage applied to the output. When I set the regulation to 0V, I can accurately measure the 47 ohms across the output terminals.

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 01:32:04 pm
content: Quote from: xavier60 on July 10, 2019, 03:30:51 amThe CV op-amp is outputting 6mV when the PSU is set to 0mV. Just measured that.

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 02:08:02 pm
content: what about this? according to the simulation (lol) this works down to a few millivolts. i'll breadboard that!i really don't care that it needs an opamp and an n-channel fet. i have already spent so much time on this that i'd gladly spend some bucks for a nice solution, especially if it is that stable!simulation attached. rename LM358.txt into LM358.lib and put it in the directory where Draf4.asc is.

author: edavid
author_rank: Super Contributor
timestamp: July 10, 2019, 03:19:26 pm
content: Quote from: nemail2 on July 10, 2019, 02:08:02 pmGo back and look at Jay's practical version of this circuit.Also, have you considered using a JFET? There isn't a great selection these days, but maybe a few MMBFJ202 in parallel?

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 03:43:59 pm
content: Quote from: edavid on July 10, 2019, 03:19:26 pmlol completely forgot about that one. is there a particular reason for M1 being a Trench-FET in Jay's circuit?i have breadboarded my version in the meantime and I'm quite happy. It really works good with a IRF3708 (had that one lying around) and a LM358. Sure, input offset and input offset drift got me with my circuit so the drawn current isn't exactly to the mA what I expected but near enough and certainly stable from 36mV output voltage up to 16V!So I have to tune this and take things from Jay's circuit to make this practical.Also I was going to measure the opamps output on the scope while tuning the voltage to see if tends to (or does) oscillate.One concern I still have:Given that my output voltage may start to oscillate at some point with some load (as I don't have proper electronic loads and most importantly don't know how to thorougly test a PSU) that constant minimal load may start to oscillate as well.So I was thinking to make this thing quite slow so it doesn't oscillate to buggery if the output voltage goes bonkers. Would that be practical? Or is it just part of the curse? Is Jay's R4/C1 already doing the job?Why a JFET and in what context? Why multiple of them in parallel? I only wanna sink a few mA at up to 16V (max.) :-)Thanks!

author: edavid
author_rank: Super Contributor
timestamp: July 10, 2019, 04:58:21 pm
content: Quote from: nemail2 on July 10, 2019, 03:43:59 pmYes, that's their purpose.QuoteOne component current sink that goes down to 0V.Quote1. To make IDSSselection easier if you don't have a quantity of parts to pick through.2. If using SOT23 parts, to spread the power dissipation.QuoteIn that case one SOT23 package should be OK, if you can find the right one.

author: duak
author_rank: Super Contributor
timestamp: July 10, 2019, 05:09:28 pm
content: An incandescent bulb has a lower resistance when it is cold vs when it is hot. (see attachment) This is not a constant current sink but it does draw more current at low voltages than at high voltages, a function that your circuit requires. It does not introduce offset voltages and it does work down to zero volts. It was used by Bill Hewlett of Hewlett-Packard for their first variable frequency oscillator to stabilize the output voltage in 1940. It does seem like a 50s solution, but it could be the simplest solution. I used it to solve a residual voltage problem on a DC power supply.MOSFETs could be substituted for the bipolar transistors in the present current mirror. This would eliminate any current injection into the output. The devices should be matched and tested to ensure that the sink current is actually what is expected.Connecting the emitters of the current mirror to a small negative voltage of -1 V relative to AGND would ensure that the output voltage would be zero when needed. There were some threads on this forum recently on this subject but I can't seem to find them quickly. TI and other manufacturers make some devices for this.There are a number of ways to test a power supply for stability. I would start with using power resistors of differing values for loads and try varying the voltages and currents applied while monitoring the output. I would then use a function generator to inject a low level 10 Hz square wave into IC2A-2 while repeating the above tests. If the output does not overshoot or ring, then it is basically stable.

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 05:16:49 pm
content: Quote from: edavid on July 10, 2019, 04:58:21 pmyou got my attention. like this, only with a jfet?https://www.eevblog.com/forum/projects/minimum-load-circuit-for-lab-psu/?action=dlattach;attach=747333the mosfet/opamp solution has the advantage that it behaves quite predictable and stable so i can remove the minimum load current from the display in software quite easy...how about those properties with the one-component JFET current sink?

author: nemail2
author_rank: Regular Contributor
timestamp: July 10, 2019, 05:28:38 pm
content: Quote from: duak on July 10, 2019, 05:09:28 pmThank you very much, I'm always interested in cool history lessions like that (really!). Awesome solution and I'll definitely keep that in mind. For this project though, where I'm trying to make the PSU as precise as possible (without spending thousands of bucks, though), that seems a bit too unpredictable.Quote from: duak on July 10, 2019, 05:09:28 pmI'll take a look whether there are matched MOSFETs in one case being sold, I don't really want to match them by hand, to be honest. I might try and breadboard that but I'm kinda sold for the opamp solution already, I have to admit. In my design there is a spare opamp (of a dual package) anyway since I have ditched that sense input idea and I have a suitable MOSFET in my BOM as well already. So, meeh.. :-)Quote from: duak on July 10, 2019, 05:09:28 pmI'd be really interested in those devices. I wasn't really able to find anything besides the LM334 but that does only work from about 1V upwards. I tried to connect -5V out of a TC7660 to the negative pin of the LM334 to shift everything but that didn't work either. Something was drawing current but it was not the LM334. I had my Lab PSU set to Imax of 100mA and those were drawn so I guess that was BS, what I breadboarded there.Quote from: duak on July 10, 2019, 05:09:28 pmI have a cheap load from aliexpress which i already used to torture my PSU, so far so good, no issues. Injecting some signal is something I didn't do yet. What amplitude should that square wave have and what current capability?I was planning to build up an array of mosfets with some power resistors and some atmega to be able to connect and disconnect various resistors very fast and thus change load very fast without switch contact bouncing and look with a scope what the output does.

author: edavid
author_rank: Super Contributor
timestamp: July 10, 2019, 05:29:26 pm
content: Quote from: nemail2 on July 10, 2019, 05:16:49 pmNo, see attached. I didn't bother trying to dig up a J202 model. It's resistive below VP, don't know if that's OK...

author: duak
author_rank: Super Contributor
timestamp: July 10, 2019, 11:17:19 pm
content: For transient response testing I would try a 22K resistor in series with a 100n cap applied to IC2A-2. For small signal response I would set the function generator to 25 mVp-p square wave. You should see about 50 to 100 mVp-p on the output. 100 Hz may be too high because of the 100 uF capacitors on the output so you can reduce the frequency. You can then increase the function generator output signal. You should then see the rise and fall times change the waveform into more of a triangle or ramp. Depending on the load and current limit setting, at some point it may invoke current limiting and you should see the supply change to constant current mode. The transition should be clean without weird spikes or oscillations.

author: nemail2
author_rank: Regular Contributor
timestamp: July 11, 2019, 05:00:51 am
content: I'll try to get or make a function gen somewhere and eventually I'll do those tests for sure, thanks for going to detail :-)Here is the revised version 1.9 of my Lab PSU, still have to fix all the designators, though:https://github.com/mamama1/LabPSU_DarlingtonI have implemented the opamp + n-fet current sink and tuned it to about 6mA. I also inserted a pin header so I'll be able to measure the current which the circuit is sinking with my multimeter so I can calibrate it out in the software. I guess it will vary from build to build due to manufacturer tolerance of the resistors, opamp and fet.

author: xavier60
author_rank: Super Contributor
timestamp: July 11, 2019, 07:36:48 am
content: Was it possible to avoid having the preload current flow through the CS resistor?