Author Topic: EEVblog #102 - DIY dummy load - higher current?  (Read 20391 times)

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Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #25 on: January 29, 2011, 07:22:11 pm »
@jahonen: yeah i'm experiencing those oscillations(figured out whilst sleeping that the response speed of the feedback amplifier wasn't fast enough so it was causing U4B output to overshoot until U3 responds that would continue in a cycle, it was kind of a Doc moment without the head bang hehehe).
I re-read your post in page 1 and i did a quick simulation using a 100nF cap on the feedback resistor and... it appears to be working!, i'll have to do some quick math to see what cutoff freq it is.. hmm with 27K and 100nF it seems to be 60Hz~, if i raise to 330nF i lower the cutoff to 20Hz(then again, i'm calculating an RC filter to ground so i'm probably doing it wrong... but looks sound)

@achmed: yeah, i'm not worried about U4B output because with 0 input the 1V offset is below FET threshold, if i where to be worried i'd also split U4 supply.
I can't scale reference from 0 to 0.5V as i'd need to VERY reliable supply a 0.5V source, the pot is only for the simulation anyway, the real deal will use a 256-tap 10k or 50K digital potentiometer. I don't think i have an accurate enough(to be cost effective) 5V supply as it will supply the entire rest of the circuit.

¿Why add a resistor to the gate if there isn't any current flowing?

Thanks for that TI reference!, very useful it ended up calculating 27K and 3K values for a gain of 10 with no offset!, many thanks!
about the mosfet: Even if the model isn't very accurate, it will give me an idea and the rest will be taken care by the opamp(in fact, i could probably chose whatever power MOSFET i can find and it would work ok as long as gate threshold is above OP Voutmin and the Vgate for 5A is enough from my 12V supply

About the offset values, the sense output will be read by a 0-5V ADC, if the sense output has a 1v offset it will skew(i can compensate in software to zero it) and i'd end up loosing a large portion of readouts in the lower end(from 0 to 1A in the Sense resistor), if i need -5V supply it's no problem(i'll have to deal with power supply issues later in the project anyway, i'm gonna take note of those 2 parts you mention).

I've ran a quick sim with the sense amp at split +-5V and the set potentiometer with a 0.5V supply, the circuit works as expected, and now i have 0v offset in the sense amp for the ADC, i still have the 0-0.5V input that i want to amplify...


Taking jahonen low-pass filter suggestion into account thus adding a 100nF cap in parallel to feedback resistor in U3A with split supply i think i have the best-of-both-worlds, 0-5V select +direct sense output to ADC:


Is it sound or multisim is pulling my leg?, sim has occasional low freq spikes that i can't seem to capture with XSC, weirdness...
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #26 on: January 30, 2011, 04:14:18 pm »
The higher the step count and the lower the resistance the finer the resolution.
I follow you on higher step count, but why with lower resistance?, given the same step count, why does the 10K pot have better resolucion than the 50K one?, tolerance should affect them both the same...
i chose the 256-value one because that's what i can get locally and any larger than that(1024 steps for example) would be overkill and lost in tolerances downwards

Your mosfet is a capacitive load. Read through this for more info about phase shift.
http://tangentsoft.net/audio/hs-opamp.html
Forgot about that since i'm using DC signals, the resistor shouldn't do any harm so i'll add it for the end design, interesting link too.

For simple gain (no offset) TI has a calculator for that as well.
http://focus.ti.com/docs/toolsw/folders/print/gain-stage-calc.html
Ah, a simpler version of the one you psoted earlier, bookmarking it.
Nice thing that it compensates for open loop gain(didn't make a difference with my choice amp, but good to know)

You asked earlier why Vgs/Vds discrepancy.Trans conductance isn't model in the fet accurately if at all that's why.
i see, that's why i'm seeing weird Vgs values, i guess i'll have to simulate it in "Solder '11" ;)

What you’re doing is getting difficult to follow. It sounds like your trying to condition your circuit around your ADC. Why don’t you just condition your 0-0.5V sense signal to your ADC?
See.
http://www.sensorsmag.com/da-control/hitting-sweet-spot-range-adcs-1549#
You can say for example 0.5V from the opamp output is zero for your ADC input. This is the offset at the output of your U3A for a zero current. See the screen shots below for Using the TI tool for gain and offset.

Letsee if i can clarify a little on your question: i am in fact conditioning the 0-0.5v sense to my 0-5V ADC, that's what x10 gain U3 is for, with the added benefit of using the same output to allow me to use a 0-5V input for current select input(the "pot" part).
Both current select and sense will come/go to a microcontroller

i'm not following, ¿why would i want to set the full scale 0.5v as 0 for output?,i kind of find that would complicate things if i can simply put 0 to 5v as output with 2 less resistors and simply Rf+Rg
Nice url as well, good to bookmark when dealing with negative inputs(which i won't have in this circuit, that simplifies thing so i don't need floating input offsets).

Thanks for your patience and work on the post Achmed, i appreciate it a lot
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #27 on: January 30, 2011, 07:45:32 pm »
but i'd like to avoid having to use a 0 to 0.5v in the digital potentiometer Vcc, getting a reliable precision low noise 0.5v supply there would be akin to having a negative rail regulator(hence my insistence/interest on using the same *10 sense amp as the feedback input, something i could also do is add a /10 attenuation opamp after the pot, but i really don't want to go there, it would introduce more errors and design issues with such low offsets yadda yadda).

hmm i see where you went with the 0.5 min, actually a regular opamp would be somewhat higher than that, around Vss+1V. Problem with all the RR I-O amps i've found(tried to use them first) is their very high offset voltage and limited supply range(the latter i can ignore since the sense amp would be +5V~).

Hmm i was forgetting about the upper swing when using +5V as opamp supply, i'll never get full swing(not with the opa277 at least best case is v+ -1V), so it's either another power rail for that part OR using the same +12V supply for FET gate... but it's a design quirk i'll have to look into i think later....

letme process the numbers........
why do you use a gain of 9?, ahh... it is to get 4.5v at full scale....
but same thing applies as i said before, to get 0.5v i'd need a r-t-t ultra low offset amp(rare) OR split supply(which negates the need for this), so i'd need to experiment with higher values but i end up with the 0.5V problem for the potentiometer...
i see where you go, the difference due to offset is negligible in the rest of the components tolerances when using *9 or *10

yeah i'm not planning to meaure anything below 100mA(or for that matter, i'm pretty sure anything below 0.5A, but i'd like to be flexible and future proof).
so, let's say i use the sense amp for the ADC with 0.5v offset, or even 1v to be safe..., so it's 1 to 4v, etc etc.
how do i reliably get a precise 0.5v with less than 5mV regulation supply for the digital pot?, do i use a precision voltage reference?(so i'm trading a standard negative inverter for a precision reference?, a quickie search shows some AD references, ofc i wouldn't dream of getting one of those exotic devices here....)

bleh i also discovered that 3K ain't a precision commercial value, closest is 3.01K but with 27K/3.01K is 9.97 and skews result a little too much for my taste.... i'll have to find some combo with what i can scrounge...(trimpots only go to 5%....), it also doesn't helps that all 0.1% resistors i can find are SMD 0805 or 0603, that sucks....

hehe yeah, everyone has their own "design taste" to account, i'm approaching the limits for simulation i'll have to start building a prototype soon...

sound like your project also has lots of headbutting with low-value precision stuff
 
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #28 on: January 31, 2011, 08:28:40 pm »
it's not much inability as cost-effectiveness, i could supply anything from mauser/digikey, i'm just not willing to pay 40$ to 100$ of shipping for half the components net value because the project cost would skyrocket.

the current translation sounds much too complicated and i really don't want to get myself into having to finetune with input impedances and current converters, i rather deal with straight voltage values.

i could trim from 5V, but that 5V supply will have all the logic and load will vary, and i feel it will introduce too much error plus all the manual nuisance of having to trimpot for 0.5V at full scale so i'm gonna try the opamp feedback to noninverting and see if i can deal with transients and runaway oscilations.
I've been searching for ultra low voltage references, there are almost none and all of them start at 1V(apart from one programmable source from maxim, but it uses SOT23.....)

i'll use the OPA277 variants where precision is needed as i can get them here, LF353 and similar for blunt work(if needed at all), will use split supply as generating it is trivial compared to the math nuisance of maping the ranges(at least for me).

the problem now are the voltages, 7660 variants only support 10V, for the fet drive i'm gonna need from 10 to 15V or more(still can't decide, testing needed), i was hoping on using that same 15v for the feedback amp but with the 7660 is impossible, and +-5V won't cut it, so i'm gonna need a precision low dropout reg for ~7V

and to make matters worse, i want to make this battery powered so i'm gonna have to deal with all kinds of step-up/down converters from the 9V supply(step up for fet drive, linear LDO for logic, and for 7V i haven't got the slightest idea....), i fear i'm gonna have to forget about battery power for now and use a LM317 to supply the feedback amp from the 15V supply :S or use split transformer to generate +-9V, hmmm that sounds doable...
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #29 on: February 01, 2011, 01:42:00 am »
for FET i'm looking at IRFP048, IRFP064 but first prototype units will probably use a ST STW20NB50 that i jacked from the main switcher of large PSUs, 20A continuous, +-30 Vgs rating
but Multisim does not have it's model so i don't know for certain how many gate voltage i'll need to drive it to 5Ax20V, if i take the datasheet output characteristic, for 20Vds and 5A i shoudln't need more than 6V.... but multisim says otherwise(i wonder who's bullshitting me..., guess i'll discover it in protoboard).

ah so you're using the 12K and 3K to divide the 5Vcc and make 0.5v then you use the AD8629 to isolate it, nice idea, i thought about that too but kept thinking it couldn't work due to output offset, but then again if i chose a simple RtR opamp output...., then i could also use a precision 1v voltage reference or something of the sort to get over noisy Vcc(same with U2).
where do you get the 2.5v "ref3125"?, and i'm sure R7 and R8 need to be precision 1% or 0.1% parts as well which worries me

the idea is to test things by module, but i worry what would happen when all is put together, the 5V line feeds the LCD, backlight, microcontroller, some of the opamps(and some parts i haven't even begun to design yet for interfaces and testing), then the 12~15V line feeds the main FET, a 12V line for the HSF etc etc
i guess with proper decoupling i shouldn't worry(lcd and backlight should be the hungriest at 140mA~), i could also use a ULN/tight regulated linear reg off the bridge rectifier for precision voltages....

So far then, from your feedback i can guess 3 variations:
v1: "the simpler" with feedback amp, 2 opamps total with split +- 7v rail, prone to oscillations and weird stuff
v2a: "elegant", 0.5V precision supply, divider with opamp follower into digipot input, then another opamp for sense amp into ADC with split +-7V rail, 3 OPs total and precision resistors
v2b: same as a, but using 0.5~0.8V offset to 4~4.5v(the OPA277 datasheet specs that output for 10K load is V-+0.5v and V+ -1.2V whiuch leaves me with 0.5 to 3.8V, best case from graph is -1V), no split rails but i tradeoff dynamic range and software "complexity"(which is somewhat trivial), smae 3 OPs

I live in Argentina, distributors here suck(the ones that have stuff have minimum order ammount to ship -at your expense-) and are in the stoneage, the current component selection is based on what i can get from them should i chose to go into production(otherwise id just be ordering free samples of every device i need disregarding supply), Sense resistor is proving to be nigh impossible to get(but i found one from my fathers electro-stash, a old Dale 0.1R 0.1% unknown power resistor).
i can't even get a SMD breadboard, let alone almost any smd component or 1% resistors
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #30 on: February 01, 2011, 02:19:39 pm »
that AD8638 opamp looks awesome(i'll have to run a simulation with that opamp), i'm pretty sure it would be perfect for my device, but i can't get it here, AND it's SMD(i'm staying clear of SMD) and it's impossible to get free samples as well. That's why i'm using the opa277 variants, even when they're not RtR
and i have a ton of problems sourcing 0.1R, they're all SMD :(
 

Offline johnmx

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #31 on: February 09, 2011, 02:34:37 am »
Eliminateur, check my recent post about my current dummy load:

https://www.eevblog.com/forum/index.php?topic=2487.0
Best regards,
johnmx
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #32 on: February 09, 2011, 12:39:35 pm »
john, very nice and complete!.
I see that you went for a setup with an amplifying NI opamp U3 and then the main opamp in U4, how did you control oscillation problems?(or you didn't experienced any?)
i should get me some MAX430 from the local stores, looks awesome for my need AND it's DIP!.
¿Why R2 between amps?
¿and C9 in the feedback loop?(or is it part of a RC filter between the 2?).
that TC962/TC7662 looks nice and higher voltage than the 7660 i have, taking note of it...
¿how do you handle the large VI offset of the LM258, doing a quick math, with a 1mA signal you get ~375uV on U4A input, or is that way below your min current?
good thing about the fuse, hadn't tought of that!, adding....
Good protection circuitry, luckily i can dispense it since the fet can handle 270W continuous and i'm using a ucontroller to drive the circuit so i can limit it over there

umfortunately this module of the project is in the backburner for a while whilst i finish an arduino-controlled lead-acid battery charger. I also need to focus on other modules to see if i need components in common or not but i've still made some design choices to narrow down the build:

1) i'm gonna use split supply for the feedback opamp alone i think, because a 1V vout offset is way below the MOSFET threshold voltage.
2) forget about battery power, i'm gonna need an active heatsink like the one you used(gonna use an old P4 one and screw it on that) plus microcontroller, display, associated circuitry... too much hassle and cost to power it by battery. What's left is to determine the minimum supply voltage for the maximum power output and then build a "central" power distribution module
« Last Edit: February 09, 2011, 12:48:31 pm by Eliminateur »
 

Offline johnmx

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #33 on: February 09, 2011, 10:59:23 pm »
- The main purpose of R2 and C9 is to stabilize the circuit. The same for R20 and C22, but in this case it’s desired to have a slower response on the limiting circuit (U4B) than in the current control circuit (U4A). That’s why R20*C22 > R2*C9. If you don’t follow this rule then the whole circuit will probably oscillate when it reaches a power limit situation.

- All components are through-hole. I believe MAX430 is now obsolete, but you can use any equivalent part.

- 1mA is too low for this circuit. The potentiometer has a mechanical counting dial on it with a resolution of 20mA.

Let me give you an advise, keep your circuit as simple as possible. If you turn it into something very complex then the chances to not finish it will increase a lot. I prefer to have a limited but working project than an unfinished high-tech top project.
Best regards,
johnmx
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #34 on: February 10, 2011, 01:52:53 am »
John,
thanks for the clarifications and the advice, i want to keep things as simple as possible too, this module on my project will be simpler than your device as i won't have the protection circuitry you added.
i'm gonna have to add those R/C networks to stabilize the system as well, yeah the MAX430 is obsolete, but that means there's a high chance i'll be able to get it here as a "state of the art" device :D. Umfortunately, Maxim seems to have abandoned DIP packages for precision parts, so there aren't any more chopper-stab opamps from them in DIP, and TI chopper-amps are very old "outdated" designs that need external caps and have limited supply and horrible output swing.
I wonder how the MAX430 deals with single-rail...
Linear has some nice devices but they refused to send free samples to my country for "small volume" projects with all kinds of BS red-tape(wankers)

I don't know if i mentioned this before but this is gonna be a part of my laptop PSU tester, and that has some hurdles even if i don't go into feature-creep(which i want to avoid!!!), at least i've set the goals and began doing design choices to mold it, so i'll be opening other threads with request for assistance when i'm up against the wall hehe ;)

 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #35 on: February 28, 2011, 02:03:11 pm »
Back again on this project part...
i've found(no idea where, it was there on my desktop :D) this circuit of a current source, built it in multisim just for kicks and it works -as expected-, but i have some questions about it that maybe you guys can enlighten(and so i can apply to the "big daddy").
What i understand is the divider choice, to get 495mV from 5V adjust equals the drop on Rsense@1.5A, so far so good...:

1) What's R4 for?, i'v tried removing it and makes no difference in loop gain, or is it part of a snubber with C4?.
2) Why does it put C3/R3 in series with FET, and why those values in particular?(should i calculate it as an RC filter, but C isn't to ground..., if i calculate it as a normal Rc filter, it gives a cutoff of 153kHz)
3) why is C4 across output and NI input?
 

Offline johnmx

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #36 on: February 28, 2011, 02:43:06 pm »
You already asked those questions before. Those components are there in order to stabilize the circuit.
You need to read some info about loop stability. Check the following docs:
http://www.en-genius.net/site/zones/acquisitionZONE/technical_notes/acqt_092407
Best regards,
johnmx
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #37 on: February 28, 2011, 08:16:07 pm »
hmm yes.. i thinki did, for the other circuits, but i don't understand why those exact values and for what frequencies they where thought for...

i've *tried* to digest that note you pasted and it's way more complex for my rusty state than i'd hoped, far too complicated and theoretical :(

thanks for it anyway, i'll have to take a further look into it
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #38 on: March 29, 2011, 03:34:38 am »
oook, back in the saddle!
i finally moved my ass and built the damn thing on a protoboard and the power part in a universal board with kelvin connections and all, but i have to say the results are somewhat dissapointing....
letme go by parts:
1) attached is the circuit i'm building now
2) i find it very hard to calibrate/make it do what i want, maybe it's my crap DMMs and i don't have a precision Amperimeter at hand and the 10A range is horrid...
3) even with kelvin connections, the measure i do on the sense resistor and the ones on the other end of the kelvin cables is way off, almost by +5mV @ 17mV in the rsense!.(which explains why i had to fiddle the loop gain)
I was guessing some sort of ground loop/return issue, so i "joined" the load ground to the protoboard and moved the setting potentiometer closer to the input and that kind of solved the large difference i was seeing...
this will be a problem when laying out the PCB, with return paths and stuff, and how to keep the kelvin connection as close as possible, but i have no idea how to do it efficiently because i'm not doing differential sense, so only 1 pin(the high side of the resense) goes to the 4277 that has split supply, the ground... returns via ground, so how do i take advantage of the 2nd Kelvin connection(the gnd one)?
should i connect it closest to the R10 ground?

4) after twiddling with those ground problems and tweaking the trimpot i'm seeing a close relation between the potentiometer voltage and the load current, looks stable also...

5) i'm not seeing problems with the mosfet threshold(the schematic shows a IRFP451 but i'm using a IRFP460), but my lab supply OCP started kicking at around 1.5A so i couldn't push it more, sense resistor is ice cold at that current..., i could even go to +-12V since Vgs never goes above 5~6V, i'm sure that at higher Load voltages i'm going to need higher gate, i only tested at 5V and 12V

6) seeing the adjustment problems, cable loses and the inherent tolerance of the sense resistor, i'm going to have to use a trimpot in the final circuit for the X10 path, right now it's measuring 26.9K for the 27K 5% one i put and 3.59K in the trimpot, so instead of x10 i'm seeing

i'm also attaching a photo of the little monster for your reference...

So here i am, wondering how to calibrate this, how to deal with the PCB routing and how to make best use of both kelvin connections(as right now the gnd one is kind of unused) and all that!
 

Offline scrat

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #39 on: March 29, 2011, 04:05:15 pm »
A few things come up to my mind...

Why did you use that opamp configuration? U1B is not used as a differential amplifier, nor an integrator, nor a real low-pass... U1A is not a real integrator... I think a common PI would have worked.
Why is there an RC parallel on the gate? It introduces a zero in the transfer function...
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #40 on: March 29, 2011, 04:32:56 pm »
Scrat:
U1b is a x10 amp for the feedback path, since i'm using a 0.1ohm sense resistor and need to maintain 0-5V(0-5A in turn) control voltage, thus needing to amplify the feedback path.
I don't follow you on U1A integrator... U1A is the same as Dave's original circuit, it's a negative-feedback amplifier i think....
the R4/C4 RC i took it from suggestions made before and other circuits i found(including the one i posted in an earlier post and johnmx circuit), to stabilize the loop, wheter or not the values i chose work.. it's another issue... i haven't found a way to effectively calculate which are the best values(johnmx posted an article but i found it very hard to follow)
 

Offline scrat

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #41 on: March 29, 2011, 05:16:29 pm »
I was thinking of using only differential proportional+integrator, but there is the problem related to the low voltage from the current sense, which is why you put the gain there. If your idea was to filter and amplify the feedback signal, you should have also put a capacitor to ground (same as C1) at the + pin of U1B.

The differential configuration (that can still be used for the 10x amplifier) could have solved a possible drop on the sense connection, although it is quite strange what you get (R4 is 10000x the sense resistance, and ground is specifically wired).

BTW, U1A is there as an integrator, since the feedback is through a capacitor, and if you calculate the gain from U1B output, it results 1/(R4*C5)*integral(input). It seems that the loop gain is stable, not taking into account the R2-C4-Cgate (gate equivalent capacitance is variable).
One machine can do the work of fifty ordinary men. No machine can do the work of one extraordinary man. - Elbert Hubbard
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #42 on: March 29, 2011, 07:11:07 pm »
Nice pointers scrat.

i hadn't considered decoupling the NI input of U1B, i'm going to add it as well, .01uF or 47nF should do it i think...

the R4 is on the schematic but not on the protoboard(in simulation i didn't notice any difference with or without R4, i should add it as well to the built circuit).
in my original circuit, C5 also wasn't there, i borrowed it from johnmx current sink circuit.

how would you do a differential config for the Rsense with x10/variable gain?, i've been looking at some circuits using opamps and i'd need to balance and match the 4 gain resistors and have to deal with the decreased impedance as well(unless using some specific differential amp like the INA106 that has fixed +10 gain, then again if i have to do gain adjust i'm screwed with that device).
Even without going differential, should the ground point being close/same as R10 do the trick?

Yeah i don't know why i'm getting such large difference with a straight soldered single-core wire, i'll redo the ground path to do single point ground today when i get home and add the missing R4 and the decoupling cap to ground.

BTW, forgot to add, tested the sink from 0 to 30V and the current didn't even blink, Vgate wasn't any different as well(i thought it would change, but i guess not).
i'm going to lower supply to +-12V as well, after all the only reason i'm using -rail is because the OPA4277 isn't RtR so U1B would never go to 0V.
I'm still seeing differences in set value to current measured even after setting the trimpot, but i can't be sure if it's because of the DMM inaccuracy in 10A range or those funky differences (side note: i need to get a good dmm, but no cash at hand atm).

Edit: After these issues it occurred to me:
1) Since the initial idea is to feed the output of U1B to the on-board ADC of a micro, thus having a direct current readout and i could calibrate via software, problem is that if the U1B output =! to the Vshuntx10(or to be more precise, straight Current across Rsense) i have no way of knowing what the "real" value is, as i'm using U1B as the sense amp as well which could lead me to over/undercompensate and have false data....
So... why don't i calibrate the sink in "best guess" scenario, without much precision issues and don't use U1B as sense?, then...

2) I use a low-side current sense like this one: http://focus.ti.com/docs/prod/folders/print/ina220.html which as a bonus has I2C output, Vbus sense, etc etc and thus use that value as true feedback for the current sink(in software doing something like: rise Sink current by ".2 amps"(or whatever value) -> small delay -> read INA -> use difference as feedback, readjust), trick would be to do a feedback calculation that won't end in oscillation....

i don't know why i hadn't come up with that idea before... probably because i thought i wouldn't encounter these issues with the feedback amp being so imprecise

« Last Edit: March 29, 2011, 07:56:45 pm by Eliminateur »
 

Offline Eliminateur

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Re: EEVblog #102 - DIY dummy load - higher current?
« Reply #43 on: March 30, 2011, 03:47:24 am »
Well, i'm back home and twiddled a bit..., oh and i brought my old fluke 8020A

here's what i did:
1) added the 1K R4 resistor, added the decoupling cap to NI of U1B(0.1u)
2) the low side ground return of the rsense(the black wire in the "joined ground") i put it directly to the PSU gnd terminal, thus ensuring the most direct path for current return, i also dejoined that ground position.
3) changed Vcc/Vss to +-12V
4) calibrated the trimpot using the fluke as amperimeter, kind of hard as the value moves slow and all the sudden jump some mA.
5) connected a croc from the DUT GND to the insertion point for GND on the protoboard(single point ground on the left), this cable is much longer than the Rsense ground return of course

now, if i leave the Kelvin GND connection open(meaning no GND connection between power supply and DUT GND), current draw peaks hard up to 1.5A and moves everywhere
but depending on what part of the protoboard ground plane i connect it to, the current sinked also changes!, and the more current i draw, the more pronounced the difference is(close to 100mA @1.5A).
I put the fluke between that wire and protoboard ground and i measured around 50uA at full scale, surely that ammount of current shouldn't induce a ground path return potential on the trimpot?

in the end i left it connected to the trimpot GND for cal purposes...

but i'm still baffled, i should do a quick PCB of this and replace the caps and resistors(if i ever get around to knowing how to use a PCB design soft as i have a some nonstandard sized components)
 


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