Yet another DIY Electronic Load - Page 1
Forum section: electronics / projects

author: pinoccio
author_rank: Contributor
timestamp: May 29, 2019, 03:44:41 pm
content: This is my planned concept of a simple DIY Electronic load ...any remarks or suggestions are highly welcome.I'm posting it here for the community to possibly check for issues I might have missed or even not thought ofand for inspiration to others.I will publish the final schematic and resources once I make at least a working proof of concept on a breadboard/perfboard (will surely take some time).Heavily inspired by:https://juangg-projects.blogspot.com/2019/02/arduino-based-electronic-load.htmlhttps://www.instructables.com/id/DIY-Adjustable-Constant-Load-Current-Power/Similar thread:https://www.eevblog.com/forum/projects/diy-electronic-load-project-thread-d/Main specs:- 24V max, <6A max, (withstand continuous 5A@12V; used heatsink limited)- generic Arduino Nano controlled- use of external ADC & DAC modules for improved resolution/precision and simplification- temperature limit control and active (pwm) fan cooling- simple UI - small 0.96" 128x64 OLED screen, one button and a rotary encoderThe code / UI part is not ready nor being prepared yet. But that's a relatively** easy part...Planned vision:- simple menu, operated by a rotary enc. + its button, and 1 extra button- display will show: mode and current+set: A, V, W, T, +Temp.- modes: CC, CP, Capacity, *Resistance?- settings for respective modes: Amps, (m)Watts, Capacity, Time?- protection settings for max limits to shut off at: A, V, W, T, +Temp.- maybe storing last settings in EEPROMI want to design a reliable circuit first...So here is a draft:https://ibb.co/zFKLnpvSorry for the usage of many connectors and headers instead of direct lines or specific parts and modules, but the circuit is designed directly so that a PCB can be made easily (spot the logo in the schematic).The main part is built around a logic level MOSFET driven by OP amps - a well-known conceptFirst block (U1.1) drives the fet according to sensed voltage from a shunt (array of 1R/1W --> more precise 0.1R@10W), which is meant to provide 1V@10A -read via a 10k resistor (R11)[necessary?], and a the set voltage on its other input.C1 is supposed to improve stability[maybe someone will comment if this is necessary].The desired current (input voltage) is controlled by the second block (U1.2) -a voltage follower with a approx. 1/9 divider (R12+R13) - that's supposed to go up to 1/9 * 5V = 0.555V --> so up to 5.5Amps max load. This is driven either from the MCU by the MCP4725 module 12bit DAC (#IO3), via a 1k resistor (R16) -no low pass filter needed because of using the DAC, or an optional external source (H2)[someone may comment on options or the protection diodes for details], selected by a switch (P4).The current reading is obviously fed into the MCU - but first it's 4x amplified by the third block (U1.3) for improved resolution. That amplified voltage (0.555 * 4 = 2.22V @max current of 5.5A) is fed to a ADS1115 module 16bit ADC (#IO1).A optional connector (H1) for external reading can be placed too.And to make a use of the fourth block (not really necessary), we buffer the connected voltage sensed by a 1/5.3 divider (R17+R18) - that defines the max limit of 26.5V on the input transformed to 5V on the divider. This is then fed (#IO2) also to the ADC module, and an optional external output also (H3).This all needs just some fine calibration in software naturally.So to make it a complete description of the core part, the tested source connects to P1. Those optional P2 (for verification/test multimeter) and P3 (for a fuse) can be omitted.Temperature (on the heatsink) is sensed by a simple NTC in a divider configuration (I may modify this later), and fed to the ADC module (#IO4). To simplify this part and avoid calibration + calculations, I'm planning to measure the voltage at room temperature and when dipped in water boiled to 75C, then just map() it in software to pwm %.A digital voltage sensor can be used instead, just mind the max temp. specifics.The heat-sink fan will be pwm driven too (about 35-75C -> 20-100%). I hope it can work in a basic Arduino pwm mode (@490Hz I belive) without too much noise.Rest of the wiring consists of a simple 5V power input with basic protection and filtering, and the Nano with a rotary encoder + 1 button. The external modules and a display are attached to connectors, all running on I2c (with pullups if needed: R20+R21).I have not calculated the required heat-sink, but the coarsely estimated 70W will require some decent dissipation, the fan should help to reduce its size thou.(EDIT: not sure if the single IRLZ44 is suitable for the job and won't get a bit overloadedat that planned maximum power, might be safer not running it continuously at more than 2A in this setup)Thanks

author: pinoccio
author_rank: Contributor
timestamp: May 29, 2019, 03:45:21 pm
content: --- reservation for followup / updates ---

author: Cliff Matthews
author_rank: Unknown
timestamp: May 29, 2019, 05:16:34 pm
content: Serious doubts here. IRLZ44 is likely not the device you want running in linear mode at 70w continuous. Edit: If you want 4 jelly bean devices you could modify something like this..

author: pinoccio
author_rank: Contributor
timestamp: May 29, 2019, 07:42:27 pm
content: Could you elaborate on that a bit more? We might learn something here... What is the problem running this MOSFET in its linear/ohmic region at 5A with a solid heatsink? It's not exactly obvious from the datasheet. Will it get so extremely hot?I'm not an expert in thisOne of the goals was a simple design and construction. So I'd like to stay with a single transistor.Are there any more suitable MOSFET options for this purpose?Btw. the 70W was an estimate for an appropriate heatsink to be safe, at 12V 5A the FET has 11.5V * 5A = 57.5W to dissipate. Provided my calculations are rightI'll have to have a better look at your circuit to understand how it works and spreads load across 4 ...but the basic idea is obviousThe modification would be to use several fets, also multiply the driving op amp (U1.1 in my case), and also the shunts. That complicates the current sensing too. I'm not sure it would be ok to sense at just one shunt and assume they all will be balanced without further circuitry...For now, I still prefer the single piece configuration.One more thing for possible improvement of my circuit might be some stabilization for the op amps, like RC network... I'll have to look up and figure something out maybe.

author: Cliff Matthews
author_rank: Unknown
timestamp: May 29, 2019, 09:48:28 pm
content: Member Gyro wrapped it up in nutshell "Most switching mosfets get unhappy if used in their linear region and can fail due to localised hot-spots on the die. It's normal to significantly de-rate them for linear operation, regardless of heatsinking"This thread gives the issue at little more light:https://www.eevblog.com/forum/projects/disapating-750w-of-mosfet-heat-for-under-$100/

author: OM222O
author_rank: Frequent Contributor
timestamp: May 29, 2019, 10:53:27 pm
content: IXYS makes linear fets, specifically for this application. have a look at their part selection.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 30, 2019, 02:02:04 am
content: Hi,I see that you have used an LM339. This is a quad comparator. A comparator is not the same as an op-amp. A comparator compares the two input signals and gives, essential a logic signal that indicates which one is more positive than the other. You need to use op-amps in this circuit.If you want to understand how to design the analog portion of the load:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/Regards,Jay_Diddy_B

author: pinoccio
author_rank: Contributor
timestamp: May 30, 2019, 11:49:53 am
content: Oh damn, you're right!I know I need op-amps, not comparators... Bought some LM324N-s too... Not sure how I managed to put those 339s in the schematic not noticing the obvious. Sorry, might be I just forgot to change the part name.Btw. that's is a fine thread you linked...I'll have to study it for a whileOk, next steps will be redesigning the analog portion - correct op-amp (LM324 should be fine as this circuit doesn't need rail-to-rail output), find a more suitable mosfet (logic level; maybe I'll just try the MTP3055VL from Daves video), and at least duplicate the power part fet+op-amp combo to spread load (I can sacrifice the unnecessary voltage buffer to get another driver op-amp and not need another package).Thanks

author: pinoccio
author_rank: Contributor
timestamp: May 30, 2019, 12:06:09 pm
content: Found some information on Linear MOSFETs:And a nice electronic load project:http://www.kerrywong.com/2013/10/24/building-a-constant-currentconstant-power-electronic-load/...interesting idea those paralleled mosfets(so I could drive 4 with 2 available op-amps in my circuit)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 30, 2019, 01:11:49 pm
content: The earliest use of a MOSFET for an active load that I've seen was in the 1983 Siliconix MOSPOWER design catalog. Most everything is an offshoot of that circuit. Note the use of the 100 kohm resistor to provide local feedback. Some use a small capacitor there. Without that local feedback, the circuit is prone to oscillation and unpredictability, especially if something faster than a 741 is used.

author: Yansi
author_rank: Super Contributor
timestamp: May 30, 2019, 01:14:47 pm
content: Lol, why you call it an off-shoot? This circuit can think up from zero almost any sensible engineer.To OP: At least add a series resistor to gate (~100ohm).

author: pinoccio
author_rank: Contributor
timestamp: May 30, 2019, 02:06:21 pm
content: Something like this?Is it ok to change gate resistor to 1k in this wiring when connecting two parallel MOSFETS (likehere) ? I don't know yet how to design proper RC for stabilization here :/Not sure about the C5 cap - connect to ground, or source?(EDIT: minor bug in circuit - sticky 339 ;P )

author: Yansi
author_rank: Super Contributor
timestamp: May 30, 2019, 02:54:44 pm
content: No. C5 shall go agains pin 4 not ground.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 30, 2019, 04:07:27 pm
content: Quote from: Yansi on May 30, 2019, 02:54:44 pmNO! what he did is correct. to prevent oscillations, you feed back the output to the inverting input rather than smoothing the oscillations from the feedback!I would also get rid of that 100k resistor between gate and source of the fet. it is not needed in newer designs.I created a thread and got some very good feedback on it. if you purely want the details about stability, jump to page 2, otherwise there are some good info about electronic loads in general. you can also use the same idea for range switching.https://www.eevblog.com/forum/projects/current-source-feedback-capacitor/on a side note: you can use BJTs / darlington pairs as a cheaper alternative to linear fets. they tend to be better in linear applications imho.

author: MarkF
author_rank: Super Contributor
timestamp: May 30, 2019, 04:48:20 pm
content: Modified circuitPlus some alternate resistor divider choices

author: MarkF
author_rank: Super Contributor
timestamp: May 30, 2019, 04:58:42 pm
content: Quote from: OM222O on May 30, 2019, 04:07:27 pmContradiciton!His circuit is wrong

author: OM222O
author_rank: Frequent Contributor
timestamp: May 30, 2019, 06:05:49 pm
content: Quote from: MarkF on May 30, 2019, 04:58:42 pmyes, looking at the schematic again, he has connected output to groundmy badalso the best divide by 10 is 3k and 27k, perfect ratio.

author: MarkF
author_rank: Super Contributor
timestamp: May 30, 2019, 07:53:24 pm
content: Quote from: OM222O on May 30, 2019, 06:05:49 pm2K and 18K resistor divider works too. I like the lower total resistance.Gut feeling that the higher current will be more immune to noise.I was trying to target a total resistance less then 10K but there isn't an exact match using standard values.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 30, 2019, 08:26:03 pm
content: that would be 300 ohm and 2k7 or 200 ohm and 1k8.that is an insane amount of power draw for a voltage divider! keep in mind that a dac / op amp is driving that. noise won't be an issue with 20k or 30k overall resistance.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 30, 2019, 08:37:40 pm
content: Hi,quite often in these discussion the paralleling of MOSFETs comes up.I presented a study of why one op-amp per MOSFET is (much) better. You can find the analysis here:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/75/Most, if not all, commercial electronic loads used one op-amp per MOSFET.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 30, 2019, 08:40:34 pm
content: U4.2 should be an op-amp (LM324) not a comparator (LM339).Jay_Diddy_B

author: MarkF
author_rank: Super Contributor
timestamp: May 30, 2019, 08:45:32 pm
content: Quote from: OM222O on May 30, 2019, 08:26:03 pmYes. That's why my table only goes to 620 and 5.6K. (i.e. 0.8mA)I considered that as the lowest overall resistance.

author: pinoccio
author_rank: Contributor
timestamp: May 30, 2019, 09:30:15 pm
content: Thanks for lots of great info and suggestions.Tried to implement some improvements...Newer version:I'd still like not to overcomplicate this projectI thought it's not good to go too low with the divider exactly because of the current. But maybe my values are now a bit too muchThe ratio is even higher with 4 fets because I only need to go up to about 2.5A per fet group. So the fets should be now safe running below 2A each.Quote from: OM222O on May 30, 2019, 04:07:27 pmRegarding the price, I just bought 5pcs MTP3055VL from ebay for US $2.80 shipped, so no big deal even if some die in testsAnyhow, would that darlington be just a drop in replacement with minor tuning? Worth the linear characteristics?Quote from: Jay_Diddy_B on May 30, 2019, 08:40:34 pmYes, I wrote about the typo at the and of mypost

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 30, 2019, 09:42:15 pm
content: Hi,You have now changed the names of the parts to LM324, but you still have the LM339 pin numbers:The LM324 pin numbers are:Regards,Jay_Diddy_B

author: MarkF
author_rank: Super Contributor
timestamp: May 30, 2019, 10:14:02 pm
content: No. Only one MOSFET per op-amp. I used the IRFP064 in my load.I would use the 2K and 18K resistor divider and adjust your sense resistors for the max current you want.You might want to average the sense resistor voltages to get a better load current measurement.The MOSFETs are not going to sink the exact same current.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 03:07:23 am
content: yes, you can drop in BJTs/ Darlington pairs as direct replacement, no tuning required!this is a terrible design for multiple reasons:H2 (Ext.drive) is basically pointless! there is a diode (D2) with non linear AND non constant characteristics with quite horrible temp co in series with it, which creates a variable voltage drop, throwing out all accuracy all together!Ditch the current sense amplifier weirdness all together! you can't mix signals like that. just use a proper current sense IC on VCC, instead of P2 (Ext. Current measur.)what is the MCU that you're using? the DACs on those can be quite bad. an external DAC + ADC is always preferred.DO NOT PARALLEL two fets from one op amp inputs, ESPECIALLY if you want to replace them with BJTs/ Darlington pairs. base threshold voltages would be different enough that one will be basically off, when the other one is carrying all the current!10 resistors in parallel give you quite good power dissipation on those resistors, but a higher wattage equivalent resistor is often cheaper and takes less space on the PCB. if you use a proper current source IC, you can just use a high power 1ohm resistor and not care about temp co, since current is actively monitored and corrected for in the software.why do you want to do division by 19? that makes no sense and makes all calculations very difficult? I can't see any advantage in that.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 03:22:02 am
content: I also had a look at the fets that you bought. they are awful for linear applications, even kept at 25C (that is ice cold btw!) they are only good for up to about 10 volts for your 2.5A amp goal!add some de-rating as they get hotter and you're down to nothing!this is where a proper linear mosfet like the IXTQ14N60P shows its advantages. just loook at the SOA for yourself. it starts at 10V and 1A! it's good for up to 200 volts at 1A DC! that's about 200 watts! to be safe just assume 100V but even that alone is 100 watts! PER FET that is! even at 2.5A you can push 70 volts! again, add some head room and say 50 volts, that's still miles better than what you have.They cost 3.16GBP a pop though, which is quite a bit more than what you paid for. I think they're worth it however. again, in the hobbyist price range, a few parallel darlington pairs would do better, however you can't just parallel them like you did with mosfets as I mentioned in my previous reply.

author: MarkF
author_rank: Super Contributor
timestamp: May 31, 2019, 04:24:14 am
content: You might want to consider this alternate control circuit.Note that if you select external control and nothing is connected to the control inputs, R13 will pull down the op-amp input and turn OFF the load.

author: pinoccio
author_rank: Contributor
timestamp: May 31, 2019, 09:37:09 am
content: Great remarks and ideas flowing hereSo to sum up recent recommendations:- use just one op-amp per transistor- reconsider transistor - proper linear MOSFET or BJT/Darlington- correct package pinout, doh- sense both branches / for average current----Quote from: Jay_Diddy_B on May 30, 2019, 09:42:15 pmYeah, I just changed part name and didn't pay attention to the pinout %) Will fix that, but for principle discussion, the circuit is clear...Quote from: MarkF on May 30, 2019, 10:14:02 pmokQuote from: MarkF on May 30, 2019, 10:14:02 pmWill think about that. So to have 0.5V driving voltage for max load of about 2.5A, means just reducing the resistor array to half (0.2R).Quote from: MarkF on May 30, 2019, 10:14:02 pmI also thought something like this might be necessary. Hopefully just averaging via those two resistors will work reliably enough. Btw. is 100R not too low to avoid mutual influence?Quote from: OM222O on May 31, 2019, 03:07:23 amniceQuote from: OM222O on May 31, 2019, 03:07:23 amI'm considering the modified control circuit from previous post.Quote from: OM222O on May 31, 2019, 03:07:23 amI'd like to read some more info on this.Why some constructions I've seen online, and based my concept on, use this with (presumably) no problems... It's supposed to improve resolution for ADC (we're reading 1V at max load here), but frankly might not be necessary. Or I could just turn it into a buffer. Or even play with the ADC module gain, but that means tweaking all inputs.Quote from: OM222O on May 31, 2019, 03:07:23 amseefirst post... Arduino Nano (so 328p) with ADS1115 ADC and MCP4725 DAC.Quote from: OM222O on May 31, 2019, 03:07:23 amJust a quick search on ebay - you get 100pcs of 1R/1W/1% metal film resistors for less than US $2.5, or 1pcs 0.1R/50W/?% for less than US $1.5 ...so there's no point considering priceJust other factors like maybe tolerance, construction/PCB design, personal preference, etc.Quote from: OM222O on May 31, 2019, 03:07:23 amTo set the max. driving voltage (thus load current) to 0.265V -> respective amps (sensing at 0.1R shunt), while having good resolution using the full 5V scale (or most of it, that op-amp won't go rail-to-rail).Quote from: OM222O on May 31, 2019, 03:22:02 amYep, I realize that... well, maybe not that it is actually so badBut I just picked the same one as in Daves video example (see first post).Seems like I will have to use something else in the end.Quote from: MarkF on May 31, 2019, 04:24:14 amI definitely will++Something to consider using BJTs:https://www.eevblog.com/forum/beginners/bjt-vs-fet-in-dummy-load/

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 02:07:55 pm
content: to get around the base current error, you would use a darlington pair. they are usually rated for a gain of 1000 at minimum which is about 1mA per 1A that they pass. that's 0.1% error! plenty good enough in my books, especially given that you are using LM324 with up to 3mV input offset voltage. 3mV divided by 0.1R (your shunt resistors) is 30mA errormuch worse than the base current error. that is also another reason to use a higher value resistor such as 1R. then 3mV offset error will be 3mA instead (3mV/1R = 3mA). if you desperately want low voltage operations (not quite sure why you would want to load something less than about 1V?) or very low currents (less than 10mA,at those low currents the op amp will supply all the current, not the source that you are testing, since the darlinton pair might not turn on) you could use a seperate channel using a mosfet (doesn't have to be linear since it's only used for low voltage / low current ranges) but if you want to do anything reasonable (at least 100mA load / more than 1V supply) that won't be needed.regarding the current sense circuit you used:a proper mixer must be inverting:all sorts of weird things happen when you do a non inverting mixer. In theory it works fine, but I'm not comfortable doing it, just in case murphy gets me with that specific setup. all in all, a current sense amplifier (if you want to thinker the least, something fully digital like the INA219, but there are tons of different versions with / without integrated shunt and special features, just search T.I. current sense amplifiers section for example) will give you a better reading, since the 0.1R shunts that you use are going to heat up and decrease in value, therefore your current value can be off by that amount. no sense in using a 16 bit ADC when your numbers are all over the place by design. a current sense amplifier will also give you a total current value, rather than some odd averaging method. it's just better in every way than your current method (probably cheaper too!).A side note on the ADC: If you haven't bough the ADS1115, order an ADS1219. 1$ price difference but much much better in a lot of ways. you should also consider some filtering for the ADC inputs, I don't see any on your design.Quote from: pinoccio on May 31, 2019, 09:37:09 amplease tell me you're not powering the DAC from the same 5V supply as the arduino1) if you use the usb supply, it's actually closer to 4.75v rather than 5V2) if you use an external supply (and LDO or similar) they can have quite bad accuracy too. also any switching noise anywhere on the 5V line will change your voltage reference which is bad!that is why you must use DACs with proper voltage reference ICs.There still isn't a point in doing weird stuff like division by 19 to get exactly the full range. you can do division by 20 for example and lose just a small amount of the maximum range, but making the math a lot easier.

author: pinoccio
author_rank: Contributor
timestamp: May 31, 2019, 03:51:15 pm
content: Some improvements possibly...Never worked with op-amps really, so the summing amp is just a first blind shot for now...

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 04:05:36 pm
content: much better! you also won't have an issue with the inversion since the ADS1XXX family are actually bipolar (ADS1115 is effectively 15 bits, 16th bit is the sign bit. same for ADS1219: it's a 23 bit ADC + 1 sign bit). you just need to be careful at the software stage about positive and negative being inverted.

author: MarkF
author_rank: Super Contributor
timestamp: May 31, 2019, 04:50:15 pm
content: The summing op-amp configuration you now have for the current sense is INVERTING.This WILL require a +/- power supply for the op-amp. A single positive power supply will not work.You might also consider the IRFP064 or IRFP250 MOSFETs. Much bigger package (TO-247) and power handling.Edit- And since you raised your max sense voltage to 2.5V, you will need a higher op-amp supply voltage than +5V.Probably +/-12V would be a standard.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 06:34:01 pm
content: Can you please make a copy of your schematic and make it public? I think there is still a lot left to be desired in terms of component selection.Just let me know:Are you willing to do SMD soldering?Are you using the atmega 328p chip or an off the shelf aruino nano?

author: JeanLeMotan
author_rank: Contributor
timestamp: May 31, 2019, 07:50:01 pm
content: I ended up using the IXTQ42N25P mosfet in my load. It has a SOA for DC and I tested it with 150W so far (30V 5A) without any issue.https://www.distrelec.biz/en/mosfet-250-42-300-to-3p-ixys-ixtq42n25p/p/17111086?queryFromSuggest=trueMy first tests were with a IRFZ44N and the much beefier (according to the datasheet) IRFP3006 but both kept popping like balloons above 12-15V and 2-3A, way below their "spec" according to the datasheet. The issue was that I was using them in the linear region which is not covered by the datasheet. So not good for a load.Do you intend to go down to 0A? Due to the opamp offset voltage of the LM324 (3mV max, 2mV typical) and your shunt value (0.5 Ohms) you'll be able to get down to a min of 6mA.Your choice of shunt resistors will probably mean that they will share a lot of the load with the MOSFET. At 5A and 10V they will dissipate 25W, half of the total load power - so you have to consider 50-100W resistors and keep them really cool.I really recommend going down to 0.1 Ohms for the shunt - but in this case the LM324 might be too noisy and the offset voltage will be amplified by 10 (so for a 3mV offset voltage, you get a min current of 30mA through the load).

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 08:25:02 pm
content: That 5A will be shared between 4 resistors! That's just over 1amp per resistor. So less than 2 watts power dissipation per resistor. No need to go with 100 watt resistors! Even the most basic smd resistors can handle that without an issue. I've usually use the 1ohm vishay resistors (2515 package, sorry I don't remember the exact part number, but it's something lile WCS or similar). That will also help the fets to some extent with higher loads. Some air flow and large ground / power planes keeps them cool enough.

author: pinoccio
author_rank: Contributor
timestamp: May 31, 2019, 09:17:30 pm
content: Originally I meant to keep this project simpleFound some of the recommended IXYS mosfets on ebay for good money, like IXTH75N15 for 2 bucks, so probably that will be the next swap in my circuit.Powering will be another question, might have to inevitably go 12V, but to hold on to simplicity, I hope I can still use single supply & virtual ground.Btw. my plan was using a small SMPS module. Yeah, module everything...same for the ADC and DAC, and the Nano of course. That's why I designed it with those connector headers. Plugin and go, simple, cheap, quick construction.Anyhow, reading all this, maybe the op amp is not the best choice too...Quote from: OM222O on May 31, 2019, 06:34:01 pmYes, I can make a public easyeda project copy soon.I'm ok with SMDs as long as it's not very small, like 0805 is still manageable by hand.Quote from: JeanLeMotan on May 31, 2019, 07:50:01 pmI went from 0.1 to 0.5 shunt because of previously posted suggestions... Still not sure it is the best for this project, but hey, we have a nicely evolving conceptActually I think I messed up this part in the last circuit version and the divider should be 1/4 for 5A limit.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 10:04:51 pm
content: WSC25151R000FEA is the resistors that I was talking about, unfortunately it's only a 1W resistor, so its not suitable for this application, however you can use the larger package which is the WSC45271R000FEB. 2watts and plenty good enough for this application.also a very versatile op amp is the MAX4238.please send me your schematic and I will apply the changes.Also for current sensing, you can use the INA286 with a 10m\$\Omega\$ current sense resistor which will be pretty cheap at the end.you can also buy high side current sensing modules if you want to go that route.

author: Yansi
author_rank: Super Contributor
timestamp: May 31, 2019, 10:33:47 pm
content: Why would you add this overpriced sense-amp, if you can already measure the current from the bank of shunts in the source electrode of the mosfet?Also, to the OP: 5V is very insufficient as a supply voltage for the OPamps that control the mosfets.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 31, 2019, 10:45:24 pm
content: Quote from: Yansi on May 31, 2019, 10:33:47 pmover priced? it's only about 2$ ! and it's way more accurate than the current method he is using. it also eliminates the requirement of a dual supply which adds more than 2$ cost for sure. same can be done with a low offset amplifier like the MAX4238 or OPA333 and an external 4x100k laser trimmed resistor (you can't use generic resistors for an accurate difference amplifier). which will be more complex AND more expensive. there is a reason these parts exist. unless you don't care about accuracy and you're happy about 12mV accuracy (3mV offset x gain which is 4). in which case upgrading the op amps or the resistors makes little to no senseINA220 is a suitable / cheaper but less accurate alternative.

author: JeanLeMotan
author_rank: Contributor
timestamp: May 31, 2019, 10:50:20 pm
content: Quote from: OM222O on May 31, 2019, 08:25:02 pmMy math was off, you're right. At 5A and 0.5Ω that's 12.5W total going through the shunt resistors. So 3.125W per resistor. That's still a lot for this application.At 5A in my load the shunt resistors go up to 55-65°C in a 20°C ambient. I had to drop the max current from 5A to 4A for this reason.With a 50ppm temp coefficient, that's a change of ~0.28% over the whole range which doesn't sound that bad - but the bigger issue is that it heats up other components around including the copper traces from/to the shunt and those have a much much higher temp co. (0.4% per °C)That's with 0.1Ω total, 4x series parallel 5W 4527 resistors:https://lcsc.com/product-detail/Low-Resistors-Current-Sense-Resistors-Surface-Mount_Ever-Ohms-Tech-MA452750FR100FZ_C252911.htmlFor 0.5Ω you would get 5 times the power dissipated and 5 times the heat so the temp will go much much higher.Btw - for the WSC45271R000FEB the datasheet shows a temperature rise of 120°C at 2W dissipated!On top of that there's power derating: at 120°C you can only dissipate 1W.I don't see how you can do 5A with 0.5Ω and such small resistors unless I'm missing smth obvious. Someone else from this forum recommended having a shunt resistor rated at least 5-10x the power dissipated which for this load would mean a 60-100W shunt.I'll definitely go for a smaller shunt in my next rev and I'll get a proper manganin 0.01Ω one.

author: Yansi
author_rank: Super Contributor
timestamp: May 31, 2019, 10:54:30 pm
content: $2 is quite expensive, for the rest of the circuit compared and actually, quite unnecessary here.For what would you need a dual supply? The current sense resistors are already present! And no, you won't improve accuracy with the INA.If the thing would be properly designed with a current sense voltage amplifier within the current regulator loop, then the extra divider-buffer whatever opamp would not be even needed and current already available for direct measurement with ADC.Making the shunt very small is also not great, as it degrades accuraccy (offset and tempco errors).Instead of slapping there completely unnecessary part with another heating element to sense current, use a suitable precision opamp, such as MCP6072 or any similar cheap one to do the job. And please be that kind an do not lie about the INAxxx prices.

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 12:13:07 am
content: jean, your math is still offhe is passing a maximum of 2.5A per branch (i.e mosfet). each mosfet has 2 resistors in parallel, so 1.25A per resistor. using ohms law we have P= I2*R = 1.252*1 = 1.5625 wattsYansi, then please explain why literally any commercial product uses a huge low value resistor with a differential amplifier (much more expensive than the INA ones I mentioned) in order to achieve this? they often have 6 or more branches, each with a separate op amp driving them and a set of shunt resistors. for example this one (BK Precision 8500):and here are the multiple mosfets each with their own driving stage:you can clearly see the small SOIC (or maybe TSOP) package next to each fetyou can also see quite beefy power resistors soldered vertically (green package)the reason this is done is is the "shunt" resistors are optimized for power and help the fets with power dissipation as take some load off the fet. however they get warm and they usually have about 50 PPM/C temp co which is quite bad, therefore the main low value shunt gives you the correct power draw with great accuracy and no need for summing amplifiers etc.The reason a dual supply would be needed is summing amplifiers must be inverting like I explained before, if you don't want weird things to happen which implies dual supply requirement. it is also quite bad in terms of accuracy when used with cheap op amps. if you want to spend more on op amp and a precision resistor network, again it would be pointless. just use a proper chip like the INA series at that point.Since you brought up me lying about the INA series prices as well, here is the prices on mouserI'm not paid by T.I to promote their productsit would be best to not spread misinformation if you have no idea what you are talking about. giving false advice is really bad, so be sure to do your research before recommending something

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 01, 2019, 02:26:22 am
content: Hi group,Let me sharemyidea of what the BK8500 load schematic would look like, based on studying the photographs:There is one op-amp per MOSFET. This is essential to make sure each MOSFET shares the current. Each MOSFET has its own source resistor.There is a low value shunt and a low drift differential amplifier that is used to measure the total current.HP 6060 STYLEThis is a partial schematic from an HP 6060 load. It uses 8 MOSFETs in 60V 60A 300W load.There are two op-amps per MOSFET. The OP270 was chosen for low noise and low offset, 75uV. The MC34072 was chosen because of its ability to drive capacitive loads. Each MOSFET has its own current sense resistor.A summing amplifier U5 is used to obtain the total load current.Dual RailsBoth of these commercial loads used dual supply rails.Regards,Jay_Diddy_B

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 02:39:38 am
content: thanks john! I wasn't sure if they are doing high side or low side current sensing, but I just assumed high side. both are equally valid methods but the idea still is that the low value shunt is used for actual current monitoring, not the summing the the voltage across source resistors and assuming they stay at 1\$\Omega\$ or whatever their value is

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 01, 2019, 03:05:23 am
content: HiHere is a picture from the BK8601 teardown:The source resistors are 0.05This is a 60A load with 8 MOSFETs. 7.5A per MOSFETMaximum dissipation is 7.5 x 7.5 x 0.05 =2.8WThe op-amps are NE5532. These are low noise. The offset specification isn't supper good 0.5mV typ 4mV max.MOSFETs are IRFP250NRegards,Jay_Diddy_B

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 04:17:23 am
content: the IRFP250N is surprisingly common with commercial loads, despite lacking any DC SOA specifications.OP plans to go upto 2.5A per fet, so the source resistors can be a bit larger.In terms of op amps chopper,rail to rail op amps seem best for these applications, but understandably they are more expensive.

author: MarkF
author_rank: Super Contributor
timestamp: June 01, 2019, 04:38:56 am
content: Quote from: OM222O on June 01, 2019, 04:17:23 amI used a TLC272ACP op-amp in my DC Loadhttps://www.eevblog.com/forum/beginners/a-load-off-my-mnd/msg2151265/#msg2151265

author: pinoccio
author_rank: Contributor
timestamp: June 01, 2019, 07:20:25 am
content: public project:https://easyeda.com/pinoccio/Electronic-Load-DIY-open

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 08:20:56 am
content: Quote from: OM222O on June 01, 2019, 04:17:23 amOM222O, could you please kindly stop pushing your bullshit with those high side current sense amplifiers? They don't even have enough bandwidth to begin with. They are anything but useful for designing a good stable load with good transient characteristic.IRFP250 from ST for example specifies DC SOA. But please be aware, that even specifying it does not mean it is fully characterized and no secondary breakdown will happen at higher voltage.Also, specifying DC SOA for 100W does not mean one can get 100W from it, proper thermal derating has to be accounted for. The typical practical limit for a TO247 package with isolation pad is about 25W (10W for TO220), and about 40-50W when screwed directly on large enough heatsink with good thermal compound.

author: pinoccio
author_rank: Contributor
timestamp: June 01, 2019, 09:19:32 am
content: Btw. for me ebay is the cheapest source of components, so that is the price reference. Checked today- INA286 starts at GBP 5.19 / US $6.54 with free shipping. Mouser (or other big suppliers) might be at 2,54 € / $2.92, but adding 20€ / $30.00 shipping is unacceptable.And we are slightly drifting away from a simple circuit too...I'd prefer staying with the current concept for now.Regarding precision/resolution, for ahobbyist project, the possibility to step current by 1mA is totally sufficient IMHO. Don't even need to go so close to 0, 30 is fine I guess, but about 10mA would be nice if it can be achieved reasonably easily. Don't know if that is possible with op amps.Taking another look, can't we just omit that current sensing op amp completely and feed directly the ADC module (2 inputs are available) and sum in SW?That would also get us back to single supply, right?.As I already mentioned, I can go with those cheaper IXYS mosfets. If there are no objections, like they're not logic level, that could become the final version (regarding power transistors).

author: JeanLeMotan
author_rank: Contributor
timestamp: June 01, 2019, 10:18:14 am
content: Quote from: OM222O on June 01, 2019, 12:13:07 amForgot there are 2 branches.In any case 1.56W per resistor is still a lot for the one you recommended and its temp will rise by about 100°C even with the 4527 package.As I said - I'm using a 20W 0.1Ω shunt split across 4x 4527 SMD resistors and for 5A I had big issues dissipating that heat. No way I could do 5A continuous, so I dropped the max to 4A and it's still getting hot.Here you're proposing a 0.25Ω shunt (2 resistors paralleled per branch, 2 branches paralleled) for the same 5A load - so 2.5 times more power dissipated than in my load. That will get incredibly hot!Once you include derating due to temperature & continuous operation SMDs are just not worth it for shunts at these currents.My advice is to go with beefier TH. Or add way more SMD series/parallel resistors to spread the heat and get a bigger surface area. Or add little cute heatsinks for each SMD resistor. Here's the thread for my load:https://www.eevblog.com/forum/projects/jlm-electronic-load-0-30v-0-4a-1ma1ma-resolution/

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 10:50:27 am
content: Do not use SMD resistors for power dissipation, unless you really must. There is no way to dissipate much past one or two watts to a PCB, always use through hole stuff for power. Will also save you same board space and heat spreading to surrounding components.5A over 0.1ohm, thats 2.5W which should be sort-of fine even for 3W wirewound THT resistor (when a bit of airflow is present, better to get a 4W one though). Beware, these suckers can get nasty hot, I think their rated power is getting the surface temperature sometimes well past solder melting point.And also, I do not recommend using those plastic resin encapsulated resistors. They can't dissipate heat well. Nothing beats a proper wirewound ceramic one (unless struggling for very low inductance).

author: pinoccio
author_rank: Contributor
timestamp: June 01, 2019, 11:36:07 am
content: You can calm down guys...I never planned to use smd resistors for the shunt in the first place.Most probably I will go with an overkill of 10 or even 25W wirewound in aluminium package each! And they will end up screwed to the heatsink too. At a price below $1/pc (the bigger ones are actually cheaper), I don't need to bother with derating and temperature issues here

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 12:36:05 pm
content: Wait a minute... what is your current goal? 5 or 10 amps? I think it is quite unnecessary (and might be even contraproductive) to use such overrate shunt resistor.It should be possible to design a current sense circuitry requiring sense voltages such low, that the overall heat dissipated won't be over 3W - which is manageable using a single THT resistor of a proper sizing.If you want to go really precision, I'd recommend using 4-wire shunt resistors, such as from Isabellenhütte (PBV-0R01). These can be also heatsinked.It depends on your goals - if precision is the goal or simplicity and cheapness perhaps? If you do not care about transient response behavior and loop bandwidth, then you may even use those overpriced high-side sense amplifiers - but I do not see a single reason for them. Noone is using them in DC loads (for good reasons).

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 03:10:11 pm
content: Quote from: Yansi on June 01, 2019, 12:36:05 pmI love how you call them overpricedyou clearly haven't used any chips over 2$ in any projects. some voltage refrences or even resistors can easily cost more than 10$ depending on the application. most commercial products use much more expensive differential amplifiers like I mentioned above, costing more than 5$for example the schematic provided by john for the BK 8500. it uses the LT1013 which is about 6.6GBP from mouserthe *over priced* INA series are actually cheaper alternatives with less accuracy, but still good enough for this applicationhere is a much better version of the design:https://easyeda.com/theepicn008/electronic-load-improved-I switched over to low side sensing (100mV/A). that is quite good but I would personally add some extra parts to improve the accuracy in the mA range. it would be a bit more complicated and you mentioned keeping things simple so I didn't do it. it requires a fet and another resistor in the feedback path of U5 to adjust the gain from x10 to x100.-voltage sense circuit got a few improvements-accuracy is improved by at least a factor of 10 due to much better op amps (2uV maximum offset!)-some general clean up of the schematicSome notes:I changed the name of the nets: IO0 to IO3 are connected to the ADC (actually there is no IO3, I replaced it with the setpoint). DAC output is IO4.External input / input switch had the 1k resistor in the wrong place.I switched to 1206 for the most part. change them back to axial if you want to use those.You don't need sil pads between the fets and the heat sink, since their drains are electrically connected anyways. if you are connecting other things such as voltage regulators, etc. be sure to use sil pads for those.The IXYS fets would be fine, you don't have to use logic level fets. the IXTH75N15 can pass about 10A with a Vgs of 5V at around 2V Vds. it seems like a reasonable part if you can keep the case cold enough. they only specify it for case temp of 25c so I'm not sure about derating at higher temps.with the parts you have, I'm not sure you could have gotten down to 1mA accuracy (not even 1mA resolution). It's because of your dac. with a 5V input, the resolution is about 1.2mV, which means a resolution of 1.2mA for your load 2*(0.0012mv /4)/0.5\$\Omega\$ . that is also assuming the DAC is perfect, which it's not.it has +-2 LSB INL and some DNL (I forgot sorry) which means you can be off by 4 codes, which is 4x1.2mA = almost 5mA. I think that's still reasonable for you, but let me know if you want to improve that.this time I will be called out by Yansi for *My bullshit* and for *Use of overpriced op amps*... again, I'm not paid by maxim to promote their chips. you can easily buy the max chips from RS (don't use ebay for parts ... they are often clones or salvaged parts, hence the cheap prices). here is an screenshot in case I'm lying about the prices againalso for resistors, There are some smallish 15 watt resistors that would be perfect for this application:500m\$\Omega\$ 15 watt:https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0159714/1\$\Omega\$ 15 watt:https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/7547676/I would personally go with the 500m\$\Omega\$ since it ends up being cheaper.This is as simple as I could make the circuit without adding any special features. let me know if its still too complex or if you want additional features such as multiple current ranges etc.

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 06:30:20 pm
content: Quote from: OM222O on June 01, 2019, 03:10:11 pmJeesus crist. The OP can't tell a difference between LM339 and 324 (nothing wrong with that, he'll learn) and you still trying to push overpriced crap. Yes, for the purpose here, it is overpriced.Get the OP to do his first steps with cheap jellybean parts that get the work done quicker and cheaper, instead of insisting on buying completely unnecessary components.

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 07:19:40 pm
content: OP wants something useful at the end, besides if you look at the schematic there is no INA part. I used max4238 for every part.If you suggest building the circuit with cheap trash, the end result will be cheap trash. besides "Good practices" are a major part of learning. nobody uses summing amplifiers for current sensing in the industryquite honestly I had no idea how to stabilize the current loop until a few weeks ago. every time I asked people just suggested redesign or random value components such as resistors or capacitors between x or y which wasn't helpful at all. Jay_Diddy_B posted some simulations and analysis of how the ideal situation would be and how to set the correct values to get proper phase margin and gain margin, which elements attribute more to the instability, etc. that was extremely helpful and I learned a lot. being a beginner and not knowing something isn't bad. that's why we ask questions here. giving vague / wrong advice and forcing your opinion on others on the other hand, is very bad. unless you have some constructive feedback, don't post aggressive comments such as : " YOU STILL TRYING TO PUSH OVERPRICED CRAP". so far all you have done is to tell everyone they are wrong without giving actual solutions. he provided a public schematic, so if you have any ideas, copy that and make the modifications.you also contradict yourself by stating:Quote from: Yansi on June 01, 2019, 08:20:56 amif the goal is to make something with jellybean parts, who cares about bandwidth anyways? it's not a high frequency application and in most cases transients are a non issue for a load. 100 to 200Hz BW would be plenty good enough! do you want to over complicate current sensing and increase bandwidth? be my guest!I'm not sure what your suggestions / solutions are for this issue?Unfortunately this aggressive comments and strong opinions seem to be getting more common here.Learning how to do something properly is a lot more important than "just using jellybean parts that get the work done quicker and cheaper". maybe for a one off project that needs to be done ASAP and needs to be in use, getting something dirt cheap to work is a priority, but the OP seems to be more interested in learning how to design a proper load as simple as possible using existing modules.

author: markce
author_rank: Regular Contributor
timestamp: June 01, 2019, 09:43:27 pm
content: Now wait. This is not going to help Pinoccio. You're both right in some way, but Pinnoccio is still at theQuotestage.My approach would be to use basic proven components, from a reliable source and a simple schematic to start with. Can't be that difficult?

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 10:04:01 pm
content: Quote from: OM222O on June 01, 2019, 07:19:40 pmYou are full of nonsense. There is enough jellybean parts which one can make FAST amplifiers for cheap. Based on what do you think that only expensive stuff is the fast and good one?100Hz bandwidth load is not enough. For discharging batteries, maybe.Quote from: markce on June 01, 2019, 09:43:27 pmI second that. But mind you I am not the one pushing specialized components, that are of not much help to a beginner, other than cleaning his/her wallet.

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 10:23:48 pm
content: Quote from: markce on June 01, 2019, 09:43:27 pmI actually had a look at ebay. MAX4238 goes for a minimum of 5$ therejust wow! the same part on RS Slovakia is I think 1.2 euros. even cheaper on mouser. understandably so, mouser charges shipping for low value orders but I think above 30GBP components, the shipping is free. I recommend he checks all the suppliers before assuming ebay is always the cheapest source.Quote from: Yansi on June 01, 2019, 10:04:01 pm"Based on what do you think that only expensive stuff is the fast and good one?"based on the fact that you haven't came up with any ideas that are cheaper than my suggestions and making me seem like a shill for different companies. all of my recommendations are based on ACTUALLY USING THE PARTS in my own projects. my second suggestions was a single op amp with 3 resistors, costing about 2 euros overall. can you be please so kind as to introduce a cheaper solution?also the use case for a hobbyist is exactly the sort of things such as discharging batteries to get their capacity or loading down a power supply to check their ripple. this is not a professional equipment that requires high bandwidth. again, I recommend you post a proper solution / some calculations / simulations that help the OP rather than calling people names. Have a nice day

author: MarkF
author_rank: Super Contributor
timestamp: June 01, 2019, 10:47:43 pm
content: HOW QUICKLY THIS BASIC CIRCUIT HAS GONE OFF THE RAILS.PUT IT BACK IN YOUR PANTS BOYS AND GO TO YOUR CORNER.ONE MANS JUNK IS ANOTHER MANS TREASURE.Quote from: pinoccio on June 01, 2019, 09:19:32 amIf you have the inputs, I think monitoring each MOSFET individually is an idea worth investigating.However, I would buffer the sense resistors with a voltage follower to your Arduino.Go back to your max sense voltage of 500mV. You would have a hard time applying your load to a 3.3V power supply with a 2.5V sense voltage.Stick with the 2K and 18K voltage divider for the control input and 0R100 to 0R200 for the sense resistors.I think a 150 mOhm 5W sense would be best. It would give you a max 6A load current capability. Your target(6A @ 30V)(6A @ 24V)?I would change to the IXYS MOSFETs you mentioned. I would look for the larger TO-247 package just for the extra heat dissipation.I would not be putting my project at risk of getting counterfeit parts by buying the power MOSFETs from eBay.Pay the extra to Mouser or Digikey and get a genuine part.The +5V supply for all your op-amps seems to be a little low to me. I would use +12V.Edit- Totally missed that you will need some gain before sending the sense voltage to the ADC.I arbitrarily choose a gain of 5 but you might want to go as high as 10.Here is my idea:

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 01, 2019, 10:49:53 pm
content: Hi,I don't want to join the discussion on the price the parts. But, I will join the conversation if I see that some of the advice given is wrong.I will review the circuit posted here:Link:https://easyeda.com/theepicn008/electronic-load-improvedLet us look at one of MOSFET circuits:We have an IXYS IXTH75N15 MOSFET been driven by a 5V op-amp. We also have 0.5source resistor.If we look at the MOSFET datasheet:We can see that for 5A we need about 4.5V of gate source voltage.If we have a drain current of 5A, and therefore a source current of 5A. With a 0.5source resistor. The source will 2.5V positive with respect to ground. If we add the 4.5V Vgs, the output of the op-amp needs to be 7V.How can we get to 7V with a 5V supply?Regards,Jay_Diddy_B

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 11:08:37 pm
content: I have already warned about insufficient power supply voltage at #38, but it seem it got lost in OM222O's push for exclusive parts.One way to get away with just 5V supply would be to use a bipolar transistor as the power part. There are plentiful choices out there for rather cheap, compared to grumpy mosfets, that do not have the DC SOA fully characterized and will blow unexpectedly.For example: TIP3055, BD249, or if one wants some speed action: 2SC5200, MJL3281 or for a very robust ones: MJL21193, etc...(PS: avoid 2N3055 at all costs!)

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 11:14:28 pm
content: Just an idea for the BJT solution

author: 2N3055
author_rank: Super Contributor
timestamp: June 01, 2019, 11:23:58 pm
content: Quote from: Yansi on June 01, 2019, 11:08:37 pmWhat's wrong with me?LOL!Seriously, for this purpose good bipolars are dedicated audio types. They are expensive though..

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 01, 2019, 11:26:41 pm
content: Hi,As well as the load I documented in this thread:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/I have also build a few more. Here are a couple of examples:This one uses the LT1635, which is a version of Bob Widlar's famous LM10. It is a combination op-amp and 200mV reference.This one is a quad load. It uses a jumper block for setting the current range of each channel. It is intended for testing multi-rail low voltage power supplies. It use the LT1014. The LT1014 can be replaced by a LM324, but the LM324 will have larger offset voltages.The schematic shows the common section and one of the four channels.I have not built one with microprocessor control.Regards,Jay_Diddy_B

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 11:28:46 pm
content: Quote from: 2N3055 on June 01, 2019, 11:23:58 pmLol, nothing against you, but against "manufacturers" providing today's substandard 2N3055 transistors, that simply aren't up to spec.And yes, those MJLs I have suggested are audio amps.

author: 2N3055
author_rank: Super Contributor
timestamp: June 01, 2019, 11:44:01 pm
content: Quote from: Yansi on June 01, 2019, 11:28:46 pmNot to mention old ones were diffusion types with massive crystals with massive SOA. Slow but robust. New ones are epitaxial, faster but not robust. They are different enough that sometimes are not good replacement for the old ones when repairing some circuits..

author: OM222O
author_rank: Frequent Contributor
timestamp: June 01, 2019, 11:50:01 pm
content: john is right! I mentioned the fet being able to achieve 10A at 5V Vgs, forgetting that there will be about 1.25 source voltageAlthough the current through each fet will be 2.5A, not 5, it's still pushing the limits of the gate voltage. the data sheet isn't really fine enough to tell whether 2.5A is possible at 3.75V Vgs. I also tried downloading the spice model for the IXTH75N15, but there is only a pspice model which I don't have installed. However I attached it, if somebody has it installed, they can run the simulation and report the results.I also had a look at about 10 to 15 IXYS fets, they have very similar admittance graphs. if this doesn't work, you have to give up on linear fets. hold off on ordering them until someone can confirm 2.5A at 3.75Vgs is possible, or you can also just buy one and test it for yourself if that's within your budget.Edit: OP initially mentioned a 5A rating, but has been vague about if he meant 5A per fet, or 5A total. 5A per fet will not work with linear fets at all with a 5v supply.The texts were copied from the original schematic and they indicate a 10A goal, however the resistor divider used for the set points was division by 4 which indicates a 5A total ratingI made some modifications to the text to correct that.Edit#2: the OPA188 or OPA2188 seem reasonable replacements for the MAX4238 in the load control section. they can go as high as +-18V, but it's ok to power them from 0 and 12V (assuming you have a 12V supply somewhere). They are also available on RS, only slightly more expensive.

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 12:05:50 am
content: Quote from: 2N3055 on June 01, 2019, 11:44:01 pmYou sure about the diffusion type? I have heaps of locally made power transistors similar to 2N3055 that has also those huge dies (4x4 or even 5x5mm), but I think the catalogue I have clearly states the are epitaxial.

author: MarkF
author_rank: Super Contributor
timestamp: June 02, 2019, 01:56:27 am
content: Quote from: Yansi on June 02, 2019, 12:05:50 amWOW.WE CAN'T EVEN STAY ON TOPIC!

author: 2N3055
author_rank: Super Contributor
timestamp: June 02, 2019, 09:12:03 am
content: Quote from: MarkF on June 02, 2019, 01:56:27 amSorry boss, won't happen again..Thing is that I guess we are saying that if you want to make a bipolar transistor load good choice are audio transistors. They have good SOA and decent speed. Powerful transistors like 2N3055 are robust enough, but not fast enough to make a good load.@ yansi I have few very old diffusion type 2N3055H made in YU.. They have better SOA. But any 2N3055 made in last 20-30 years will be EPI.

author: MarkF
author_rank: Super Contributor
timestamp: June 02, 2019, 05:31:16 pm
content: Quote from: 2N3055 on June 02, 2019, 09:12:03 amNot trying to be boss.Just frustrated by all this side noise. And I believe the OP is too. Scan back through the topic for the last time he posted anything.I updated my last circuit to add gain to the current sense op-amps to the Arduino.Why don't we talk about something more relevant, like protection for the MOSFET drive op-amps and the Arduino in case the MOSFETs let out some magic smoke?My Load doesn't have a MCU but I did use 6.2K resistors to MOSFET gate and feedback to try protect the op-amp.It hasn't been a problem yet. I thought that would be too much on the gate. But even using an external square wave for switching the load looks okay.

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 05:41:19 pm
content: 6.2k ... that's just ridiculous. Look at the time constant of that Rg and Cg. But, as long as you are just designing load with 100Hz bandwidth to discharge batteries, then good enough.

author: OM222O
author_rank: Frequent Contributor
timestamp: June 02, 2019, 06:50:09 pm
content: Quote from: Yansi on June 02, 2019, 05:41:19 pmcan you please be so kind as to let us know what else you use your loads for? clearly not discharging batteries and testing power supplies!people give advice, yet you tell everyone they are wrong.either give constructive comments or stop whining about everythinghe said he tested it and it worked fine, I tend to agree. even 10k resistors are usually fine for this application!

author: MarkF
author_rank: Super Contributor
timestamp: June 02, 2019, 06:56:23 pm
content: MOSFET = IRFP064F = 2 * 3.14159 * 6.2K * 10000pF = 2567HzJust how fast does it need to be?

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 08:39:19 pm
content: Quote from: OM222O on June 02, 2019, 06:50:09 pmI am just wondering, how you are going to test a power supply transient response with a load that has a loop bandwidth of couple tens of Hz? You know, loads are also commonly used in pulsed mode. 100Hz loop bandwidth ain't cutting the mustard even for testing basic off-line SMPS circuits.It may be surprise for you, that static load testing for any (closed-loop controlled) power supply ain't enough.Hence why I think ya should make the bandwidth large enough, or at least as good as it'll get.Design decisions such as 6kohm gate resistors for 10nF mosfets are just wrong.You have in this thread even a couple of circuits from commercial solutions, so why not inspiring there?Also, there is this good appnote fromJim Williams: AN133, A Closed-Loop, Wideband, 100A Active Load.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 02, 2019, 09:14:36 pm
content: Quote from: Yansi on June 02, 2019, 08:39:19 pmYansi,There was no mention in the OP message that he was building a load for doing transient load tests on power supplies. It is possible that high loop bandwidth is not a requirement. Loop stability is always a requirement.I presented a fairly extensive coverage on the loop stability of this type of load in this thread:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/I have obtained bandwidth of around 70kHz for a 5A load and 350kHz for a 1A load.(To get the high bandwidth, in the low power load, I used relatively small MOSFETs to get around the problems caused by large Cg. We do not need to deal with the Miller capacitance, Cdg, because the voltage across it should be constant.)I did not attempt to protect the op-amp from MOSFET failure in these designs. This is common practice in commercial loads. There is no evidence of op-amp protection in either the BK or the HP 6060 loads.If I wanted to protect the op-amp, I would do something like this:I am showing that the protection circuits I have added, the transorbs and the small signal diodes have not really compromised the control loop, especially for a static load. I would do not do this for a dynamic load.I obtained a loop bandwidth of 10kHz without any effort, with the option to increase bandwidth at the expense of phase margin.I used the LT1014, which is an LM324 with better offset and similar GBW product.The MOSFET was chosen from being in the LTspice library.Note:I have included the lead inductance in the model. As a countermeasure for the lead inductance, the snubber circuit is required. The snubber circuit can be found in the BK and HP loads.I have attached the LTspice model.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 02, 2019, 09:32:59 pm
content: Hi group,I just want to comment on the Jim Williams AN133 circuit:This load was designed to do a specific task. It was designed to test low voltage, around 1V, high current power supplies. It was designed to be very fast, high bandwidth. The design is limited to low duty-cycles. The maximum duty-cycle is 1.5% at 100A.By limiting the duty-cycle and the on time, Jim was able to use MOSFETs that are much smaller than would be required for 100% duty-cycle applications.To get the waveform fidelity shown in the application note:Jim had to use 10,000uF of low impedance capacitors.This picture shows the effect of adding just 20nH between the load and the power supply:Great work for the intended application, but not a good general purpose load.Regards,Jay_Diddy_B

author: MarkF
author_rank: Super Contributor
timestamp: June 02, 2019, 10:14:09 pm
content: Quote from: Yansi on June 02, 2019, 08:39:19 pmTo start, I have NO need to test power supply transients.Second, you keep spouting 10s of Hz bandwidth. Show me the math!

author: 2N3055
author_rank: Super Contributor
timestamp: June 02, 2019, 10:43:23 pm
content: Maynuo 9812 has 25kHz max pulse repetition rate, and can achieve slew rates of 2.5A/usec.That is sufficient for testing "normal" PSU, LED drivers, and such.For testing special fast drivers you need something faster, like that in Jim Williams appnote Yansi linked.You can use load for static test, like battery discharge, or testing power supply thermaly.You can use load to pulse test output of PSU to see regulation, recovery time, to see if it's stable (no overshoot or undershoot, no oscillation) etc. For that, load has to be faster than DUT's regulation loop, preferably order of magnitude faster, so it will look to DUT as something called "step response" (a term from regulation theory). That "step response" will reveal how the DUT responds to such a rapid load change. It will show DUT step response function that tells us a lot about it's regulation, stability, speed etc.That is why active loads have limited use if they are very slow.Which is not problem if you don't need dynamic load testing.But a word of advice: Jay_Diddy_B made his load, he has shown design, calculations, explanation of loop stability, what it is and how it is calculated. There is literally nothing to be said more on the topic after his very detailed postulation. So use his design for analog part and just replace signal generation part. He is extremely nice gentleman and if you get stuck I'm sure he'll help you if he can. He has already posted few posts here..

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 10:55:02 pm
content: Jay_Diddy_B: No offense sir! But noone prevents anyone to use a different mosfet with apropriate heatsinking, different current range, yet keep he same circuit tricks from there, like for example the gate buffer stage, feedback network + compensation type, etc.

author: OM222O
author_rank: Frequent Contributor
timestamp: June 03, 2019, 12:39:43 am
content: Quote from: Yansi on June 02, 2019, 10:55:02 pmyou are so ignorant and delusional! he has explained EXACTLY WHY he did what he did and RAN ACTUAL SIMULATIONS compared to your bluh bluh bluh, I am such a great engineer and I KNOW EVERYTHING, SO YOU ARE ALL WRONG AND WHAT I SAY IS RIGHT. responding to you is worse than banging my head against a brick wall, because at least the brick wall breaks at the end, but you keep on goingthere are no rhyme or reason to your comments, rather you just keep saying everyone is wrong. please design a load with full schematic and post it, then add comments as to why you designed it the way you did. otherwise stop shitposting and wasting everyone's time. no wonder you have 2.3k posts!there's actually a very good saying about arguing with people like you which unfortunately I can't use here ...I'm really not sure what you're trying to compensate for, but nobody really cares how great you are. please keep your attitude for yourself and only post constructive comments.there needs to be more people like john who actually take time out of their day to give good, detailed explanations which have been extremely helpful MULTIPLE TIMES, at least to me personally. I think others agree too.

author: wilfred
author_rank: Super Contributor
timestamp: June 03, 2019, 06:55:40 am
content: Quote from: Yansi on June 02, 2019, 10:55:02 pmJay_Diddy_B has contributed some of the best quality posts on this forum that I have seen. I'm glad you don't want to offend him. I don't want you to either.

author: MarkF
author_rank: Super Contributor
timestamp: June 03, 2019, 09:41:06 pm
content: For a reference, here is an enhanced version of the HP 6060 Load put forth byJay_Diddy_B

author: pinoccio
author_rank: Contributor
timestamp: October 04, 2019, 01:03:32 pm
content: This project is not abandonedI was a bit overloaded with the mass of info and felt a little demotivated too, so took a break from this one...Well, there is always something to improve in a circuit, better ways to do it, change a part for better performance, better this and that. But if I don't set and hold on to some boundaries, it's going to be an endless spiral of improvements and the project will never become something real.I appreciate the efforts and time of everyone who posted something relevant in this thread. Especially those who contributed improved versions of my circuit or provided alternative ones, and actually explained how and why...I liked the modified version from OM222O as a concept, even though I may not build it as the first prototype, took some ideas thou. Same for circuits from MarkF, Jay_Diddy_B, Yansi ...Btw. never planned to test transient response etc. Might be nice, if the final circuit was somewhat capable if I learn more about that stuff in future. But for a beginners project, this is currently not considered.Well, actually any further info and insight about dynamic loads, transients, loop bandwidth, stability, op amp protection, and so on - is obviously welcome ... For future study and also other beginners who might read thisNow here is my next version:available also at the public project page:https://easyeda.com/pinoccio/Electronic-Load-DIY-open- my IXYS fets arrived- swapped op amps for possibly better ones//both ebay so ...hopefully no knockoffs or faulty rejects- changed sense resistors to 1/3 R and related modification to divider- 12V supply- played with the current sensing part...Tried to design the current sensing summing amp, so that it can still work with single supply... well, not really sureIf this is not a reasonable way to go, other options are:- proper dual supply- separate direct sensing of each channel (got the free ADC port)- dedicated current sense module high/low side -like INA* or similar

author: Inconel-oh-el
author_rank: Contributor
timestamp: October 06, 2019, 05:56:18 am
content: I'm designing a fairly similar load at the moment. I'm not sure at all why Yansi pushed for low side current sensing. The shunt resistorswillheat up, and thatwillaffect accuracy (especially with .33R/2.5A). While testing a power supply for a period of time, you may notice the reading changing- you then have the hassle of determining if it is the supply or the load causing that.Adding a summing amp/potentially dual rail supply is an inelegant, brute force solution. High side current sensing would add a single op-amp and low value current sense resistor before the load, completely decoupling the current measurement from the current passing through the load. It is much simpler, more accurate, and also allows you to add any number of parallel paths later on without requiring a new PCB.

author: 2N3055
author_rank: Super Contributor
timestamp: October 06, 2019, 10:27:31 am
content: Quote from: Inconel-oh-el on October 06, 2019, 05:56:18 amLow side, current sink design topology is industry standard that pretty much all professional loads use. Low side sensing allows current sensing without large common mode voltages, making current sensing more accurate and with larger bandwidth. Self heating of shunts is universal problem that is addressed by using lower resistance shunts with better tempco.There are many pro load schematics on the net. Study them, and you will see what they use..

author: Inconel-oh-el
author_rank: Contributor
timestamp: October 06, 2019, 02:34:46 pm
content: Quote from: 2N3055 on October 06, 2019, 10:27:31 amI've looked at several, I'm not disputing the low side current sinking. My design follows the same, as well as using low value shunt resistors, and thus an amplifier for each. Pinoccio's design is uses .33R so that no amplifier is needed for regulation, keeping things much simpler (and I don't see anything wrong with this for the intended purpose). However 2W (and especially 8W if he intends for 5A) will heat the resistors and cause readings to drift... I don't think anyone recommends building instruments that give you faulty, untrustworthy readings, no matter how simple or low accuracy they need to be.Adding a high common mode range amp like the INA286 as recommended earlier in this thread (CMR of 80V) is still much simpler than one amplifier per resistor, and then trying to add those signals/using additional ADC's on the MC. Multiplexing may also work, but the point still stands. Obviously local pricing may vary, but the INA286 is not the only suitable option.

author: pinoccio
author_rank: Contributor
timestamp: October 08, 2019, 02:44:26 pm
content: ..next iteration- slightly lower shunt- split current sensing with pre-amp (but increases complexity)- simple op amp protectionedit:updated the schematic pictures:- fixed error/s- added snubber- lower caps for better bandwidth possibly- other minor editsCan the resistor heating be such an issue here?Now the projected load is about 5A total, so 2.5A each branch, with dual shunt resistor we get 0.78W each... so I plan to use 5W aluminum shell -those should not heat up too much having such headroom.Another possible improvement would be a better DAC /with more resolution, maybe DAC8571 (16bit). But I'll leave that for later as this is planned as a module plugged into a header, so it can be replaced any time.

author: pinoccio
author_rank: Contributor
timestamp: October 09, 2019, 01:08:06 pm
content: ...updated pictures in the previous post (just small corrections and updates)

author: thomasx
author_rank: Contributor
timestamp: May 16, 2024, 12:28:10 pm
content: HiI do realize I am posting in a 5 year old thread. But there's good and interesting info in here from people with apparently the right skill setI have some linear MOSFETs on order to start building my own electronic load. However, they are a week away or so, and eager as always I want to at least try this out in a simple setup right away.Looking at the very simple setup in this post in the beginning of the threadhttps://www.eevblog.com/forum/projects/yet-another-diy-electronic-load/msg2448360/#msg2448360and checking my "stock" of components I find a BUZ10 that seems to have fairly good specs, looking at Output and Transfer Characteristics.https://www.mouser.se/datasheet/2/389/stmicroelectronics_cd00000673-1204298.pdfFor initial tests I am thinking of sinking 1-2 amps at 5-12 volts, and then to go on from there with 12 volts and see how far I can get, with about 85Watts as maximum load, meaning that at about 7 amps the DUT voltage will start to drop to maintain the output power. But according to the SOA, the BUZ10 will give up before then. Step two I guess will be to try with two parallelled BUZ10, with an op-amp each of course.Any thoughts on a simple test like this, with the BUZ10?