Electronics > Projects, Designs, and Technical Stuff
Programmable Electronic Load, 0-5A
OM222O:
--- Quote from: JeanLeMotan on February 24, 2019, 09:08:27 am ---
So just adding a low value resistor in the source of the MOSFET?
I also saw this design that uses a BJT transistor: https://youtu.be/9auu8hH4IPM?list=PLUMG8JNssPPzbr4LydbTcBrhoPlemu5Dt&t=457
Thanks a lot for the advice.
--- End quote ---
Scully has some great videos detailing the operation, but unfortunately some janky ways of reaching the same means :D
Best way to achive your desired result is to use a quad op amp, each controlling one mosfet which is connected to a separate source resistor. this way you can:
1) drop the power rating of both your fets and resistors -> cheaper parts
2) use way less parts -> more space on pcb + cost savings
I would strongly suggest you look into the illusive ADS1219, the bigger brother of ADS1115. also you can use X2Y capacitors for your adc input filter for less noise and higher accuracy. costs almost the same as the ADS1115 modules available on amazon but not as popular, so you should but it from mouser or digikey, but I can get about 100uV accuracy or better depending on the application. you can copy my ADC design from this picture:
In terms of mosfets, you can use logic level fets which should give you a better response ... not sure if the 1.4A limit on 1V is caused by the transconductance of the mosfet or not, but it's well worth trying an IRLZ44N. Others have mentioned some high linear SOA IXYS fets which are really expensive in my opinion. you can get better cooling and save cost if you go with 4 cheap fets rather than 1 or 2 expensive ones. also because of higher surface area of separated load, they will be a lot easier to cool (I could get about 20 watts using two passively cooled fets, if you have a fan (which you do) your load capacity will increase drastically) !
3D printed case is a nice idea but will soften and loose shape if your load gets hotter than about 60C which the chances are it will! even if you use ABS it's not much better. a good idea would be to make a silicon mold of your printed parts and creating resin castings from them which are a lot more durable!
one final note would be to use a proper dac instead of a PWM signal ... it depends on your budget to determine if it's worth it or not but a simple first order RC filter you used has some major flaws. you should preferably use a two stage second order filter if you want any decent settling of the voltage and level of accuracy. I strongly recommend a DAC if you can fit it in your budget.
here is a video of an dummy load I made, but it's fully analog as I will be monitoring currents and voltages externally but it was designed for the same purpose of testing battery capacities. you can add the digital parts as you like:
Sorry for the quick and lackluster answer, I'm out of time for now. if you want more details I can answer them later. hope you found this useful and good luck with the project!
JeanLeMotan:
--- Quote from: OM222O on March 03, 2019, 01:13:09 am ---Scully has some great videos detailing the operation, but unfortunately some janky ways of reaching the same means :D
Best way to achive your desired result is to use a quad op amp, each controlling one mosfet which is connected to a separate source resistor. this way you can:
1) drop the power rating of both your fets and resistors -> cheaper parts
2) use way less parts -> more space on pcb + cost savings
--- End quote ---
I'm changing the schematic to do just that now. However I need to use OP27 (or TL072) so that I can adjust the offset voltage - as I need to get down to 0 amps. This prevents me from using a quad opamp, unless there are quad opamps with offst adjustment?
--- Quote from: OM222O on March 03, 2019, 01:13:09 am ---I would strongly suggest you look into the illusive ADS1219, the bigger brother of ADS1115. also you can use X2Y capacitors for your adc input filter for less noise and higher accuracy. costs almost the same as the ADS1115 modules available on amazon but not as popular, so you should but it from mouser or digikey, but I can get about 100uV accuracy or better depending on the application. you can copy my ADC design from this picture:
--- End quote ---
Thanks for the hint - I actually searched for a higher resolution cheap I2C ADC but somehow missed this one.
I'll consider it for a future revision.
--- Quote from: OM222O on March 03, 2019, 01:13:09 am ---In terms of mosfets, you can use logic level fets which should give you a better response ... not sure if the 1.4A limit on 1V is caused by the transconductance of the mosfet or not, but it's well worth trying an IRLZ44N. Others have mentioned some high linear SOA IXYS fets which are really expensive in my opinion. you can get better cooling and save cost if you go with 4 cheap fets rather than 1 or 2 expensive ones. also because of higher surface area of separated load, they will be a lot easier to cool (I could get about 20 watts using two passively cooled fets, if you have a fan (which you do) your load capacity will increase drastically) !
--- End quote ---
I did test a lot of logic level MOSFETs and they all died above 12-20V and 1-2A. Their SOA just doesn't go that far.
Maybe with proper opamp balanced load between them (as opposed to simply paralleling them) they would be ok.
The linear mosfet I went for is ~5 euros on digikey so yes, a bit expensive but definitely cheaper than the time I spent trying out logic level mosfets (and killing them) :)
--- Quote from: OM222O on March 03, 2019, 01:13:09 am ---3D printed case is a nice idea but will soften and loose shape if your load gets hotter than about 60C which the chances are it will! even if you use ABS it's not much better. a good idea would be to make a silicon mold of your printed parts and creating resin castings from them which are a lot more durable!
--- End quote ---
Good point. I'm using PETG which should start softening at higher temps than PLA. Since I have temperature control and a fan I can prevent the load from reaching this temperature.
To be honest I don't want to open another chapter - case production. Switching between electronics and software is already confusing enough.
--- Quote from: OM222O on March 03, 2019, 01:13:09 am ---one final note would be to use a proper dac instead of a PWM signal ... it depends on your budget to determine if it's worth it or not but a simple first order RC filter you used has some major flaws. you should preferably use a two stage second order filter if you want any decent settling of the voltage and level of accuracy. I strongly recommend a DAC if you can fit it in your budget.
--- End quote ---
I spent some time searching for DACs but in the end I settled for a PWM for these reasons:
- Higher precision than a (reasonably cheap) DAC. I have my pwm at 18 bits which is overkill, but I'd need a minimum of 13-14 bits for 5A with 1mA precision. If you factor in noise and other issues I think 16 bits become useful.
- It's very linear
- Relatively easy to filter. With the filter used now I get a few mV of noise which translates in... hmm I just did the math and for a 0.1 ohm shunt, 1 mV represents 10 mA... Not good. I need to measure again, filter more or get a DAC :)
Thanks for the advice and info!
JeanLeMotan:
I'm working on rev2 schematig, incorporating the advice received so far:
- Opamp controlled MOSFETs, up to 3 of them (arbitrary limit)
- Removed reverse polarity - but will ad it back with opamp control
- Added a voltage ref and a CMOS buffer for PWM control - but I will add a DAC if PWM doesn't cut it
- +/- 12V generated on board using a cheap module
- 3v3 regulation on board so that I need only a 5V input voltage.
Here's the WIP schematic:
agaelema:
The use of PWM DAC is very interesting and many (maybe old ::)) programmable voltage references use this technique. I think the biggest tradeoff is the settling time when compared with a conventional DAC.
I don't know if I read about this on the forum or some other site, but there are some ways to minimize the ripple without to much affect the settling time.
Look at this article: https://www.edn.com/design/analog/4459116/Cancel-PWM-DAC-ripple-with-analog-subtraction
I did some simulations on Multisim and the result is very exciting. I also tested on a breadboard and it really works.
JeanLeMotan:
I redesigned the reference voltage part using a DAC and eliminating the need for offset trimming in the opamps.
What I ended up with is a 16 bit dac with an output of -26mV to 512mV.
The negative voltage is created with opamp U9.3, but I'm not sure if there will be issues with this topology. I simulated it and it worked just fine but since it's smth I 'designed', maybe there is something I missed.
The schematic is attached with some voltages marked.
The other part I redesigned is the load control: this time I'm using a precision quad opamp - OPA4277 - controlling up to 4 mosfets, each with its own shunt resistor. There is another shunt resistor used for the ammeter opamp.
Let me know what you think.
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