Help with higher end digital dummy load. Understanding opamp/fet choice - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 03, 2013, 04:59:22 am
content: I was hoping someone could help me clear up some questions about electronic loads/current sinks that involve using a fet along with an opamp. I really want to take daves dummy load to a higher technical level. Ive been working with electronics for a while but its mostly been all digital. Ive even built a simple dummy load based on daves design using a micro instead of a pot.The first question I have is the differene in transistor choice. I have seen a lot of projects using a standard logic level N Fet. In my searching I have come across a few articles about using fets as current sinks, but these all cover using a J Fet, is there any reason to use J instead of N? Ive also seen a load similar to daves using two 2n3055 power transistors and am curious if that is a better path than FETs all togather?Im looking to build my load with a 0 to 10 amp rating, hopefully with a stepping resalution of 1ma. I will be measuring the current with a hi side 100v gain sense amp, and 3watt .005ohm shunt resistor, instead of the low side .1ohm method commonly used. To drive the fet/opamp I plan to use a 12bit spi dac instead of PWM.Right now I have a few mcp6043 opamps which say they are rail to rail input and output, This is crucial to get the load to go to 0amps right? I also have FQP30N60l fets. Im assuming neither of these parts are the best choice for precision, but jist how good of a combo are they for what I want to do? Ive been reading up on opamps and understand the very basics, but im still not comftorable designing with them. Can someone explain to me what wrapping the fets source in the amps negative feedback loop acually does? This is how every fet based load ive looked at works.Lastly a while back I saw an adjustable load project based on two like 50 dollar fets somewhere online, this guy wasnt concerned with price at all. Does anyone have a link to that project I cant find it anywhere now!

author: IanJ
author_rank: Unknown
timestamp: July 03, 2013, 06:19:14 am
content: Could have been mine.......the LAST thing I look at for my projects is the cost.http://www.ianjohnston.com/index.php?option=com_content&view=article&id=78:dummyload2Expensive op-amp and fet's........1 * LM8272 op-amp2 * IXTK46N50L N-type MosfetIan.

author: JJalling
author_rank: Unknown
timestamp: July 03, 2013, 07:43:12 am
content: Quote from: rwgast_lowlevellogicdesin on July 03, 2013, 04:59:22 amHi Ian,Do you really need that kind of resolution? How do you plan to get 10.000 steps from the 4096 steps the 12 bit dac (hopefully) gives you?BR Jonas

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 03, 2013, 07:58:36 am
content: @ian, your project looks very nice but it wasnt the one I was reffering too. The project I was thinking of had a lot of pictures I remmber each fet had a massive heat sink, and the web page had a white back ground.@jjalling, ian is not the OP, I am. I guess the res really depends on the amount of voltage... I was more wondering if the opamp/fet would be accurate enough to handle 1ma steps.

author: poorchava
author_rank: Super Contributor
timestamp: July 03, 2013, 08:09:29 am
content: There's a tiny problem with FET's, as I learned some time ago: they DO NOT have positive temperature coefficient always. The temp.co of Rds(on) changes from negative to positive together with Vgs. Which means that a low drive + high current + mosfets in parallel may produce a thermal runaway.Look at this:http://www.infineon.com/dgdl/Infineon+-+Application+Note+-+PowerMOSFETs+-+OptiMOS%E2%84%A2+-+Linear+Mode+Operation+and+SOA+Power+MOSFETs.pdf?folderId=db3a3043156fd5730115c7d50620107c&fileId=db3a30433e30e4bf013e3646e9381200

author: JJalling
author_rank: Unknown
timestamp: July 03, 2013, 08:18:48 am
content: Quote from: rwgast_lowlevellogicdesin on July 03, 2013, 07:58:36 amAh, I managed to mix up the posts - sorry.Yes, it depends on the voltage. But you won't get enough resolution from a 12-bit dac to get your wanted 1mA resolution on a 0-10A scale. Or am I missing something crucial here? Please correct me if I'm wrong.BR Jonas

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 03, 2013, 10:45:02 am
content: Im not sure what I was thinking, 1ma would be the res at 4.096 amps, so a 10 amp load would be 2.44ma. Im fine with this for now, like I said I was wondering if the opamp and fet had the right characteristics to build a higher presicion dummy load. I was really hoping someone would be able to explain more about the task the opamp plays when the fet is in its feedback loop and what qualitys to look for when shopping for parts.

author: shailesh3t
author_rank: Regular Contributor
timestamp: July 03, 2013, 01:33:12 pm
content: Hi am too in the process of building a dummy load, and i found this one to be a nice one but i think for u, u need a higher resolution dac and adc, else the one designed by D jones too is goodif am not wrong the design goes upto 3 amps the one made by Jjosteram using INA 219 to read the curent of the load,MCP 4725 as DAC to set the voltage on the LM324 to control the IRFZ44n as this is the one available in my island, hardly get all parts u need in Mauritius so i do things with the parts i can find on ebay an in the island n many seller dont ship to my countryand finally i will be using arduino uno to control the DAC and INA 219 ( current , voltage, power reading @ 12bit) and an lcd to display the settings n readings n a rotary encoder to set the current.am still writing program for it as for instance not much time as my wife just got a baby so most of time with himelse am not an expert in programming n electronics am still learning n most of time i use libraries made by some experts, like D jones, JJoster,Mjlorton n ...else in some other thread u were asking for a power supply, i suggest u make one ureself as i do think its cost less and it can be customized according to you,i have built one using arduino and a switch mode power supply using tl 494 made by an another guy , still under construction as no much timemainly i did the programming and used the cct from the guyhere is a video of itlink for joster dummy loadhttp://www.google.mu/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CEQQtwIwAQ&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DWSQeZdkbquI&ei=bCfUUfeeDdGqrAeEq4HQBg&usg=AFQjCNHFeJxePfIQBnqmShWvzT4Ob_bSHA&bvm=bv.48705608,d.bmk

author: edavid
author_rank: Super Contributor
timestamp: July 03, 2013, 02:27:02 pm
content: Quote from: rwgast_lowlevellogicdesin on July 03, 2013, 04:59:22 amIf you want to build a 10A load, why are you even talking about JFETs? Where would you get a 10A JFET?Bipolars are a poor choice because a) they need base current and b) they have a high-ish saturation voltage.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: July 03, 2013, 04:42:17 pm
content: Hi rw hope this helps .Quote:- "Right now I have a few mcp6043 opamps which say they are rail to rail input and output, This is crucial to get the load to go to 0amps right?"Well getting the outputs to adjust down to zero requires opamp input's range to go to zero .(also there could be a very small output residue remaining even when input is at zero caused by the input offset voltage of the op-amp).Quote:- "I also have FQP30N60l fets. Im assuming neither of these parts are the best choice for precision, but jist how good of a combo are they for what I want to do?"Choice of Fets for this application is not really anything to do with precision . The most critical job the Mosfets have in this application is to dissipate the power as heat. So your choices will be dictated by the power dissipation you require . The two you chose only have to-220 case styles so they can only dissipate small to medium loads . If you want better heat dissipation look to Fets with case styles like TO3-P with better dissipation.Anything above about 40 Watts and you will also need to be thinking about fans aswell unless your using really large heatsinks.Two major factors in determining the precision you will get is determined by the value and type of the sense resistor you choose (simply put :- A larger value sense resistor gives you a larger feedback for a given current change)there is an obvious trade of between sense resistor value and sense resistor power dissipation which in turn effects it's precision due to temp coeff .Quote :- "Ive been reading up on opamps and understand the very basics, but im still not comftorable designing with them. Can someone explain to me what wrapping the fets source in the amps negative feedback loop acually does? This is how every fet based load ive looked at works."Look around the web theres plenty of good stuff that explains this much better than I can do in a limited few words here but I will try.I hope this is what you meant to ask.Basically what your doing is measuring the voltage across a 'current sense resistor' and feeding this back into the opamp inv input ,Your op-amp then 'compares' this to whatever voltage you have set on the non inv input pin , and if it is below this set value then the opamp output will go higher and so turn the Fet on harder ,so this will then let more current flow through both the Fet and sense resistor ,this extra current will raise the voltage across the current sense resistor .This loop will continue until the voltage across the sense resistor is equil to whatever voltage is set on the op-amp non inv input .(if the device on the main input cannot supply enough current to raise voltage across sense resistor to be equil to what was set then the Fet will remain switched hard on) . To try to summarize it :- your creating a Voltage regulator that is trying to regulate the voltage accross a fixed sense resistor (and by ohms law if we are regulating the voltage across a fixed resistor we are also regulating the current through it).Quote :- "Lastly a while back I saw an adjustable load project based on two like 50 dollar fets somewhere online, this guy wasnt concerned with price at all. Does anyone have a link to that project I cant find it anywhere now!"Your not referring to mine that I posted here a few weeks ago are you ?https://www.eevblog.com/forum/projects/fully-portable-150w12a-e-loadcurrent-reg-with-multiple-ranges/the 2 main Fets FQA70N10 where only £2 each,the IRFP3206 that is used for reverse polarity protection is ~ £3 I think .The most expensive thing I chose in that project was the 10 turn pot which are about £8 each on EBAY .total build cost of that prototype was about £45 as it is. But considering it's fuctionality an and portabilty and that the cost of commercial 150W loads is many hundreds then thats cheap to me. (on the low range it's got a setting precision of 10uA and I often use it as a current source)