LL-NFET based dummy load is not working?? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 26, 2014, 05:36:13 am
content: Ive been working on building a dummy load the last few days, using the common FET design in daves video. The only difference is I am using an Arduino to and a 12 bit DAC to drive the fet. This schematic is basically the same as mineexcept the + on the opamp is connected to my DAC not a filtered PWM output. I am using a different opamp, an MCP6043 single channel Rail to Rail opamp,http://ww1.microchip.com/downloads/en/DeviceDoc/21669D.pdf. Everything seems to work the way it should except the fet and the load. Im a slight variation of the fet in the schematic,http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdf. I have tried voltages from 100mv up to 5v on the FETS gate with the + side of a fresh 9v battery connected to its drain. The fet is just not drawing any current what so ever from the connected load. Ive also tried 4 of the same kind of fet!

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 26, 2014, 01:31:33 pm
content: Im not sure why I forgot to mention this in the last night, when the opamp is connected in a voltage follower arrangement, or as an amplifier the output voltage is exactly what it should be. Once I connect the - side of the opamp to the FET's source pin and the opamp output to the FET's gate, the voltage from the output to the gate is 0, no matter what I pump in to the opamp from the DAC.I also set up a simple switching circuit last night to turn an LED on using the mosfets, and tested all 4 of the mosfets, and they all lit up the LED at under 2vs. So the mosfets arent shot.

author: retrolefty
author_rank: Super Contributor
timestamp: April 26, 2014, 01:45:24 pm
content: What is the load resistance you are using? 9 volt batteries have pretty high internal resistance and voltage can sag badly under larger loads. Also standard arduino pwm outputs (when using analogWrite() function) are 8 bits of resolution, not 12 bits. I would be concerned about not using the low pass filter as not sure of the source and sink current capacity of the LM358 to drive the mosfet gate capacitance at the PWM switching frequency?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 26, 2014, 03:10:31 pm
content: Like I said I'm not using arduino pwm, I am driving an mcp6043 opamp with a mcp4922 12 bit Dac.I'm not sure what you mean by load resistance, I just have a 9v hooked up to the fet and ground, with a meter in series to test current draw of the fet.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 26, 2014, 03:40:03 pm
content: Quote from: rwgast_lowlevellogicdesin on April 26, 2014, 05:36:13 amFirst, your op amp supports a maximum of 6V so I hope you're not using that 9V battery with it.Next, you mention hooking the + side of the battery up. You did hook the - side up to ground, right? I ask because you would get the exact result that you're seeing if you didn't.Finally, when you get to the point were you actually get the op amp to drive the FET, you will likely find that the output oscillates. You'll find several threads that discuss this on the forum. It seems that Dave somehow got lucky with his components and didn't need any frequency compensation. Everyone else had to do something like the attached to stabilize the circuit.

author: retrolefty
author_rank: Super Contributor
timestamp: April 26, 2014, 03:49:35 pm
content: Quote from: rwgast_lowlevellogicdesin on April 26, 2014, 03:10:31 pmOk, your drawing showing the arduino pin D3 driving the circuit is what I based my assumptions on. Sorry i should have read your comments more closely.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 26, 2014, 04:21:50 pm
content: I have the 9v + connected to the gates drain, and its negative side connected to ground. The opamp is powered by the arduinos 5v supply.As far as oscillation goes I've checked with a scope, this isn't the issue yet. There is no voltage coming out of the output of the opamp, something is broken with the feedback loop, but the feedback loop is wired just like the schematic above.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 26, 2014, 04:41:00 pm
content: If you have another op amp handy--one that can handle 9V--then you might try it with it hooked up to 9V. Something like an LM358/LM324 or a TL07x/TL08x, for example.The problem might be that the op amp, when powered by just 5V, can't pull the FET's gate up high enough.Edit: Also, don't expect to be able to draw much current from a 9V battery. For testing, you would be better off using something like a AA cell. If you do use a 9V battery as the current source then don't use thesame9V battery to power the op amp.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 26, 2014, 05:24:20 pm
content: According to the data sheet the fet should be on and able to pass about 15 amps right at 3v?http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdf

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 26, 2014, 10:29:03 pm
content: Yep, I agree that itshouldwork.You might want to draw up a schematic of exactly what you have and post it.A picture of your circuit might help as well. Maybe someone can spot something mis-wired or something.

author: sleemanj
author_rank: Super Contributor
timestamp: April 26, 2014, 10:39:46 pm
content: Are you sure you have not accidentally swapped the inverting and non inverting inputs.Check and report the voltages at the inputs, and outputs of the amp, including v+ and v- relative gnd.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 26, 2014, 11:39:34 pm
content: Hi,You need to add a few more parts to the circuit. This to control the gain in the circuit and prevent oscillations.The schematic is in this thread:https://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/msg170808/#msg170808You can also look at this this thread.https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/msg288313/#msg288313Regards,Jay_Diddy_B

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 27, 2014, 01:27:43 am
content: 12 bit DACs vREF is a 4.096 reference, so each step is 1mv. DAC is set to 1800 which should output 1.8v.I also switched the load to a double AOpamp VoltagesVCC=5.05VDD=0.00V+ is at 1.804vV- is at 1.341vVout is 5.05vMosFet VoltagesGate is 5.05vDrain is 1.34Source is 1.34I will hand draw a schematic, I really need to learn a pcb package

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 27, 2014, 01:35:20 am
content: Quote from: rwgast_lowlevellogicdesin on April 27, 2014, 01:27:43 amYou could get these conditions if the power supply feeding the load is not able deliver 1.8ATry reducing the voltage from the DAC to 1.00V, and measure the voltage across the 1 Ohm resistor. It should also be 1 V.Jay_Diddy_B

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 27, 2014, 01:44:30 am
content: I got it somewhat working now, my problem was with the sense resistor. Now my problem is I cant draw over 300mah out of the double A, I added a 220 pico farad cap from V- to Vout with a 100ohm series resistor between the gate and Vout, to help oscillation.

author: sleemanj
author_rank: Super Contributor
timestamp: April 27, 2014, 01:48:15 am
content: Quote from: rwgast_lowlevellogicdesin on April 27, 2014, 01:27:43 amSooo.. what exactly is the problem?Your set point is 1.8, it's achieving 1.34, your sense resistor is 1R, so you are getting 1.34 Amps out of it, and the amp is driving the fet as hard as it can. Looks to be working to me.You can see that your Drain voltage is at 1.34v, and your drain is connected directly to your DUT (9v battery), therefore you can conclude that the battery is sagging massively because it simply can not provide that current you are demanding of it.Oops, missed you'd change the DUT.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 27, 2014, 01:50:11 am
content: Yep, you are asking too much of the poor AA battery. You'll likely need something else to test your circuit at higher currents.So far as schematic capture goes, you might download either the Tina-TI simulator or LTspice. You can draw schematicsandsimulate them.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 03, 2014, 03:58:11 am
content: I just wanted to say I was having a lot of problems with this. It turned out that my FETS were messed up. Im not sure what was wrong with them, but I ordered 6 new ones and everything seems fine.Im having a problem testing bigger loads though, I cant seem to get it to draw over 2.7 amps from anything! I have tried LithiumIon 18650 cells rated for 30 amp discharge, a lantern battery, an 8 cell 2400mah NiMh pack. Most recently I connected an ATX power supply to the load, one red wire and one black wire, measuring 5v. The PSU says +5v 40amp on the sticker! I have a heatsink on my fet and have even tryed leaving the fet/heatsink in the freezer for 30 mins before using it to test a load. I thought I was just running in to battery limitations but shouldnt the ATX psu be capable of sourcing enough current to fry my fet?

author: sleemanj
author_rank: Super Contributor
timestamp: May 03, 2014, 04:09:40 am
content: Quote from: rwgast_lowlevellogicdesin on May 03, 2014, 03:58:11 amYou probably just need to drive your fet harder, increase the supply to the op amp assuming the op-amp can handle it (if it can't, get an op-amp that can) so that it can push the gate higher.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 03, 2014, 05:18:18 pm
content: Well the opamp I'm using right now will only go to 5.5v, I really need a 10v capable opamp anyways to hit may 10 amp goal. I'm looking at a rail to rail opamp that takes 3.8 to 12v. My concern is it is 5.5mhz not 1mhz like the amp I'm using now. Will this cause oscillation? My dummy load also settles really slow too, it takes about a second or two for the voltage across the sense resistor to stabilize after the encoder is turned, will a faster opamp fix this, or is this a capacitance issue with the fet? I have a pull down on the gate.

author: Monkeh
author_rank: Super Contributor
timestamp: May 03, 2014, 05:33:12 pm
content: Quote from: rwgast_lowlevellogicdesin on May 03, 2014, 05:18:18 pm.. No, you need to drop the value of the sense resistor. Desperately.

author: tonyarkles
author_rank: Regular Contributor
timestamp: May 03, 2014, 05:40:01 pm
content: Quote from: rwgast_lowlevellogicdesin on May 03, 2014, 03:58:11 amHere's your problem: Your sense resistor is 1ohm. At 2.7A, there's going to be 2.7V across it, which means that the source of the FET is also already at 2.7V. Your opamp only has a 5V rail, so as the current increases, you have less and less headroom to turn the FET on.As others have said, drop the value of the sense resistor by quite a bit. If you used a 0.1ohm instead of a 1ohm, you could get to 27A before you run into the same problem (you might run into other problems first, but you won't have *this* problem).

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 04, 2014, 06:09:21 am
content: Hmmm ok well if I use a smaller sense resistor that will throw off the resolution, in the end im hoping to get up to 10amps with 1mA notches, if I switch to even a .5ohm resistor it will end up turning each division in to 2mA per 1mv. The only thing I can think of is to use a relay to switch sense resistors once i get to the 2amp mark. Im using a a 12bit dac with a 4.096 vref. The dac has a built in amplifier so I can multiply the output by 2 if need be, here is a schematicI guess im just asking what the best way to keep 1mA accuracy is, while getting over two and a half amps.

author: sleemanj
author_rank: Super Contributor
timestamp: May 04, 2014, 09:02:08 am
content: I didn't realise you were using such a large sense resistor. In which case as the poster above implied, you have forgotten that the source voltage of the mosfet is lifted considerably by that sense resistor and thus the drive voltage (relative to ground) to the gate needs to be higher to keep it suitable relative to the lifted source - it's "Volts Gate Source" not "Volts Gate Ground".Quote from: rwgast_lowlevellogicdesin on May 04, 2014, 06:09:21 amThere is nothing to say you can not modify either the sensed voltage, or the set voltage before you feed it into the fet-driving op-amp. I did exactly this in my own dummy load because I wanted setting to be 1v/amp, but sensing to be 0.1v/amp.So use a 0.1R sense, and either:a) multiply the sense value by 10 with another op-amp before feeding into the input of the fet driving op-amp; ORb) divide the set value out of your dac by 10 with a resistor divider before feeding into your fet driving op-ampof course this does introduce places for errors, but careful trimming in a) of the gain, or in b) of the divider, can sort that out.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 04, 2014, 02:07:30 pm
content: Quote from: rwgast_lowlevellogicdesin on May 03, 2014, 05:18:18 pmThe reason for the slow settling is the 100nF cap you are using for compensation. That cap is limiting the bandwidth of the circuit. It does this to prevent oscillation. It can likely be made smaller, however, so you might tweak its value and watch for oscillation as you do.