OpAmp/MosFet Based Current sync/source, Question about mosfet gate voltage? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 21, 2014, 01:48:19 am
content: Last night I built an adjustable current source, it basically looks like Daves dummy load only the mosfet drain goes to the loads ground side and the loads positive is connected to the voltage rail. kind of like this schematicI would like to control a source/sync circuit with a 1 bit DAC. The thing is I cant really figure out how to derive the output current of the mosfet/resistor combo based on the DAC or POT(in an analog setup) based on the mosfets gate voltage. I would think with a 1 ohm resistor, I would need to pump 1 vot out to the opamp in order to get 1 amp. Only this isnt correct, last night using a .1ohm resistor it took 433mv on the DAC/POT to make 1 amp across the resistor.Can someone help me figure out how the gate voltage relates to the output current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2014, 02:39:30 am
content: As long as the op-amp supply is sufficient to turn on the MOSFET, and it is not oscillating, the voltage drop on the resistor must equal the input voltage.Tim

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 21, 2014, 03:44:39 am
content: Well I checked the gate with a scope, it is not oscillating, here is a picture of of mV across current sense resistor vs, amps produced by mosfet. Everything seems fine thereand here is mV out of the POT, vs amps generated by mosfet

author: madshaman
author_rank: Frequent Contributor
timestamp: May 21, 2014, 04:19:17 am
content: Based on the circuit you showed, you shouldn't get the result you're getting.Usually this is because the schematic and circuit are different. It happens to everyone.Recheck everything.Do the voltages at the opamp inputs match? If the inverting input of the opamp is at the reference voltage, and there is no significant oscillation, then the current through the sense resistor must obey ohm's law.Is the sense resistance really what you think? My first guess is that the wire and solderless breadboard traces from your inverting input to ground makes the sense resistance more like half an ohm, in short, your sense resistance is too small and being swamped by other sources of resistance...Try measuring the resistance between the inverting input and ground.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 21, 2014, 04:41:13 am
content: Im acually using a 1ohm 10 watt in these pictures just to make testing easier. I will test out the opamp inputs when I get some time, when I hook the opamp up as a voltage buffer everything works as it should, so if the non inverting and inverting inputs dont match when I throw the fet it, what could be the problem. I can tell you right now they dont just by looking at the meter on the + input and the meter across the resistor hooked to the - input.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 21, 2014, 11:08:31 am
content: ok so I took the big 1 ohm resistor out and replaced it with a 1% metal film 1ohm .25watt. Then I turned the pot until I was at about 100mA current draw according to my meter, here are my voltage readingsat opamp + 155.5mvat opamp - 167.8mvat opamp out 2.118across resistor 100.6mvAMP out 100.00mAI dont if this would matter but i am usin g this opamphttp://ww1.microchip.com/downloads/en/DeviceDoc/21669D.pdfand this fethttp://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdfIve also experienced this issue using a 12 bit DAC. Its driving me up the wall im trying to do 1 to 10amp precision source/sink with 1mv steps, and I have this issue every time I build the circuit, wether it be to source or sync!

author: RR
author_rank: Contributor
timestamp: May 21, 2014, 12:54:27 pm
content: Quote from: rwgast_lowlevellogicdesin on May 21, 2014, 11:08:31 amThese two should be same by your schematic in first postDo you have same ground? Good connection? Tried different opamp?

author: madshaman
author_rank: Frequent Contributor
timestamp: May 21, 2014, 01:14:33 pm
content: Quote from: RR on May 21, 2014, 12:54:27 pm+1Also, looking at your setup and equipment, the other values you measured look sane.It looks like there's there's additional voltage drop along the path from the inverting input to ground, look for it: try measuring voltage across points in that path where you think there should be none (e.g. measure the voltage from resistor's ground terminal to ground etc.)Your opamp *is* happily doing its job, it's ensuring that the voltage at its inverting input matches the set point by driving the mosfet's gate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2014, 11:30:14 pm
content: I don't know what your pot is doing but 167mV it ain't.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 22, 2014, 01:10:14 am
content: The pot reading is correct, why do you doubt it? That meter has just been compared against a few TI ref50 series .05% accuracy voltage references, and its not to far off. Im curious why you say the pot can not be outputting that? It is a Chinese 10 turn across ~3.4 volts.I did find the issue, it was the ground strip on the breadboard, the PSU ground was plugged in on one side of the breadboard and the the 1ohm resistor on the other side. Between the top and bottom of the ground strip, my meter measures 1.8ohms. I just plugged the two grounds in next to each other and everything worked fine.It seem like almost every funny issue I end up having turns out to be a breadboard thing...