Constant current dummy load. Please review & suggest.

ronv

Joined Nov 12, 2008
3,770
@ronv
but the opamp's non inverting i/p reaches ground when pot knob is turned down ? effectively the opamp o/p should go to something that makes sense resistor voltage = ground also.....
Not all op amps can go all the way to ground. The two you have listed cannot. When the + input is ground the output will still be about 1.5 volts.
i have chaged the schematic as per suggestion, please see.
Don't forget the resistor in the output of the op amp or it may oscillate.
 

Thread Starter

IcedFruits

Joined Jan 15, 2014
97
^
LT1014 datasheet says on 0-5v the o/p swing min is in the range of tenths of mv.
and K2749 datasheet says gate threshold is 2-4v @ Vgs of 10V.
So they should work well, no ? mosfet will be well switched off @ mv level.

however, i m a bit confused with op07 datasheet, it only says +-12v swing on dual supply etc. so i dont understand what the swing will be with 0-12v supply ...... ideas ?

Don't forget the resistor in the output of the op amp or it may oscillate.
u mean putting a resistance in feedback path ? working on that, studying the low pass filter in the feedback path like DickCappels suggested.....
 

ronv

Joined Nov 12, 2008
3,770
The LT1014 and the 324 & 358 all swing very close to ground. The op07 will only go down to plus 2 volts so it won't work.
Take a look at the schematic I posted to see where to put the resistor in the output and the cap and resistor in the feedback path. The resistor between the output and the gate isolates the op amp from the FET gate capacitance. The cap & resistor in the feedback path limits the frequency response. Between the 2 the circuit will be stable. You can see the effect when you get it running is spice if you input a square wave and look at the current response in the load resistor.
 

bance

Joined Aug 11, 2012
315
In the data-sheet look at electrical characteristics > output characteristics...

For a supply voltage of 15V output voltage swing is 13V.
 

Thread Starter

IcedFruits

Joined Jan 15, 2014
97
ok, thanks for explaining.

any i studied ronv circuit (here) a bit, and it seems to be similar configuration is being used by many people online.

i read somewhere that r3 is used to match the offset voltage as in unity gain amp, c1 is there parallel to feedback path, to dampen higher frequency oscillations, but i don't understand the purpose of r4 (pull down resistor?) & r2(why is that even needed there?).

and also what is the basis for calculating c1, r4 & r2.

please pour in some theories..... :)

# i am going to go shopping tomorrow probably, please suggest some more common & suitable low offset, ground reaching opamps. will hand over the list to storekeeper and see what they have there for me.
 

Thread Starter

IcedFruits

Joined Jan 15, 2014
97
so i visited the shop, they had LT1013, but it was bit of expensive - i could have ~15x LM324 at that price. anyways, just gave it up and bought a few 324 & OP07s.

tested them today, OP07 o/p reaches 1.3 minimum and max of ~11v, with supply of 0-12v (ic gets a bit warm with ~12mA consumption). it's offset was really very low, my DMM did not even show any figure (except - sign when near @ 1.5v input range), i assume its as per the datasheet, below 60uV.

now i really want to use OP07 to read sense resistor, which i m going to up to 0.75E, but guess i will need to feed the op amp -ve supply in place of ground, cause OP07 seems to defy all opamp rules when reaching below 1.7v...... for the rest of opamps, 358 is probably sufficient (DMM shows 0.3 - 0.5mv offset for 358).

so, what are the easy ways to generate -ve (-3v to -5v for my case may be ok ?) voltage from +ve voltage ?
 

ronv

Joined Nov 12, 2008
3,770
i read somewhere that r3 is used to match the offset voltage as in unity gain amp, c1 is there parallel to feedback path, to dampen higher frequency oscillations, but i don't understand the purpose of r4 (pull down resistor?) & r2(why is that even needed there?).

and also what is the basis for calculating c1, r4 & r2.

please pour in some theories..... :)
R3 & C1 form a low pass filter that will help eliminate high frequency oscillations.

R4 eliminates what is called crossover distortion found in the 358 and 324 op amps. In this circuit it would show up as small high frequency oscillations. It also helps the op amp get closer to ground.

R2 - The op amp has problems driving the capacitance of the FET. This resistor isolates it from the FET. The circuit will oscillate without it because there will not be enough phase margin.

I used LTSpice to optimize R3 and C1, but I think you can just make the time constant of them twice as long as the time constant of R2 and the capacitance of the FET.

If you check the fine print (Notes) in the data sheet for the 324 you will find R4.[/QUOTE]

# i am going to go shopping tomorrow probably, please suggest some more common & suitable low offset, ground reaching opamps. will hand over the list to storekeeper and see what they have there for me
I'm not sure why you are worried about the offset. Are you going to run a lot at low currents?
Probably the easiest way to get a negative supply is to use a little wall power supply. Without knowing what is available it is hard to say on the op amp. You can go to the Digikey site and look for rail to rail op amps then check the offset voltage.
 

Thread Starter

IcedFruits

Joined Jan 15, 2014
97
I'm not sure why you are worried about the offset. Are you going to run a lot at low currents?
Probably the easiest way to get a negative supply is to use a little wall power supply. Without knowing what is available it is hard to say on the op amp. You can go to the Digikey site and look for rail to rail op amps then check the offset voltage.
Why not just use the 324? I just made a dummy load using a 324, can wind it right down to zero no problem at all.....
thanks for explaining them so nicely, i think i finally realized it can well be done with 324.

~> was previously worried about what effect it will bring with low currents and high offset op amps, but to think 2mV or ~10mA less control with 0.25 E sense resistor is acceptable ..... anyways , now i have decided to go for 0.75E sense resistor.
~> then thought about measurement error for sense resistor if connected to micro-controller directly, as atmega analog pins have 32k resistance with ground i read somewhere, so thought of using a precision opamp buffer ..... but after calculation, it does not seem to be a bother as much 1/ (1/0.75 + 1/32,000) = 0.7499824223.
~> will have to use buffer only for reading the test voltage of load line, i think, cause of the high resistance of voltage divider, even if i use a 2k+8k then the 2k will effectively become 1.88k, i.e. (2.00/10 - 1.88/9.88) = 0.97% error in reading.

and yah it will be running a lot low currents in the level of 100mA, since i will be testing out some rechargeable batts once in a while. (capture load input voltage & current over time, to see how much capacity left, or how much load the batt can drive for what duration etc)

please pardon me for querying so much, i want this one to be a bit of instrument grade and sophisticated enough for permanent use. :D

will update the schematic with all the modifications soon, and hope to reach a design conclusion by this weekend. thanks again. :)
 
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Thread Starter

IcedFruits

Joined Jan 15, 2014
97
updated modified schematic at first post, please see.

some changes apart from opamp suggestions:

# voltage reference now is lm385.
# comparators added for shutdown purposes - over temp & over current
# 100k pot added for fine tuning.
 
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bance

Joined Aug 11, 2012
315
I think your design is a little overly complicated, I'm sure some/all of the comparator work could be handled directly by the micro.

Also I'm concerned that a single mosfet will handle the power requirements....

HTH Steve
 

Thread Starter

IcedFruits

Joined Jan 15, 2014
97
i added comparators, cause if i don't want to run loop at the micro controller at the fastest, it may be late to throw reaction at shutdown transistor. i will probably run at 200ms loop delay, saves energy and also eliminates the LCD character fluctuations.

apart from that hardware comparator reacts faster than microcontroller programmed execution anytime.

so it will be like this, i will put certain voltage at -ve pin of comparator through adc pins of microcontroller. when thermistor value / sense resistor values go above the pre defined limit, comparator will activate transistor.

but i m not sure in 1N4148s are necessary there or not , may be i should keep them to avoid reverse current flow through other components, when either of them gives high output ?

about mosfet heat dissipation, i had xfx gt 8800 this gfx card (~105 watts) whch died some years ago and i pulled out the heatsink, it has got some weights of metal, 2 heat pipes and lots of fins. also is cooled by a fan.

so, thinking of using it.
 
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EinarA

Joined Sep 13, 2014
5
I don't know where the OP is with this project, but the circuit in post 1 still has several errors. R13,16,18 are too large by a factor of ten. Vref needs bypassing, 1 to 10 uF. Connect the load adjust pot to Vref not +5. The attempt at coarse fine adjustment will not work well when the pots are at minimum setting. C5 is placed wrong ,it goes from pin 1 to 2 and is probably too low in value. The comparator output circuits are wrong, the resistor goes to V+ not Gnd. Since they are open collector you can connect them in parallel with Q3 and remove D1,2,3 and the two resistors. Don't know the value of the thermistor so can't tell if R12 is right.
 

Thread Starter

IcedFruits

Joined Jan 15, 2014
97
op is currently having a hard time assembling and aligning all the components on a 3"x4" board, that the max size eagle free allows.

thanks for ur feedback, i saw that lm385 configuration didnt work, actually picked that up from this datasheet, and it simply doesnt work. checked some other lm385 datasheets, couldn't find the same configuration in those either. so, standard 1.24v is the way to go i think. and i m yet to check the resistor values, so thanks for ur suggestions.

already there is a big 100uf capacitor there, i will try to put lm385 & 78L05 near that, or will do as u suggested.

C5 placement was a mistake from my side, now it is corrected. regarding that comparator o/p, i will remember it ! & thermistor value is 10k.

thanks again for taking interest in my project & becoming a member in the way.

(^-^)ゝ
 

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