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My constant current load - 90W , 3A 30V

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Spikee:
OLD first post in this topic:

--- Code: ---[b]Schematic:[/b]

[url=https://dl.dropbox.com/u/100819959/Schematic%20Prints.pdf]https://dl.dropbox.com/u/100819959/Schematic%20Prints.pdf[/url]

[b]Info:[/b]
I use a Atmega328 running Arduino IDE to program the code.
I have an external rotary encoder to set the voltage to the dac ( 0-2048) and i instruct it to do a gain of 2.
So the 12bit dac MCP4728 outputs a set 0-4.096V .
The 12bit dac output goes to the first opamp (U4A).
The gain is set by a 15 turn 1k pot to achieve ~12V at 4.096V input.
R5 and C9 act as a low pass filter. I use S2 to switch between linear mode and pwm mode.

 The second opamp U4B is an regular voltage follower. The output has an pulldown resistor R4 (10K) to keep the gate of the mosfet low when needed.

J2 connects to the source of the mosfet for feedback. J1 connects to the gate.

S3 selects if A0 of the atmega is connected to measure voltage or not.
Because if if I > 5A than it wont like it. [[u]text in schematic is wrong[/u]]

The mosfet , 100W power resistor is mounted on a fan assisted heatsink.
U7 measures the temperature of the heatsink.


feedback is appreciated.

--- End code ---
Spikee

Updates:

Update 1 : breadboard - https://www.eevblog.com/forum/projects-designs-and-technical-stuff/did-i-forget-anything-constant-current-load/msg166741/#msg166741

Update 2 : Measurements + photos - https://www.eevblog.com/forum/projects-designs-and-technical-stuff/did-i-forget-anything-constant-current-load/msg166933/#msg166933

Update 3 : Mosfet matching - https://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/msg168787/#msg168787

Update 4 : status + pics - https://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/msg168847/#msg168847

Update 5: made a youtube video for you guys:


Update 6: new youtube video , fixed opamp problem:


Update 7: new youtube video , testing 1ohm vs 0.1 ohm load resistor + running the load at 5V 10A.


Update 8: Big progress , 4 IRFP064N on a big CPU cooler running 63A peak + video
https://www.eevblog.com/forum/projects/did-i-forget-anything-constant-current-load/msg195585/#msg195585

Update 9:
Schematic :
https://dl.dropbox.com/u/100819959/constant%20current%20load/Constant%20current%20EEVblog_Spikee.pdf

Harvs:
Some thoughts:
-   The voltage regulators have no bulk capacitance following them. I would probably put a 10uf tantalum or 100uF electro on their output as well.
-   I don’t see a connection between the 12V bar on the op-amps and the output of the 12V reg.
-   I don’t see any bypass capacitance on the uC.  A 100nF ceramic cap close to the power pins will reduce noise.
-   You’ve connected both AVCC and VREF directly to the power pins.  At the very least AVCC and VREF should be decouple from the main supply noise through an LC circuit as shown in the ATmega328 datasheet.  You have a very hard time trying to keep digital noise out of the ADC as it is, so you need all the help you can get.  Further, if you are using a filtered 5V rail for the ADC just be aware that your accuracy is only going to be as good as your regulator.
-   I would swap D1 out for the fast UF4007, 1n4007 is generally too slow for switching applications. Cost difference is negligible.
-   The LM35CZ power source is best being powered by the filtered analogue power (reduce noise).
-   Instead of switching the input voltage from the analog pin, why don’t you just put a voltage divider (two resistors) between the input voltage and uC input?  A switch as a protection measure is almost certain to get you into trouble one day (e.g. a voltage spike.)
-   The gate driving op-amp needs some compensation to stop it becoming unstable.  If you have a look at my design posted a couple of days ago on here, you can see a resistor isolating the op-amp from the high mosfet gate capacitance and adding a capacitance between the op-amp output and the feedback path creating a dominant roll off in the op-amp response.
-   Can you talk more about what overall specs you’re trying to achieve, and I might be able to provide more feedback on the over parts and voltages in the design.  There’s a few areas that look like they might not do what you’re intending (but that depends on what your intentions are…)

blackdog:
Hi Spikee,

I do not see were you sense your current...
Use a 0.1 Ohm resistor 3 a 5 Watt resistor in your Source.

I made al little schematic for you, its a part of my Electronic Load.


I hope this wil help you :-)

Kind regarts,
Blackdog

Spikee:
I sense the current using connector j2 (who is connected like your schematic on a big heatsink off the pcb) .

I use the NXP buk652R7-30c fet 30V max  200W @25 deg. Celsius.
My Resistor is : Ohmite TEH100 W 1ohm
it is mounted on a ELV LK-40 heatsink capable of 100W when assisted whit the normal fan. 0,73 K / W @ 6.5 CFM
I bought a new fan that does 24 cfm at max speed (delta FFB0412SHN ) 6W 13000 rpm.

My specs are just to try some supply's of mine. My main Agilent psu does 30V @ 3A and it would be nice to fully test it.
My design got some inspiration from jjoster and dave.

The test pcb is beeing manufactured as you speak [for free :) ] i will find / fix the problems after it gets back (a week or so) .



U7 is actually on the heatsink, not on the board.
U1,2 are on a heatsink , i just didn't make it 3d.

Reply's :

I added the switch (S3) because i rarely would be going over 5 amps.
And i will replace the output cap of the voltage regulator whit a bigger one.
The LM36 just gives a ballpark figure of the temperature. I don't need it to be super exact.
The opamp (one lm324N ) is connected to the top rail of 12V and bottom rail of GND.
I will replace the 1n4007 whit  a faster equivalent . I will test if the gate capacitance gives me a hard time. If so i will solve it accordingly (bodge  it) . The ADC could give me a hard time because of noise but i will have to experience it first hand and solve my mistake than.

blackdog:
Hi Spikee,

If you use a 1 Ohm resistor and the design is for 5A, you wil lose 25 Wats in this resistor!
And, the minimum voltage you can use 5A wil be around 6V.
Also wil the opamp U4B have not enough drive voor 5A and 1 Ohm resistor.
The drivelevel wil be: Voltage drop Sensresistor, Wires, Gatedrive voltage.
This wil be around 10V.
The output commonmode voltage of the LM324 on the + site is around 3V.

Kind regarts,
Blackdog

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