EEVblog #577 - Precision 1A Current Source Part 2

Dave gets his precision 1A current source circuit working on a breadboard.

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[XFolf]

I remember as a kid, not being able to afford anything more than the time to recycle bits from dead electronics, I'd sit for hours hand matching sorting resistors by actual value for when I'd need an exact value... today's resources are amazing.

[mchgartenwohnung]

Dear Dave, as already mentioned below you should play with the gate resistor. Also you should add a GateDrain (Not Gate Source!) capacitor in a small value. (47p-100p).

If you want to do a PCB:
Also you already you want to use a excellent opamp you should consider using the reference as reference (datasheet: typical application) and the opamp not only as a impedance converter but as regulator/amplifier. You already use a excellent part, so use the incredible Gain-Bandwith product you have provided by that part. But be aware that the offset for your opamp increases when you have to high rail voltages. (Refer datasheet)

One more thing to tweak precision, but handle these advises with care:
For the ground connection of the power paths you should do a star-grounding, NOT returning over the sense pin. The sense pin is not meant for current.
Instead I would recommend to use your spare OpAMP as impedance converter for the negative sense terminal and drive the reference ground.

Greetings from Germany.

[Yocairo]

As a beginner electric engineer I found this video very informational.

[manytoolsmike]

thanks Dave. I learn so much from the way you walk through, step by step, your thought process on the bread board.

[microcontrollersandmore]

Great vid!  The "how to make a circuit that does XYZ" type videos are always appreciated by those of us with a software / digital electronics background!
-MCM

[AlexandreKandalintsev]

A compensation network is needed when putting a pass element like mosfet here. This is because it adds a phase lag. Of course you can just throw random caps to attenuate high frequencies and hope it will work well. But it does not guarantee stability. Only freq. response analysis over a range of temperature and loads (and supply voltage!) can ensure stability problem is solved. Also, gate resistance looks way bigger than it normally is. Most schemes use 10-100 Ohm gate resistors. Larger values may not actually solve problem, it hides it. This scheme is a perfect example of it: 12k was not enough, it needs a cap in the right place. Another thing is that electrolytic caps are bad in compensation loops as they have poor frequency response, bad tolerance and dry out.

[dearjulio]

I love these "fly on the wall" videos. Learned heaps. Nice to see how you trouble shoot things. Thanks Dave!

[loud]

These intermediate level videos are the best :-)

[pete]

This is awesome, I watch this blog all the time and just out of pure coincidence I was about to looking for a precision 1amp current source when I opened up my laptop earlier today!!

[metaforest]

bravo, Dave I've always shied away from testing precision circuits on a plug-board like that.  Thank you for showing that with just a little care, one can get meaningful results and some confidence that the properly-spun PCB WILL meet spec. 

[FirstName-]

Bloody good video Dave, lots of insightful info there for us juniors!!  

[edmoore]

So sad that you had to point out that Linear app note designs might not be tested to work. Jim Williams would be turning in his grave.

[redtails]

Great video, great demonstrations! Each time I was screaming "well instead of talking about it, try it!" and you did, great!

[ArnoldSzathmary]

I would never try to do this kind of stuff on a breadboard. Mind the inductance of the wires. They are pretty much unpredictable.

[rogeronslow]

The 12k gate resistor and the input capacitance of the mosfet creates a pole in the transfer function. The phase shift associated with the pole is causing the oscillation. The solution is to decrease the value of the gate resistor. This will move the pole much higher in frequency.

[park]

Great Dave, I'm searching for a none "burning" solution for weeks, unfortunatly all those parts are currently out of stock ! Damn...

[caltonpizzaco]

Great video. It's good to see that you registered that version of Dave CAD. I know he's particular about people using the pirated versions!

[ChrisGammell]

Have a real similar circuit on the BenchBudEE for current output. I put a PTC resettable fuse on the high side V+

[ljay]

the solderless breadboard you are using is "riddled" with inductance and stray capacitance. You need to hard-wire the circuit and keep the circuit tight.

[lejink]

i dont understand wtf ur talking about half the time..but i enjoy listening to u