EEVblog #567 - Precision 1A Current Source

[Sam__]

To remove any possible noise from the PSU you could test it with a battery. Then if it still oscillated you'd know the error was in the topology of the breadboarded circuit.

[Barny]

Sorry my english is realy bad.
And I'm a noob to.

Is it possible that the light (LED and fluorescent tube) makes some nasty stuff?
There are some long unshildet wires flapping around.

[gszo123]

Maybe testing the µcurrents

Or maybe he's testing the new and completely revolutionary no-prefix-current, which can measure currents in the range 1 mA - 10 A. Just like with the µCurrent, the user can plug in a test current on one side and get a voltage reading on the two other  terminals, with the scale 1 V/A. I hear the no-prefix-current features a passive design, which has a heavily reduced complexity compared to the µCurrent and doesn't even require a battery.

Hah, youre right, I ve got leaked schematics freshly stolen from EEVBlog Corp. for the new no-prefix-current. Seems like very robust mil-spec design. Dave's always worked on serious stuff

[TiN]

Dave butchery with poor TSSOP. Need some TSSOP-8 adapters? We have plenty of them here in Taiwan on local radio hobbyists shop.

I had built multi-channel current loads, even with simple LM324 usually things work pretty okay, but to be exact, i was using NFETs (IRF3205 for "low" power channels 0-30A and FB180SA10P for 0-50A). I was doing that for computer power supply ATE, driving 12 channels from 8bit DAC. Caps and proper passives are critical tho, but even on breadboard all should work fine.



Later i got rid off sense resistors, and was just using cable from power input board to FET (around 20-40cm AWG12) as a sense resistors. After calibration DAC/ADC, got accuracy around 1% at most of levels over 3A (again, that was designed for 20A range).



If need, i can post thread covering this ATE build somewhere in forum, just let me know where

P.S. Long time wanted ask you, from previous videos you have all nice gear, Agilent scopes, but mostly using Rigol gear. Why so, is there some deal with vendor, so u tied to that when doing videos? I'd use best available tools for experiments, if there is a choice. Just curious..

[kg4arn]

Thanks Dave,

I really like this kind of video.
Stirs up a lot of thought and insightful discussion.
Stretches my brain to think about what is going on and how to fix it.

Ed

[Frant]

I had built multi-channel current loads, even with simple LM324 usually things work pretty okay, but to be exact, i was using NFETs (IRF3205 for "low" power channels 0-30A and FB180SA10P for 0-50A). I was doing that for computer power supply ATE, driving 12 channels from 8bit DAC. Caps and proper passives are critical tho, but even on breadboard all should work fine.

I don't see any advantage of the circuit shown in the video compared to the well-known one with a MOSFET.

[TiN]

Well, having precision vref may help to have stable current over time, but i think still need opamp to drive FET/BJT from voltage reference.
Main point is not to keep fixed voltage over current sensor, but to keep fixed current flow thru sensor.
I had same mistake before, trying to fix voltage, but quicky find out that when resistor/FET heating up - resistance changes as well, and current drifts, if drive curcuitry have only
voltage feedback.

But issues Dave had on video are not related to this issue.

[jesuscf]

Dave,

After looking carefully at the breadboarded circuit I believe pin 8 of the LTC6655 is floating.  Shouldn't it be connected to GND?  The data sheet says: "GND (Pins 3, 5, 8 ) : Internal Function. Ground these pins."

Jesus

[LaurenceW]

Hmm, at the risk of appearing to give up on the LTC6655 device a little early, what about the MAX6225? I have a couple of these rigged as precision and untrrimmed voltage references. They are ROCK STEADY even on a breadboard, AND are available in good 'ole 8 pin DIPs

[smashIt]

ha!  i'm building a current-source for a power-LED with a max output of 1,4A and 5V supply-voltage at the moment   

the idea was simple:
led on +, then a mosfet (1010N in my case) then a shunt (0,22 ohms) to ground
voltage across the shunt goes into the inverting input of an opamp (lm358) and the noninverting comes from a voltag-divider
all neatly distributed across my breadboard

boy was it a failure 

when set to 0 the the led was lit
when set to max it didn't deliver the 1,4A
the values of the voltage-divider didn't fit the output
it even seemed that the opamp generated an offsetvoltage at its inputs 

what were the problems:
breadboards don't like high currents!
under load ground of the opamp und ground of the shunt drifted apart (~0,1V)
so i soldered the high-current stuff on a separate board  and kept the rest on the breadboard

the output of the opamp couldn't go low enough to switch the fet off
and it didn't go high enough to drive 1,4A through the fet
so i build up a voltage doubler and inverter powered by a single 555 
now the opamp is sourced by -3V and +7V

finaly the damn thing does what it's supposed to do 

[clifford]

Dave: I think a bypass cap between the force and the sense pins should do the trick. (The control loop becomes unstable because the transistor is to slow.) I've tried it in the simulation provided by Jay_Diddy_B and it seems to works there:

[ivan747]

Maybe testing the µcurrents

Or maybe he's testing the new and completely revolutionary no-prefix-current, which can measure currents in the range 1 mA - 10 A. Just like with the µCurrent, the user can plug in a test current on one side and get a voltage reading on the two other  terminals, with the scale 1 V/A. I hear the no-prefix-current features a passive design, which has a heavily reduced complexity compared to the µCurrent and doesn't even require a battery.

Sounds terrific! I'm gonna start saving for the Kickstater campaign... oh wait... 

[jesuscf]

Dave: I think a bypass cap between the force and the sense pins should do the trick. (The control loop becomes unstable because the transistor is to slow.) I've tried it in the simulation provided by Jay_Diddy_B and it seems to works there:

Once again.  The oscillations may be caused by the SPICE engine in LTspice and not the circuit topology itself.   Try this:

a) Right click on the line ".tran 10m startup", un-check "Start External DC voltages at 0V", click Ok.  The line should be now ".tran 10m".  If the power supplies start at 0V shouldn't the initial conditions for inductors and capacitors be also zero?
b) Click "Tools"->"Control Panel"->"SPICE" and change the default integration method to "Gear"; click Ok.
c) Run the simulation again.  Presto! No more oscillations.

Always beware of oscillations in SPICE simulations!

Jesus

[Jay_Diddy_B]

Hi Jesus and group,

While is possible that the oscillations result from the LTspice simulation algorithm.

If you take a circuit like a Wein bridge oscillator, in the absence of noise, initial conditions and transients the oscillation will not start.

If I take my model and I add the SPICE directive: .options method=gear

And change the transient simulation to .tran 10m



No oscillations are seen.

If I take the same model and then add a small step, only 10mV at time = 1ms, the circuit bursts into oscillations:






Would you expect this circuit to work? Running the decoupling capacitors into an op-amp configured for unity gain. Assume the op-amp is connected to suitable rails.



Jay_Diddy_B

[cellularmitosis]

Try substituting the ceramic caps for electrolytics, or add a very small series resistance (0.5-1R) in series with the ceramics. Sometimes their ESR is TOO LOW, forms an underdamped LC tank circuit with the PCB trace/wire/package/whatever inductance, causing ringing on transients and oscillation.

I was just reading about that earlier today!  http://www.acoustica.org.uk/t/3pin_reg_notes3.html

[nitro2k01]

Dave butchery with poor TSSOP. Need some TSSOP-8 adapters? We have plenty of them here in Taiwan on local radio hobbyists shop.

I'm pretty sure a certain PO box in Baulkham Hills wouldn't say no to a little loot.

http://www.eevblog.com/about/ (Scroll down a bit for the address.)

P.S. Long time wanted ask you, from previous videos you have all nice gear, Agilent scopes, but mostly using Rigol gear. Why so, is there some deal with vendor, so u tied to that when doing videos? I'd use best available tools for experiments, if there is a choice. Just curious..

Maybe he thinks Rigol gear is more expendable in case he would goof up completely? Just a guess.

[EEVblog]

Hmm, at the risk of appearing to give up on the LTC6655 device a little early, what about the MAX6225? I have a couple of these rigged as precision and untrrimmed voltage references. They are ROCK STEADY even on a breadboard, AND are available in good 'ole 8 pin DIPs

The LTC6655 is also rock stable on the breadboard as just a voltage reference, no problems there.

[EEVblog]

P.S. Long time wanted ask you, from previous videos you have all nice gear, Agilent scopes, but mostly using Rigol gear. Why so, is there some deal with vendor, so u tied to that when doing videos? I'd use best available tools for experiments, if there is a choice. Just curious..

I have explained this several times. There is no deal, or any pressure from the manufacturers at all. My Agilent scope sits on my working bench, which is an absolute mess and no room for shooting nice videos. My Rigol sits on my cleaner teardown/general video bench where I shot this video. So whatever bench I work at I use the nearest scope to hand. Same with the meters.

[TiN]

Ok, sounds fair, thanks for reply. I'm still new here..

[Oracle]

Rigol have some firmware issues... It's a trap for young players, that's why I invest lot of money on test gears... at least I can say that my instruments are telling me the truth.

[clifford]

Dave: I think a bypass cap between the force and the sense pins should do the trick. (The control loop becomes unstable because the transistor is to slow.) I've tried it in the simulation provided by Jay_Diddy_B and it seems to works there.

Once again.  The oscillations may be caused by the SPICE engine in LTspice and not the circuit topology itself.   Try this:

Fair enough. (I have only tried the spice simulation because it was already there and have not followed the discussion regarding it or have given it much thought.)

I nevertheless think that bypassing the force->sense path will fix the problem. (I'm not sure how much current the force output can source (could look it up in the datasheet) and what frequency/amplitude the oscillation was. It must source enough current to provide the necessary small signal feedback using the 1.25 Ohm as working resistor via the cap.. Its worth a try.)

[SeanB]

IIRC they were a mailbag thing he got. They are pretty common, you get them from all the major component suppliers and places like ADafruit and Sparkfun. Look for SMD breakout boards.

[trackman44]

Would't it be fun if there was a SMD breadboard? Just for SMD parts? That would be awesome. It would solve the SMD to DIP package woes. I'm just throwing an idea out there, hopefuly someone will pick it up and run with it

Will

[Rutger]

Hello Dave - I am interested to know where you source those little SMD breakout boards from on which the LTC6655 was soldered ?

Search for 'MSOP-8' on ebay and you will find plenty, most boards can be used for both MSOP-8 & TTSOP-8

[TiN]

Was making these couple years ago for own use at prototyping:)

SOT23-8 + SO8 + TSSOP8, but wider than DIP