EEVblog #577 - Precision 1A Current Source Part 2

[EEVblog]

Part 1:
Dave gets his precision 1A current source circuit working on a breadboard.
LTC6655 http://cds.linear.com/docs/en/datasheet/6655fc.pdf
VPR221Z Precision 4-terminal Z-Foil resistor from Vishay: http://www.vishaypg.com/docs/63116/vpr221z.pdf
LTC6655 Voltage Reference: http://cds.linear.com/docs/en/datasheet/6655fd.pdf
OPA376: http://www.ti.com/lit/ds/sbos406f/sbos406f.pdf

[Chipguy]

Where can you actually buy the precision resistors with customer specific value ?

I know you can buy trimmed SMD precision resistors from Digikey, they say "Enter your desired resistance in the Web Order Notes".
http://www.digikey.com/product-search/en/resistors/precision-trimmed-resistors/66806

However I can not find a source for the TO220 ones

[just_fib_it]

Where can you actually buy the precision resistors with customer specific value ?

You can get them from Vishay directly: http://www.vishaypg.com/foil-resistors/how-to-order/

Caveat: I haven't actually ordered from them, so I don't know if they'll talk to hobbyists.

[Co6aka]

Re the opamp buffer, why not turn it into a second order low pass filter and do away with the huge cap?

[calexanian]

Where is the ground rail of the op amp coming from? The negative sense line or circuit ground? Might be worth trying both.

[casinada]

You had the Overcurrent limit for 1.1A on the power supply but it didn't trigger when you shorted the 12K resistor around minute 25 on the video. Is that protection working? 

[Bored@Work]

You had the Overcurrent limit for 1.1A on the power supply but it didn't trigger when you shorted the 12K resistor around minute 25 on the video. Is that protection working? 

If you look carefully the Rigol's display changes from 1.006 A to 1.100 A the moment he does that around 24:36. Part of the reason he got the awful waveform might because of that current limit.

[BravoV]

Murphy got him ?

[Chipguy]

Where can you actually buy the precision resistors with customer specific value ?

You can get them from Vishay directly: http://www.vishaypg.com/foil-resistors/how-to-order/

Caveat: I haven't actually ordered from them, so I don't know if they'll talk to hobbyists.

Thank you, that's the page I needed.
I have searched the entire Vishay website but didn't find this page with all the helpful information on it about to actually get them.

I got a project at work where I need some of these resistors along with some low thermal voltage relays
http://www.meder.com/fileadmin/products/de_datasheets/8805271800d.pdf

I let you guys know how it went with Vishay, first I need to get some quotes where I expect something like 30-50 bucks for each resistor, we will see....

Yeah, I started the last 3 sentences with "I", dunno why... I know it's rude, but it's 6 AM in the morning here and I am still tired 

[poorchava]

There is also Isabellenhütte, who also produce exceptional resistors.

[Icarus]

Great video.
Does it also slow down the circuit when we add 470uF cap to the system ?

[EEVblog]

You had the Overcurrent limit for 1.1A on the power supply but it didn't trigger when you shorted the 12K resistor around minute 25 on the video. Is that protection working? 

Ah, that could have an effect, although I think it would still oscillate without the resistor.

[Dr. Frank]

Well done, Dave.

Creating precision current sources is always a sort of Fine Arts.

The T.C. of the Vishay shunt resistor can be estimated from your video, when the output did not oscillate any more.

The first stable and maximum reading from the 34461A at 20:57 is 1.000635A, 1.000677A, decreasing due to heating of the shunt, stabilizing to 1.000627A at 21:30. (In first order, I assume that the 34461A does not drift.)

Therefore, the Vishay shunt drifts by 50ppm when loaded with 1.25W.

Specification states 4ppm/W, that relates to the "typical" +/-0.2ppm/K (-55..125°C) in the parameter table 1 in the datasheet.

Therefore, it should typically drift 5ppm only, but the real drift obviously is 10 times that value.

That means, the real T.C. is about 2ppm/K, still within the spread of the specification.

Let's calculate that a little bit differently.
Somewhere in the video, you estimated a shunt temperature of 60°C, i.e. a 40°C temperature rise.
That would give a smaller T.C. of 50ppm/40K, around 1ppm/K.

So, these Vishay shunt are really excellent, but their blatant advertising in the datasheet, i.e. printed in fat letters: typ. 0.05ppm/K @ R.T., is still a little bit exaggerated, by a factor of 20.

Frank 

[nitro2k01]

A silly suggestion: Now that you're no longer using the direct configuration, instead of putting a huge-ass cap over the resistor sense terminals, wouldn't it be possible to do (some or most) this filtering in the feedback loop? Either in the opamp voltage follower or even by putting a smaller cap between the sense and the force line. Am I wrong in thinking this would help stabilize the loop?

[EEVblog]

A silly suggestion: Now that you're no longer using the direct configuration, instead of putting a huge-ass cap over the resistor sense terminals, wouldn't it be possible to do (some or most) this filtering in the feedback loop? Either in the opamp voltage follower or even by putting a smaller cap between the sense and the force line. Am I wrong in thinking this would help stabilize the loop?

Yes there are other ways.
The big arse cap was simply the easiest solution to try first, and it worked.

[calexanian]

I still can't help but feel it should be more accurate. I am still thinking about where the ground for the reference is coming from.

[han]

pardon me,
just a suggestion
-input buffer for sense pin in 4 pin sens resistor,and low pass filter for mosfet

[Skimask]

19:38 - "Not even half a bee's dick"

Worth a decent belly laugh...

[c4757p]

I still can't help but feel it should be more accurate. I am still thinking about where the ground for the reference is coming from.

I'd put money on this. The LTC6655 is drawing its quiescent supply current through the sense line of the resistor - all 5mA of it!

Output current: 1.000631A
Voltage: 1.000631A (1.25R) = 1.2508V
Error = 800uV
Calculated resistance = 800uV/5mA = 160mOhm.

Not unreasonable, I think, for the sense lines.

It's the right polarity, too, as the current will lift up the LTC6655, causing it to put out a higher reference relative to the actual bottom of the resistor.

[han]

Proposed schematic,
-use 2 buffer opamp since the sense wire should not have any current on it...
-low pas filter for MOSFET to reduce oscilation, without big cap on the sens input pin.

[nitro2k01]

Han, what's the point of driving the reference's ground pins lie that? You already have the opportunity to filter the overall loop at two places, and adding the extra opamp might help create instability, and will certainly add an offset voltage from the opamp, no matter how small. But if you want to go this way you might want to drive only pins 3, 5, 8, which are, at least presumably, connected internally to the bandgap reference's bottom side. (See the block diagram on page 10 of the datasheet.) But driving the bandgap reference low side over the chip's internal ground might also create problems.

Instead of your suggested connection, I would probably connect pin 4 (called device ground in the datasheet) to the system ground, ie the low current pin on the shunt. And then pins 3, 5, 8 straight to the low sense pin without any buffering. It seems to me like this exactly what these pins were meant to be used for. So my suggested schematic becomes:

[han]

 i didn't sure  my self that will work prefectly, but i have few reason for using the op amp,
1. I didn't sure witch pin that used for V ref Ground reference (pin 4 only? or 3,5,8)
2. I didn't want to separate pin 4 to pin 3,5,8 since in the datasheet i didn't find any clue where is the band gap reference ground is, and what effect will occur if we separate the ground .
3. If the ground is jointed there is a problem with the current sink

[Co6aka]

Wouldn't configuring the OpAmp sense buffer as a second order lowpass filter reduce or reduce the potential for noise to be introduced into the loop? (The filter's FC would of course have be chosen wisely.) Though the first order RC filter on the MOSFET gate slows the loop down, and seems necessary, it doesn't "solve" a noisy loop.

I've done a lot of work on AGC loops in receivers, which are usually just an after-(non)-thought by designers, and have found that keeping undesirables out is better than filtering them out, so... Thinking the same concept might apply here. 

[YoavR]

Hi!

A small question about the efficiency of the circuit:

This circuit supposes to work on battery power, so does the 1.3W-5W constant power consumption is the best solution?
Is there a way to have a more power efficient circuit?

Thanks
Yoav

(This is my first post, so if I'm breaking any rules - SORRY !!!)

[Marco]

You can have a switcher DC DC converter with a control loop to make sure Vcc remains just high enough to get X volt across the MOSFET, but it will obviously add noise.