High precision, good accuracy Current Source Schematic

[_maxim_]

I am looking for a schematic diagram of a compact, temperature compensated, stabilized, high precision (accuracy within 0.5% or less) current source.
The target current should be easily set to 100/150 mA, no matter if discrete or integrated solutions must be used.
This project will be related to supply current to the Hall Sensor.

Thanks for any addressing.

 

[yungman]

It should not be hard, just use a simple precision op-amp and a MOSFET.

The op-amp is in non inverting configuration with

1) +ve input as voltage control. Input voltage call Vref
2)The output of the op-amp drive into the gate of the N-MOSFET.
3) A resistor of say 10K connect from the source of the FET to the negative input of the op-amp.
4) A 200pF cap from output of the op-amp back to -ve input for stability.
5) A precision resistor of 0.1% or better from the source of the FET to ground. This is the resistor that set the constant current. The drain is the constant current source. The resistor called Rs.The current is equal to Vref/Rs. getting 0.5% is easy. You want better, get better Rs and better op-amp. You want the other polarity, use P-MOS. I am too lazy to draw, scan, upload the drawing. It should be easy enough following step by step. Any question, post back.

 

[_maxim_]

It should not be hard, just use a simple precision op-amp and a MOSFET.

...
I am too lazy to draw, scan, upload the drawing. It should be easy enough following step by step. Any question, post back.

Thank yungman!

Now you've intrigued me and I really want to see the schematic from your own handwriting
Seriously, can you advise me some schematic from the web or suggestions about components?

And what about the temperature compensation? This is a quite critical aspect.

 

[yungman]

Op-amp like OP-27 is not bad. But look at Digikey:

http://search.digikey.com/scripts/DkSearch/dksus.dll

Look at Analog Devices, Linear Technology.

The thing you need to look for low offset voltage, that is part of your error. offset current and bias current is not important.

If you really want to be picky, get 4 terminal resistor for Rs. 4 terminal resistor has two terminals( sense terminal) that hook up to the two end of the resistor body to eliminate the voltage drop on the lead that give you error.

What you ask is not that critical, make sure you have at least 2 volts across the Rs, then the little offset error of the op-amp is not even important.

Temperature compensation? get low drift component and you'll be fine. You can get some really precision resistors! Look into Vishay and particular Caddock resistors.

 

[jim hardy]

National's AN31 is a handy one to print and keep someplace in hardcopy. It's a collection of ad-hoc op amp circuits. It's so old the opamps they suggest are mostly outdated, but what a great "How To" resource.

www.national.com/an/AN/AN-31.pdf
"The Handyman's Secret Weapon"

current sources page 13
adjustable capacitance multiplier page 24

 

[_maxim_]

Thanks guys,

I see the light.

From TI Op Amp Circuit Collection I've found this schematic:

http://desmond.imageshack.us/Himg834/scaled.php?server=834&filename=precisioncurrentsource.png&res=medium

In needs a negative Vin, while there is only a +5 Vcc available.
Also, I think the temp regulation is not so accurate here.
Any more idea?

 

[jim hardy]

looks from its datasheet like your sensor is ~2 ohms ?

with only +5volts available there's not much headroom for opamps, but I've used LM324's there.


see if this looks attractive to you

http://www.ti.com/lit/ds/symlink/lm117.pdf
bottom of page 19 current limiter
of course you'd use a fixed resistor , maybe with a small trim..

that 117 regulator should be okay with 5 volt supply, the regulator needs 3 volts across its I/O terminals, and 1.25 more for control terminal
and your 2 ohm sensor will drop 0.3, leaving

5 - 3 - 1.25 - 0.3 = 0.45 so you haven't a lot of room, but enough by the book.

LM117 says temperature stability is 1% over whole range -55 to +150 (chart page 6) so it'll be important to select a stable current measuring resistor. Dont eat into your voltage margin with long connecting wires.

Yungman's expertise is leagues above mine here.

old jim

 

[_maxim_]

Hi again Jim,

looks from its datasheet like your sensor is ~2 ohms ?

Yes, the HGT-3030 from Lakeshore has 2 Ω as approx input resistance.

with only +5volts available there's not much headroom for opamps, but I've used LM324's there.

see if this looks attractive to you

http://www.ti.com/lit/ds/symlink/lm117.pdf
bottom of page 19 current limiter
of course you'd use a fixed resistor , maybe with a small trim..

I guess you're talking about this circuit (906316):

that 117 regulator should be okay with 5 volt supply, the regulator needs 3 volts across its I/O terminals, and 1.25 more for control terminal
and your 2 ohm sensor will drop 0.3, leaving

5 - 3 - 1.25 - 0.3 = 0.45 so you haven't a lot of room, but enough by the book.

This has been a mistake of mine. It looks like the output voltage is 3V3 rather than 5V, so maybe this makes things a bit more tricky. On the other hand, we can also provide a supplementary line of 5V, with very few additional components, I guess.

LM117 says temperature stability is 1% over whole range -55 to +150 (chart page 6) so it'll be important to select a stable current measuring resistor. Dont eat into your voltage margin with long connecting wires.

Sounds good.

Yungman's expertise is leagues above mine here.

old jim

Yungman is more than welcome!

Just for speaking, what do you think about the following schematic from Maxim, which uses an op amp, a transistor, and a resistor ?

Here there is an application note for DS4303/DS4305, but I am puzzled about the ADJ procedure as well as about how the BJT can operate within its linear range...

Any further consideration is greatly appreciated.


Maxim

 

[yungman]

measuring resistor. Dont eat into your voltage margin with long connecting wires.

Yungman's expertise is leagues above mine here.

old jim

Your are too humble, I don't think so. But thanks

Alan

 

[yungman]

I see you need current source from the top. This is the circuit I come up really quick. Double check whether I make any mistake:
150118[/ATTACH]"]

The reason I kept pushing for MOSFET is because it does not draw any gate current and is a true current source with high output impedance. Using simulate current source implemented with a voltage source has a disadvantage of slow change with changing load impedance and is really not low impedance output.

The idea of the drawing is a differential amplifier with reference to the top sense lead of Rs. The resistor Rf and R1 should be in single match package. So is R3 and R4. I think Vishey sell these precision resistor pair specially for this. I just put the ratio of 1:1, other ratio is possible. You have to work out the detail. One issue is the +5V supply which is a little low for this circuit. It is a little low for even the other ones here!

The adjustable battery is the current control where I=V/Rs. The op-amp voltage follower is used to buffer the sense lead of the Rs so it does not draw any current by the differential amplifier. It is important to choose an op-amp with low offset voltage drift as it contribute to temp. error coef. Rf, R1, R2 and R3 should be under 10K, the capacitor should be about 200pF for stability. The 1K resistor from the output of the op-amp to the gate is to isolate the output from the gate capacitance.

Again, double check my work as I am having fun and it's your neck! :tongue2: :rofl:

 

[jim hardy]

I'm sorry maxim - it's page 19 of the document which is page 20 of the PDF file.

How do you copy those drawings for posting?

Anyhow look at bottom of page with number 19 at bottom which is page 20 of pdf document, for little circuit " precision current limiter"

it's just the IC and one current measuring resistor...

i will read your 4303 datasheet and see what it is.

Basic approach is to set a reference , measure current with a precise resistor and wire an opamp to keep them equal.
Only two difficult parts are a stable resistor to measure current, and a stable reference voltage for desired current.

LM117 has a decent reference and opamp internal to it.

Does this need to be computer controlled, adjustable by a knob, or set it once and leave it there?

I've never worked with 3 volt opamps but they abound. One of them and a Mosfet should work, i think, if we give it a good reference like the MAX6520 and get a good current sampling resistor.

http://www.maxim-ic.com/datasheet/index.mvp/id/1533

If i can figure out how to draw something and post it, will do.

Only trouble i see with your circuit from Maxim appnote is the measured current is referenced to V+ while the 4303's output voltage is referenced to circuit common - so if done exactly as shown current would fluctuate with supply variations.

Would you and Yungman check on my thinking?


having trouble getting this to post... hope it appears

 

[_maxim_]

I see you need current source from the top. This is the circuit I come up really quick. Double check whether I make any mistake:

Interesting!
I like the MOSFET in the output stage, as well as two op-amp!
No too complicated, despite the fact the V is only 3V3 or 5V.
I_out should be 100 mA, thus using 3.3V as input voltage and Rs = 33 Ω low tolerance, I should be on the right side.
What is not clear for me is the limit of V=5V as an issue you mentioned earlier.

But ok! now is the time to find components.

 

[jim hardy]

Here's a current injector i built years ago.

Yungman - Were R3 shunted with a 2 terminal reference and A1 changed to a 3 volt rail to rail amplifier, would we be there? Tweak R values of course.

This worked great with LM324 but i had to give it higher supply, it needs 1.5 volts headroom to V+ on inputs , though they'll sense down to negative rail.

 

[_maxim_]

I'm sorry maxim - it's page 19 of the document which is page 20 of the PDF file.

How do you copy those drawings for posting?

Just a screenshot, save it as jpg/png image and upload to an external sharing server.

Anyhow look at bottom of page with number 19 at bottom which is page 20 of pdf document, for little circuit " precision current limiter"

So, this guy here?

it's just the IC and one current measuring resistor...

i will read your 4303 datasheet and see what it is.

Basic approach is to set a reference , measure current with a precise resistor and wire an opamp to keep them equal.
Only two difficult parts are a stable resistor to measure current, and a stable reference voltage for desired current.

LM117 has a decent reference and opamp internal to it.

Does this need to be computer controlled, adjustable by a knob, or set it once and leave it there?

This stage is only for providing a stable current to the sensor Hall. The output from that sensor will be processed by a PC. No needs to be remotely controled, just set ot once.

I've never worked with 3 volt opamps but they abound. One of them and a Mosfet should work, i think, if we give it a good reference like the MAX6520 and get a good current sampling resistor.

http://www.maxim-ic.com/datasheet/index.mvp/id/1533

If i can figure out how to draw something and post it, will do.

Only trouble i see with your circuit from Maxim appnote is the measured current is referenced to V+ while the 4303's output voltage is referenced to circuit common - so if done exactly as shown current would fluctuate with supply variations.

Would you and Yungman check on my thinking?

having trouble getting this to post... hope it appears

It looks ok for me.

 

[jim hardy]

oops i'll digest yours now

be back tonite, need the daylight for car repairs...

old jim

 

[jim hardy]

maxim - that's the one. Simple to the max, isn't it ??

old jim

 

[yungman]

Here's a current injector i built years ago.

Yungman - Were R3 shunted with a 2 terminal reference and A1 changed to a 3 volt rail to rail amplifier, would we be there? Tweak R values of course.

This worked great with LM324 but i had to give it higher supply, it needs 1.5 volts headroom to V+ on inputs , though they'll sense down to negative rail.

Both this and my circuit use a MOSFET, I am worry about the gate voltage eat up a lot of the supply voltage. Some MOSFET need about 3V between gate and source to work, there goes 3 out of 5V! I think both work if you can get it to work for 5V supply no matter what. I think the MOSFET can be replaced with a darlington hybrid with a p-JFET and a PNP. JFET is to really lower the gate current to get more accuracy and if the drain of the FET drive the PNP base, you don't loss any current at the process. Amps are easy to find now a days. I have been designing using 6V single supply op-amp that can work down to 1.8V single rail. A lot of them are rail to rail. It is the pass transistor that I am worry about.

 

[yungman]

Interesting!
I like the MOSFET in the output stage, as well as two op-amp!
No too complicated, despite the fact the V is only 3V3 or 5V.
I_out should be 100 mA, thus using 3.3V as input voltage and Rs = 33 Ω low tolerance, I should be on the right side.
What is not clear for me is the limit of V=5V as an issue you mentioned earlier.

But ok! now is the time to find components.

There are plenty of op-amp that work down to 1.8V single supply! That's not the problem. The uncertainty for me is the gate source turn on voltage is about 3V, that eat up your supply voltage. Look into it in more detail and see whether you can make it work for 5V. Also a hybrid darlington using a p-JFET and a PNP might help as the gate source voltage is in opposite direction and it might even help. You have to do the calculation, I am only giving a suggestion. Try look for low threshold P-MOSFET.

Maybe it's not a problem. If you can find a P-MOS with operating gate source voltage of -3V, you still have 2V on Rs to play with. Say if the op-amp output is at 0V for max current, the source will be at +3V, so operating voltage for constant current output at the drain can be from 0 to about +3V. Maybe it's not a problem.

 

[jim hardy]

I think the MOSFET can be replaced with a darlington hybrid with a p-JFET and a PNP.

i think i used a n mosfet, could just swap opamp's +&- inputs for p installed other way..

here's an interesting transistor..
http://www.onsemi.com/pub_link/Collateral/ENN7508-D.PDF
see curve Ic versus Vce, pages aren't numbered it's about middle of second page.

will those 3 volt opamps drive as much as a milliamp?

never mind, just looked at one.
http://www.ti.com/lit/ds/symlink/lmv711-n.pdf

old jim

 

[_maxim_]

This forum is fantastic, and you're really lovable persons!

So, being inspired by your good advices, I put together the latest project.

It uses a low power Differential Amplifier (AD8276B) supplied by a Voltage Reference (ADR3425), a low bias current Op Amp (AD8603) for a feedback loop, then an external current source BJT (T1, to be defined).

http://img861.imageshack.us/img861/8749/schematicdiagramsourcec.jpg

Please, have a quick look to that and let me know if makes sense.

Later on I will write here my considerations and calculations for the component sizing, based on datasheet and app notes from Analog Devices.

Of course, any idea, suggestions or criticisms are welcome!

maxim

 

[_maxim_]

About the schematic above, here I have put few comments, hopefully updated by volunteers:

The AD3425 serves as voltage reference to provide a stabilized 2.5V (V_REF)

The AD8276B is used here as difference amplifier, where V_REF is applied to its non inverting input (+) to control the amount of output current, I_O. The inverting input (-) is grounded. The output of the AD8267B is used to drive the transistor (T1 to be defined) since a current of a few hundreds of mA is required.

The AD8603 op amp is used in the feedback loop of the circuit; it has low bias current (< 1 pA), low offset voltage (< 50 μV), and low temperature drift (< 4.5 μV/°C ) .

So, since the AD8276B has R_F1/R_G1 = R_F2/R_G2 = 1, the output current Io depends only from the ratio V_OUT/R₁ = V_REF/R₁. The accuracy of R₁ is critical, so it should have 0.1% tolerance or less.
Considering V_REF = 2.5V and I_O = 100mA, R₁ value is 25 Ω.
R_HALL is ~ 2 Ω

Minimal conditions have to be met, fixing Vs = 5V:

1. V_HALL = I_O × R_HALL must be within the AD8603 op amp input range (−0.3 V , 5.2 V)

2. V_OUT = I_O × (R_HALL + R₁) must be within the AD8276 SENSE pin voltage range (−10.2 V, 7 V)

3. I_O × (R_HALL + R₁) + 2(−Vs) − 0.2 V to 2(+Vs) − 3 V must be within the AD8276 output voltage range (−7.5 V, 7 V)

First 2 conditions are OK. I am puzzled about the last one.

 

[jim hardy]

lets see here

my simple thinking goes thus:

AD8276 will hold his inputs equal

which dictates that since his inverting input sees Vout/2



and his noninverting sees (2.5+Vhall)/2

equating those yields

Vhall = Vout - 2.5
so your circuit will hold 2.5 volts across R1 giving the constant current you seek.


Now a sanity check to make sure feedback is proper direction:
Assume Rhall drops a teeny bit
Vhall accordingly decreases a teeny bit, so (Vout - Vhall) got larger
however the decreased voltage at noninverting input lowers pin6 voltage, driving Vout in proper direction to restore balance at inputs.

Looks Great so far, said the blind man !

At 100 ma i see Vout = 0.1 * (2 + 25) = 2.7 volts, {corrected - jh}
and if hall sensor varied between zero and [STRIKE]0.5[/STRIKE] 5 ohms Vout need only range between 2.50 and 3.0 volts

That amplifier doesn't have a lot of output current capability but datasheet says it can drive to within 0.1 volt of supply with 10 k load so you might get away with a TIP120 darlington driver , 3 Volts + 1.5 for Vbe leaves 0.4 volt headroom.
Yungman is way better than me at selecting a transistor.

Using a PNP darlington emitter side up and swapping Mr 8276's pins 2&3 would more nearly center his pin 6 voltage at mid-supply. Perhaps with just a tad of emitter resistance for loop stability...
You guys will find a better driver I'm sure, there are even IC's made specifically for boosting opamp drive capability.

Your third concern i think is from their warnings to assure headroom,
which looks okay from my simplistic approach above.
Inputs and Vout are near mid-supply and pin 6 has ~ 1/2 volt headroom even with a darlington's two eb junctions. PNP darligton driver (TIP125?) would give about 1.3 volt ..



I have to talk my way through a circuit before attacking its equations.

A most elegant looking solution, my friends.


old jim

 

[jim hardy]

hope i didnt say something socially wrong - we "Aspies" can do that unwittingly.

fixed my voltage blunder above..

 

[_maxim_]

Back after a lot of calculations... maybe I am wrong but I've the impression that swapping pins 2 & 3 nothing changes other than the polarity of Vref:

Vout = [V(+IN) – V(-IN)] x gain

So if you tie Vref to pin 2 instead of pin 3, Vout will be a negative voltage (assuming negative supply).Going back to the original configuration, now I need help to refine calculations.

Let say I want to drive 300mA but keep the same conditions, I would have to increase Vref or R1.
If I change Vref to Vref = Io * R1 = 0.3*25 = 7.5V, this is outside the range of AD8276 on a 5V supply.

The other option is to change R1. So for example, I could change R1 to 10 Ohms, that means Vref = 1V gives 100mA and 3V gives 300mA, and I think it meets all the input range limitations...

Then a TIP120 has a Vbe(on) of 2.5V... Why not use a regular NPN BJT like the 2N1711?
It has a Vbe(sat) of 1.3V (max) with IC = 150 mA & IB = 15 mA

What do you think about?

 

[jim hardy]

Back after a lot of calculations... maybe I am wrong but I've the impression that swapping pins 2 & 3 nothing changes other than the polarity of Vref:

Vout = [V(+IN) – V(-IN)] x gain

So if you tie Vref to pin 2 instead of pin 3, Vout will be a negative voltage (assuming negative supply).

wow, is my face red. i took an old shortcut and it bit me - adding an inverting stage behind an opamp often let's you just swap input pins, and i assumed, without thinking it through, that just swapping 2&3 would do the job..
:Rats. I'm old enough to know better.
I extend my apology.
Eternal vigilance ...

Soo,, much chagrined - This time i did my algebra.
What i missed before is that to swap that internal opamp's input pins , i'd have to swap pins 1&5 in addition to swapping 2&3.
That'd leaves us as follows:
Pin 2 = +2.5 volts, Vref
Pin 3 = 0v
Pin 1 = Vhall + I_o*R1
Pin 5 = Vhall
NOW internal opamp sees at his Inverting input: (Vpin5 + Vpin2)/2
or, Inverting = (Vhall + Vref) /2
and at his Noninverting input: (Vpin1 + Vpin3)/2
or, Non-inverting =(Vhall + I_o*R1 +0)/2
Equating those yields
(Vhall + Vref) /2 = (Vhall + I_o*R1 +0)/2 ;

(Vhall + Vref ) = ( Vhall + I_o*R1)
or,
Vref = I_o*R1 ;;
I_o = Vref/R1 which is what we want.



Now sanity check for direction of feedback:
with a PNP driver and both pairs of those pins swapped

IF I_o increases by Δi,
Non-Inverting input goes up by (Rhall + R1) * Δi
but Inverting input goes up by just Rhall * Δi
inputs now see difference R1*Δi , with Non-inverting more positive

Since Non-inverting input is now greater than Noninverting, AD8276's pin 6 moves positive
which moves PNP's base positive, nearer emitter shutting it off, restoring balance.
So a PNP could work if you need one.
I remember waking up in middle of night thinking i should check on this, but for some reason didn't get it done.

You are right. I didnt finish my thinkling and caused you extra work.

_____________________________________________________________

Now to your question -

Let say I want to drive 300mA but keep the same conditions, I would have to increase Vref or decrease R1.
If I change Vref to Vref = Io * R1 = 0.3*25 = 7.5V, this is outside the range of AD8276 on a 5V supply.

Indeed everything is supplied by +5, your ADR3426 would be hard pressed to deliver 7.5.

Your circuit as you drew it will keep I = Vref/R1
so as you said

The other option is to change R1. So for example, I could change R1 to 10 Ohms, that means Vref = 1V gives 100mA and 3V gives 300mA, and I think it meets all the input range limitations...

Agreed.
The internal opamp's inputs need to be kept 1.5V below supply, ie 3.5 volts.
So Pin 5 must stay below 7 Volts, okay there.
And (pin 1 + pin3)/2 must stay below 7 volts, okay again because Rhall is only 2 ohms and i think you're planning on less than an amp though it.

For the AD8276/AD8277 to measure correctly, the input voltages at the input nodes of the internal op amp must stay below 1.5 V of the positive supply rail and can exceed the negative supply rail by 0.1 V.

AD8276 datasheet page 14
And you can't go negative with just a +5 supply.

Then a TIP120 has a Vbe(on) of 2.5V... Why not use a regular NPN BJT like the 2N1711?
It has a Vbe(sat) of 1.3V (max) with IC = 150 mA & IB = 15 mA

The 15ma Ib worries me. That's a heavy load for your AD9276.
Note his datasheet says , page 4 "output characteristics", short circiut current limit is 10ma. Graphs of output voltage page 9 figs 19-21 have 1kΩ as lowest load. I don't think he's intended to deliver much current.

But with minimum hfe of 100 at 150 ma it should work...

But... Just in case;;
The TIP125 datasheet

http://www.fairchildsemi.com/ds/TI/TIP125.pdf
fig 2 page 3
shows Vce of ~0.75 volt
and Vbe of ~1.25 volt
when Ic = 250*Ib.
That would place Mr AD8276 in the enviable position of sinking very modest current(~ a milliamp) with output voltage well above mid-supply, about 4.25 volts.
With the 2N1711 he could have to source 15ma at (Vhall + VRef +Vbe) = 0.2 + 2.5 + 1.3 = 4.0 volts.
That leaves only one volt room to +supply. That's right at his limit, see fig 21 of AD8276 datasheet.

We should get a cross-check from Yungman on this point.

That's my thoughts.
Let me know how it looks to you.

Thanks for catching my error in previous post. If this one looks cross-threaded anywhere please challenge me on it. You know i'd much rather admit my mistakes than have them cause you aggravation.

Thank you , kind sir !

 

[_maxim_]

Hi Jim,

No worries about possibly error, it's the same for me.
Is always a pleasure to talk to nice people like you!

So, I would actually use a TIP125 transistor as final stage, it convinces me.
Please, help me to fix the polarization network at its best and to choose the proper power supply / Vref for the whole circuit.

Your contribution is greatly appreciated.

 

[jim hardy]

Thanks for your kind words.

Well i always seem to have "just one last correction" ..

"""That would place Mr AD8276 in the enviable position of sinking very modest current(~ a milliamp) with output voltage well above mid-supply, about 4.25 volts."""

SHOULD have read
"" That would place Mr AD8276 in the enviable position of sinking very modest current(~ a milliamp) with output voltage [STRIKE]well[/STRIKE] slightly above mid-supply, about [STRIKE]4.25 [/STRIKE] 3.75 volts.""

--------------------------

The other option is to change R1. So for example, I could change R1 to 10 Ohms, that means Vref = 1V gives 100mA and 3V gives 300mA, and I think it meets all the input range limitations...

Are you saying you want that range of adjustment ?

Soounds easy enough. You already figured R1.
A potentiometer across output of the reference and a voltage follower amp to buffer it would be straightforward approach. AD makes a dual version of AD8603 , the AD8607 so it'd be same package count.

Please, help me to fix the polarization network at its best and to choose the proper power supply / Vref for the whole circuit.

If you like i'll try to sketch something tonite.
How much room do you have for power supply?
What physical arrangement do you envision ? Will you make a circuit board or is this a one-of-a-kind hand wired?

What range of current adjustment would you like to have?


old jim

 

[_maxim_]

Thanks for your kind words.

They're deserved.

Well i always seem to have "just one last correction" ..

"""That would place Mr AD8276 in the enviable position of sinking very modest current(~ a milliamp) with output voltage well above mid-supply, about 4.25 volts."""

SHOULD have read
"" That would place Mr AD8276 in the enviable position of sinking very modest current(~ a milliamp) with output voltage [STRIKE]well[/STRIKE] slightly above mid-supply, about [STRIKE]4.25 [/STRIKE] 3.75 volts.""

--------------------------

Are you saying you want that range of adjustment ?

You know, the sensor need 100mA of polarization current. Up to 300mA is for working condition, so...

Soounds easy enough. You already figured R1.
A potentiometer across output of the reference and a voltage follower amp to buffer it would be straightforward approach. AD makes a dual version of AD8603 , the AD8607 so it'd be same package count.

So far, so good!

If you like i'll try to sketch something tonite.

I do not want to ask you to waste your time with me, although I'd appreciate it a lot!

How much room do you have for power supply?

Let say I have the option to choose the proper supply. Started from 5V because it was obvious (other divices provide 5V lines), we can change this value freely. Only the Vref voltage I wish were provided by Mr. ADR34xx for stability.

What physical arrangement do you envision ?

Only the sensor will be stored inside a probe for high magnetic fields. I think I would keep its wires c.a. 20-40cm long.

Will you make a circuit board or is this a one-of-a-kind hand wired?

Yes, this is the next step. I have to make a PCB as small as possible :)

What range of current adjustment would you like to have?

old jim

I guess 100-300mA is enough.

You're my friend now !

All the best, jim!

 

[jim hardy]

Sorry for delay, just don't get to everything anymore.
Beware of that "smartfortress virus" it's out again, turns off your antivirus and tells you you are infected , and blocks most things you can do to get rid of it. Cant even start up in safe mode if it gets far.
Microsoft antivirus recognized it yesterday, a couple hours late for me.
If you get it google smartfortress and it'll take you to a microsoft link that kills it.
I had to run their killer program and then 'restore' to a few days ago because of damage it did.

Anyhow ---
.
Okay - your scheme is so good it doesn't need much change i think.

All I did was three things:

1. make Vref variable to give 0-300ma adjustment range with 10 ohm R1.
To that end,
Added Radj a ten turn pot with locking dial. That let's you set current by a ten turn dial with good resolution.
http://search.digikey.com/us/en/products/3590S-2-502L/3590S-2-502L-ND/1088592
http://search.digikey.com/us/en/products/H-22-6A/H-22-6A-ND/499307?wt.z_cat_cid=Dxn_US_US2011_Catlink

Added a buffer voltage follower amp A2 to Radj. 500 and 2500 ohm divider on output make his output 0-3 volts for 0-2.5 in. If you use instead ADR3430, omit those resistors.
A2 can be another AD8603, or half an AD8607 with other half replacing your 8603
http://www.analog.com/en/all-operat...plifiers-op-amps/ad8607/products/product.html

The buffer keeps the resistance of your potentiometer from appearimg in series with Mr8276's internal 40K resistors.

2. Added provision for "Criss-Cross" jumpers at inputs of Mr AD8276.
That let's you use either NPN or PNP driver.
Install horizontal jumpers for NPN like your 2N1711 or one of Yungman's Mosfets
Install diagonal jumpers for PNP like my TIP125.

3. Showed three dots instead of drive transistor.
Make two sets of holes on you board, one set for TIP and another for your 2N1711.
That way you can use either NPN or PNP or MOSFET driver, emitter up for PNP and emitter down for NPN.


http://s232.photobucket.com/albums/ee289/oldjimh/?action=view&current=ForMaxim1001.jpg

NOW while typing i realized that you may not need infinite adjustment.
If not, consider this:
Omit buffer and Radj, go back to 2.5 volt Vref.
Make your R1 measuring resistor be three 25 ohm resistors and a switch that let's you select one, or two or three of them in parallel. That'll give you 100, 200 or 300 ma.

And in keeping with Detecive Columbo's "One Last Thing";;

Yungman's sketch in post 10 had a capacitor to stabilize things. Always a good idea.
That additional stage behind Mr8276 increases his open loop gain , but only if we use PNP driver.
Sooo we should add a zero ohm resistor (jumper) between top of driver transistor
and Vsupply , to be replaced with just a couple ohms to reduce loop gain in case PNP exhibits instability.
Can add that to sketch.

What's your thoughts so far?

 

[jim hardy]

i see size is a matter. That ten turn pot is big.

BTW while you're experimenting, try that LM317 circuit from post 14

1.25V/.1amp = 12.5 ohms

 

[_maxim_]

Sorry for delay, just don't get to everything anymore.
Beware of that "smartfortress virus" it's out again ...

So happy to be an "old" Linux user who uses windows only for playing chess... :tongue2:

Okay - your scheme is so good it doesn't need much change i think.

All I did was three things:

1. make Vref variable to give 0-300ma adjustment range with 10 ohm R1.

To that end, Added Radj a ten turn pot with locking dial. That let's you set current by a ten turn dial with good resolution.
http://search.digikey.com/us/en/products/3590S-2-502L/3590S-2-502L-ND/1088592
http://search.digikey.com/us/en/products/H-22-6A/H-22-6A-ND/499307?wt.z_cat_cid=Dxn_US_US2011_Catlink

Added a buffer voltage follower amp A2 to Radj. 500 and 2500 ohm divider on output make his output 0-3 volts for 0-2.5 in. If you use instead ADR3430, omit those resistors.
A2 can be another AD8603, or half an AD8607 with other half replacing your 8603
http://www.analog.com/en/all-operat...plifiers-op-amps/ad8607/products/product.html

The buffer keeps the resistance of your potentiometer from appearimg in series with Mr8276's internal 40K resistors.

That's ok for me.

2. Added provision for "Criss-Cross" jumpers at inputs of Mr AD8276.
That let's you use either NPN or PNP driver.
Install horizontal jumpers for NPN like your 2N1711 or one of Yungman's Mosfets
Install diagonal jumpers for PNP like my TIP125.

Simply so smart!

3. Showed three dots instead of drive transistor.
Make two sets of holes on you board, one set for TIP and another for your 2N1711.
That way you can use either NPN or PNP or MOSFET driver, emitter up for PNP and emitter down for NPN.

Yes, I see.


http://s232.photobucket.com/albums/ee289/oldjimh/?action=view&current=ForMaxim1001.jpg

NOW while typing i realized that you may not need infinite adjustment.
If not, consider this:
Omit buffer and Radj, go back to 2.5 volt Vref.
Make your R1 measuring resistor be three 25 ohm resistors and a switch that let's you select one, or two or three of them in parallel. That'll give you 100, 200 or 300 ma.

And in keeping with Detecive Columbo's "One Last Thing";;

Yungman's sketch in post 10 had a capacitor to stabilize things. Always a good idea.
That additional stage behind Mr8276 increases his open loop gain , but only if we use PNP driver.
Sooo we should add a zero ohm resistor (jumper) between top of driver transistor
and Vsupply , to be replaced with just a couple ohms to reduce loop gain in case PNP exhibits instability.
Can add that to sketch.

What's your thoughts so far?

This circuit is a wonderful solution!
I will keep you informed about progress.

i see size is a matter. That ten turn pot is big.

BTW while you're experimenting, try that LM317 circuit from post 14

1.25V/.1amp = 12.5 ohms

Right. I will try also the LM317' solution: easy and compact.