How Can I Design a Digitally Controllable Constant Current Source?

[jim hardy]

23.5 ohms each way.

Here's what i think happened...

That valve is a solenoid with lots of inductance.
When one attempts to suddenly stop current through an inductor , the inductor opposes that . You've heard of "inductive kick "?
I expected to see a warning for this in the valve datasheet, but didn't. Closest thing to it is that note i cited above. I emailed manufacturer asking about it, no answer yet.

Anyhow i think we have ruint the MOSFET and LM324.
Mosfet died, most likely from inductive kick but possibly from overheating.
LM324 died when mortally wounded mosfet let current flow out its gate into pin 1 . Observe LM324 can only sink 20 milliamps
so our path around R1 is through a burnt up LM324.
Observe on LM324 datasheet at http://www.ti.com/lit/ds/symlink/lm324.pdf
page 2, "amplifier schematic"
output pin goes to a PNP straight to V-, the "rake" symbol , with no resistor to limit current.
If it shorts we have path around R1: of out Mosfet gate edit into LM324 pin1, then straight to AGND.

Good thing you put the voltage divider in there !

Fix is three stage:
1. Eliminate inductive kick by connecting a common everyday diode , like 1N4001 or 1N4148, across valve terminals.
Be sure cathode (stripe) is on +24 side, anode to Load side.
That'll protect next mosfet against inductive kick.
2. Eliminate overheating of Mosfet.
Observe in mosfet datasheet at
http://www.supertex.com/pdf/datasheets/VN3205.pdf
curve "safe operating area" on page 3
it specifies case temperature of 25degC, room temperature
and at 100 miliamps only allows 8 volts drop
i'm sure we are overheating it
we need a bigger mosfet and a heatsink

3 (not mandatory but i'd do it) Protect LM324 against overvoltage on 24V supply
Move LM324's V+ from +24 to a +5 or +12 supply terminal coming from Mr RIO.
We only need to get out of LM324 1.65 volts for R1 and ~1.5 volts for gate drive to Mr Mosfet, total = 3.15 which LM324 should do from a 5V supply.

Now this one confuses me a bit

One thing though: I put the multimeter terminals across OUT and V+ with load connected and I see 26.2V. I disconnect and I get 25..9V. Meanwhile I am still having the voltage drop of 1.65V across the SET 10 ohms resistor. Pin 2 of Op-AMP is still 0.265V as before.

1. Is SET 10 ohms R1 or is it part of the voltage divider ? Maybe you'd put up a current schamatic, I'm still going on this one which doesn't mention SET 10 :

2.

Pin 2 of Op-AMP is still 0.265V as before

meaning we get current through R1 when valve is disconnected?
Where does that 26.5 milliamps come from? I'm guessing through a fried LM324.

Don't despair - we have so far succeeded in letting the smoke out of only cheap parts.
And we lucked out with MBG's voltage divider - THANKS MBG !

Does this make sense?

 

[jim hardy]

just so they're easier to find
LM324 datasheet http://www.ti.com/lit/ds/symlink/lm324.pdf
MC3403 datasheet http://www.onsemi.com/pub_link/Collateral/MC3403-D.PDF
PVQ valve catalog page http://content.smcetech.com/pdf/PVQ.pdf
Supertex mosfet datasheet http://www.supertex.com/pdf/datasheets/VN3205.pdf
IRF mosfet datasheet http://www.vishay.com/docs/91015/sihf510.pdf

 

[Lexilighty]

Thanks Old Jim. I will implement the suggestions when I get to the lab this morning. I am so grateful for your willingly offered expert advice. I've learned s!*t! :)

 

[Lexilighty]

And I will surely post an updated schematic once I get access to a scanner. And yes, the set resistor is the ohm bottom one. Is there a power MOSFET with heatsink that you'd recommend?

If I change the supply pins to either +/-5 or 12V, will we get enough 24V to drive the solenoid? I thought the solenoid works at a voltage of 24V and currents of between 100mA and 165mA? I am not clear about this.

 

[jim hardy]

If I change the supply pins to either +/-5 or 12V, will we get enough 24V to drive the solenoid? I thought the solenoid works at a voltage of 24V and currents of between 100mA and 165mA? I am not clear about this.

1. It's only the V+ supply pin for the LM324 that you'll move,
solenoid still goes from +24 to OUT(mosfet drain).
So it still gets its full voltage.
LM324 doesn't need 24 volts, all it need is enough to turn on Mr Mosfet.
Mosfet will turn on when gate is 1.5 volts > than source
and at 185 ma source will be at 1.65 volts, that's the voltage across R1
in electrical circles we use the phrase "voltage with respect to (a point in your circuit)", abbreviated "wrt"
and all wrt means is your meter's negative lead is placed on that point

so at 165 ma you'll have
top of R1 wrt AGND 1.65 volts
source of Mr Mosfet 1.65 volts wrt AGND because electrically that's the same point as R1 top
gate of Mr Mosfet about 3.15 volts wrt AGND , the 1.65 across R1 plus the ~1.5 to turn on mosfet

Since LM324 doesn't need to make much voltage why give him a blank check ? He only need 5 volts to make 3.5,
and if he melts inside there'll only be 5 volts that can break out and wreck something else.
SO 5 or 12 volt supply should be enough for him.

2. I will look for a mosfet.
Exact part is not critical,
at 100 milliamps i figure it'll dissipate about a watt
that's 14 volts across solenoid and ten across mosfet, ten volts X 1/10 amp is a watt
so look for a N channel in a case like this, it's called TO-220
you can probably find some in old dead computer power supplies or VCR's

with ratings of at least an amp and 50 volts,
that's what your little TO-92 was rated for
but not 50 volts and an amp at same time, see its "safe operating area" curve.Then bolt a couple square inches of aluminum or copper to it , or find TO-220 heatsink.
I've been known to split and flatten half inch copper plumbing pipe for heatsink, works great. A thrift store aluminum cookie sheet is another source of aluminum sheet
Be aware good contact between heatsink and transistor case is important. Sand the heatsink smooth on the contact side and bolt it firmly.
Most any silicone grease in the joint will help.
Be aware heatsink will likely be at Drain voltage so keep it away from chassis...
unless you're lucky enough to find a mosfet with isolated tab. Then you can bolt it to chassis.3. Your perseverance ir commendable and rewarding to see.
I'm embarassed to have missed the mosfet power dissipation issue
and I'm guilty of assuming the solenoid had internal provision to suppress its inductive kick .
In higer circles "inductive kick" is called "flyback", a term left over from tube television days .

Thank you for your patience

We learn by doing. And you're doing great !

 

[Lexilighty]

so look for a N channel in a case like this, it's called TO-220
you can probably find some in old dead computer power supplies or VCR's

I found an NTE966 and luckily got a heatsink from a burnt circuit I smoked couple of weeks back. I know if functions as a current regulator (see page 2). On page 3, it says VIN, GND and VOUT as the terminals. I am assuming these are Gate, Source and Drain respectively.

One more thing, it does an AMP but not up to 50V. You think this is still good enough?

 

[jim hardy]

Here's a robust mosfet that's available at Radio Shack for a couple bucks , half that mail order...
i figure you need to get going quickly.

IRF510
http://www.vishay.com/docs/91015/sihf510.pdf
http://www.radioshack.com/mosfet-irf510-transistor/2762072.html#&tab=tab2

looks like it needs a volt more gate voltage to turn on than does your VN3205
so the LM324 may or may not drive it with only 5 volt supply. But 12 would work great.

this one would probably work too though i couldn't find a very good datasheet on it
http://www.radioshack.com/nte2985-mosfet-power-n-channel-60v-30a/55052956.html

 

[Lexilighty]

The NTE966 won't do?

 

[jim hardy]

I found an NTE966 and luckily got a heatsink from a burnt circuit I smoked couple of weeks back. I know if functions as a current regulator (see page 2). On page 3, it says VIN, GND and VOUT as the terminals. I am assuming these are Gate, Source and Drain respectively.

No, it's not a MOSFET at all
it needs 7 volts across itself so won't give you full current

from the fine print in that figure on page 2

the output voltage compliance would be the input voltage less 7 volts.

 

[Lexilighty]

Oh, the datasheet's page 2 deceived me. I will go pick up the IRF510 at a local store in my city this afternoon.

 

[jim hardy]

Here's NTE's N-channel mosfet selector guide. TO220's are a couple pages in.

http://www.nteinc.com/Web_pgs/MOSFET_N_Ch.html

TO3PML package would work too

 

[Lexilighty]

I tried this with the IRF510 but I kept shorting the transistor somehow. Gate - Source resistance is almost zero all the time. I replaced the transistor with another IRF510 extra once and I can barely measure any voltage across OUT and V+. So I replaced this with the spare Supertex I was using before and the voltage drops across each opamp input is back to 1.65V and the voltage across OUT and V+ is 23.16 after taking my V+ supply to 30.89V. I have the V+ of the opamp now wired to +5V of the RIO and all GND are on the RIO. As meBIgGuy early on suggested, I put a 10k resistor between opamp output and GND and I got 1.451 voltage drop across it. I know the 23.16V comes from (30V of supply - 5V of RIO - 1.65V across Opamp input - 1.451 V across 10K resistor. Bear in mind that I am using the MC3043 opamp as opposed to the 324.
But here is a shocker, my valve gets powered but when I vary the voltage across the analog out of the RIO, I am not observing a change in air flow across the valve. Something beyond me.

 

[jim hardy]

I tried this with the IRF510 but I kept shorting the transistor somehow. Gate - Source resistance is almost zero all the time.

dont take offense at this
the Supertex has gate on middle lead, left to right S G D
the IRF has gate on outside , middle is drain, left to right G D S

it'd be real natural...
let me decipher the rest of your post it's a lot of observations to tie together...

 

[jim hardy]

I replaced the transistor with another IRF510 extra once and I can barely measure any voltage across OUT and V+.

so the IRF510 isn't conducting ?

So I replaced this with the spare Supertex I was using before and the voltage drops across each opamp input is back to 1.65V and the voltage across OUT and V+ is 23.16

Sounds like the Supertex conducts but IRF doesn't. And sounds like the circuit worked with Supertex...at least long enough for you to measure equal voltage at opamp inputs.

As meBIgGuy early on suggested, I put a 10k resistor between opamp output and GND and I got 1.451 voltage drop across it.

that's confusing - it would seem the opamp was trying to turn the mosfet OFF but couldn't... 1.45 volts to gate of mosfet whose source is at 1.65 should turn it off...
and that agrees with this observation

But here is a shocker, my valve gets powered but when I vary the voltage across the analog out of the RIO, I am not observing a change in air flow across the valve.

It sounds like we are unable to turn ON the IRF
and unable to turn OFF the Supertex...

Need to split system logically in two so perhaps we can narrow down problem...

but when I vary the voltage across the analog out of the RIO, I am not observing a change in air flow across the valve

Is opamp trying to control mosfet? Let's look.
BTW which mosfet is in there now ?

What is voltage at opamp output with RIO output at zero?
pin1 =
What are voltages at opamp inputs then?
+in pin 3=
-in pin 2=

What s voltage at opamp output with RIO output at max (calling for 165 ma ) ?
pin 1 =
What are voltages at opamp inputs then?
+in pin 3 =
-in pin 2 =

Hopefully that'll tell us which direction to go looking for trouble .

 

[meBigGuy]

Sorry to just pipe in occasionally. I'm not sure you are correctly looking at the issue of the solenoid voltage.

Regarding the voltage across the solenoid. It will not be 24V unless you are trying to drive max current. The 24V voltage is split between the MOSFET/R1 and the solenoid.

The full 24V supply is alway dropped across the solenoid/MOSFET/R1. If you are set to 0 current, there will be 0 volts across the solenoid, and 24V across the MOSFET. As you increase the current, the voltage across the solenoid increases (per ohms law, E=IR where R is solenoid resistance).

So, as you increase the current, the voltage across the MOSFET/R1 will DECREASE as more voltage is dropped across the solenoid.

Sorry if that was already understood.

 

[jim hardy]

Sorry if that was already understood.

Hey THANKS for piping in.

Most of us think in pictures and communicate in words.
Party A describes in words the picture in his mind,
Party B receives said words and from them paints a picture in his mind;
How closely do those two pictures resemble one another ? Depends on a lot of variables...
I'm very prone to mis-communication, mildly Asperger i think...So your observations and input are helpful.

I've been thinking about this last night.
Hypothesis:

I tried this with the IRF510 but I kept shorting the transistor somehow. Gate - Source resistance is almost zero all the time.

Were the IRF installed assuming it has same pinout as the NN3205, which it doesn't,
Source would be at OUT , bottom of solenoid
Drain would be at opamp output pin 1
Gate would be at R1.
The mosfet's internal diode would be forward biased allowing solenoid current to flow into opamp output
probably wrecking opamp
and if it burnt pin 1 open internally, that'd put 30 volts source to gate exceeding its 20 volt rating
and the next mosfet wouldn't pass any current until its gate-source junction failed.

that hypothesis sort of agrees with this observation

I tried this with the IRF510 but I kept shorting the transistor somehow.

and with this observation

I replaced the transistor with another IRF510 extra once and I can barely measure any voltage across OUT and V+.

and I'm not quite sure what this one means

Gate - Source resistance is almost zero all the time.

so let's see whether he finds opamp still working and build from there?

From PF homepage:

PF values productivity
• Disciplined to remain on-topic
• Honest recognition of own weaknesses
• Solo and cooperative problem solving

He's SOOO close to success on this thing...
Again - THANKS, mBG

What're your thoughts ? Do you think this thing could be oscillating? Maybe look for AC across solenoid?edit duplicate post deleted

 

[Lexilighty]

Is opamp trying to control mosfet? Let's look.
BTW which mosfet is in there now ?

Right now, I have the VN3205 on the board. And BTW, I took cognizance of the connector pinouts of the IRF510 as against the VN3205, big Jim.
Everything I am testing for now is with the VN3205.

What s voltage at opamp output with RIO output at max (calling for 165 ma ) ?
pin 1 = 1.468V
What are voltages at opamp inputs then?
+in pin 3 = 1.659V (RIO out is at 1.65V)
-in pin 2 = 1.735

Hopefully that'll tell us which direction to go looking for trouble .[/QUOTE]

Alright.

Note, I am powering the Opamp from the RIO +15V and GND connectors. Voltage across R! is 1.698V

 

[Lexilighty]

More so, I saw the following, prompting me to think the shunt resistor should be across the motor and V+...and ground the source ?

 

[jim hardy]

What s voltage at opamp output with RIO output at max (calling for 165 ma ) ?
pin 1 = 1.468V
What are voltages at opamp inputs then?
+in pin 3 = 1.659V (RIO out is at 1.65V)
-in pin 2 = 1.735

Great info! I should have known you'd catch the pinout but one has to ask to eliminate the question. No offense meant.
Good point on +15 for opamp, too.

From your readings
Opamp sees higher voltage on his - in than on his +in, so he should be driving his output as low as he can.
1.468volts doesn't sound real low,
but
a mosfet with its source at 1.698 volts
and his gate at 1.468 volts
shouldn't conduct.

-in pin 2 = 1.735... Voltage across R! is 1.698V

R1 and pin 2 are joined by a wire ? that's a lot of difference... out of curiosity, where is negative lead of 24 volt supply physically connected?

Okay, it looks like opamp is trying to turn OFF the mosfet when we call for full current,
Does he try any harder when we call for zero current?
What is voltage at opamp output with RIO output at zero?
pin1 =
What are voltages at opamp inputs then?
+in pin 3=
-in pin 2=

Then let's help him and see if mosfet will turn off.
Jumper drain(or opamp pin 1) to AGND
and read again
out pin 1 =
+in pin 3 =
-in pin 2 =

volts ACROSS solenoid, and we'll check for AC in case we'e oscillatng
volts DC =
volts AC = ,, if there's enough to measure get frequency if your meter is so equipped.

now remove jumper from drain and repeat solenoid measurement
volts DC =
volts AC = ,, if there's enough to measure get frequency if your meter is equipped.

That'll tell us if mosfet is capable of following directions he gets from opamp. And hopefully whether he's oscillating.

Old troubleshooter technique - when nothing looks bad, make everything prove itself good.
I'm curious of that 1.4 volts at opamp out
and the 1.735-1.698 = 37 millivolts difference between R1 and pin 2

all GND are on the RIO.

Is there measurable voltage between physical bottom of R1 and GND ?

 

[jim hardy]

More so, I saw the following,

Those parts across the motor are to absorb "flyback"
the resistor/diode combination let's you control how much kick there'll be by choosing resistor value
the diode/zener does same thing

we ought to have a diode across our solenoid, it'll both damp oscillation and save the mosfet when current is shut off quickly.We could measure current above mosfet by something called "high side sensing"
i don't think we need to get that extreme, though.

This should turn out to be something simple.
Mother Nature always makes us work hard for our lessons.
We humans seem to learn from our mistakes instead of from good examples like we should. At least i had to.

old jim

 

[Lexilighty]

Okay, it looks like opamp is trying to turn OFF the mosfet when we call for full current,
Does he try any harder when we call for zero current?
What is voltage at opamp output with RIO output at zero?
pin1 =1.492V
What are voltages at opamp inputs then?
+in pin 3= 0.048V
-in pin 2=1.7632

volts ACROSS solenoid, and we'll check for AC in case we'e oscillating
volts DC =
volts AC = ,, if there's enough to measure get frequency if your meter is so equipped.

Jumper drain(or opamp pin 1) to AGND
and read again
out pin 1 = 0.010V
+in pin 3 =0.036V
-in pin 2 =0.003V

volts ACROSS solenoid, and we'll check for AC in case we'e oscillatng
volts DC = 23.34V
volts AC = 0.6mV,, if there's enough to measure get frequency if your meter is so equipped.

Is there measurable voltage between physical bottom of R1 and GND ? 0.001V

R1 and pin 2 are joined by a wire ?

Directly beside the pin2 on the bread board.

BTW, pin 1 and GND are now 1.391V, piin 2 and GND now 1.594. Pin 3 is 0V with Vref of RIO at 0V and it's 1.654V with Vref of RIO at 1.654.

 

[Lexilighty]

I noticed something strange just now. The carbon-film resistor I have been using is 1/4W. Currently, at max voltage from RIO, we are passing 0.27W through it. Does this matter?

 

[Lexilighty]

Our Circuit is finally working. I realized I was not meeting the wattage requirement of the shunt resistor. I was passing a max voltage of 1.65V through a 10 ohms carbon-film resistor. That makes 165mA across the resistor at full throttle from Mr RIO. Power dissipated across R1 is 0.27W. Carbon-film resistor I was using was 1/4W.

It dawned on me to calculate this few minutes ago. So I replaced the 1/4W 10 ohms with a 7 ohms 15W and bingo! The circuit now works like a charm. There’s still a lot of oscillations in the circuit though. I figure a few caps would do to keep gentle transistor drain current stable. Cheers to perseverance. Mother nature always has a hard way of making us learn. Beats simulation, hehe!

 

[jim hardy]

What is voltage at opamp output with RIO output at zero?
pin1 =1.492V
What are voltages at opamp inputs then?
+in pin 3= 0.048V
-in pin 2=1.7632

Great Scott !
Pin 3 went down meaning Mr RIO requested Mr Opamp to lower current.
Pin 2 didn't decrease meaning we still had current ?.
Neither did pin 1.
Looks like Opamp didn't try any harder to shut off current.

How about when w gave him some help ?

Jumper drain(or opamp pin 1) to AGND
and read again
out pin 1 = 0.010V
+in pin 3 =0.036V
-in pin 2 =0.003V

Aha, the plot thickens
Pin 2 went down, inferring we successfully turned off current through R1.

So the mosfet will turn off current if directed by opamp?
Are you thinking what I'm thinking - Mr Opamp isn't changing his output at all ?
(by the way - for now only run current zero or max because Mr Mosfet will overheat at mid-range currents)

meBigGuy suggested a long time ago a 10K resistor from pin 1 to ground.
That is called a "pulldown" because it helps pull the output voltage toward zero.
We went to the extreme and used a zero ohm pulldown resistor. It looks like that worked.

But my test wasn't bulletproof - we might have bypassed current around R1 again.

If you're not out of patience
would you lift drain wire from opamp pin 1 and briefly hook it to source, top of R1?
Does that drive R1 to zero volts?
Does it stop air out of solenoid?

Then take the gate wire off source.
Select on your DMM 200 milliamp scale
connect meter between that gate wire and AGND

If you get current out the gate wire, the mosfet is shot again

if you get no current out gate and solenoid air stops
then opamp is not living up to its promise of "output voltage can include the negative rail".

We'll try a 1K pulldown, or replace opamp.
I never used a 3403 and its specsheet is less than crystal clear
says in first page text that Vout can include negative supply
but page 4 note 5 says it needs a 10K pulldown resistor to do it.
Page 3 section "Output Voltage range " suggests Vout might go to only ~1.5V shy of negative supply and that's just what we are observing.

Fix may be as simple as a resistor from opamp pin 1 to gnd.
If it needs 10K i'd use 3K.

Does this make sense to you, Lex ?
What do you think, meBigGuy?
I've been known to criss-cross my logic - part of that Asperger i think.
Eternal vigilance is the price of accuracy !

Pin 3 is 0V with Vref of RIO at 0V and it's 1.654V with Vref of RIO at 1.654.

Good - the expensive RIO and divider are okay

BTW, pin 1 and GND are now 1.391V, piin 2 and GND now 1.594.

Just can't seem to get that pin 1 below 1.4volts , can we?

If you want, just try 1k from pin 1 to gnd. Above tests though should remove any lingering doubts about mosfet.

old jim, trying to be thorough
thanks for your patience

 

[jim hardy]

I noticed something strange just now. The carbon-film resistor I have been using is 1/4W. Currently, at max voltage from RIO, we are passing 0.27W through it. Does this matter?

It will not last long. If it smells bad it's probably failing.

Great news on your success - open resistor all the while?

 

[jim hardy]

i'm having difficulty with system it seems to ignore "post reply"

oh well

congratulations on your success
can you describe the "oscillations" ?

Feel your Mosfet the VN2035 will overheat at currents around 100 ma
volts X amps must stay well below 0,56

 

[jim hardy]

Cheers to perseverance. Mother nature always has a hard way of making us learn. Beats simulation, hehe!

You made my day ---- Thank You !

if you've not discarded that carbon 10 ohm resistor,

how many ohms does it read now?

Shutoff current of your mosfet should be a milliamp or less
so your resistor should be 1500 ohms or more?
a pulldown resistor is probably still a good idea
as would be a stout mosfet on a heatsink

 

[Lexilighty]

how many ohms does it read now?

Shutoff current of your mosfet should be a milliamp or less
so your resistor should be 1500 ohms or more?

Actually, it's discarded :(

Right now, I am using a 7 ohms 15W wirewound resistor. Not entirely a bad choice!

And btw, I am using my last IRF510 after I have burned the remaining two I got yesterday. It's glued like a bad habit to a heat sink.

I will try with a 3k resistor to circuit ground as you have suggested.

 

[jim hardy]

All that for a resistor... I'm a little embarrassed--- seems you mentioned someplace early on that it was 1/4 watt.
but we were moving in right direction. And it was fairly methodical.

One should use resistors at half their rating.

Mother nature always has a hard way of making us learn

I've been learning from my mistakes for almost 70 years now and have built up a great repertoire of them.
How're you coming along ?

old jim

 

[Lexilighty]

Something stranger keeps happening though. I returned from an outing and tried to test once again and it seems the transistor, IRF510 is gone again. So I replaced it and I can vary flow across my solenoid valve from 1.17V to 1.65V using the 7 ohms. Bear in mind I am using the IRF510 and it has a heat sink.
I have a 2.9k resistor between pin 1 and GND, a diode is sitting on the +ve terminal of the valve to prevent inductive kick, and 0.1uF metallic film caps between pin 1 and GND/pin 4 and GND to shut off ripples.
Voltage across pin 1 and GND now 10V, pin 2/3 and GND now 1.298V and RIO supply is 1.295V. I have 1.273V across R1. So I am somewhere good on the opamp side.
But on the question of varying the voltage in the solenoid valve, I am doing more than the manufacturer required specs of 100mA and 165mA at 24V but the solenoid works. Maybe there is something I am missing.
At 7 ohms, my volt from RIO to drive 100mA to 165mA for the valve should be 1.15V to 0.665V but it seems this valve is requiring more volts. Hopefully I have not damaged the valve.
Again, your expertise will be appreciated, big Jim.
Thank you for your good insight all the while.

 

[jim hardy]

metallic film caps between pin 1 and GND/pin 4

that one should come out. Opamps don't like capacitive loading very much
try it between 3 (+in) and gnd instead. That'll take noise out of input signal.

Where does your 24 V supply negative wire connect to gnd ?
Physically run your finger along the wires that valve current must take to get from R1's bottom to -24.
It is important for that current to NOT flow right adjacent opamp's V- pin.
Run one wire from bottom of R1 to AGND
and another wire from bottom of R1 to 24V negative. That one carries load current back to supply , separated from opamp,
so that load current cannot inject noise into opamp.

1.273V across R1

=185 ma ? 185/165 = 12% high..
How close to 7 ohms is that resistor? Can you measure it? 112% of 7 ohms is 7.84 ohms... probably it's closer than that.

Voltage across pin 1 and GND now 10V,

That one i don't understand at all yet. 4 or 5 volts should have him fully on.
Is there any AC in it ?
Ahhhh never mind - i'l bet he IS fully on.
Measure volts across MOSFET S-D when he's got ten volts on his gate: should be less than a volt, probably less than 0.1 volt
In other words, your mosfet puts full voltage across valve and there's no more available.
But Mr Opamp thinks you need just a little more so increases gate drive.

Go to about half current and repeat
Pin 1 (opamp out) =
Pin 3 (RIO requested current) =
Pin 2 (Measured current) =
i bet you find everyone happy
and it's good that you don't have enough supply voltage to hurt valve. Adjust it to just a little more than you need for operation.

Sanity check
Is mosfet hot? Is opamp hot ?
Valve should get warm, 24 volts X.18 amps = 4.3 watts
Observe he is specified for 10% hysteresis. So you'll get 10% variation in current depending on direction of approach.

a diode is sitting on the +ve terminal of the valve to prevent inductive kick,

I trust it's connected between the valve's electrical ends ! _(clarity)

This thing is going to work great. Keep refining it in teeny steps.
Measure valve's resistance and write it on side. If valve gets hurt by overheating its resistance will go down as insulation fails. Five years from now it'll be nice to know what it was new. For extra precision, write temperature too.

 

[jim hardy]

Wow didnt think that one was going to post.
had to log out and back in, still took three tries.
Something is haywire with system. It just doesn't obey the "post reply" button.

I'll be back tomorrow.

 

[Lexilighty]

Where does your 24 V supply negative wire connect to gnd ?

It's a 0V and not -24V. The 0V is grounded to same place the GND of the RIO is.

Can you measure it? 112% of 7 ohms is 7.84 ohms... probably it's closer than that.

It's 7.4 ohms actually.

Valve should get warm, 24 volts X.18 amps = 4.3 watts
Observe he is specified for 10% hysteresis. So you'll get 10% variation in current depending on direction of approach.

It sure does feel really warm after a few minutes of operation.

I trust it's connected between the valve's electrical ends ! (clarity)

The cathode is on the positive end of the inductive load while the anode connects to OUT. Is that wrong?

Measure valve's resistance and write it on side. If valve gets hurt by overheating its resistance will go down as insulation fails. Five years from now it'll be nice to know what it was new. For extra precision, write temperature too.

Will do this tomorrow. I'm home now. Again, thanks for yor help, most excellent Jim. You are God-sent!

 

[jim hardy]

The cathode is on the positive end of the inductive load while the anode connects to OUT. Is that wrong?

That's just right ...

from earlier post

At 7 ohms, my volt from RIO to drive 100mA to 165mA for the valve should be 1.15V to 0.665V but it seems this valve is requiring more volts.

Why do you say the valve is "requiring" more volts ? Do you mean volts across R1 or across valve?
Might it just be that your machine sends it more current than it needs? You said at one point your supply was adjusted up to 30 volts.

What happens when you make your RIO send whatever current is required to make 165 milliamps? You could put your DMM in series with valve on current scale and measure directly.
What is voltage across R1 at measured 165 ma?

That'd be a good check to do anyway because your device depends on R1 having stable resistance at its operating temperature. So it's good that you have a 15 watt resistor now, it shouldn't heat up much at all.

Have fun exploring this thing you've built . I really enjoyed the project. Thanks for letting me play !

 

[Lexilighty]

You said at one point your supply was adjusted up to 30 volts.

It's been at 24.53V since I got it working.to ground

I think the problem is with the opamp. It keeps generating unequal voltages across the inputs when I run the circuit for a sufficiently long time (for example, right not pin 2 to GND is 0.288V when pin 3 to GND is 1.652V (RIO is also 1.652V). I have probably lost the opamp. I also get a very hot valve afterwards. But I do have the 10k resistor between opamp V+ and ground. I had to change this opamp yesterday after I got it working and burnt and it looks like something is about not right with this circuit.