Sensor Monitor

[Willbe]

hi willbe.


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Hi, Mr. G:
Thanks; I'll print this out 'cause I know it will take me a few shots.

 

[andy_king]

Hi Willbe, thank you very much for this detailed explanation. Things are making much more sense now

I replaced the 2 diodes in series with a 2V zener diode and along with the 2 source sinks connected; I get a nice 10V across the 10K resistor and 12V with the ground when simulated. A problem arises when I tried varying the input current (between 0 and <1mA); without the current sinks connected, the output voltage between the upper resistor pin to the ground is 2V at ~0A, 7V at 0.5mA and so on(exactly what is required), however with the current sinks on, it never go beyond 11.5V! Can you kindly tell me why this happened/if I’m doing something wrong?

Thanks a million!

 

[Willbe]

with the current sinks on, it never go beyond 11.5V!

Hi, Mr. K:


100(11.5/12) = 96% accurate, but a half volt in this circuit seems like a huge error.

I suspect that the problem is that the source impedance of the voltage source output is very high (10 kΩ) for a voltage source but I don't see how you're having this problem now that the protection circuit zener is at 12 V.

You should post what you have with everything connected and hook up current meters to the protection diodes. I would first like to confirm where the 10 kΩ resistor current is going.

But, one band-aid [Revision B] is to hook up an opamp configured as a voltage follower to the top of the resistor, then hook up the upper protection diode to the opamp's [~zero impedance] output.

The lower end of the 10 kΩ resistor is a low impedance point so I think this will be OK.

 

[andy_king]

Hi Willbe,

Please find attached simulation results with 0.5mA as source. Not following up what you mean by hooking the Opamp part, can you explain it a bit more?

Cheers

 

[andy_king]

I also attempted the Op-Amp approach, which seems to avoid me the hassle of worrying about drawing current from sensor. Which does work nicely, but outputs 0 - 10V, is there a way to shift it to the 2 - 12V region? I remember you mentioning the use of a level shifter in the feedback loop before, can you tell me more about it? I can't find right source of info about it on the Internet. In this case, what do you reckon I should do about limiting the output voltage from going above 14V and lower than 0V, in case anything go wrong in the circuit? Thanks again

 

[Willbe]

Hi Willbe,

Please find attached simulation results with 0.5mA as source. Not following up what you mean by hooking the Opamp part, can you explain it a bit more?

Cheers

Hi, Mr. K!
The anode of D13 needs to be grounded. If that doesn't improve things substantially we can try an opamp.
Is D10 now a 12v Zener?

I usually make two copies of a schematic and one I mark up as each wire is connected, even for simple schematics.

The opamp would be used as a voltage follower (=buffer=current amplifier) for the top of the 10kΩ resistor.
https://en.wikipedia.org/wiki/Operational_amplifier_applications
I don't yet think we'll need it.

 

[andy_king]

Hi Willbe,
The cathode of D13 is already grounded, that is meant to be the diode at the bottom of the +1V source you drew before, unless I misunderstood it's connection. D10 is 12v Zener with 1mA nominal current.

Sounds like a good approach to me, I reckon it is much easier to do on paper than on simulators, shall I make a clearer schematic, especially the wires?

Thanks for the link, I think I'll just stick it on then to avoid any issues with the potential load that will be connected to the output.
Thanks

 

[Willbe]

This is not going to work because D11 conducts anytime the 10 K resistor top voltage is below (12-0.7) volts.
That's why you're getting 9.5 V with only 1/2 mA inputted.

If you use an opamp it will fight with Zener D10.

I can't believe how much trouble this upper output protection diode is giving me!!!

Back to the drawing board for me. . .and this time I will more thoroughly think out what I'm doing.

Sorry about this.

 

[Willbe]

Here's your new schematic. Even with a thick pen it still looks ratty.

Hopefully by going through and explaining the entire circuit to you [and to me] I have found all errors. . .

Every component and its function along with voltage and current levels should have been addressed below.

OK.

Ia is a 0 to 1 mA current source that puts 0 to 10 V across 10 K resistor R1, since (1 mA)(10 K) = 10 V. Two volt Zener D4 shifts this voltage to 2 to 12 V and R2 supplies keep-alive current for D4.

If Ia is really an ideal current source then D1 is not necessary. In the real world it prevents the 12 V at the top of R1 from forcing current backwards into Ia. So I guess that makes this a real-world schematic, but not optimized for cost or performance or parts availability.

The keep-alive current through two volt level-shifting Zener diode D4 could be anything > 0 mA. Since Imax in R1 is 1 mA let this keep-alive current be 5 mA and so R2 = (15-2)/5 mA = 2600 ohms.

For the time being let R3 also equal 2600 ohms to supply keep-alive current for two volt Zener diode D5, which is part of the undervoltage protection network. If OUT - should try to go below (2-0.7) volts then undervoltage protection diode D2 will conduct and start stealing current from D5.

R4 supplies keep-alive current for 12 volt overvoltage protection network 12 V Zener diode D6. For the time being let this current also be 5 mA and so R4 = (15-12)/5 mA = 600 ohms.

If the voltage at the top of R1 is < or = 12 V then the overvoltage protection diode D3 is off. If the voltage at the top of R1 goes above (12 + 0.7) volts then D3 turns on and along with Zener D6 tries to resist the overvoltage.

During normal operation the current through D2 and D3 is 0.00000 A.

If the output over/under voltages are from ideal voltage sources nothing can resist them. If these sources, however, have non-zero source impedances like in the real world then how well these out-of-range voltages can be resisted by these protection networks depends on the current through R2, R3 and R4.

If this doesn't work you should hire these two guys. . .
https://www.youtube.com/watch?v=Ewu1laA2nmI

 

[andy_king]

Thank you very much for this Willbe, it is very clear and simple for me.
I tried to simulate it, and got the followings:

at 1mA, the voltage diff between; OUT + & OUT - is +10V, OUT + & GND is +12.1V (perfect) and +2.08V between OUT - and GND
at 0.5mA; +5V, +7.07V and +2.07
at 0A; 0V, +2.07V and +2.07V (perfect!)
Between 2mA (testing overvoltage) and 9mA; +10.7V, +12.7 and +2.08 (Great)
at 0.1A+; all start to going up past those previous take numbers.
at -0.1mA; -0.5V, +1.57V and +2.07V (Excellent)
at -0.2mA; -1V, +1.07V and +2.07V
at -0.3mA; -1.5V, +0.56V and +2.06V
at -0.5mA; -2.5V, -0.44V and +2.06V
at -1mA; -5V, -2.94V and +2.06V

So as far as I can tell, the undervoltage protection is active, but not hooked up to the main output somehow.
However, the overvoltage is brilliant and so is the circuit in general. Great work Willbe, thanks

 

[Willbe]

Thank you very much for this Willbe, it is very clear and simple for me.
I tried to simulate it, and got the followings:

at 1mA, the voltage diff between; OUT + & OUT - is +10V, OUT + & GND is +12.1V (perfect) and +2.08V between OUT - and GND
at 0.5mA; +5V, +7.07V and +2.07
at 0A; 0V, +2.07V and +2.07V (perfect!)
Between 2mA (testing overvoltage) and 9mA; +10.7V, +12.7 and +2.08 (Great)
at 0.1A+; all start to going up past those previous take numbers.
at -0.1mA; -0.5V, +1.57V and +2.07V (Excellent)
at -0.2mA; -1V, +1.07V and +2.07V
at -0.3mA; -1.5V, +0.56V and +2.06V
at -0.5mA; -2.5V, -0.44V and +2.06V
at -1mA; -5V, -2.94V and +2.06V

So as far as I can tell, the undervoltage protection is active, but not hooked up to the main output somehow.
However, the overvoltage is brilliant and so is the circuit in general. Great work Willbe, thanks

Good; you can fuss with the keep-alive resistor values to change how much this thing resists out-of-range voltages.

 

[andy_king]

Sure, but my only concern is the undervoltage protection, which seems to be somehow isolated than the output when testing with currents at -0.2mA and beyond

 

[Willbe]

Sure, but my only concern is the undervoltage protection, which seems to be somehow isolated than the output when testing with currents at -0.2mA and beyond

Is the undervoltage the only thing currently not working? Sometimes multiple faults have a single root cause.

I'm not sure how you have this circuit hooked up with your undervoltage simulator. Can you post your schematic with all voltages and currents shown for this undervoltage section?

I also now notice that R2/D4 by themselves resist undervoltage in addition to level-shifting, so you would still get some undervoltage protection even with R3/D2/D5 removed. As a troubleshooting aid you might want to temporarily disconnect the cathode of D2 from the cathode of D4.

 

[andy_king]

Hi Willbe,
Please find attached simulation results. Disconnecting the R3/D2/D5 part, doesn't make any difference, when tested with different ranges of input current. (especially < 0A)

The only other thing I noticed not working (may it is actually working, but something to do with diodes breakdown?) is when applying source current more than ~100mA the overvoltage protection, starts to leak and the output voltage starts going 13.5V+ (18V at 1A)

 

[andy_king]

Eureka! It was the matter of connecting both over- and under- voltage protection terminals to the same output line (OUT +)! Since OUT- voltage never changed at all, with all the variation in current, so I tried taken it out and connecting D2 to OUT+ before D3 and it works nicely! Please find attached simulation result when tested at -3mA input.

I think the only thing left that needs sort of clarification is the circuit output behavior at input currents > ~100mA and < -7mA, where it tends to go way out of the limits. What do you reckon is causing that?
Cheers

 

[Willbe]

You're losing me.

I have only designed/analyzed this circuit for 0 to +1 mA input. Why is the source at -3 mA?

I thought over/undervoltages are supposed to come from the right side of the screen, from external sources with non-zero internal resistances.

The voltages to the right are readings, not sources, right?

On second thought, I think for a real world D4 you will need D2/R3/D5.

For clarity, can you show the out + and out - as terminals on the right of the screen?

I'm callin' those Exorcist guys!!!

 

[ericgibbs]

Andy,

You could consider this option.

Its based on a 0 to 1mA current loop being converted and shifted to +2V thru +12V.

As willbe says, dont throw us any more curves..

 

[andy_king]

I just tried some extremes of the sensor's output current, in case anything go wrong on the sensor side. the voltage

Under/over voltage protection are for the purpose of ensuring that the output signal won't go up/below that 1.3V - 12.7V region, which will cause damage to the rest of the circuit (the output goes to an A/D converter, that might get damaged, if the voltage goes below/up that acceptable A/D Vin range of 0V-14V). I only need one output pin from the I-to-V that its signal is between 2V to 12V with reference to the ground for the A/D, following me so far?

The voltages to the right hand side, are just readings (DVM).

Am I making sense, or we not following each other?

Leave out those Exorcist guys for now, maybe later?

 

[andy_king]

Hi Eric,

That's it, works exactly as required! Thanks a lot man. Can I ask about how did you come up with the resistors configuration and the Op-Amp choice? As I tired it that myself before and couldn't get the right configuration. Any online references you recommend maybe?
Thanks a million

I will stop asking any further questions now

 

[ericgibbs]

Hi Eric,

That's it, works exactly as required! Thanks a lot man. Can I ask about how did you come up with the resistors configuration and the Op-Amp choice? As I tired it that myself before and couldn't get the right configuration. Any online references you recommend maybe?
Thanks a million

I will stop asking any further questions now

hi andy,
Look at this extract from a pdf, it covers the NI opa configuration.
I would advise you to download the complete AN31 application notes pdf if you work with opa's.

I use the CA3140 and CA3240 dual opa often, its a good all round amp to have on the shelf.
Goto www.datasheetarchive.com for the datasheet


EDIT: added a screen shot of the project board amp, as per the previous circuit pdf.

I have only taken the Vout to +5V on my PC adc, for the screen shot, it does limit at +13V for higher inputs than +1.2V from the loop sense resistor.