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Blocking Voltage

Blocking Voltage

Blocking Voltage

(OP)
Hi,

My application consist of two parts. The first part of the circuit puts a 0-20mA signal through a 500 ohm resistor for an equivalent voltage of 0 to 10 VDC.  The second part of the circuit consists of a voltage divider that accepts a ±10 VDC signal.  My circuit is part of a larger data acquisition system that can accept both a 0-20 mA signal and a ±10 VDC signal.  In between the two circuits I placed a diode to allow the voltage from the current circuit to pass, but not allow the voltage from the voltage input to pass back to the current circuit.  The problem is that when I place a negative voltage on the voltage input is passes through the diode.  My only immediate solution is to put a mechanical switch in place of the diode.  The switch will either be manual or a relay.  I tried using a solid state Analog switch which worked, except for the when the signal was negative.   Is there a device I can use that could act as a switch to allow the current signal to pass and block both the positive and negative signal.  The device must be able to be controlled by a logic signal, I tried using an SCR which worked, but the only problem is that it only triggered when the current input was around 8 mA, I need to cover the whole range of 0-20mA.  Any help would be greatly appreciated.

Thank You

RE: Blocking Voltage

The only way I can see to solve this problem without redoing the entire circuit would be to put in place a relay system that trips when you have the negative voltage input. Sorry if this of little help to you.

RE: Blocking Voltage

Why not just use a multiplexer to switch between the two?  

Steve Owens @ FinishLinePDS.com

RE: Blocking Voltage

Or just put the diode on the 0-10V VINT line.  Move D5 to node above R58.  Will that solve your problem?

RE: Blocking Voltage

(OP)
Good Afternoon Everyone,

I will look into the multiplexer to see if that could work.  Earlier I tried a Maxim  MAX362 analog switch.  Unfortunately that did not block the negative signal from going into the current circuit.  

Melone, as far as putting the diode on the 0-10V line, that would still allow positive voltage to pass to the current circuit and completely block the negative voltage which is needed depending on the application.  I need to able to read a bi-polar voltage signal as well as the 0-20mA input.  Both inputs will never be used at the same time by a user.

Thanks,
morizabal

RE: Blocking Voltage

Which direction is your 4-20ma current flowing?  And, please indicate whether you mean conventional positive current flow or electron flow.
Can the 4-20ma current be reversed?

RE: Blocking Voltage

Remove IC21B from the circuit, increase R63 a bit and instruct the user to provide a jumper between the voltage and the current input when the current input is used.

Beside this I would recommend to redesign the circuit around IC23B. Dividing a voltage that way (10k and 50 Ohm) may lead to large errors due to offset voltage and variable resistor instability

RE: Blocking Voltage

Hi, not a great circuit best to redesign it. You seem to be trying to drive the the voltage input whith the current output! Remove the diode and replace with a resistor, insert an equal resistor in the voltage input line, it will at least work then (after a fashion).

RE: Blocking Voltage

Some general comments:

>  The series diode is a poor idea, since you've converted the current signal to a voltage and now you have to subtract a diode drop that's load dependent, so your measurement accuracy takes a nose dive.

>  The bipolar input should be converted to single ended, otherwise there's no way to selectively shut off the input.  Another op amp with the reference set to -10V and a gain of 1/2 will shift everything to 0-10V.

> The summer should be redone with the two sources going the + and - inputs of the second op amp.  A couple of npn or open collector TTL gates can then be used to disable the inputs.

TTFN

RE: Blocking Voltage

(OP)
Thanks for all the insight.

I though I should give you guys a bit more detail on why I designed this circuit.  My application uses an A/D board which is specifically made to read Load Cell or other Wheatstone bridge transducers.  The A/D board requires a differential signal of about ± 50 mV.   The A/D board is a third party product so I can't re-design it and I cannot change the range.  My application requires me to be able to accept a high ±10VDC voltage and 0-20mA input.  The voltage divider drops the ±10VDC to a range of 0 to ±50mV.  This ±50mV signal is used to provide the offset voltage to the common mode of the A/D board within the required voltage range.  The current input merely converts the current to a voltage where 20mA = 10 VDC.  I would prefer not to use any jumpers because we don't want the user to take the system apart.  If you guys have any better design ideas I would greatly appreciate it.

Regards, Mario

RE: Blocking Voltage

Only 1 circuit can be active at a time, right?

RE: Blocking Voltage

(OP)
Melone,

Yes, either the user will be inputing a voltage signal or current signal, but never at the same time.

Mario

RE: Blocking Voltage

Can you use a toggle switch?

RE: Blocking Voltage

morizabal, would you mind answering my previous question?
I won't offer any suggestions otherwise!

RE: Blocking Voltage

(OP)
Hello,

Lewish:  Current will flowing in a positive direction, from the source to ground causing a positive voltage on the 500 ohm resistor.  The current will not be reversed.

Melone:  We will use a switch as a last resort due to limited space in our front panel and other mechanical restrictions.  



RE: Blocking Voltage

(OP)
CBarn,

I followed your advice on using a resistor.  I changed the circuit to the following configuration



The circuit worked without any major problems.  My question is though,

Can an OP-AMP (IC24B) be damaged by applying a voltage at its output.  That was the original idea behind the diode.  The circuit works, but is this a good solution, can the op-amp be damaged over time?

Thanks for all your help.

Mario

RE: Blocking Voltage

The 10K series resistor simply acts like a load, so in the worst case, you'll have 20V across a 10K, which is 2mA, and should be well within the capability of the op amp to supply.  

Note that the series resistor makes it so that you are not applying a voltage directly to the output of the op amp and you never did.  

TTFN

RE: Blocking Voltage

Is your input supposed to handle both 4-20mA AND -10-10V inputs on the same connector?

RE: Blocking Voltage

(OP)
Melone,

Same connector but different terminal

RE: Blocking Voltage

You haven't mentioned what output voltage you expect from the circuit (ie. what is the approx value the pot is set to).

RE: Blocking Voltage

(OP)
Pebe,

The pot is used for calibrating the circuit.  I will inputting a known 10 VDC value and adjusting the pot until I get the desired reading.  I will be adjusting the pot until I get a reading of 3 mV/V.  (The A/D boards are calibrated to mV/V)  This is an equivalent of 15mV of voltage on pin 5 of IC23B, this is an equivalent resistance of 15.075 ohms for the pot.  I know that due to resistor tolerances that there will be an error present between the two circuits which I will have to correct using software.  Eventually my plan will be to replace the pot with a fixed resistor and using software for calibration.

Mario

RE: Blocking Voltage

With both inputs on a connector (and not on a screw-type terminal strip) using a jumper as suggested on Jan 1, 2003 isn't a good solution.

I also try to avoid jumpers inside of a device because of the reason you already mentionend (and because this would require additional documenation whereas jumpers on terminal strips are documented in the overall schematic of the equipment).

But keep in mind that the  solution you have choosen now is even more prone to introducing errors by offset and tolerance than the first approach.

Scaling down a signal and amplifying it again is never a good idea. So I would recommend to analyse the errors resulting from you design and compare it to the specification.

You have choosen a precision opamp with very low offset and offset drift, but you should also consider that radiated electrical noise can be rectified at opamp input stages and look like a dc input voltage to the opamp.

RE: Blocking Voltage

(OP)
electricuwe,

I mentioned previously that voltage and current inputs are on a screw terminal connector on separate terminals.  I will be using low TCR 0.1% resistors (except for the feedback resistors) to minimize errors.    You mentioned that it is not a good idea to scale down a signal and ampliflying it again.  Since my A/D boards will not accept a high level signal, what would be a better approach?


Morizabal

RE: Blocking Voltage

Hi Guys, I think he was on the right track using a "Analog switch" BUT, the type he used was incorrect....

analog switches can be biased 2 ways, the dallas I'm not famil. with BUT, 4016 and 4116. 4016 is single ended only, BUT the 4116.... can be placed in a +/- supply config, and will NOT lockup when used with - voltage...

hope this helps. Gawd, hope I have the numbers correct:)

RE: Blocking Voltage

Can you just remove the voltage input from the circuit altogether?  Make the current input your only input connection.  When the input signal is 0-20mA, the circuit is fine as it is.  When the input signal is +/-10V, remove R63 (either by removing a jumper, turning off a relay, digital pot, etc.).

RE: Blocking Voltage

(OP)
a31Ford,

I will check out the 4*** devices you mentioned, for now I will be using the resistors that appear to be doing the job.  Like I mentioned on a previous post I will be using software error correction which I have tested and it works fine.  I tested the circuit for stability, and once its warmed up it will remain very stable over time with no drift.  If anyone has a better idea for converting a ±10 volt signal to a ±30mV differential signal any suggestions would be appreciated.  

Thanks To All
morizabal

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