Blocking Voltage

[morizabal]

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

 

[msprayberry]

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.

 

[steveowens]

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

Steve Owens @ FinishLinePDS.com

 

[melone]

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

 

[morizabal]

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

 

[Lewish]

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?

 

[electricuwe]

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

 

[cbarn24050]

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).

 

[IRstuff]

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

 

[morizabal]

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

 

[melone]

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

 

[morizabal]

Melone,

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

Mario

 

[melone]

Can you use a toggle switch?

 

[Lewish]

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

 

[morizabal]

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.

 

[morizabal]

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

 

[IRstuff]

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

 

[melone]

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

 

[morizabal]

Melone,

Same connector but different terminal

 

[pebe]

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