Multiple inputs on 0-5VDC analog transducer

Hello, I've never worked with voltage output transducers before. I've only used current transducers (4-20mA).

I am working with an old telemetry system that uses an 0-5VDC voltage output transducer (SSI p51-75-g-a-p-5V-000-000). I want to "piggyback" onto the output of this transducer and feed the output into a new telemetry system (CLICK 0-10VDC input module) without disrupting the input to the old system.

Can I simply connect the new input module in parallel with the existing connection? or am I missing something here?

Thanks!

ryangriggs said:

Hello, I've never worked with voltage output transducers before. I've only used current transducers (4-20mA).

I am working with an old telemetry system that uses an 0-5VDC voltage output transducer (SSI p51-75-g-a-p-5V-000-000). I want to "piggyback" onto the output of this transducer and feed the output into a new telemetry system (CLICK 0-10VDC input module) without disrupting the input to the old system.

Can I simply connect the new input module in parallel with the existing connection? or am I missing something here?

Thanks!

Click to expand...

I would think that you could, so long as you don't overload the transducer. Considering the fact that a typical voltage analog input has very high impedance, that shouldn't be a concern. So long as you don't create a ground loop, I would expect no problems with paralleling the transducer output to two inputs.

You most likely can - Bit_Bucket's information is good - although "can" and "should" are not the same thing.

If you want to split the signal into two separate devices, you *should* do it via a dedicated loop isolator/signal splitter. Just separates things up and prevents things like the ground loops mentioned above and any issues on one system affecting the other.

That said, sometimes the application can't justify the cost when it comes to "can" vs "should", and in this case, it's definitely not a "making my eye twitch just thinking about it" compromise.

I don't think the analog input on the Click is isolated. Make sure grounding the low side doesn't cause an issue.

in addition the 0-5VDC output may be going into a resistor on the input device and that device may be reading the signal through a voltage divider.
Putting the output into a 2nd device may change the readings on both.
And neither one would be correct.
it would be better to use an isolator and protect both systems.

1. Loading.
Voltage inputs are relatively high impedance. So loading the voltage output will not be a problem. Any error will appear as a slight (probably fractions of a percent) drop in a steady indicated reading when the 2nd input is connected.

2. Ground loop
If the Parallel connection to a 2nd AI card in the same rack, the probability of a difference in ground potentials is very small.

Even two single ended AI cards in the same rack will not see a ground potential difference because all channels on a single ended AI card use a common ground reference.

A paralleled input has the same ground reference no matter it terminates.

3) Half scale resolution
Your 0-5V signal will only use half the resolution, but so what?

I'd say wire it up in parallel and use it.

The output of this sensor is 1-5V, not 0-5V.
It is 3 wire. The - of the power supply for the sensor must be at the same potential (tied) to the - supply of the Click input module.
There is isolation between the Field and Logic side in the Analog module, so there isn't a need to also common the - side of the PLC CPU power input, but there's no harm if it is. The same supply can be used to power all 3.

GaryS said:

in addition the 0-5VDC output may be going into a resistor on the input device and that device may be reading the signal through a voltage divider.
Putting the output into a 2nd device may change the readings on both.
And neither one would be correct.
it would be better to use an isolator and protect both systems.

Click to expand...

A working transducer should maintain the correct output voltage regardless of the load impedance, so long as it is not overloaded. That's what instruments do.

Thanks everyone. To clarify, the transducer is powered by a 5VDC power supply integrated into the *old* system. I can't change this, so the unit must continue to be powered this way. However, the CLICK PLC is powered by its own 24VDC power supply.

So if I measure the potential between the old PSU's GND terminal and the CLICK's power supply GND terminal, and it reads zero in both AC and DC ranges, am I safe to assume there won't be a ground loop created?

FYI the CLICK Analog 0-10V module (https://cdn.automationdirect.com/static/specs/c004ad2.pdf) is not isolated. The signal grounds are internally connected to the PSU ground.

Also, the earth ground and 110VAC neutral lines are common to both power supplies. Would this make any difference?


As a follow-up, if the grounds are at different potentials, what signal isolator module (reasonably priced) would you recommend for this application? I can support either 0-10V or 4-20mA inputs with the CLICK PLC, but the old system requires 0-5VDC signal.



Thanks!

am I safe to assume there won't be a ground loop created?

Click to expand...

If all else fails, isolate. See link

https://www.automationdirect.com/ad...t__F__Accessory_Type_ms="Signal+Conditioners"

So the FC-33 takes a voltage or current input (dip-switch selectable), and outputs current. In this case I could use the Current Input module in the CLICK PLC and keep the Voltage output going to the old system.

Does that sound right to you?

ryangriggs said:

So the FC-33 takes a voltage or current input (dip-switch selectable), and outputs current. In this case I could use the Current Input module in the CLICK PLC and keep the Voltage output going to the old system.

Does that sound right to you?

Click to expand...

You could, also some of the other models will give you a voltage output.

Bit_Bucket_07 said:

A working transducer should maintain the correct output voltage regardless of the load impedance, so long as it is not overloaded. That's what instruments do.

Click to expand...

No. An instrument's voltage output puts out a voltage and expects that what it connects to will not load the output/input circuit. Loading the circuit would create an error of some magnitude.

A voltage output is supposed to be a low impedance output. What it connects to creates two resistances in parallel. The higher the resistance of the voltage input, the less impact it has on the total circuit resistance.

But a voltage output can be 'loaded down' by connection to a relatively low impedance voltage input, with a resulting low reading error.

Those who lived through the days of analog volt meters learned that the meter's coil 'loaded' a circuit.

Most voltage inputs nowadays are at least 1M ohm if not greater, which usually does not load a transducer/transmitter output. A pH probe is a high impedance output, and requires a better-than-average high impedance input to get a pH signal that isn't measured with error due to loading.

A current loop has some ability to correct for resistance in a circuit, like you suggest, "maintain the correct output". A current loop output will change its output to adjust for load changes, within the ability of the loop power supply to drive the desired current through the loop resistance.

Some years ago, there was a thread here involving a level transmitter stopped reading at about 75% of scale and would go into initialization mode. It turned out that the resistance of the mile of copper wire between the transmitter and the receiver/power supply was high enough to let the loop operate below 16mA (75% of the 4-20mA span) but the power supply could not push more than 16mA through the loop resistance.

A current loop driver monitors the loop current and adjusts it to the correct value, adjusting for the loop resistance.

A transmitter's voltage output is not 'adjusted' by monitoring the output circuit, it is created according to whatever its measured process variable is.

Dan

danw said:

No. An instrument's voltage output puts out a voltage and expects that what it connects to will not load the output/input circuit. Loading the circuit would create an error of some magnitude.

Click to expand...

I disagree.

danw said:

But a voltage output can be 'loaded down' by connection to a relatively low impedance voltage input, with a resulting low reading error.

Those who lived through the days of analog volt meters learned that the meter's coil 'loaded' a circuit.

Click to expand...

I've been working in this field since before digital meters existed. I certainly understand the fact that the load resistance of a d'Arsonval voltmeter affects the circuit being measured. That's why an instrument with a voltage output must be capable of reacting to various load impedances to maintain the appropriate output voltage to coincide with the measured variable, just as an instrument with a current output must. Of course, the ability to compensate is limited, and neither a voltage output, nor a current output can be regulated when the instrument's output is overloaded. High load resistance will overload a current output and low load resistance will overload a voltage output.

danw said:

Most voltage inputs nowadays are at least 1M ohm if not greater, which usually does not load a transducer/transmitter output. A pH probe is a high impedance output, and requires a better-than-average high impedance input to get a pH signal that isn't measured with error due to loading.

Click to expand...

Nonsense. Any value of input resistance loads the instrument's output.

danw said:

A transmitter's voltage output is not 'adjusted' by monitoring the output circuit, it is created according to whatever its measured process variable is.

Click to expand...

I maintain my disagreement with this assertion. An instrument incapable of compensating for it's output load resistance would be useless, regardless of whether the output is voltage or current.

Most DC Power Supplies these days regulate their outputs to maintain the correct voltage under differing load conditions. What makes you think that an instrument with a voltage output is incapable of doing so?