Reading sinking inputs using a chip that only supports sourcing inputs

Thread Starter

MrSoftware

Joined Oct 29, 2013
2,015
Can anyone suggest a solution to this challenge? I'm using a MAX22190 to read 24v sensors that source current. the MAX22190 inputs sink the current. I also have a requirement to read inputs from sensors that sink current, however the MAX22190 does not source current, so I need a creative solution. Also there are 4 of these MAX chips (32 inputs) on a board, so I'm looking for a simple solution with as few parts-per-input as possible. Does anyone have any suggestions?
 

Picbuster

Joined Dec 2, 2013
1,027
Can anyone suggest a solution to this challenge? I'm using a MAX22190 to read 24v sensors that source current. the MAX22190 inputs sink the current. I also have a requirement to read inputs from sensors that sink current, however the MAX22190 does not source current, so I need a creative solution. Also there are 4 of these MAX chips (32 inputs) on a board, so I'm looking for a simple solution with as few parts-per-input as possible. Does anyone have any suggestions?
24V sensors source current needs more clarification.
Is it 4-20mA @ 24V ?
if so: some sensors do provide the 24V in 2,3 or 4 wire systems.
if not: you have to create a power supply limited to 20mA per channel ( two transistors and two resistors I assume that 24VDC is available)

Please supply information when complete different sensors are used.

Picbuster
 

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Thread Starter

MrSoftware

Joined Oct 29, 2013
2,015
Thank you for the replies! This is my first foray into industrial sensors so I'm probably blissfully unaware of some important details, which I hope to learn. ;)

I don't think a pull-up will work in this case as the input chip triggers by current, so I need current to flow into the inputs to trigger an ON state.

At the moment it's wired as shown below. The chip is configured for IEEC Type 2 and 3 sensors with the per-sensor current limited at 2.6mA. So in theory when a sensor sources 2.6mA into the an input, it triggers as ON state. The chip sinks the current from the sensors, and the current returns to the sensors through the Sensor Common line. I'm pretty sure this part is configured OK.

I need to add a way to handle the case where the Sensor Common line shown below becomes +12V or +24V (relative the sensor input lines), and the individual sensor input lines now become current sinks, instead of sources. Somehow I need to allow the sensor to sink current through the input line, while at the same time mirroring that current through the MAX chip input pin so it registers as an ON. Is some sort of current mirror what I'm after?

There is 24V available (as well as 5V and 3.3V). The system is to work with 12V or 24V sensors, but I believe they will all match at runtime (all 12v or all 24v). I need to find out if the sensor power supplies will be isolated from the PCB power supply..



upload_2019-8-2_9-58-14.png
 

MaxHeadRoom

Joined Jul 18, 2013
23,908
@MrSoftware Generally speaking source devices operate into sinking inputs and vice-versa .
If getting into different sensors, it is often advantageous to use 2 wire devices if possible, as these can be used with a sink or source input.
Max.
 

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nsaspook

Joined Aug 27, 2009
8,899
Thank you for the replies! This is my first foray into industrial sensors so I'm probably blissfully unaware of some important details, which I hope to learn. ;)

I don't think a pull-up will work in this case as the input chip triggers by current, so I need current to flow into the inputs to trigger an ON state.
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019LBbSAM
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019N8KSAU&l=en-US

Normally a sensor that sinks (in the activated state for fail, range, etc ...) current is helpful when an OR pull-up circuit of >1 sensor is needed (or desired for simplicity) for one input like an interlock good or safe signal that should be ON unless there is a failure or out of range type event. The Open-drain (MOSFET) or open collector (BJT) configuration will short-circuit the pull-up current that normally flows in the sinking input for ON state and switch the input to OFF state.
 
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MaxHeadRoom

Joined Jul 18, 2013
23,908
Have you any sensors in mind or is this for general applications?
The majority of industrial systems use Opto isolated inputs, some are dedicated, others can be found that support both sink and source.
The various Opto22 versions are an example of outboard sensor interface.
Max.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
2,015
The actual sensors are a mystery at the moment (I know it doesn't make sense..). Basically in the 11th hour I was handed a little piece of a much bigger project so some seemingly important details have been difficult to come by. ;)

That said: one of the more experienced hardware guys on the project found this device, which I think just might solve the problem:

https://datasheets.maximintegrated.com/en/ds/MAX22191.pdf

It will handle both sinking and sourcing sensors and give a 2.4mA output, which I could use to drive opto-isolators that in turn control the inputs to the MAX22190.
 
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