How to increase sensor's output current

[PLC]

I got stuck on my science project. I used an QTI sensor to give signal to my Pansonic PLC. My PLC can only sense 24 volt signals. I have replaced the resistor for the IR emmiter on the sensor to a 2.4K resistor. However, I found that the output current of the sensor is only 2.5mA, too small for the PLC to sense (the PLC senses 0.1A). I was wondering if there is any other way to increase the output current other than using transistors. Thank you very much in advance.

 

[Sceadwian]

What's wrong with using transistors? This is exactly what they're for..

 

[Hero999]

What's a QTI sensor? Is it an IR diode?

Is this an input or an output?

An IR emitter is an output, it's normally an LED which should be connected in series with a suitable resistor and normally only needs 10mA which the PLC's output should be able to provide.

The current through an IR sensor can be converter to a voltage, which a PLC can easily sense, using a resistor.

 

[ericgibbs]

I got stuck on my science project. I used an QTI sensor to give signal to my Pansonic PLC. My PLC can only sense 24 volt signals. I have replaced the resistor for the IR emmiter on the sensor to a 2.4K resistor. However, I found that the output current of the sensor is only 2.5mA, too small for the PLC to sense (the PLC senses 0.1A). I was wondering if there is any other way to increase the output current other than using transistors. Thank you very much in advance.

hi,
I would do as Sceadwian suggests, use a transistor as a current amplifier for the QTI line follower sensor.

 

[upand_at_them]

Try decreasing the resistor value. If the emitter is being run at 5V then 2.4K is definitely too high. If it's being run at 24V (post a circuit, because this isn't clear) then a) you still have room for more emitter current and b) if the emitter is being pulsed you can go even higher...check the datasheet for max steady/pulsed currents.

 

[Hero999]

I Googled for QTI sensor, it's just a reflective IR sensor.

Do you have a datasheet or part number from the one you're using?

What PLC are you using?

The chances are, you don't need an amplifier or a transistor, just a couple of resistors.

You should use a voltage input on the PLC, not a current sensing input and use a resistor to convert the current to a voltage, especially if the PLC input impedance is really high.

 

[Slorn]

Hi PLC

The best thing to do in my opinion is to build a photodiode amplifier with an opamp (operational amplifier), like the one shown in this **broken link removed**. It converts the current through the photodiode to a voltage. The conversion ratio is equal to the resistance of the resistor. Very easy and a lot more accurate than using a transistor.

Slorn
Dwengo... gets you started with microcontrollers!

 

[Hero999]

I don't see any need for an amplfier.

Here's an idea.

This assumes the PLC input has an impedance of more than 250k and voltages over 15V are considered to be high. Of course, if the PLC input impedance is much higher the "high" voltage can be nearer 24V.

 

[Nigel Goodwin]

I think you've missed the point Hero, his PLC needs 100mA drive current.

 

[Sceadwian]

I am the ghost of postings past oooooooooooo, transissstorrrrrrrrrr.

 

[Hero999]

I think you've missed the point Hero, his PLC needs 100mA drive current.

Sorry but I doubt that's true.

He's probably using the wrong input, probably a current monitoring input rather than a voltage input which should be high impedance.

No PLC I know of requires 24V@100mA on each input, if there are 10 inputs, that's a power dissipation of 24W which would cook a PLC unless it has a fan.

 

[Sceadwian]

No one said it was at each input Hero, apparently it's at the input he's using though, can we stick to what the poster is asking about rather than esoteric unknowns? Ask him questions, don't question our answers. You're plucking information and conjecture out of thin air with no response from the poster. Wait till he provides more information or you're just guessing. No one can say anything at this point without more information from them.

 

[Hero999]

I suppose you're right.

I think he's probably doing it the wrong way. I was reading between the lines, he's probably got this IR sensor and the datasheet says it has a constant current output, so he's using a current input on his PLC.

I could be completely wrong though, as often is the case, the original poster hasn't provided enough information so we're left guessing.

 

[Sceadwian]

If you're not sure then request more information and wait for a response, if you never get one then oh well, random guessing isn't gonna help.

 

[PLC]

Thanks everyone! I was amazed that I got so many responses. I’m very glad I came to the right place.

I think that using transistors will be too much of a hassle and it may be easier just to replace the sensor. Sorry that I didn’t provide details earlier. My PLC is a Panasonic FP0-C16T PLC with digital I/O points. Input is 24VDC, output is 24 VDC, 0.1A. Sensor is a Parallax QTI sensor.

The PLC didn’t specify the current requirement for the input. I was wandering if there are other possible issues with this setup.

I have uploaded two schematics: one is the original sensor and the other is my modified version. I have added a 10K resistor between Input(Red) and 24V(White). This is to make the sensor signal output digital. Since I’m supplying 24V to the sensor, I have changed the 470 ohm resister for the IR emitter to a 2.4K one.

As for the current sensing or voltage sensing inputs, I have no idea which one I’m using. However, I believe my PLC has only one type of input.

Sensor: **broken link removed**

PLC: https://www.electro-tech-online.com/custompdfs/2010/04/PanasonicFP0.pdf

 

[JimB]

My PLC is a Panasonic FP0-C16T PLC with digital I/O points. Input is 24VDC, output is 24 VDC, 0.1A. Sensor is a Parallax QTI sensor.

The PLC didn’t specify the current requirement for the input.

The PLC input parameters are specified on page 18 of the document in the PLC link.
The PLC digital input is an opto-isolator in series with a 5kΩ (approx) resistor.

I was wandering if there are other possible issues with this setup.

Yes there are, the sensor is incompatible with the PLC input as it comes straight out of the box.
As you seem to be happy modifying the opto sensor, wire it up as shown in my attached sketch. That should do what you want. No extra transistors needed.

Since I’m supplying 24V to the sensor, I have changed the 470 ohm resister for the IR emitter to a 2.4K one.

Good.

As for the current sensing or voltage sensing inputs, I have no idea which one I’m using. However, I believe my PLC has only one type of input.

Look at the table on page 17 of your link, your PLC has 8 digital inputs and 8 digital outputs.
In the context of this thread, the inputs are "voltage sensing".

JimB

 

[Hero999]

Yes, I knew that it didn't require 100mA which was the output current, not the input current.

The only thing I missed was it obviously doesn't need a pull up/down resistor which I should've known.

 

[PLC]

Thank you for your suggestion. I’ll take apart the sensor and do what you have suggested. I’ll report the result when I’m done. Hopefully I won’t damage the parts.

I will read all the materials carefully in future. Thanks.