Author Topic: Powering Sensor from constant current source (24 Volt IO)  (Read 700 times)

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Offline robint91Topic starter

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Powering Sensor from constant current source (24 Volt IO)
« on: February 03, 2020, 10:35:30 pm »
I'm making a special 2-wire sensor that can be power by the parasitic current that can flow in EN 61131-2 type 3 input. More info about this interface you can find here, https://download.beckhoff.com/download/document/Application_Notes/DK9222-0909-0008.pdf The standard define at which voltage level a 24 Volt digital input should trigger.

The idea is to connect my sensor in between the +24Volt line and a type 1/3 digital input.
My sensor can consume max 2 mA and by modulating the input voltage I can send an 0 or 1.

So the summery is the following,
The digital input terminal can supply 2 mA minimum and the minimum voltage is 5 Volt. So I have 10mW of power to use for my processing and sensors. It the idle (0) state I have around 40mW.

So I have 10mW when I put a logical 0 and 40mW when I put a logical 1 on the line

Now I have a CCS811B which is a nice TVOC/eCO2 sensor. In it's low power mode it consumes around 1.3 mW average.

I checked the current consumption and it does consume for 1.5 s around 25 mA. This happens every 60 s, which that averages to the above number.

Now my problem is how to bridge the power gap. For 1.5 s I need to supply at least 50 mW.

Any one an idea?

I was thinking about a small SMT supercap but the internal resistance of those is just way to big to be usefull. Small battery is not really an option because of the limited cycles...

Currently I have a TPS62175 generating 1.8 V from the 5 to 24 V supply.
 

Offline sam[PS]

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Re: Powering Sensor from constant current source (24 Volt IO)
« Reply #1 on: February 28, 2020, 06:15:05 am »
What about storing much higher voltage on a capacitor (than would not need to be supper) That would make the circuit much more complex but could work if the bom cost and/or size is not too much an issue. You just need to design two high efficiency smps, one to boost the voltage to charge the cap then a down converter for the uptime. Of course top notch efficiency would be paramount so probably bigger self than you might want. That's just an idea i haven't done any math on it so it might just be a stupid idea.

Just my 2cts...
 

Online Zero999

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Re: Powering Sensor from constant current source (24 Volt IO)
« Reply #2 on: February 28, 2020, 11:57:20 am »
I checked the current consumption and it does consume for 1.5 s around 25 mA. This happens every 60 s, which that averages to the above number.

Now my problem is how to bridge the power gap. For 1.5 s I need to supply at least 50 mW.

Any one an idea?

I was thinking about a small SMT supercap but the internal resistance of those is just way to big to be usefull. Small battery is not really an option because of the limited cycles...

Currently I have a TPS62175 generating 1.8 V from the 5 to 24 V supply.
I don't see why you need a super capacitor.

If you use a low drop-out regulator to get 1.8V from 5V, then a volt sag of 3V, assuming a dropout voltage of <200mV, would only require a 12.5mF capacitor, which could be charged from a suitable 24V to 5V voltage regulator. A had a quick look in Farnel and found a  15000µF, 6.3V, which should do the job.
https://uk.farnell.com/panasonic/eca0jhg153/cap-15000-f-6-3v-20/dp/1848531
 

Offline jbb

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Re: Powering Sensor from constant current source (24 Volt IO)
« Reply #3 on: February 28, 2020, 05:45:07 pm »
You mention:
“The digital input terminal can supply 2 mA minimum and the minimum voltage is 5 Volt.”

I think that’s 5V drop across the *PLC input*, so you might have more voltage available than you think.

In any case, something on the order of 20,000uF might do well. Remember to consider tolerance, ageing and minimum operating temperature.

I also see that the sensor supply voltage is 1.8V absolute minimum, so you probably need to leave margin (maybe 2.2V?). Also, the 25mA figure is typical; what is the maximum?
 


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