Using 5VDC sensor as analog input

[dieselvette]

I would like to use a relative humidity sensor which operates on 5 Volts DC, and has a linear voltage output of 0.75v to 3.75v based on %RH.

1. What's the best/least expensive way to supply the sensor with 5 VDC?

2. Can the PLC interpret the input voltage if the sensor is powered from a separate source? If so, how? Could this be accomplished by "grounding" the neutral of the PLC power supply, to the negative of the 5 VDC, so that both have the same reference?

Nic Van Engen
Electrical Technician

 

[itsmoked]

1) Depends on what is natively available already.

2) Yes you just tie the two grounds together so the sensor and the PLC are working from the same reference.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

How much current does the sensor require? If it is less than a few hundred mA you could hang one of the 78x05 series regulators off the 24V PLC supply.

 

[richs]

Hiya-

Just a quickie to expand upon ScottyUK's fine suggestion.

Well, not JUST a 7805ish regulator. A dropping resistor
to bring the input voltage down to a "reasonable" level
under max. current draw (think like 8VDC some regulators
drop out at 7VDC in), a couple of bypass caps. (0.01 uF)
would be nice too. A heat sink might be nice too.

Alternatively, you could go with a "wall wart" power
supply available from many surplus houses if you want.
I've seen them in my local "junk shop" places. Sometimes
it's just easier to use something "found". Most wall
wart and "brick" supplies have their ratings stamped on
the case. You might even have something around the
place already.

I guess that what I'm saying is that even if you do
send the broken electrical/electronic device to the
recycle process, it might be worthwhile to keep the
power supply assoicated with it.

I've got a few 5VDC 1A "brick" supplies that came from
old HP probes that were customer returns. They come
in handy to be sure.

Cheers,

Rich S.