Monitoring DC Voltage using 4-20mA??

Hey, i am looking to monitor a solar array using a GE Micro PLC. The problem I have is this, I just a newbie to the PLC world and cannot think of any way of monitoring this using 4-20 and am not sure cutting the 250 ohm resistor and using 0-10v will work(i am sure it will not). I am still looking at DC-DC converters but not sure if that will even work. any ideas

What's the voltage of your solar array?
See links below for signal converters.

http://www.miinet.com/products/sg_signal.shtml

https://www.ohiosemitronics.com/

The Solar Array is charging a 12VDC system. The Micro controler i plan on using is also a 12vdc but i might still use a 24vdc and put it on a battery back up as well so 12V for sure and possibly 24v in addition.

Obviously you don't want to use mA as an input for a solar system.
If you can remove the 250 Ohm resistor across the input you will have a 1-5 Volt input.
If you measure the impedance and add twice that value in series you will have a 3-15 volt input.
Instead of using series resistors you could use a zener diode to drop the voltage
e.g. a 10 Volt zener would give you an input range of 11-15 Volts (better resolution)
Hope this helps
Roy

noob,
You will need a voltage divider (two resistors) to set your voltage range and a signal conditioner to change the voltage signal to a current signal.

Voltage divider:
1/2 watt resistors will work fine for this as the dissipation is just under 1/4 watt; I'd probably go to 1 watt units for that extra measure of safety and reliability. I'd also look at 1% tolerance (or better) resistors for accuracy.



For the signal conditioner, check out:
http://www.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners/FC-33

Alternatively, you could get a 0-10v card for your PLC and forgo the signal conditioner - one less thing in the cabinet, and it might be cheaper.

@Roy, A zener diode is not a linear device, and will only limit the voltage to its zener voltage; it will not scale it. A voltage divider is the way to go on this one.


Good luck, and let us know how your project turns out.

-rpoet

you can use something like this

phoenix contact
2810078 MINI MCR-SL-U-UI-SP-NC



or you can use a 2.5k pot. at 2.5 k I think the range would be like 10-50 volts dc.

you can adjust the pot down to lower the voltage range.

you can use ohms law to figure out what you need

resistance x current = voltage

2500 x .004 = 10 this would be for 4ma

2500 x .020 = 50 this would be for 20 ma

if you need lower than this try 1500 ohms of resistance

1500 x .004 = 6

1500 x .020 = 30

Lets not forget isolation, folks

If there is more than one device involved here, and
if the analog inputs to the PLC share a common (most do)
then
using simple techniques will not work, because you will essentially be bridging all the commons via the analog input module.

Better to buy an isolating signal transmitter (Phoenix, Action Pak, etc.)

diat150 said:

you can use something like this

phoenix contact
2810078 MINI MCR-SL-U-UI-SP-NC



or you can use a 2.5k pot. at 2.5 k I think the range would be like 10-50 volts dc.

you can adjust the pot down to lower the voltage range.

you can use ohms law to figure out what you need

resistance x current = voltage

2500 x .004 = 10 this would be for 4ma

2500 x .020 = 50 this would be for 20 ma

if you need lower than this try 1500 ohms of resistance

1500 x .004 = 6

1500 x .020 = 30

Click to expand...

jdbrandt said:

If there is more than one device involved here, and
if the analog inputs to the PLC share a common (most do)
then
using simple techniques will not work, because you will essentially be bridging all the commons via the analog input module.

Better to buy an isolating signal transmitter (Phoenix, Action Pak, etc.)

Click to expand...

you are correct. In my world we are using the same supply voltage that we are trying to monitor. if you arent you will need to use a isolating signal transmitter.

Some here have proposed using isolators to convert to 4-20 mA. The problem with this approach is you will have a continuios drain on the battery, probably in the range of 16 - 20 mA. This is not a lot for a mains powered unit but for solar powered it's a bit more significant.
Far better to use a voltage input module.
If the PLC is running off the battery we can assume the input module negative is referenced to the battery negative therefore it's a simple matter of conditioning a voltage input to accept the battery positive.
e.g. if the input module has an impedance of 100k and the battery voltage is nominal 12 adding 200k in series will give you an input range of 3 - 15 Volts.
Substituting a 10V zener in replace of the 200k resistor will give you an input range of 11 - 15 Volts
(all assuming your input module is 1-5 VDC/4-20mA with a removable shunt)
Roy

I is a noob said:

The Solar Array is charging a 12VDC system. The Micro controler i plan on using is also a 12vdc but i might still use a 24vdc and put it on a battery back up as well so 12V for sure and possibly 24v in addition.

Click to expand...

A bridge rectifier is good for this, Tie the + terminal to your load and one AC terminal to each of your supplies, the load will draw from the supply with the highest voltage.
I like those square potted bridges because you can bolt them directly to a panel which acts as a heat sink.
(the negative terminal is not connected)
The name for this is "Auctioneer Diode", goes to the highest bidder.
Roy