adccommunitymod (AutomationDirect) asked a question.
Created Date: March 25,2019
Created By: Boat_bum10
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I am looking to monitor DC amperage draw on a 12V load (actually 16 identical loads). Rather than a hall effect I'm hoping to use a lower cost shunt to measure amp draw. The load is a pump that draws about 2.5 amps, so with a 5amp/100mV shunt I'll be in the 50mV signal range. If I use a P2-16AD-2 input module, which reads 0-10v, will there be any chance of reading the 50mV or is it likely to be washed over with noise and such? I'd like to read within 0.5amp load draw. I have to verify is that the pump is actually running, (not just getting voltage but circulating water) and amp draw sounds like the easiest. These are an open circulation pump, basically a boat bilge pump sitting in a tank. Given the remote location and lack of plumbing, I don't have an easy way to add a flow switch or other means to monitor the pump is spinning up. If I can get some degree of accuracy, I can monitor if the pump is actually moving water. Air or a plugged intake they draw around 1amp, normal operation 2-2.5amps. Or does anyone know of a low cost dc current monitor that puts out a 4-20ma signal? I can do dc shunts for $30ea, maybe less.
Created Date: March 25,2019
Created by: Boat_bum10
I am looking to monitor DC amperage draw on a 12V load (actually 16 identical loads). Rather than a hall effect I'm hoping to use a lower cost shunt to measure amp draw. The load is a pump that draws about 2.5 amps, so with a 5amp/100mV shunt I'll be in the 50mV signal range. If I use a P2-16AD-2 input module, which reads 0-10v, will there be any chance of reading the 50mV or is it likely to be washed over with noise and such? I'd like to read within 0.5amp load draw. I have to verify is that the pump is actually running, (not just getting voltage but circulating water) and amp draw sounds like the easiest. These are an open circulation pump, basically a boat bilge pump sitting in a tank. Given the remote location and lack of plumbing, I don't have an easy way to add a flow switch or other means to monitor the pump is spinning up. If I can get some degree of accuracy, I can monitor if the pump is actually moving water. Air or a plugged intake they draw around 1amp, normal operation 2-2.5amps.
Or does anyone know of a low cost dc current monitor that puts out a 4-20ma signal? I can do dc shunts for $30ea, maybe less.
Created Date: March 25,2019
Created by: Boat_bum10
By looking at the P2-16AD-2 specs it appears the signal com's and 24v - supply are all tied together, so using DC shunts I would have to tie the 12v battery power (pump power) and 24v regulated I/O power supply negatives together for a common reference, which will add noise to the system? The 12v battery system varies from 12-14.5 volts and has solenoids, brushed motors, and engine alternators all running on it, so not a particularly quite system.
Created Date: March 25,2019
Created by: RogerR
Most shunts that I have seen are between the positive DC supply and the load. The voltage potential across the shunt is measured.
I think that each shunt measurement would need its own common isolated from the others. That common point would not be at ground potential.
Created Date: March 25,2019
Created by: Tinker
If you only need "some degree of accuracy " at only 2.5A ( and low voltage) you probably don't need high precision commercial shunts. One question is how much voltage drop can you afford, a volt would only be 2.5W lost in the shunt and should give you pretty good resolution with a 16bit 0-10V input, however, 1V is a pretty large fraction of 12V so that might not be desirable. But there might be somewhere between a point between 50mV and 1V that would be a good compromise, maybe 250mV which would correspond to the actual current with an implied decimal point that would require a 0.1 ohm shunt, perhaps 10, 1ohm resistors in parallel.
10V / 2^16 does mathematically ~ 150uV resolution, so 50mV still gets you ~ 333 counts which doesn't sound horrible, but I would worry about noise.
Created Date: March 26,2019
Created by: g.mccormick
Put in a signal conditioner inline.
https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/signal_conditioners/dc_voltage,_current_or_potentiometer_input/fc-b34
Created Date: March 26,2019
Created by: MikeN
Using that signal conditioner will also let you keep the power supply that is running through the shunt separate from the PLC IO/analog input power supply. Hopefully saving some noise. If you do use a 100mv shunt, looks like the conditioner supports the perfect input for you to convert to PLC analog signal.
Created Date: March 26,2019
Created by: Tinker
But if someone was going to spend $130 for a signal conditioner (plus the cost of the shunt) wouldn't they be better of just buying a current transducer e.g. https://www.automationdirect.com/adc/shopping/catalog/sensors_-z-_encoders/current_-a-_voltage_sensors_(ac_-a-_dc)/current_transducers/dc_current_transducers/dct100-42-24-f
Now for that model the OPs current is pretty low, but with only 2.5A he could probably use fairly small wire and wrap say, 10 turns in the core and get up in the middle of the low range.
Remember, the OP did say "Low cost "
Created Date: March 27,2019
Created by: Boat_bum10
Thanks for all the idea. After looking at the signal conditioner, I found these https://www.adafruit.com/product/1164 which should amplify the shunt signal for less cost. Looks like I could configure for 2v signal per 1amp through the shunt. This is a going on a boat that doesn't require UL listing, though it would be preferred. The DCT100 looks to be a very accurate solution, but the cost is still a bit high for my application.
With the DCT100, multiple wraps through the coil to create a greater signal, then just scale by the number of wraps? Just curious in case I need one on another project.