In order to track motor current vs. time I plan on using one of these $70 Omega units http://www.omega.com/ppt/pptsc.asp?ref=OM-EL-USB-1… and then sync the data with the trip data from the Garvin Edge 205. I can measure the current using a shunt resistor using these directions provided by Dr. KJ:
DC Current Measurement
Place a shunt resistor in the current path and measure the voltage over the resistor.
Note: This technique can only be applied to DC currents, not AC
The value of the shunt resistor must be chosen carefully as follows:
Step 1. Find the maximum current
You may know this value (in Amps) already, but if not it can be found from the power rating of the motor (or whatever the load is):
(Amps) = Power (in Watts) / Voltage
In your case: I = 30 A
Step 2. Find a suitable value for the resistance
Make sure that the resistance is not too high
Too much voltage will be dropped over the resistor, taking power away from its actual purpose (in this case driving the motor). As a guide, try to avoid dropping more than 1% of the available voltage over the shunt. In the example above 1% of 24 V is 0.24 V
If the resistance is too low
The voltage seen by your A/D will be too low, so the current measurement will be less accurate and noisy or stepped. Avoid this by ensuring that at least 0.1 V appears over the shunt at maximum current. In the example above the voltage over the shunt should ideally be between 0.1 V and 5 V, when the maximum current is flowing. Experimenting with resistance values R in the formula:
Voltage(V) = I R
leads to the choice of 0.01 Ohms, which will develop 0.3 V at 30 A.
Step 3. Calculate the Wattage required for the resistor
We don’t want the resistor to burn out, so fit a suitably high wattage one using the formula:
Power(Watts)= I2 R
In your example, Power = 30 x 30 x 0.01 = 9 W, so you’ll fit a 9 W resistor. Once this is connected up you can configure the acquired voltage to be scaled directly in amps instead of in voltage.
OM-EL-USB-3
VOLTAGE DATA LOGGER
Range: 0 to 30 Vdc
Resolution: 100 mV
Accuracy: ±1%
GENERAL
Memory: 32,000 voltage readings
Logging Interval: 1 seconds to 12 hours
Operating Temperature Range: -25 to 80°C (-13 to 176°F)
Alarm Thresholds: High/low alarm thresholds selectable in software
Start Date/Time: Selectable in software
Status Indicators (LEDs): Red and green
Power: 12 AA 3.6 V lithium battery (included)
Battery Life: 1 year typical (depends on sample rate, ambient temperature and use of alarm LEDs)
Weight: 57 g (2 oz)
Dimensions: See dimensional drawing
OM-EL-USB-4
CURRENT DATA LOGGER
Range: 4 to 20 mA
Resolution: 0.1 mA
Accuracy: ±1%
GENERAL
Memory: 32,000 current readings
Logging Interval: 1 seconds to 12 hours
Operating Temperature Range: -35 to 80°C (-31 to 176°F)
Alarm Thresholds: High/low alarm thresholds selectable in software
Start Date/Time: Selectable in software
Status Indicators (LEDs): Red and green
Power: 12 AA 3.6 V lithium battery (included)
Battery Life: 1 year typical (depends on sample rate, ambient temperature and use of alarm LEDs)
Weight: 57 g (2 oz)
Dimensions: See dimensional drawing
