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Measuring low current on solenoid valve

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youbing
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Joined: November 23rd, 2017, 10:51 pm
Country: China

Measuring low current on solenoid valve

Post by youbing » November 23rd, 2017, 11:09 pm

Hi to all, I've been looking for an answer since the beginning of the year. I've digged alot internet, talked to friends who majored in this , but I still have some doubts about this.

For a period of time, I'm working on a project and we need to measure, with precision, the power comsumption of a group of solenoid valves. We need to measure the power comsumption of 4 valves individually and the power comsumption of 16 valves together. All of them will have flyback diodes attached. Seems pretty straightforward, but I still have some doubts. Lets focus on the problem of measuring each valve independently. First, the specification of the valves and acquisition system:

All of them will consume between 1.5W to 15W, but most important is the measurement of the valves that consume between 1.5W and 4W, so we'll handle this first. Power supply: 24 VDC. I'm using a NI - SCXI 1102B (using a NI SCXI 1303 connector block), which measures between -10 and 10V with gain = 1 and between -0.1 and 0.1, with gain = 100. It is connected to a SCXI 1000 chassis and then with a DAQ PCI 6259 - 16 bits resolution.

Question

We are going to measure the power of the individual valves by measuring both voltage on the valve and current through it. Since our DAQ maximum measurable voltage is +-10V, we'll use a voltage divider in parallel with the valve in order to measure the voltage and then programatically convert it to 0-24V DChttp://www.kynix.com/Detail/669784/DC.html. Pretty straightforward. The problem is the current measurement. Firstly I was considering using a shunt resistor in series with the valve on the low side of the circuit (between valve and ground). Doing some simple math, P = Vi, one may assume that, for a 1.5W valve, its steady state current would be i = 1.5/24 = 62.5mA and its "equivalent resistance" would be R = 24/62.5m = 384 Ohms. Then, I needed to estipulate a value for this shunt resistor. Firstly I thought that i needed the lowest resistor I could find, in order to not interfere with the valve operation, so I've decided that a 1Ohm 1% shunt resistor would do. The power dissipated on this resistor would be P = i²R =~ 3.9mW, which is, i think, a low dissipation. But the voltage drop would be V = Ri = 1*62.5mA = 62.5mV, which is really low. I guess that won't be a problem, since, according to the manual of the SCXI 1102B, for a gain = 1, the maximum measurable voltage is +- 0.1V. But then for a 4W valve, the current would be 166,67mA, which yields, for a 1 Ohm resistor, 166,67mV. I'd then need a smaller resistor in order to lower this voltage drop OR I could use the gain = 100 and measure between -10 and 10. By lowering the resistance, say, for 500mOhm, the dissipated power would be: P = 13.88mW, way higher than the previous 3.9mW. So maybe this is not an optimal solution?

Then i've learned that, in fact, it is good to increase the resistance instead of lowering, some people saying that 1kOhms for a mA measurement is essential. Sure, makes total sense when we think about the read voltage, which would be in the magnitude of Volts and not mV and also the power dissipation, which would be really low, but for the solenoid valve, its "equivalent resistance" is 384Ohms, wouldn't this resistance intefer with its function, i.e. the valve would not open at certain conditions?

Then I've learned about the SparkFun ACS 712 but, as explained here, for my application, the sensitivity is not good enough, and I'd need to filter the noise in order to extract the information. http://forum.arduino.cc/index.php?topic=81605.0
EDITED - I've made a mistake: P = Ri^2, sorry.
Which would be the best solution? What should I consider?

Thanks all, guys!

Andrea C
Keithley Applications
Keithley Applications
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Re: Measuring low current on solenoid valve

Post by Andrea C » November 26th, 2017, 12:39 pm

In my opinion, you should try for smallest current sense resistor that you can get away with.

Keep in mind, your 24V from power supply will be applied across the I*R drop of your current sense resistor and your solenoid value. The more the I*R is, the less voltage for your solenoid.

A DMM product such as Keithley model 2000 has "voltage burden" spec for the current measure range. For 100mA range, voltage burden is max 30mV. This means the internal current shunt resistor in the DMM will not cause more than that voltage burden, not more than 30mV.

Also, can your power supply measure and report current? Consider the model 2280 Power Supply. They have internal 6.5 digit measure feature and can do pretty good job with moderate currents.

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