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Short Circuit Detection/Current Sensing

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seasons555copy

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Hi,
I am trying to figure out a cheap way to determine if the load that I am controlling is working properly, open circuited (disconnected) or shorted (fault).
The load is one or two 24V AC solenoids each of which requires 7.2 VA.
Now my problem comes in that I have to detect the current/status of each of the solenoid groups. There will be a total of 16 groups which must be sensed seperatley because if 1 of the 16 solenoids fails, the power to only that solenoid should be stopped while allow the other 15 to operate normally. This could be expensive.

The power to my circuit is produced from the same 24VAC that is powering the solenoids.
Because of this I am not sure if a conventional OP-AMP approach would work. And if it would how could I accomplish this.
I have looked at the Allegro hall-effect based current sensors but they are about $5 each.

I would not like to spend more than a dollar or two for each channel that I need to detect.
I am also open to other solutions to produce the desired results other than a current based approach.

I should also mention that I am going to be using a pic with either 16 or 24 ADC pins on it for this purpose. Although I could get one with 32 if needs be.

I hope the above is clear enough, if not I'd be happy to clarify anything.

Thanks in advance for your advice.
 
Thanks for your reply.
I was under the impression that to use an OP-Amp power from the same source as the current being detected that there needed to be some sort of isolation. If this is not the case, great!

Also can you recommend a good OP-Amp that will be able to handle the input rails being 35V or so above that of its power source? Or is there a way to get around this as well?
 
I actually think those are very good questions. So I am going to do a schematic that explains how to sense 24V ac current with a 5V instrumentation amp. Hall sensing, or an isolation amps are preferred solutions for this, but it most certainly can be done for cost sensitive applications. I had to prepare some oscilloscope shots and what not to illustrate my points so it is just taking me a minute.
 
Another way to get isolation when measuring AC current, if needed, it to use a current transformer.
 
Yes, I am using the mains at 60Hz.

I look forward to looking at your schematic and oscilloscope shots whenever they are ready.

Thanks for all of your help so far CafeLogic!
 
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Thanks, That was very helpful.

I have one question though, you said that 1% accurate resistors should not be used, rather that 0.1% resistors should be. This makes sense however if I did use 1% resistors what would the error be? I would assume that the error could be as large as 4% or would it be larger?
 
It depends on the sense voltage VS. common mode voltage. The circuit in the post is kind of an extreme example. There we have a maximum instantaneous current of about 420ma. With the 0.2ohm sense, that would develop a maximum sense voltage of about 85 mV. After it is divided down by 11, we are left with about 8mV total. 420ma is produced when the common mode voltage is about 32V. Let's say there is a 1% mismatch. Instead of 10k/1k and 10k/1k, it is 10k/1k and 10.1k/1k. The top of the sense resistor (10k/1k: 32.085V) divides down to 2.917v. The bottom (10.1k/1k: 32v) divides down to 2.883v. So, instead of 8mv, we got 34mv, a 400% error. Now, if you used a 2 ohm sense resistor, maximum sense voltage would be 850mV, making about a 30% error under the same circumstances (you would need an amp with less gain, about 16 would be good for that).

You don't have to get 0.1% resistors, there are a few ways to handle it. An easy way is to get a bunch of 1% resistors and set up bins, like for 10k, a 9.989k bin, and a 9.991k bin, then measure the resistors and place them in the appropriate bin. They don't need to be exactly 10k, the resistor in divider 1 just needs to be the exact same resistance as the corresponding one is divider 2. Another method would be to just be fairly close, then calibrate the difference in software.
 
Using this amp instead (gain of 10): **broken link removed** would mean you would only need to divide by 2, that would make it a bit more tolerant of common mode error.
 
I do have one last question. Have you considered running the solenoids from DC so that you can just use typical current sensing techniques?
 
It depends on your specific situation and specific solenoids. The inductance of the coil provides impedance to AC (that naturally increases when the core moves into the coil). With DC, this impedance is only present when the DC is first applied. Consequently, the current would build up to a much higher level with DC. Therefore you have to compensate by operating at a lower voltage or adding series resistance. Off the cuff, I would guess your average 24V AC solenoid should be operated with no more than 12V DC. Of course, YMMV and you would need to verify that the solenoid does not get any hotter than while being operated on AC. When you set it up so that the same amount of current is dissipated in the coil, the net result will be that you will have about the same holding torque but significantly less pull in torque. The acceptability of that, of course, it dependent on your application.

There are some simple DC circuits you can Google that partially solve that problem (A resistor in parallel with a cap, both in series with the coil, if I remember correctly). Although I have never tried, I would imagine the ideal way to do it is a PWM circuit similar to those used to control stepper motors. That would give you complete control over both pull-in torque and holding torque. For example, you could jam it with 48V until your desired current was reached then drop down to a 10% duty cycle.

All of that may very well be inappropriate for your application, it was just a thought.
 
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