I am working on a project where I have to measure the variation of current with the variation in another parameter. However this current is in microamps and I was wondering if there was a way to amplify this current using a ULN 2803 ic. Changes in current in microamp range cannot be detected with ACS712, which is why I need to amplify it.
Hello Pranavps35,
I suggest that you should convert the current
to a voltage. We do that with our friendly
current to voltage converter called a resistor.
The current flowing through a resistor drops
a proportionate voltage across its terminals.
You can then either read that or amplify it
to meet your readout needs.
Herb
We probably need more details, so we know whether something like herbschwarz's comment is viable, or not. For instance, would introducing a resistance, somehow, be:
- Impossible due to physical constraints?
- Alter functionality in some detrimental way?
- Be impractical because of a noisy environment?
- etc.
It would really be a splendid thing if you read: How To Use This Forum
Or, find the first post in the Forum, and read it.
Well I already have a voltage sensor. The thing is in my experiment, The resistance, current and voltage drop keeps changing with the parameter I mentioned. Hence it is important for me to measure current seperately than convert voltage into current. Which is why I need to amplify the current.
Pranavps35:
Well I already have a voltage sensor. The thing is in my experiment, The resistance, current and voltage drop keeps changing with the parameter I mentioned. Hence it is important for me to measure current seperately than convert voltage into current. Which is why I need to amplify the current.
Please read about the "X-Y Problem" at: https://forum.arduino.cc/index.php/topic,148850.0.html#msg_1118343
A resistor has normally a pretty constant resistance. The current and voltage are related to each other and the resistance following Ohm's law.
All we know now is that there's apparently a very small current that you try to measure (small enough to be easily affected by EMI), and some rather mysterious "parameter" that apparently messes up everything you try.
So why don't you start by explaining what this parameter really is, and why it may be causing trouble? Because without that information there's not much we can do.
But if you really just want to amplify a current, use a transistor. The good old BJT transistor is doing exactly that: amplify a current. The humble BC547 can do 200-300 times amplification by itself.
I am working on a piezoresistive material. This material's resistance changes when you apply a force on it. The parameter I was talking about is this force. Which is why I need a way to amplify the current output from material. The voltage drop is not difficult to measure using an Arduino. However, measuring the current has been a problem for me.
Thank you.
Why do you want to measure the current if you already know you can measure the voltage drop? They're directly related to one another.
Just wire your piezoresistive material as voltage divider. Use a second resistor with a value close to the value of interest of your piezoresistive sensor for maximum sensitivity in that area.
Pranavps35:
I am working on a piezoresistive material. This material's resistance changes when you apply a force on it. The parameter I was talking about is this force. Which is why I need a way to amplify the current output from material. The voltage drop is not difficult to measure using an Arduino. However, measuring the current has been a problem for me.Thank you.
And, what current are you referring to? I don't know a lot about the science of piezoresistive material, but I do know that for a current to flow, there needs to be a path for it to flow in. So, are you talking about an external current, or are you talking about measuring some sort of current internal to the material?
This material behaves differently when you apply a force on it. I have practically tested it to find that both voltage and current change when a force is applied on it. Which is why I want to measure both of them.
And current in this case is current passing through the material when connected to a power source.
Well when the resistance changes, the voltage and current change accordingly. Measure one and you know the other.
wvmarle:
Well when the resistance changes, the voltage and current change accordingly. Measure one and you know the other.
Boy, that's a load of circular logic. The original question was, basically, how to measure the current. The only way to know the resistance, in this case, is to measure the current. Dope slap!
So, how do you measure the current, in this instance? As the OP stated, because the current is in the micro amp range, some amplification is needed. How do you do that? Probably with an OpAmp. If it's possible to insert a series resistor in one of the power lines, then use an OpAmp to amplify the voltage developed across the resistor. Or, even better, use an IC dedicated to this sort of thing. Such as the INA199, which is the only one, still in production, that I have any experience with. It's a dinky little thing, though -- not sure if that's a buzz-kill. There are others--in larger packages--mostly, though, all surface mount.
But, try Googling "Current Sense Amplifier".
Pranavps35:
I am working on a piezoresistive material. This material's resistance changes when you apply a force on it. The parameter I was talking about is this force. Which is why I need a way to amplify the current output from material. The voltage drop is not difficult to measure using an Arduino. However, measuring the current has been a problem for me.Thank you.
Is it posssible, and are you allowed, to describe what the project is actually doing ?
You never know, if you described the project to the forum, someone might suggest a solution.
ReverseEMF:
Boy, that's a load of circular logic. The original question was, basically, how to measure the current. The only way to know the resistance, in this case, is to measure the current. Dope slap!
And the normal way of measuring a current is... to lead it through a resistor and measure the voltage across that resistor!
OP has revealed by now that what they really want to measure is the resistance of that material. They even mentioned they can measure a variable voltage drop, and that they have an external voltage source. Well, that all leads to the good old voltage divider.
Now it can still be that OP has withheld important information, but we don't know that. Anyway, classical XY problem.
By the way, an OpAmp amplifies voltages, not currents. So to measure that current and amplify it with an OpAmp you still need that resistor to turn it into a voltage...
wvmarle:
And the normal way of measuring a current is... to lead it through a resistor and measure the voltage across that resistor!
As I said.
wvmarle:
OP has revealed by now that what they really want to measure is the resistance of that material. They even mentioned they can measure a variable voltage drop, and that they have an external voltage source. Well, that all leads to the good old voltage divider.
Only if you mean, a sense resistor in series with the piezoresistive material.
wvmarle:
Now it can still be that OP has withheld important information, but we don't know that. Anyway, classical XY problem.
As I said.
wvmarle:
By the way, an OpAmp amplifies voltages, not currents. So to measure that current and amplify it with an OpAmp you still need that resistor to turn it into a voltage...
As I said.
I kind of understand what your goal is, however I'm not sure why an additional resistance in the circuit would diminish your results.
If you know the voltage (i.e. the power supply) and you know the value of the fixed resistor, can you not back calculate the change in resistance in the piezo element?
I'm assuming you are making measurements when different forces (actually impacts) are applied to the piezo element.
John
ReverseEMF:
Only if you mean, a sense resistor in series with the piezoresistive material.
A fixed series resistor, measure the voltage in the mid point. Doesn't matter which side you measure, the result remains the same. As said, standard voltage divider, nothing special about it.
wvmarle:
A fixed series resistor, measure the voltage in the mid point. Doesn't matter which side you measure, the result remains the same. As said, standard voltage divider, nothing special about it.
Well, actually, there is something "special" about it. A sense resistor, I suppose, could be considered a portion of a "Voltage Divider". It does, indeed, divide voltage. But, it also subtracts voltage--taking it from an existing voltage. Basically, it parasitically consumes some of the existing voltage, for the sake of measuring the current flow in whatever that voltage is across.
The typical concept of a voltage divider, is a series pair of resistors, placed across a point in a circuit, to divide the existing voltage down. It doesn't need to consume any of the original voltage to do this, and it's not in series with the original circuit--like in the case of the sense resistor--but is, instead, in parallel with the original circuit. It will, however, parasitically consume current -- and even, the consumption of that current, may cause a rise or drop in the voltage, the divider is applied across, but that's secondary to the parasitic current.
So, it's not really the same.
Now, if you're advising the use of a voltage divider to measure the PRM's current, I see no way for that to work.
Also, it does matter which side you measure from, in both cases. The result is *not * the same. You will, possibly, be able to use math to convert from one result to the other, but in the case of the series sense resistor, measuring any other way, but across the sense resistor is a ridiculous proposition. Also, often, a voltage divider is used to reduce a larger voltage down to a voltage in the common mode input range of some device, such as the input of an ADC, or an OpAmp, or MCU input. Connecting these things to the other side, would cause component failure. So, not sure what you mean.
Are you thinking of a Wheatsone Bridge?
I was thinking of a standard voltage divider - the common way to measure an unknown resistance such as also used for NTCs and FSRs.
A Wheatstone bridge may also work, depending on the actual (change in) resistance. It's a common way resistive load cells are measured.