How much current (part 1)?
Someone send to me a question, about how much current my OLED board consumes at all. Well, after measure with a multimeter the answer is: 660μA (aprox.) with display all ON and contrast at 0xFF (max). But this question make me thinking about how measure that current myself, without a multimeter.
When I’m in graduation (some good years ago) I made a little circuit to measure how much current my power supply project output. The circuit is based on “high-side current-sense” methodology. See my hand-drawn circuit above:
This circuit is a classic high-side current-sense, where the voltage drop across Rs resistor is isolated by a operational amplifier (op amp) in a differential configuration with unitary gain. The many implementations of technic were based on discrete components or semidiscrete circuitry. In their simplest form, such high-side monitors require a precision op amp and a handful of high precision resistors.
The resistor value should be low (like mΩ scale), to minimize power losses, but don’t be too low, because stability problems. And don’t forget about power dissipation across the resistor.
One common approach for high-side measurements has been the use of the classic differential amplifier, which is employed as a gain amplifier.
So, after reading a lot of theory, I’ve going to search my integrated differential amplifier.
Integrated Differential Amplifier
- Tiny SOT23-5 package;
- Low cost;
- 3 gain options (20V/V, 50V/V and 100V/V);
- +2.7V to 28V range of operation;
- Consumes only 30μA;
- 0.18% full-scale accuracy with 100mV Vsense input (This is equivalent to only 0.18mV input offset voltage);
The MAX4372, in a tiny SOT23-5 package is a very good device to make a current sense device. You can set the full-scale current reading by choosing the device:
- MAX4372T: 20V/V Gain;
- MAX4372F: 50V/V Gain;
- MAX4372H: 100V/V Gain;
What means 20V/V Gain? If you have 1A current flow through a 100mΩ Rs resistor, you gave only 0.1V drop. But with 20V/V, you multiply this and obtain 2V. Obviously? The design goal with Gain is thinking in terms of full scale design. If your A/D converter uses 3.3V was reference, you can consider this to check what the max current value you can measure with determined gain value.
A test circuit
I’ve designed a test circuit to test my idea. Basically is a MAX4372T with a 100mΩ Rs. I’ve mounted it in universal board, and my idea is connect it to my USB Low Pin Kit:
To test them, I’ve used a PIC18LF2520 and a great character LCD from Electronic Assembly. See my test circuit working:
The pictures show that the global idea works great. Now I need some work to improve stability and other features.
The next step is create a USB device for read, store and show the current measurements: