Posts Tagged ‘pcb’
Some people ask me about how to do hand soldering on smaller parts, like on my OLED Display Board. It uses a “TAB” (tape automated bonding) or “COF” (chip on flex) style flex tail mated with a “COG” (chip on glass) display. Normally, TAB connector is soldered directly to corresponding pads on your PCB using a hot-bar soldering machine.
I don’t have that hot-bar soldering machine, so my hand’s can make the job. The first time I’ve done this soldering, I was a bit scare about damage the connector. My only tip is “don’t spend much time over the fragile contacts.”
For help us, I’ve recorded my last OLED soldering. Maybe can help some people about SMD soldering (as some internet videos help me some years ago).
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:
Finally I’ve finished this project. It’s a USB Charger for Lithium Polymer Battery, that’s very compact and high powerful batteries that I use in most of my projects.
In my recent GUI Bot project I’ve used the famous MAX1555, a greatest IC in a tiny SOT23 package. One thing about MAX1555 that I don’t like is the current limitation it has (about 100mA). I know it’s for USB low speed compatibility, but who knows someone that don’t have a high speed USB host surround? Because that I use the greatest MAX1811. It’s a great IC too, but can deliver up to 500mA! See one of my first post to check more details.
That’s the finished work:
If anyone like this circuit, email to firstname.lastname@example.org.
Well, my board comes to me yesterday and I’ve soldered all parts. The board is really small, very compact, very robust. I’ve made a little test only to check if are something wrong, but not. Everthing works well. Very good for version 0.
See the photos:
Now, I (and you, please contact me if you like it) can test and make nice devices, without boring with crystals, capacitors, reset, USB it self. You can attach it on a breadboard or your own prototype PCB with your circuit.
I’ve created a kind of pin card, for rapid pinnout reference. I got the card idea from Mbed project. It’s very useful, with all pins and ports plus the information about device peripherals.
Think the nice things you can do… I want to make a test with USB Mass Storage Device, yes, a Flash Drive. I’ve a little project where this really help me with configuration storage, etc.
For test, I’ll show us some application example, like a USB RFID reader.
If you like this board and want to get one, email me to talk about. It’s a zero version, so some errors could exist.
Eagle files here.