Current. Too little of it, and you can’t get where you’re going, too much and your hardware’s on fire. In many projects, it’s desirable to know just how much current is being drawn, and even more desirable to limit it to avoid catastrophic destruction. The humble current shunt is an excellent way to do just that.
To understand current, it’s important to understand Ohm’s Law, which defines the relationship between current, voltage, and resistance. If we know two out of the three, we can calculate the unknown. This is the underlying principle behind the current shunt. A current flows through a resistor, and the voltage drop across the resistor is measured. If the resistance also is known, the current can be calculated with the equation I=V/R.
This simple fact can be used to great effect. As an example, consider a microcontroller used to control a DC motor with a transistor controlled by a PWM output. A known resistance is placed inline with the motor and, the voltage drop across it measured with the onboard analog-to-digital converter. With a few lines of code, it’s simple for the microcontroller to calculate the current flowing to the motor. Armed with this knowledge, code can be crafted to limit the motor current draw for such purposes as avoiding overheating the motor, or to protect the drive transistors from failure.
In fact, such strategies can be used in a wide variety of applications. In microcontroller projects you can measure as many currents as you have spare ADC channels and time. Whether you’re driving high power LEDs or trying to build protection into a power supply, current shunts are key to doing this.
The passive component industry — the manufacturers who make the boring but vital resistors, capacitors, and diodes found in every single electronic device — is on the cusp of a shortage. You’ll always be able to buy a 220 Ω, 0805 resistor, but instead of buying two for a penny like you can today, you may only get one in the very near future.
Yageo, one of the largest manufacturers of surface mount (SMD) resistors and multilayer ceramic capacitors, announced in December they were not taking new chip resistor orders. Yageo was cutting production of cheap chip resistors to focus on higher-margin niche-market components for automotive, IoT, and other industrial uses, as reported by Digitimes. Earlier this month, Yaego resumed taking orders for chip resistors, but with 15-20% higher quotes (article behind paywall, try clicking through via this Tweet).
As a result, there are rumors of runs on passive components at the Shenzhen electronics market, and several tweets from members of the electronics community have said the price of some components have doubled. Because every electronic device uses these ‘jellybean’ parts, a decrease in supply or increase in price means some products won’t ship on time, margins will be lower, or prices on the newest electronic gadget will increase.
The question remains: are we on the brink of a resistor shortage, and what are the implications of manufacturers that don’t have the parts they need?
Through-hole assembly means bending leads on components and putting the leads through holes in the circuit board, then soldering them in place, and trimming the wires. That took up too much space and assembly time and labor, so the next step was surface mount, in which components are placed on top of the circuit board and then solder paste melts and solders the parts together. This made assembly much faster and cheaper and smaller.
Now we have embedded components, where in order to save even more, the components are embedded inside the circuit board itself. While this is not yet a technology that is available (or probably even desirable) for the Hackaday community, reading about it made my “holy cow!” hairs tingle, so here’s more on a new technology that has recently reached an availability level that more and more companies are finding acceptable, and a bit on some usable design techniques for saving space and components.
Through-hole resistors come on tape that we’re now calling bandoliers. Since [Spencer] is selling a boatload of his RC2014 backplane computer kits on Tindie, he’s been chopping up a lot of resistor bandoliers. It’s a boring and monotonous job.
Fortunately, a lot of people have had a bandolier cutting problem over the years, and there are some hobbyist-grade robots that will do this work for you. One of the more popular robots tasked for bandolier cutting is a laser cut robot. However, if you already have a laser cutter, why not just use the laser to cut the bandoliers? It’s brilliant in its simplicity.
[Spencer] spent a little bit of time designing a template to turn his laser cutter into a cutter for through hole resistors. No, he’s not trimming the leads — this is just a device to cut resistors into groups mini bandoliers of a handful of resistors. The tool is made out of plywood, with a smaller top piece held down with magnets to keep the resistors aligned.
The entire template is up on Thingiverse, and it’s great if you need to cut hundreds of resistors to kit dozens of projects. If you’re only doing one or two, scissors will be the way to go, but if you’re cursed with the monotony of trimming hundreds there’s no better way to get things done than to put a robot to work.
Like any reasonable person, [daqq] decided it would be fun to “solve one of those nasty [electrical engineering] puzzles/exercises where you start out with a horrible mess of wires and resistors and you are supposed to calculate the resistance between two nodes.” You know, just an average Saturday night. At the time, he was also fascinated by Charlieplexing – an awesome technique that either allows one to control multiple polarized components, such as LEDs, simply by connecting them in a specific way. After toying with the idea for a while, [daqq] found that using just Charlieplexing would be“a horrible mess” but he didn’t stop there. Drawing inspiration from Charlieplexing, he came up with the idea to connect things in such a way that every node is connected by one connection to every other node – a complete graph from a topological view point (this makes so much more sense visually). From here, he was able to set pins to HIGH, LOW, or INPUT and gather all the data needed to solve his linear system of equations.
Now, there is a balance to everything, and while this system can determine the resistance of .5*N(N-1) resistors using just N wires, it also a memory and computation hungry method. Oh well, can’t have it all. But, while it’s computationally hungry, [daqq] got it working on an ATMega32, so it’s not an unmanageable feat. And, let’s not forget to mention [daqq’s] wonderful writing. Even if you don’t know linear algebra (or would rather forget), it’s a good read from a theory perspective. So good, in fact, that [daqq] is getting published in Circuit Cellar!
If you aren’t old enough to remember programming FORTRAN on punched cards, you might be surprised that while a standard card had 80 characters, FORTRAN programs only used 72 characters per card. The reason for this was simple: keypunches could automatically put a sequence number in the last 8 characters. Why do you care? If you drop your box of cards walking across the quad, you can use a machine to sort on those last 8 characters and put the deck back in the right order.
These days, that’s not a real problem. However, we have spilled one of those little parts boxes — you know the ones with the little trays. We aren’t likely to separate out the resistors again. Instead, we’ll just treasure hunt for the value we want when we need one.
[Brian Gross], [Nathan Lambert], and [Alex Parkhurst] are a bit more industrious. For their final project in [Bruce Land’s] class at Cornell, they built a 3D-printed resistor sorting machine. A PIC processor feeds a resistor from a hopper, measures it, and places it in the correct bin, based on its value. Who doesn’t want that? You can see a video demonstration, below.
Resistors are one of the fundamental components used in electronic circuits. They do one thing: resist the flow of electrical current. There is more than one way to skin a cat, and there is more than one way for a resistor to work. In previous articles I talked about fixed value resistors as well as variable resistors.
There is one other major group of variable resistors which I didn’t get into: resistors which change value without human intervention. These change by environmental means: temperature, voltage, light, magnetic fields and physical strain. They’re commonly used for automation and without them our lives would be very different.