Electrical Engineering

Here are some companies who will fab boards from uploaded Gerber files which can be produced in Fritzing. I have no idea about the quality of these services but I’m collecting them for future reference.

When there are four marking bands, the first three bands specify the value. The first band’s color value is multiplied by 10. The second band’s value is added to that. The third band’s value is the exponent that 10 is raised to which should be multiplied by the previous total.

For five band resistors, the first three compose a three digit number, the fourth is the multiplier, and the fifth is the tolerance. Note that the tolerance is usually separated with more space than the other bands.

Example:

The 4th band of a 4 band resistor indicates its accuracy. Gold at 5% seems like the most common.

A unit without a tolerance band should still be implicitly within 20%. Gold and silver are by far the most commonly used tolerances, gold being almost ubiquitous in my experience. Much less commonly gold and silver can also be order of magnitude signifiers - .1 an .01 respectively.

Digikey has a nice chart and calculator.

What if you get a very weird resistor, bigger than usual, and it has a yellow band outside of the gold band? For example, I have one that is BRN GRN BLK GLD YEL. I don’t know for sure but that seems like it may be one of these wire wound non-inductive resistors.

Sounds easy, right? But I’ve had moments of confusion so here’s a conclusive summary and some examples of how this works with my meter (and many others).

The values that the meter shows are in the units that match the dial’s setting. So if it’s set at the 2K setting, that will be in "K" or kilo Ohms. If you have it set on 20M that will read in mega Ohms. If you have it on the 200 setting, the one with the audible low resistance continuity signal, it will read directly in Ohms. The values on the settings represent the maximum value that range can accommodate; so the 2000K setting can handle a 2 mega Ohm value but will produce kilo Ohm readings.

Let’s say I have a resistor that has YEL VIO BLK BLK BRN or possibly (reversed) BRN BLK BLK VIO YEL. But we’re not sure if the violet is really gray or blue or who-knows-what. It’s helpful to start with a best guess, in this case 4-7-0-(10*0) +/-1% Ω.

How should that appear on a meter? It would be nice if it came up as 470 Ω. But it mostly will not. My typical meter has settings for "20M", "2000K" (i.e.2M), "200K", "20K", "2K", and "200". If I suspect my resistor is 470 Ω then I should go to the smallest setting that is larger than the value. 200 is less than 470 so that won’t work; "2K" is larger than 470 so that’s the one to try. If the color band guess was correct, the reading should be something like ".468".

How about another: BRN GRY BLK YEL BRN, or maybe BRN YEL BLK GRY BRN? Maybe the GRY or YEL is wrong — hard to tell being microscopic and badly printed. The first interpretation should be "180 * 10,000 +/-1%" or 1.8M Ω. The second way to read it is "140 * 100,000,000 +/-1%" which is 14G Ω, a bit high.

Let’s assume the 1,800,000 Ω or 1800 kOhms. This is just under the 2000K setting so use that. The reading comes in between 1350 and 1430. That’s not what was expected! The problem with this measurement is that I’m holding the leads of the meter in my hands and my body has a certain (pretty high) resistance that for values like this are non-negligible. After connecting the resistor to clips so that it can be measured without touching it, the reading bops around from 1783 to 1805 which is what is expected.

How about BRN ORG BLK BLK BRN, possibly BRN BLK BLK ORG BRN. This one seems most reasonable to assume 130 Ω. Setting it upon the meter, I get a "1 ." which means it’s not even close. Maybe the other way was right after all — 100 * 1e3 which is 100k Ω. This means that the dial must be set to "200K". Then the reading comes up "100.2" which seems to confirm it.

What about BRN RED BLK BLK BRN or BRN BLK BLK RED BRN. The first way is 120 Ω and the second way is 10K Ω. On the 20K setting I get 10.01 which confirms the second reading.

Note that HF chokes can look exactly like resistors. I had some with 4 bands BRN BLK BRN SLV that were definitely not 90-100 Ω. They measured more like 3 Ω. So if your resistance is absurdly low for the markings, consider that the component may not be a resistor.

In series, resistors just add up in a pretty intuitive way.

In parallel resistance takes this form.

In GeoGad this is easily computed. Here are two 8 Ω resistors plus a 4 Ω.

Answer is 2 Ω.

Here is how to compute the equivalent resistance of five 470 Ω resistors in parallel.

Answer is 94 Ω.

Since polarity is a diode’s one job, they are definitely polarized.

These are also called breakdown diodes. They conduct in one direction unless the voltage is over a threshold when their blocking stops functioning. This allows them to protect sensitive low voltage components from surges of high voltage. The symbol for zener diodes is like a regular diode but there are little extensions off the cathode bar; ironically, these extensions are isomorphic to an S and not a Z.

Often shown on datasheets as Vf (forward voltage). This is the voltage lost from the power supply’s or battery’s nominal value. I’ve been told that these are reasonable values.

That is a load of bollocks and you should either not care what it is, or measure it yourself.

Normally the LEDs I acquire come from random cannibalized sources and I do not know the Vf drop. Amazingly, you can check it empirically by turning it into a de facto solar panel. Hook the LED up to a multimeter on DC low range (2V on mine - not 20V or 200mV, etc). Hold the LED very near a very bright light (normal 7 watt household LED light bulb works fine) and the multimeter should reflect the voltage drop useful for LED resistance calculations.

Put negative lead of LED on negative terminal of the battery. Measure the voltage from the positive lead of the LED to the positive lead of the battery. Subtract this value from the voltage between the two battery terminals. This will be the LED’s voltage drop.

It’s good to know this based on the rating as they were purchased. But if you don’t know it’s reasonable to assume that the reference current is usually 0.020A i.e. 20mA. 25-30mA is usually maximum continuous current.

LEDs and diodes generally need a resistor (or some kind of resistance) to keep the circuit from essentially being a short.

The charge a capacitor can store is the capacitance times the voltage. Somehow Farads times Volts equals Coulombs. That means that Farads times Volts divided by seconds equals Amps.

The energy in a capacitor is the capacitance multiplied by voltage, squared, divided by two.

Capacitors in parallel are equivalent to the sum of the individual components. The total capacitance in series is the inverse of the sum of the inverse capacitances. So if you have a 240pF capacitor and a 560pF the total capacitance would be calculated from this equation.

In general it is a good practice to add decoupling (aka "bypass") capacitors in parallel (the bypass) with the power supply to every IC to protect it from fluctuations. For ICs, a sane value is usually 0.1 μF. Capacitors are also useful for smoothing out chops in DC current produced by a diode rectifier (where 2 diodes guide the current one polarity, and then 2 other ones take it for the reverse polarity).

Note that while it may seem sensible to think of a capacitor as something with capacitance, they may additionally have other properties. Equivalent series resistance or ESR is the resistance of the capacitor itself. This resistance can produce heat and shorten the life of the component. It would seem to be a property that should, in some critical situations, be carefully considered.

Capacitors sometimes have polarity, usually when their values get large. This is convenient because their packages are larger and thus there is usually more room to print the polarity in some obvious way. If the capacitor is not polarized, the diagram will just show two parallel lines making a gap in the path as the symbol.

A two digit label on a capacitor is in picoFarads.

A three digit label is not just that value in pF. The third number is the number of zeros you’d tack onto the first two numbers to produce the value in picoFarads. Some examples.

A Letter after the value is the tolerance code.

This excellent capacitor resource says an absurd amount about obscure capacitor polarity and value markings.

Does your fancy multimeter have a capacitor measurement ability? Mine does. It does not appear to matter which sockets you put the leads in as long as one is on top and one is on the bottom. For testing a capacitor labeled 104, it is expected to be 0.1 μF. If I put the setting on the 2 μF setting it should read something like .096. Like with resistance, the units of the setting are what is shown and the number is the maximum value valid with that setting.

Note that you need to wait a few seconds for capacitance readings to settle down to the correct value.

Surface mount capacitors are put in packages that have designations like 0402 which is 0.4mm x 0.2mm. Or 0603 (0.6mm x 0.3mm).

There are 5 leads to an opamp. * The non-inverting input is positive. Negligible current flow. * The inverting input is negative (since negative numbers invert). Negligible current flow. * An output. * A power supply positive pin. * A power supply negative pin.

If you have a 1.00V non inverting input, and a 1.01V inverting input, the output will want to be the difference amplified to some huge gain (1e6 times or so). So in that example, 10kV; which means that it will saturate at the power V+ level producing what is in effect a comparator. In other words, if the signals are different by a tiny amount, the circuit will be made. (Not that using opamps is an ideal comparator - there are special purpose such devices).

The normal way that opamps are used is with a feedback resistor between the output and the inverting input. With this the output will be modulated so that the inverting input voltage will match the non-inverting input. Another way to think of that is if the output is just shorted right to the inverting input, the output will be set to match the non-inverting input exactly — this is called a "buffer" and is not too exciting. But it establishes the operation for when you do start fiddling with resistances of the negative feedback. Also, since no current flows to the inputs, any current flow is coming from the V+ supply, not the inputs. This means you can use a voltage producing sensor like a strain gauge without pulling any current that could damage it. The current you might need to drive a dial to read the gauge’s value will come from the V+ supply. High impedance signal, low impedance output.

Insulated Gate Bipolar Transistors. Features a very fast switching speed. Current flows from a "collector" to a "emitter". V_TH is usually between 3V and 8V. IGBTs can switch greater amounts of current than MOSFETs.

Metal Oxide Semiconductor Field-Effect Transistors. Current flows from a "drain" to a "source". V_TH is usually between 0.5V and 1.0V. Generally switch quicker than IGBTs and are usually cheaper. Better for small to medium motor control.

Brushes are also called commutators and the process where the current is reversed in the windings to keep the induction forces pointed in the right direction is called commutation. The fact that the brushes are a friction contact surface means they will eventually be consumed and need maintenance, a significant problem with this kind of motor for applications with long life cycles. Also the commuter has mass which robs from the potential payload that the energy could otherwise be applied to moving. And the commuter’s sparking is also a potential for EMF and electrical noise.

Permanent Magnet DC (PMDC) motors are the most popular brushed motors. These motors have relatively stable (permanently) magnetic fields making the ratio of speed to voltage K_V pretty much constant.

PMDCs are simple and have many useful properties, but the magnetic field diminishes over time resulting in a loss of torque and speed. This problem is exacerbated when the motor is subjected to large startup currents.

Series-Wound DC (SWDC) motors have "field windings". Instead of the magnetic field being generated by a permanent magnet, it is generated by an electromagnet. As the name implies, these magnetic field windings are wired in series to the rotor windings that produce the torque. Because an increase in current increases the magnetic field in addition to increasing the torque, SWDC motors can be much stronger than similar form factors of PMDC motors. This sounds well and good, but the problem is speed control. Because the magnetic field is changing along with the current, it’s hard to keep the speed stable.

Shunt-Wound DC (SHWDC) motors put the field windings in parallel to the rotor windings. These don’t have the torque of series wound motors, but their superior speed control properties can be important.

Control of brushed DC motors that only need to turn in one direction can be achieved with a MOFSET that can just open and close as needed with signal pulses (PWM). One problem is that when the power supply is shut off, the motor, which acts like an inductor, can generate EMF that pulls hard on the open switch, possibly damaging the controlling transistor. This is where a flyback diode in parallel with the motor can help. With that, the current can just cycle back if the motor motor inductance keeps the voltage high (with reverse polarity).

Controlling a brushed DC motor in either direction arbitrarily is trickier. You basically need four switches, two that turn on a path (in and out of the motor) for one direction’s polarity and two that do the other direction. This arrangement is called an H-bridge. The L298 is an example of an H-bridge (two of them) in an IC package. The L9110 is similar but the IC also includes the flyback diodes which generally route around the 4 transistors in an H-bridge.

A BLDC motor is basically like a traditional brushed DC motor but the commutation is done electrically, not mechanically. Without mechanical contact between the rotor and stator, this kind of motor can be more durable and efficient. In the typical BLDC configuration, current passes through windings on the stator while the rotor is composed of magnets. There are usually many more windings than in brushed designs, and these stator windings comprise many iron core electromagnets arranged around the perimeter. These electro magnets are energized in sequence to continuously pull the rotor magnets.

Although different forms are possible, most BLDC motors are 3 phase. This means that the electromagnets are divided into 3 groups and each group has its own distinct energizing schedule. So if there are 6 winding "slots" and the BLDC motor is 3 phase, there are 2 linked A poles that fire together, 2 linked B poles, and 2 linked C poles. Note that most practical BLDC motors that normal people will run into have 3 wires, one for each phase.

The main difference between BLDC motors and AC motors is that BLDC current is constant when the winding set is energized, while AC motors have sinusoidal current.

At slow speeds, BLDC can become jumpy as the rotor hops from one winding set to another. There are more expensive "slotless" BLDC motors with more windings that try to ameliorate this problem.

The number of magnets in the rotor is fixed to the number of "poles" listed on the spec sheet (maybe poles= magnetsx2). The more poles, the greater the torque. An "integral slot" motor has the number of slots is a whole multiple of the poles. The motor is called "fractional slot" when this is not the case; this configuration can help reduce "cogging" or the jumpiness of BLDC motors. For example, a 6 slot stator with 3 phases, A, B, and C is a fractional slot BLDC motor if there are 4 magnetic rotor poles. It would be integral if there were 3 or 6 rotor poles.

There are two physical configurations of BLDC motors. The most ordinary looking one is the "inrunner" where the magnets are inside. The main difference between a normal inrunner BLDC motor and similar sized brushed motors is that the former will usually have 3 wires (3 phase) and the latter 2 (+ and -). The other configuration is the "outrunner" where the magnets are surrounding the windings and shaft. The magnets are always considered the "rotor" but this is strange when the shaft, considered a part of the stator on outrunners, will look like it’s turning. I believe the real motor shaft must be mounted fixed and the "shaft" that looks like it’s turning is really attached to the outer, spinning, motor case. Here is a good example.

Stepper motors traverse precisely through a specified rotation without the need for position feedback. Common step angles for step motors are 30, 15, 7.5, 5, 2.5, and 1.8 degrees.

The main defining feature of a proper servo motor is not in the motor itself but in the system which includes a feedback mechanism. This feedback allows positional accuracy like step motors, but also adds the ability to control rotational velocity and acceleration.