A few things that you might find useful in your various electronics exploits. Please post any queries or components you want details on, but this sticky will be kept clean(ish) Equations: Ohm's Law: V = IR (Voltage = Current x Resistance) and the various re-arrangements of this: V/I = R, V/R = I Power equation: W = VI (Watts = Voltage x Current) and more re-arrangements: W = I * I * R, W = V * V/R There's a calculator here. Potential divider equation: Vout = Vin *R2/(R1+R2) Resistors in series: R = R1 + R2 + R3...etc Resistors in parallel: 1/R = 1/R1 + 1/R2 + 1/R3...etc. Capacitors in series: 1/C = 1/C1 + 1/C2...etc Capacitors in parallel: C = C1 + C2...etc. Resistance units Resistance is measured in ohms, but rather than use the omega symbol it's generally abbreviated to "R" for ohms, "k" for kilohms (1000R) and "M" for megohms (1,000,000R). To aid clarity on schematics, the letter is used instead of a decimal point, so 4k7 is 4,700 ohms, 0R47 is 0.47 ohms. Resistors Resistors, in particular, don't come in just any old value. They come in standard values, which aren't neatly arranged (eg 100, 250, 500, 1000), but rather take some values which many people think are unusual. The standard E12 series is this: 10 : 12 : 15 : 18 : 22 : 27 : 33 : 39 : 47 : 56 : 68 : 82 As you see, there are twelve values. These are then available in powers of ten, so the next range up is 100, 120, 150, etc. The reason for these particular values is actually fairly simple. These days it is easy to get these resistors to 2% or 1% accuracy, but not so long ago they were typically only 10% accurate. So a 6800 ohm resistor could in fact be as much as 7480ohms. At the same 10% accuracy, an 8200 ohm resistor could be as little as 7380 ohms. There is a slight overlap between these values, and this happens all through the E12 series if you take a 10% tolerance. So whatever value you calculate you want, there's one in the E12 series within 10% of it. There are other series available with bigger steps (E6, E3) and smaller steps (E24, E48), details here. Don't forget you can make any values you don't have by using other resistors in series and/or parallel, there's a calculator here. Next we move onto the colour code. Beginners often bemoan the colour code, seeing these cryptic bands of colour around a resistor, which supposedly mean something. The first step to understanding is to know what number each colour represents: Black = 0 Brown = 1 Red = 2 Orange = 3 Yellow = 4 Green = 5 Blue = 6 Purple/Violet = 7 Grey = 8 White = 9 For the multiplier, the colours above are used, so black is nothing on the end, brown is 1 zero (x10^1=x10), red is 2 zeros, (x10^2 =x100), etc, plus Gold = x0.1 Silver = x0.001 There are also colours to show the accuracy; Silver= 10% Gold = 5% Red = 2% Brown = 1% That's all very well, but which end do you read from? Well...this takes practice....basically you read from the end that makes sense. If the resistor has 4 bands, the layout of the bands is this: 2 colours for the "E" number, 1 colour for the number of 0s, and one colour for the accuracy. So if there's a silver or gold band, start reading at the other end An aid to reading the resistor correctly is that often the accuracy band is separated from the number bands by a slightly wider gap than is between the number bands. It does depend on the resistors though, and they may or may not have the extra gap. 5-band resistors: Maplin's 0.6W metal film resistors have an extra red band on one end to show a 50ppm temperature coefficient, after the brown 1% tolerance band. No E12 or E24 resistor starts red-brown (21), so you know to read from the other end. Other 5-band resistors show 3 digits, followed by the multiplier, then the accuracy band. In the E12 and E24 series, the third band is always black (0), so yellow-purple-black-red-red is 470 x 100 (47k) to 1% accuracy. Resistors also come in various power ratings, from 1/8W to several watts. Knowing the resistor value and the maximum likely current or voltage it will see, you can work out what power rating to use from one of the Power equations given above. Further reading: Beginners' Guide to Electronics, Part 1 - Basic Components Explained This covers all the passive components, resistors, capacitors and inductors, well worth a look. -------------- IsaacSibson wrote the original text, but any mistakes are now mine.
Variable resistors (Potentiometers) : Variable resistors can be used either as a variable resistance or as a potential divider. As a variable resistance, they control current in the load circuit. As a potential divider, they control the load voltage. (but don't forget current and voltage are linked by Ohms Law) In either situation, the potentiometer value needs to be matched to the load resistance - for example, a high-resistance potentiometer placed in series with a low-resistance fan will not work very well. A question that often crops up - What's the difference between a rheostat and a potentiometer? A rheostat is (as far as we need be bothered) a potentiometer designed to carry high currents, and will be rated 1W or more. It is used to vary the current through a load (such as a fan). Only two wires need to be connected, one coming in to the middle wiper contact and one leaving from one end contact (or vice-versa). A potentiometer can be used as a rheostat if it is (a) Low enough resistance (20-100 ohms). Aim for one with a similar resistance to the fan (R=12/Fan current) to give 6-12v control. (b) High enough wattage. For most fans, 3.5-5W is satisfactory. High power fans, you need to work out the wattage needed. The common carbon track potentiometers are only 1/4W or so and cannot be used to control fans (unless you add some electronics). Further info: Beginners' Guide to Pots
Capacitors are like very small rechargable batteries, they charge up and discharge. They charge up when the two sides of a capacitor are at different voltages and disgharge when the voltage difference decreases. Capacitors can also be used to block dc and only allow ac through which is very useful in audio systems Electrolytic capacitors (the can-like ones) have a - marked on them, this goes to the side that will always be at a lower voltage (usually ground in a pc) and can be damaged if connected the wrong way round (even explode and release evil stuff everywhere). -------------------------------- added by cpemma: Capacitors are covered in Beginners' Guide to Electronics, Part 1 - Basic Components Explained which explains the different types (ceramic, polyester, electrolytic, etc) far better than I can. A bit more info on the polarised types: Aluminium Electrolytic: The most common type of polarised capacitor. Commonly just called an 'electrolytic'. The capacitor consists of two aluminium foils separated by what is virtually a piece of blotting paper. The paper is impregnated with a special chemical liquid. The whole lot is tightly wound together and then attached to a DC power supply for the "forming" process. Anodising forms an insulating layer of aluminium oxide, which acts as the dielectric between the two metal layers, giving a capacitor of relatively high capacitance (for the size). The power supply is removed and the capacitor remains 'formed'. (If you connect a polarised capacitor the wrong way round, the insulating oxide layer is destroyed, the capacitor conducts DC and can get hot, boiling the liquid and blowing the end off ) It is important to note that this 'formed' capacitance value diminishes over time if the capacitor is not in circuit, although this takes quite a few years. The capacitor, by being in a circuit (and linked to a DC supply), remains 'formed' in use. Aluminium electrolytic capacitors do not have high insulation properties. They 'leak' small currents across the dielectric. This is not generally significant in many applications (such as power supplies, etc.) but does limit their use in long-period timing circuits, eg, with a 555 ic. You can now get low leakage versions but these only have a limited capacitance range, so tantalum types are generally used. Tantalum Dielectric: This capacitor is used when a lot of capacitance in a small space is needed - or where low leakage is required. The capacitor is basically a small bead of tantalum oxide which is porous like a piece of pumice The surface area - as you can imagine - of all the pores is tremendous. The metallisation forms over this surface area to produce a stable, low leakage but polarised capacitor. The bead has leads attached and is dipped in a resin coating. They were originally colour coded but most tantalums these days have the actual value printed on them.
Yes it can, even if you do NOT have a heart condition! Don't mess with them, a 1 farad capacitor can kill you if you are not careful. -special [k]
When voltage comes from a "noisy" source such as a transformer in a PC power supply, it is usually rectified to make DC. However, this DC will usually have spikes in it, so a regulation circuit is used to smooth the DC out so it can be used with the computer. Unregulated power is used for less sensitive components.
A typical unregulated DC power supply has a transformer to get the voltage to the level you want, a rectifier to convert the AC to DC, and a capacitor to smooth out the peaks and troughs from what was a 50Hz or 60Hz sine-wave. There's a voltage drop on all these components, and it varies with the current demanded by whatever the load is, the higher the current demand, the bigger the drop, and the less volts left for the load. So if the psu is powering your hi-fi, a loud signal won't be quite as loud as it should be and a quiet patch maybe a bit louder than planned. And even with the best capacitor smoothing, some sine-wave ripple will get through, possibly giving you "mains hum" or causing interference on sensitive circuitry. A regulated supply adds a controller to the basic power supply described above. In its simplest form, you transform to enough volts to cover the worst-case scenario (supply at minimum, load at a maximum) and then waste any excess volts across the regulator (as heat) at lower load levels. The regulator is a fixed output voltage, so apart from load variations, any variations in the mains supply can be catered for.
LEDs: Remember that LEDs have a forward voltage drop (Vf) rather than a fixed resistance value. Ball-park figures for this are: Red, Yellow, Amber, Orange, Green : 2.0-2.2v Blue, White, Bright/True Green : 3.7-5.5v but try to check with the suppliers' catalogue or a datasheet. This will also show the maximum safe current, If(max). Always use with a series resistor to limit the LED current If where: Resistor = (Vsupply - Vf) / If With two or more LEDs in series, add up the Vf voltages first. Total must be less than the supply voltage. Having got the resistor value, check what power rating is needed, with either: Wattage = (Vsupply - Vf) x If or Wattage = If2 x R (or use linear's calculator). LEDs described as "5v LED" or "12v LED" have the resistor already built in. LEDs only conduct in one direction (like other diodes), and can be destroyed if connected up the wrong way round (maximum reverse voltage is generally not much over 5v). The casing on an LED will sometimes have a flat side, which is generally the cathode (negative terminal), and the cathode lead may be shorter than the anode lead. It's easy to check with any multimeter that has a diode test position. It won't damage the led if you get it wrong on the first try and when you've got it right there should be a faint glow and a low number on the meter.
Electric Shock Dangers At the end of the day, it's good old Ohms Law that decides if your number's up. Your body normally has a fairly high resistance, several thousand ohms, and if you're stood on a poor earth, like a dry wooden floor or nylon carpet, and only touch the live wire, the current through your body will be very low and you should survive 240v mains OK. Try it with sweaty hands, stood on damp concrete or also touching an earthed metal case, and you might not be so lucky. There's a table here that reckons you can feel 1mA or more, over 10mA will contract your muscles so you can't let go, and over 100mA will stop your heart. What voltage you need to get that current level depends entirely on your body's resistance, and the resistance path to earth, at the time. In the UK we reckon 50-60v is safe (ie non-fatal ) in most circumstances (110v tools are actually running on +/-55v with a central ground) though 24v is the usual safe limit for home use. Prevention is far better than cure, so unplug equipment before messing, discharge capacitors safely, and test with a meter before touching. More Safety methods here.
