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- The Mark Burnley Files
Tips and tricks from
Mark Burnley
Updated 23 December 2004
Getting to know electronic components -
Resistors !
Whaddya Mean.....Resistors ?
There's probably more of the critters on the planet than humans, but do not fear ladies and gentlemen, we have them under complete control ... or we will have after we've looked at a few facts 'n figures about that harmless but loveable component ... the resistor.
Okay, it's a daunting task wading through the average component supply catalogue getting the bits together for your latest pro audio project. There's pictures, figures, percentages ... but what does it all mean? Lets break it down and get it sorted. Resistors are one of the basic components in electronics. There wouldn't be much electronic equipment without resistors and the principle of resistance. So what's the job of a resistor ? Quite simply, a resistor is a component which resists or impedes the flow of electrons [ hence the word Impedance ]. A current is a flow of electrons in a conductor such as a piece of wire or circuit board trace [track]. So a resistor allows us to have control over the amount of current flowing in a circuit. The greater the resistance of the current path, the less current is able to flow. Decreasing the resistance in the current path allows more current to flow.
" But what's this got to do with my compressor I'm building ?" I hear you ask.
Wel l- it's a very small answer, with a tiny bit of maths [ which you won't even notice ! ]. Here goes...
Resistance, current and voltage are all linked. When you have a resistor of "R" ohms with a current of "I" Amps flowing through it, you will measure a voltage of "V" Volts across it. And these are linked by Ohms' Law, which looks like this...
V= I * R
V is in Volts, I is in Amps and R is in Ohms.
So a resistor can be used to convert a voltage to a current, and a current to a voltage. A circuit designer makes use of this in a design. So when a point in a circuit needs to have 1mA flowing, and the power supply available is say 15V, a quick blast on Ohms law will work out what resistor value is needed.
Because V=I * R doesn't give us an easy way to get to the value of R, we can state it in two other ways...
R=V/I and I=V/R
Much easier. So our designer just uses R=V/I, where V= 15V and I= 0.001A
15/0.001=15 000 Ohms which equals 15k
A 15k resistor connected to 15V will have a current of 1mA passing through it. And put another way, if you pass 1mA of current through a 15k resistor, you will measure 15V across it.
[ There's a nice "cheat" with Ohms law. If you keep V in volts, but use R in kilohms (k), you can use I in milliamps (mA). Also, keeping V in volts you can have R in megohms (M) and I in microamps (uA) and still get sense. ]
And that's about all the "deep" theory we need to know about resistors.
And this isn't just useful for designers.....when we look at a schematic, armed with our knowledge of Ohms law and a calculator (hey, not everyone can carry rows of 000's in their head, I can't! ) we can see what sort of voltages and currents can be expected at different points in the circuit without even picking up our soldering iron and meter!
Now that we've delved as deep as we want to into the science of resistors, lets move on to actually choosing the things from a catalogue.
We'll assume that we're using "thru-hole technology" components rather than "Surface Mounted Devices", because although you can make your own SMD circuitry, most of the DIY audio projects rely on the older "leaded" components. These are much easier to work with, and are actually better suited for some audio applications than their SMD counterparts. Anyway, we've usually got a nice 1U or 2U rack case to play with- plenty of room! So this narrows the choice a bit more.......
The reason that the resistor section of a catalogue is so fat is that there are four factors which go into the choice of a resistor:
1. Resistance Value - in ohms, kilohms or megohms.
2. Composition - what it's physically constructed
of inside.
3. Power Rating - how much current can flow through
it before you smell that "cooking" smell or it bursts into flames.
4. Tolerance - how precise it is at "being",
for example, a 10k resistor.
Here's some more information on each of these factors:
1. Resistance Value
Self explanatory, well almost. The basic unit of resistance is the Ohm. Most of the projects we work on in audio, whether solid state or tube will require resistors in the range of ohms, kilohms or megohms.
Saying 1000 ohms is quite a long way of marking a component.( Because the symbol for the ohm is the Greek letter "Omega", we tend to use a capital R nowadays; makes it easier for us to type on a computer. So from now on, we'll call a 470ohm resistor a 470R resistor) A better way is to use a prefix to "ohm" which means "a thousand"- so 1000R is equal to 1kilohm or more simply 1k. Much simpler. When you hit 1000k, you're looking at 1 million ohms, so it's much easier to say 1megohm, or 1M.
One thing you'll see on schematics is the placing of a letter in the middle of the value. So for example you'll see "4k7". This just says that the resistor has a value of 4.7k. Then why not just say 4.7k? Well, there are two ways of marking a resistor. For smaller resistors they'll use the 3-band or sometimes 4-band colour code. This is a great way to mark components.( You don't even need to remember it. Most component catalogues have it somewhere. Cut it out and paste it on the wall. Even better, buy one of those "resistor colour code wheels"- it's a piece of card with wheels with the colours on- match the colours on the wheels to your resistor and read off the values....)
But larger resistors, especially "power" resistors which tend to get hot and could lose their colour bands (yes, done it myself- red band turned brown- 2k2 looked like a 220R and well....a bit of smoke...) so they have their value printed on. If our 4k7 resistor was printed "4.7k" it would be very easy for the decimal point to be rubbed off and this could lead to all kinds of trouble. Being marked 4k7 is a much more reliable way to mark them. So look out for 6R8, this is 6.8R; and 3M3- this is 3.3M.
But what values are available, and which do we need? Resistors are commonly available in values from 1R up to 10M, and most values we'll use are within this range. Values of less than 1R are available, as are ones above 10M (check out a U47 mic- 100M!)
But resistor manufacturers aren't going to make every value from 1R to 10M in 1R steps- thats 10 million values! So what happens, is that they make a range of "preferred values" which are standard ranges of values from which to pick the one for the job. The two main ranges are the E12 and E24.
Here are the 12 values making up the E12 range:
1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
And the 24 values of the E24 :
1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1
So for example, in the E12 range you can get 3.3R, 33R, 330R, 3.3k, 33k, 330k and 3.3M resistors. And it's the same for the E24 range, except that there is a greater range of values. Most of our projects use resistors from the E24 range. So when a circuit designer finds that they need for example a 385R resistor, they will use a 390R resistor which will be close enough for most cases. You can get values inbetween these ranges, but they tend to be for more specialised work, and cost more.
One tip here for odd values. If you find that you need a 2k resistor, but only have 1k8 or 2k2's, but you do have lots of 1k's to hand, you can "make up" resistors. Two 1k resistors connected in series, i.e. "end to end" will be the equivalent of adding their values- 1k+1k=2k. You could also use a 1k8 and a 200R in series for 2k and so on.... If you needed a 500k resistor, you can place two 1M resistors in parallel, i.e. solder one "piggyback" across the other, and you'll reach the value of 500k. [I'll go into this technique further in a later article about useful tips and tricks and simple equations for DIYing.] You have to allow for the circuit board layout and space, but it can be useful in situations where you just haven't got the exact value and want to get it up and running.
All component lists for projects will state the value of the resistor required. And now we can decipher the values, and hopefully even read them when we get them through the mail! I find that when I've ordered a lot of resistors, a lot of them are supplied on paper "tapes". It can be well worth spending ten minutes going through the resistors, reading their colour codes and writing it on the tapes with a pen so that when you come to "stuff" your circuit board, you've got a bit of order. And if you really don't want to read the colour codes, use a Digital Multi Meter. Even cheap DMM's now come with a pretty accurate resistance measuring range. Put some croc-clip leads or "grabs" on the meter leads, pop a resistor between them and you're off!
2.Composition
To create a resistor, the manufacture takes a material which has a "resistance" property such as carbon, a metal oxide film, a metal film or a length of metal-alloy wire such as Nichrome or Constantan, and makes it into a neat user-friendly package with a protective coating and connections to the outside world. The amount of resistance is determined by the amount of resistive material. These resistors all have different properties depending on their construction. It helps to know a bit about these properties, so lets have a quick look....
Carbon - for years these were the cheapest and most common type of resistor. Basically a tube of carbon with leads. Their value tends to drift with age and they aren't keen on heat. Spot the crumbly carbon resistors on things like older tube guitar amps and audio equipment. Their values are not that accurate either (see "tolerance" below). Also, for audio they can be noisy. They have quite a high level of self-generated noise. Carbon film resistors have slightly better specifications than standard carbon composition.
Metal Oxide - These are of similar construction to the carbon film- a thin layer of tin oxide instead of carbon is placed on a ceramic tube former. These are more expensive, but have good electrical specs, and are quite rugged.
Metal Film - Again, these have similar construction to the metal oxide and carbon film types, but have a metal film coating instead. These have very good characteristics of low self-noise, value stability, tolerance, power rating and ruggedness. They also have a very good price-to-performance ratio. They are cheap enough to be used in most applications, and many suppliers have now phased out their carbon film stock because metal film have become so popular. Metal film resistors are a good choice for most audio work, especially solid state discrete and op amp-work.
[ You can buy something called "starter packs" of 1% 0.25Watt metal film resistors from some suppliers. These have 10 of each value in the E24 resistor range from 10R to 1M and can be a good place to start, especially if you want to do a bit of modifying or designing, and it saves you money buying in bulk ! ]
Wire Wound - One of the oldest and simplest to understand designs of resistor. Basically just a length of resistive metal wire wrapped around a tube to give a certain value of resistance. Most of the wirewound resistors you'll see are for power resistors with power dissipation of 3W up to a few hundred watts! You can often actually see the spiral of the wire below the ceramic coating. They can usually handle a lot of heat, but tend to be quite bulky and expensive. Some very high-precision resistors for test equipment and scientific equipment are made wire wound with very exact values of resistance.
There is much debate about the composition of resistors and their various merits for audio circuitry. Some people swear by vintage or "NOS" (new-old-stock) carbon types for their tube guitar amps and tube studio equipment. I'm not going to discuss this here because it really is a collossal subject and open to much debate, theorising and points of view. Lets just say for now that as long as the electrical specifications of resistance, power rating, tolerance and sensible composition/pricing are met, then your project is going to work. Some stages of a circuit in a piece of audio equipment have no effect on the quality of the audio signal and are merely to set values of voltage or current to allow the active devices (transistors, op-amps or tubes) to work properly. My advice is to build the project with what you can find/afford, and at a later date try for yourself any "upgrades" to higher specification/different composition components within the signal path, and see if it makes a difference to you. Be prepared to experiment. As long as you stay within the electrical specification, you can try what you like!
It also doesn't matter if you use different makes or compositions of resistor in a circuit. If the parts list does not specifically say that certain parts need to be physically or electrically identical, then a mixture of types can be used without worry.
3. Power Rating
Again, self explanatory. But what is 'power', and how does it relate to a resistor? Well, if we look at it in simple terms, we said before that a resistor resists the flow of electrons and therefore current. What happens is that electrons "slam" into the atoms of the material in the resistor, and the energy created is given off as heat. The resistor gets hot. There will come a point when the heat generated in it is greater than can be removed. Heat is removed from a resistor by radiation- like the bar of an electric fire. When the heat can no longer be removed, "hot spots" are generated inside which can cause a number of failures. These can range from "fusing" of the resistor- the material is destroyed and is no longer a resistor, mechanical breakdown- it falls apart (or explodes!) due to heat fatigue of leads, former [core] or material, and can even range up to a small scale fire ! [personal experience]
So it's really important to choose the correctly rated resistor for the job. Even at low voltages, a low value resistor can easily exceed its' power rating. To decide what value power rating is required is very easy. It's just using a small equation again. Nothing heavy, basically zero-maths. There are two approaches. In both methods you need to know the value of the resistor, but you'll probably know that anyway. With the first method you need to know the maximum current that will flow through the resistor. The power P, in Watts, dissipated by the resistor is the current through it I, in amps, squared (i.e. I x I) multiplied by the resistance value R in ohms. Like this:
2
P= I * R
So in a circuit with a 22k resistor with 3mA flowing through it, the power in watts will be:
P= (0.003 * 0.003) * 22 000
P= 0.000009 * 22 000
P= 0.198W
So a 0.25W (quarter Watt) resistor would work in this circuit.
The other method is if you know the maximum voltage expected across the resistor. The power P in Watts dissipated by the resistor is the voltage V across it in volts, squared (i.e. V x V) divided by the resistance R in ohms. So:
2
P= V / R
So in a circuit with 15V across a 100k resistor, the power in watts would be:
P=(15 * 15) / 100 000
P= 225 / 100 000
P= 0.00225W
You can see in this case that a 0.25W resistor would work perfectly- even in a high ambient temperature, there would be no problems. There's a safety factor of over 100 times.
It's often better to "over-rate" a resistor especially in equipment which gets hot, or is left on 24-7 (like in most recording facilities), or stuffed in a rack with little ventilation. A 0.25W resistor will dissipate 0.25W safely when surrounded by air at room temperature, but will not perform so well in a warm sealed chassis! That's why resistors in tube equipment and power supplies/power amps tend to be larger, because the higher voltage supplies, higher currents and greater operating temperature, call for higher power-rated resistors to extend their working life.
[ Things like this can often be the difference between "pro" and "consumer" electronic equipment. The consumer equipment designer will choose a resistor or other component with a "close" rating; in the first example above, they'd go with the 0.25W resistor, and make a cost saving with a shorter life expectancy, compared to the "pro" designer who will purposely use a higher power-rated 0.5W or 1W resistor. It will cost more but on average create a longer "working life" .]
[With this newly gained knowledge, you may be able to work out why it's not a good idea to wire a 10R 0.25W resistor directly across the terminals of a 15V power supply!- try the second equation again with these values- don't hook up the circuit!!]
[Also, you may be able to work out why it's so difficult to find a 10M 5W resistor? What kind of voltage is needed to "warm it up"?]
So if the parts list doesn't state the power rating of the resistor, and you're not convinced a 0.25W or 0.5W resistor will do the trick, you can have a go at working out what sort of voltages and/or currents can be expected in that part of the circuit, and use the power equations to see what sort of levels you're dealing with.
[At the very least, you can actually find the maximum power level expected in any part of the circuit by checking the power supply voltages. Say in a split power supply of +/- 15V as used in many op-amp circuits, the maximum voltage is from the +15V rail to the -15V rail. This adds to 30V available from rail to rail. So any resistor in the circuit can only see a maximum of 30V. You can then decide on a safe power rating for the resistors based on this maximum. Remember that this is a maximum value, and that some components will only see 15V if connected between a rail and 0V, but it will give you an overall idea and safety margin. ]
4. Tolerance
This can be confusing at first. When you buy a 10k resistor, you want to be pretty certain it is 10k and not way off. But some applications are more crucial than others. For example, you want a "Power On" LED for your project. The parts list says 4k7, but for some reason you've only got a 5k6. That's fine, you can use it. The LED will just be a bit dimmer than expected. But say you're building a mic preamplifier with a switched gain control; if your resistor values are way off value, then the gain values will be wrong, affecting the "feel" of the control. Balanced input circuits using, for example, an op-amp are another crucial place for resistor value matching. Any difference in value of the input resistors can actually make a balanced input less effective at rejecting noise.
To help us out, resistor manufacturers measure their components at the factory and then grade them according to tolerance. Most carbon composition resistors are 5% and even 10% tolerance. So a 100k 10% resistor could read as low as 90k, or as high as 110k! A 100k 5% resistor could be as low as 95k or as high as 105k. Not so bad. A 100k 1% resistor will measure a minimum value of 99k, and a maximum value of 101k. Much better! There is not much of a price difference now, as 1% resistors are pretty much standard. For some EQ and balanced input/output and gain stages where the value and matching of the resistors are crucial, you can get 0.1% resistors. But of course you've got to pay a lot more for these. If it says 0.1% on the parts list you should use them, because they are obviously matched closely for a reason!
[There is a way around the resistor-matching problem without resorting to 0.1% resistors. In some applications, it's not the precise value of the resistors that's important, but their ratio with respect to each other. For example, if it states that two 10k 0.1% resistors are needed, and it's obvious from the circuit that this stage relies upon their ratio, for example in an op-amp gain stage or divider, then you would get just as good results from two 9.98k resistors. If you've got access to a good ohmmeter it would pay to wade through a batch of 1% resistors and see if you can find two that match! This won't work every time, but could help you out in certain situations.]
The 6k8 resistors that apply phantom to the microphone need to be matched. In this situation the matching of the resistors is more important than the actual value of 6k8. If you went through a batch or resistors and found two at 6k9 or even 7k but very closely match, then this will be fine for this application.
So how can we tell the tolerance of a particular resistor? Well, it's in the colour code...
You've probably noticed when you look at the colour code of a resistor that there are usually four or five bands in total. With the standard "3-band" code, there will be three bands placed together, this is the value code. The value code reads from left to right, the first band the tens, then units, then multiplier. If you carry on looking across towards the right, there will be a solitary band on its' own. This is the tolerance band. The colour code for this is:
1% = Brown Band
2% = Red Band
5% = Gold Band
10% = Silver Band
20% = No Band
Resistors with a printed value will say something like "5k6 10%" which is nice and easy, but some use the slightly more cryptic "letter code" which is as follows:
1% = F
2% = G
5% = J
10% = K
20% = M
And that about wraps up all we need to know about resistors. Sure, there's a lot more if you want to get deeper into it like temperature coefficients (whether the resistor values increases or decreases as it heats up), and resistors at higher frequencies start having capacitative and inductive qualities and.....but for DIY what we've gone over already is quite enough!
So to sum up, when choosing resistors from a catalogue, the most important factor is it's value. Then you can decide what composition you need. Unless it specifically says wirewound or carbon etc., I'd stick with metal film for most work. Then you need to find out what power rating is suitable. Don't forget that if you have a ready-made PCB or a copy of the layout artwork, then a clue is close at hand- check the spacing of the holes on the board for that resistor. If the holes are close you can assume a low wattage 0.25W to maybe 1W type. If the holes are wider apart, then a higher power device is intended. The tolerance decision is usually ready made for you- if you've got a metal film resistor then it will probably be 1% or 2%. If you go with a carbon it will have a greater tolerance of maybe 5% or more.
There can be some confusion with component lists for projects when it says something like "R21= 150k". Thats it. No other information. In my experience, if a component list has no mention of the resistor other than value, then you can bet that in a solid state circuit you could get away with using a 0.25W 5% one. This does not always hold true. But component lists usually add the tolerance, composition, power rating etc. if it isn't a 0.25W 5%. As most metal film resistors are now available as 1% very cheaply, these are your best bet, and you can usually get a 0.5W power rating in the same size body as a 0.25W carbon film, so these are the best all-rounder in terms of precision, noise, stability, power rating etc. For tube circuits, it usually states the power rating, but 1W or greater are usually required.
So using these rules of thumb and small equations, you can soon be on your way to DIY heaven. And remember, it doesn't actually matter if you order the wrong parts. Get some component drawers and start to build up your "stock" of parts. You can never have too many types/values of resistors, and you never know what project is going to come along next!
Many thanks to Mark Burnley of Liverpool in the UK
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