Elliott Sound Products Components - Part II

Page Created 15 Feb 2004

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1 - Introduction

The EIA (Electronic Industries Association) and other industry bodies and authorities worldwide specify standard values for resistors, commonly referred to as the 'preferred value' system. This system has its origins in the early period of electronics, at a time when most resistors were carbon composition with poor manufacturing tolerances. The idea is simple - select values for components based on the tolerances to which they are able to be made.

In the early days of electronics, resistors were hand made to suit. Once companies started making actual components, their tolerance was quite broad, and 20% tolerance was common. With improved manufacturing techniques, the tolerance fell to 10%, and later, 5%. Today it's not possible to get 10% or 20% resistors, and 1% or 2% types are the most common. Closer tolerances are also available if you need very high accuracy.

Based on 10% tolerance devices, we might work with a preferred value of 100Ω. It makes no sense to produce a 105Ω resistor, because 105 ohms falls within the 10% tolerance range of the 100Ω resistor. The next reasonable value is 120Ω, because a 10% 100Ω resistor will have a value somewhere between 90 and 110 ohms.

A 10% 120Ω resistor has a value ranging between 108 and 132Ω. Following this logic, the preferred values for 10% tolerance resistors between 100 and 1,000Ω is a roughly logarithmic sequence of 100, 120, 150, 180, 220, 270, 330 and so on (rounded to the closest sensible value). This is the E12 series shown in the first table below.  The values in any decade can be derived by multiplying or dividing the table entries by powers of 10.

In each series, values may start from as low as 0.1 ohm, and may extend to several megohms, depending on the type of resistor and its intended purpose (and cost, of course). These days, you probably won't be able to get 10% tolerance resistors even if you wanted them (although I can't imagine why). 5% is very common, and most suppliers also have 1% or 2% (usually metal film) resistors in the E24 range.

The highest number of values is the E192 series, but these are normally only required where extreme accuracy is needed. For the odd occasion where a highly specific resistance is needed, it is usually simpler to use 2 or more resistors in series or parallel to obtain the needed resistance. Few suppliers stock the E96 or E192 series, so they will be difficult for most people to obtain.

2 - E12, 24, 48 & E96 Series

Resistors are commonly available in E12 and E24 series, and somewhat less commonly in E48, E96 or E192 series. The tolerance shown in the following tables is indicative only - 1% tolerance is now very common, even when the range offered by a vendor may only be a subset of the full range available. For example, it's easy to get 1% tolerance metal film resistors in the E24 series.

 E12 1 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2
12 Values per Decade (5% Tolerance)

 E24 1 1.1 1.2 1.3 1.5 1.6 1.8 2 2.2 2.4 2.7 3 3.3 3.6 3.9 4.3 4.7 5.1 5.6 6.2 6.8 7.5 8.2 9.1
24 Values per Decade (2% Tolerance)

 E48 1 1.05 1.1 1.15 1.21 1.27 1.33 1.4 1.47 1.54 1.62 1.69 1.78 1.87 1.96 2.05 2.15 2.26 2.37 2.49 2.61 2.74 2.87 3.01 3.16 3.32 3.48 3.65 3.83 4.02 4.22 4.42 4.64 4.87 5.11 5.36 5.62 5.9 6.19 6.49 6.81 7.15 7.5 7.87 8.25 8.66 9.09 9.53
48 Values per Decade (1% Tolerance)

 E96 1 1.02 1.05 1.07 1.1 1.13 1.15 1.18 1.21 1.24 1.27 1.3 1.33 1.37 1.4 1.43 1.47 1.5 1.54 1.58 1.62 1.65 1.69 1.74 1.78 1.82 1.87 1.91 1.96 2 2.05 2.1 2.15 2.21 2.26 2.32 2.37 2.43 2.49 2.55 2.61 2.67 2.74 2.8 2.87 2.94 3.01 3.09 3.16 3.24 3.32 3.4 3.48 3.57 3.65 3.74 3.83 3.92 4.02 4.12 4.22 4.32 4.42 4.53 4.64 4.75 4.87 4.99 5.11 5.23 5.36 5.49 5.62 5.76 5.9 6.04 6.19 6.34 6.49 6.65 6.81 6.98 7.15 7.32 7.5 7.68 7.87 8.06 8.25 8.45 8.66 8.87 9.09 9.31 9.53 9.76
96 Values per Decade (< 1% Tolerance)

3 - Capacitors

Film capacitor values normally follow the E12 series. In some cases, suppliers will decide (based on what criteria I really don't know) that some of the available values are 'not needed', and they can be hard to find.

Electrolytics usually follow a limited range of the E12 series, and in the case of larger types, may not follow any particular series at all. For example, 8,000uF caps are quite common, but don't fit into any of the above tables.

Electrolytics also have rather broad claimed tolerance (up to +20% -50%), but in reality, most are remarkably close to the marked value.

Although the E6 series was once used with resistors (or so I believe - I've never seen it), it is still common with electrolytic capacitors. Although the range seems very limited, it is normally quite sufficient for the typical uses of electros - power supply decoupling, coupling capacitors, etc.

 E6 1 1.5 2.2 3.3 4.7 6.8

4 - Potentiometers

Although there are exceptions, potentiometers (pots) are usually only available in a modified E3 series. This provides a 1, 2, 5 sequence between values. At one stage, it was common to find 22k and 47k pots (for example), but these will most commonly be classified as 20k and 50k now. Given that most pots have a much wider tolerance than fixed resistors (10% and 20% are typical), it makes little sense to be too specific about the value.

You need to be aware of the tolerance of standard pots, and also understand that tracking between sections of multi-gang types is often rather poor. Log pots are worse than linear types, and it's not unusual to have 3dB variation between the two sections of a standard carbon film log or 'audio taper' stereo pot at some settings.

5 - Zener Diodes

The following is far from a complete listing, but gives a reasonable range of voltages and power dissipation. Personally, I prefer the European designations, such as BZV85C6V8 - you can tell instantly that the voltage is 6.8V from the number. Unfortunately, these are not always easy to obtain, and the 1N series are more common (in non-European countries at least). Please note that due to the amount of data in the table, it is almost a certainty that I have made one (or more) mistakes in the translation, so always check the data sheet before committing yourself to a particular device. Also bear in mind that many of the devices listed will be extremely difficult to get. 1W zeners are the most commonly available, and a method is shown below to use these at much higher power levels.

 Voltage Power 0.25 0.4 W 0.5 W 1.0 W 1.5 W 5.0 W 10.0 W 50.0 W 1.8 V 1N4614 2.0 V 1N4615 2.2 V 1N4616 2.4 V 1N4617 1N4370 2.7 V 1N4618 1N4370 3.0 V 1N4619 1N4372 1N5987 3.3 V 1N4620 1N5518 1N5988 1N4728 1N5913 1N5333 3.6 V 1N4621 1N5519 1N5989 1N4729 1N5914 1N5334 3.9 V 1N4622 1N5520 1N5844 1N4730 1N5915 1N5335 1N3993 1N4549 4.7 V 1N4624 1N5522 1N5846 1N4732 1N5917 1N5337 1N3995 1N4551 5.6 V 1N4626 1N5524 1N5848 1N4734 1N5919 1N5339 1N3997 1N4553 6.2 V 1N4627 1N5525 1N5850 1N4735 1N5341 1N4553 7.5 V 1N4100 1N5527 1N5997 1N4737 1N3786 1N5343 1N4000 1N4556 10.0 V 1N4104 1N5531 1N6000 1N4740 1N3789 1N5347 1N2974 1N2808 12.0 V 1N4106 1N5532 1N6002 1N4742 1N3791 1N5349 1N2976 1N2810 14.0 V 1N4108 1N5534 1N5860 1N5351 1N2978 1N2812 16.0 V 1N4110 1N5536 1N5862 1N4745 1N3794 1N5353 1N2980 1N2814 20 V 1N4114 1N5540 1N5866 1N4747 1N3796 1N5357 1N2984 1N2818 24 V 1N4116 1N5542 1N6009 1N4749 1N3798 1N5359 1N2986 1N2820 28 V 1N4119 1N5544 1N5871 1N5362 60 V 1N4128 1N5264 1N5371 100 V 1N4135 1N985 1N4764 1N3813 1N5378 1N3005 120 V 1N987 1N6026 1N3046 1N5951 1N5380 1N3008 1N2841

Voltages that are unavailable are easily made up by connecting zeners in series. If possible, keep the voltages of the two (or more) zeners as close as possible, or their current handling capabilities will be different, possibly leading to overheating of the higher voltage (and therefore higher dissipation) device(s). Don't assume that you can get a higher power rating by running zener diodes in parallel - unless they are perfectly matched (which is impossible), one will take most of the current and will fail. You can make a higher power zener by using two in series - for example, two 10V 1W zeners in series gives a 20V 2W zener.

For maximum temperature stability, the zener voltage should be 5.6V, as the positive and negative temperature coefficients cancel at this voltage. Below 5.5V, the junction has a negative tempco, so the voltage falls with increasing temperature. Above 5.5V, the avalanche effect is dominant in the diode, and this has a positive tempco. Voltage increases with increased temperature.

Zeners should always be operated at between 10% and up to a maximum of 80% of rated power to obtain the best (most stable) reference voltage. They also have to be derated if operated at high temperature - see the datasheet for the device you intend to use to see the required derating curve. In general, the allowable power dissipation will be half the rated figure when the diode is operating at about 110°C.

To determine the optimum current (say 25% of maximum to keep dissipation reasonable), use the following simple formula ...

I = (P / V) / 4

where I is current, V is rated voltage, and P is rated power. For example, a 27V 1W zener should be operated at around ...

I = (1 / 27) / 4 = 0.00926A = 9.26mA   (power is 250mW)

In many cases, the preferred current will be too high and will cause either excessive heating or higher than desired current drain, important for battery operated devices. However, it must be understood that the regulation of the zener is not very good until it is operating at more than the lower limit (about 10% rated power).

There will be times when you really do need a high power zener, but find that the one you need is either not available or very expensive. There is generally no real need to use high powered zeners unless space is at a premium, because the simple circuit below will work in most cases.

Figure 1 - Transistor Assisted Zener Diode

The circuit works by simply amplifying the zener current, the majority of which is fed directly into the base of the transistor. If the voltage attempts to rise, more zener current flows, thus more base current to the transistor. This causes the transistor to turn on until a state of equilibrium is reached, where the voltage across the 'composite zener' is held at the correct value (+ 0.65V base-emitter voltage, of course).

Since zeners are typically classified by their dynamic resistance (or impedance), it is worthwhile looking at the assisted version to see what sort of performance we can expect. Figure 2 shows the dynamic impedance of a zener by itself, and that of the assisted version.

Figure 1 - Dynamic Impedance

The dynamic impedance is measured by changing the current by a known amount, and measuring the voltage change. In the case of the zener, the current was varied by 100mA, and the voltage changed by 0.4975V, therefore ...

R = V / I = 0.4975 / 0.1 = 4.975Ω

The transistor assisted zener is a great deal better (as can be seen from the flatter curve). In this case, a current change of 100mA only achieved a voltage change of 25mV, so dynamic resistance/impedance is ...

R = V / I = 0.025 / 0.1 = 0.25Ω

It is possible to improve this further, but I'd suggest that if you need better performance than an assisted zener can provide, then you'd be a lot better off with a proper regulator.

Note that the transistor must be chosen so that it is operating in its continuous safe operating area, and must be appropriately derated for temperature. Expect a 100W transistor assisted zener (for example) to dissipate a lot of heat, so don't skimp on the heatsink.

For those who have looked at a lot of the ESP site, you will notice that the early version of the P37 - DoZ preamp power supply uses an assisted zener as the main power supply regulator.

6 - Conclusion

The main purpose of this section is to tie up a few 'loose ends', and add some of the harder to find (or just plain frustrating) information that you will need from time to time.

For much more info about zener diodes, have a look at AN008 in the application notes section of this site.

It is probable that more loose ends will turn up in time, and this will be added to the information here as found or needed. Unfortunately (or perhaps fortunately) the range of components is so diverse that it is not possible to cover everything.

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 Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.