|Elliott Sound Products||Components - Part II|
Copyright © 2004 - Rod Elliott (ESP)
Page Created 15 Feb 2004
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.
For example, based on 10% tolerance devices, we will 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% (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.
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 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, input caps, etc.
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.
|0.25||0.4 W||0.5 W||1.0 W||1.5 W||5.0 W||10.0 W||50.0 W|
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 the lower limit of 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 P37 - DoZ preamp power supply uses an assisted zener as the main power supply regulator.
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.
|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.|