Tag Archives: Resistors

Current-Sense Resistors Tradeoffs

Using a resistor for sensing current should be a simple affair. After all, one has only to apply Ohm’s law or I=V/R. So, all it takes is to measure the voltage drop across a resistor to find the current flowing through it. However, things are not as simple as that. The thorn in the flesh is the resistor value.

Using a large resistor value has the advantage of offering a large reading magnitude, greater resolution, higher precision, and improved SNR or Signal to Noise Ratio. However, the larger value also wastes power, as W=I2R. It may also affect loop stability, as the larger value adds more idle resistance between the load and the power source. Additionally, there is an increase in the resistors self-heating.

Would a lower resistor value be better? But then, it will offer higher SNR, lower precision, resolution, and a low reading magnitude. The solution lies in a tradeoff.

Experimenting with various resistor values to sense different ranges of currents, engineers have concluded that a resistor offering a voltage drop of about 100 mV at the highest current is a good compromise. However, this should preferably be a starting point, and the best value for the current sense resistor depends on the function of priorities for sensing the current in the specific application.

The voltage or IR drop is only one of two related problems, with the second problem being a consequence of the chosen resistor value. This second issue, resistive self-heating, is a potential concern, especially when a high-value current flows through the resistor. Considering the equation W=I2R, even for a milliohm resistor, the dissipation may be in several watts when the current is multiple amperes.

Why should self-heating be a concern? Because, self-heating shifts the nominal value of the sense resistor, and this corrupts the current-value reading.

Therefore, unless the designer is measuring microamperes or milliamperes, where they can neglect the self-heating, they would need to analyze the resistance change with temperature change. For doing this, they will need to consult the data for TCR or temperature coefficient of resistance typically available from the resistor’s vendor.

The above analysis is usually an iterative process. That is because the resistance change affects the current flow, which, in turn, affects self-heating which affects resistance, and so on.

Therefore, the current-sensing accuracy depends on three considerations—the initial resistor value and tolerance, the TCR error due to ambient temperature change, and the TCR error due to self-heating. To overcome the iterative calculations, vendors offer resistors with very low TCR.

These resistors are precision, specialized metal-foil types. Making them from various alloys like copper, manganese, and other elements, manufacturers use special production techniques for managing and minimizing TCR. To reduce self-heating and improve thermal dissipation, some manufacturers add copper to the mix.

Instrumentation applications demand the ultimate precision measurements. Manufacturers offer very low TCR resistors and fully characterized curves of their resistance versus temperature. The nature of the curve depends on the alloy mix and is typically parabolic.

New Clearance Categories and Products!

We are finally updating our clearance categories. We’ve added lots of new products to the subcategory pages including:

Click on the links above to see all the new products that have been added!

What are Current Sense Resistors and how do they work?

Efficiency has become the keyword in global trends in meeting demands for lower carbon-di-oxide emissions. Whether it is the smartening of the electrical supply grid or the electrification of our automobiles, the global trend is driving the need for electronic circuits to become more efficient. Knowing the level of current flowing through the circuit and reaching the load accurately is an important factor in gauging its efficiency for circuit designers and systems operators. This knowledge helps in maximizing operating performances of a battery, hot swapping server units, controlling motor speeds, and many more. Current sense resistors are inexpensive components that provide optimal solutions helping OEMs create more efficient circuit designs for a wide range of applications.

Current sense resistors are components helping to improve system efficiency by reducing losses. They have high measurement accuracy compared to other technologies, and they are ideally suited for helping developers measure currents precisely in automotive, industrial, and computer electronic designs.

Current sense resistors detect and convert current to voltage, using Ohm’s law. According to this law, the product of the current and the resistance value through which it is passing gives the voltage developed across the resistor. As these resistors feature very low resistance values, the voltage drops are equally insignificant, of the order of 10 to 150 mV in specific applications.

Design engineers place the current sense resistor in series with the electrical load, which causes the entire current to be measured to pass through it. As the voltage drop across the resistor is proportional to the current through it, measuring this drop provides an estimate of the load current. Measuring the voltage drop is usually accomplished through various amplifier options such as operational, differential, and instrumentation amplifiers. Selecting the right current sense resistor amplifier for a specific application involves looking at the input common-mode voltage specification. This is the average voltage present at the input terminals of the amplifier.

With the current sense resistor sitting in series with the load, they can directly measure the current. Contrast this with indirect current measurement techniques using coils. Here the voltage is induced across a coil and is proportion to the current. As a series resistor senses current directly, it dissipates power. Therefore, series resistors tend to have very low resistance values.

Current sense resistors also feature a very low temperature coefficient of resistance or TCR. This feature defines its low drift with varying ambient temperature and its long-term stability. These characteristics make temperature dependency of current measurement to be very low, while increasing the accuracy.

However, when using very low ohmic resistors of the surface mount type the resistance of the solder pad and the copper tracks of the printed circuit board can be uncertain and more than the resistance of the current sense resistance itself. This can lead to inaccuracies in the current measurement. In addition, the TCR of the tracks of the PCB can be much higher than that of the series resistor element.

Therefore, it is necessary to use current sense resistors implementing the 4-wire Kelvin principle, as these employ additional leads for measuring current more accurately.

How Do You Read Resistor Values?

Resistors range from huge multi-watt giants to sub-miniature surface mount devices (SMDs) and parts with different types of leads in between. The larger varieties do not pose much of a problem as they usually have a big-enough surface for printing the value of the resistance, its tolerance, and other necessary specifications. For smaller sizes, codes are generally used for letting the user know the details of the resistor.

Two common methods are under use for identifying resistors – color coding for resistors with leads and number coding for SMD resistors. Color coding is an easy way to convey a lot of information concisely and effectively. One of the advantages is that specifications of the resistor are visible irrespective of its orientation on the PCB – very useful for overcrowded boards. As SMD resistors have only limited surfaces, number coding is more suitable.

Color coding for resistors

Resistors with color coding come with one of two standard codes – the 4-band code or the 5-band code. The 4-band coding is used more with resistors of low precision with 5, 10, and 20% tolerances. Higher precision resistors with tolerances of 1% and lower are marked with 5-band color codes.

The colors used have their own values. For example, Black represents zero, Brown represents one, Red represents two, Orange represents three, Yellow represents four, Green represents five, Blue represents six, Violet represents seven, Gray represents eight, White represents nine, Gold represents 0.1, and Silver represents 0.01.

For tolerances, Gray represents ±0.05%, Violet represents ±0.1%, Blue represents ±0.25%, Green represents ±0.5%, Brown represents ±1%, Red represents ±2%, Gold represents ±5%, Silver represents ±10%, while an absence of color represents ±20%.

The 4-Band color coding scheme

The 4-band color coding has thee color bands crowded on one side with the fourth band separated from the others. One has to read the code from the left to right beginning with the crowded colors on the left and the separated color band on the extreme right. Starting from the left, the first two color bands represent the most significant digits of the resistance value, while the third band represents the multiplier digit. The isolated fourth band is the tolerance band. As an example, a resistor of 4.7KΩ, 5% value will have the colors bands Yellow, Violet, and Red representing 4700Ω, with a fourth band of Golden color. In cases where there are only three color bands, it means the resistor has a ±20% tolerance.

The 5-band color coding scheme

High quality, high precision resistors with tolerances of 2%, 1% or lower are represented by five color bands, with the first three denoting the three most significant digits of the resistance value. The fourth band represents the multiplier value, while the fifth stripe gives the tolerance. Some resistors have an additional sixth band denoting the reliability or the temperature coefficient.

Number coding for SMD resistors

SMD resistors usually have three or four numbers on them, depending on whether they are of 5% or 1% tolerance. The last number is the multiplier with the others representing the most significant digits of the resistance value. In some cases, an alphabet is used, representing the resistor’s tolerance. However, if the alphabet is an R, it represents a decimal at its position. For more details, refer to this web site.

Do wirewound resistors suppress noise?

Specially designed wirewound resistors are used as noise suppressors in automotive ignition systems for reducing RFI or Radio Frequency Interference caused by electrical discharges. These resistors are usually placed in the leads and or caps of spark plugs and in the rotor of the distributor.

A gasoline engine generates high frequency electromagnetic Interference or EMI. This is commonly referred to as RFI or Radio Frequency Interference that comes primarily from the high-voltage side of the automotive circuit. At these places, the ignition system produces sparks at the coil that converts the battery voltage into high-voltage pulses. These pulses appear at the distributor, which routes the high voltage to the appropriate plug. Here, the spark ignites the air/fuel mixture in the combustion chamber producing the power that drives the crankshaft. Diesel engines do not have spark plugs as the air/fuel mixture is compressed to ignite and hence, diesel engines produce negligible EMI/RFI.

The high-voltage ignition pulses have a very rapid current change that generates an electromagnetic field around the ignition system. When electricity bounds through air, it passes through the air molecules, ionizing some of its atoms. As these atoms de-ionize, they release a tremendous amount of RFI. Although the frequencies are random and appear only for fractions of a second at a time, they affect almost any type of electronic device installed nearby to some degree.

Not only do these disturbances interfere with telephone and radio communications, they can even disrupt engine functioning and ABS control electronics. This type of interference sounds like a huge amount of crisps, crackles and rattles in radio receivers in communication systems.

International legislation requires manufacturers to reduce these disturbances to an acceptable level. That means the RFI must be reduced to a level so that there is no appreciable interference with the functioning of receivers not on the vehicle itself. Interference Suppression Regulations describe the RFI damping characteristics that manufacturers are required to follow, for example, VDE 0874 to 0879, CISPR or Council Directive 72/245/EEC, and usually differs from country to country.

Manufacturers usually track down the sources of RFI and limit it either at its source or filter it out before it can reach the instruments. The simplest and easiest method of prevention is by installing resistive spark plugs, resistive leads or ignition suppressor resistors. These contain internal impedance to dampen unnecessary emissions from the ignition system. Some manufacturers resort to redesigning the grounding circuit or installing feed-through/bypass capacitors.

Conventionally, spark plug leads usually carry a resistance of 6 to 15 Kohms per meter, and that makes them poor transmitters of RFI. However, electrical ignition systems may be sensitive to varying resistances in the spark-plug leads due to different lengths and can give mixed signals to the control module. Therefore, it is preferable to have solid-core wires with noise-suppressor resistors screwed onto brass fittings at the ends. This helps to maintain an equal resistance on each cylinder.

Use of noise suppressors is the best solution for reducing RFI. These resistors are designed for specific ignition systems and have the finest damping characteristics that do not cause disturbances to the ignition pulses. It usually suffices to place the resistors in the rotor of the distributor, in the spark plug caps or in the leads.

Learn about metal film resistors

Resistors are a common passive item in any electronic assembly. They are used for restricting the amount of current flowing in a circuit; acting much as a valve does in a water pipeline. The most commonly in use are carbon, thick metal and thin metal film resistors. The film forms the resistive material of the resistor.

The axial resistor is usually a cylindrical conductive film on a non-conductive ceramic carrier. Two leads projecting from both ends of the resistance help in connecting the item electrically within a circuit. Although the appearance of a metal film resistor is very similar to that of a carbon film resistor, the former has much better properties of stability, accuracy and reliability.

A cylindrical ceramic core of high purity forms the base of a metal film resistor. Manufacturers mostly use a method known as sputtered vacuum deposition to deposit a thin metal layer on this ceramic base. This combination is then kept at a low temperature for a long period, which results in very good accuracy for the resistor. Mostly, the resistance material used is nickel chromium (NiCr), however, for special applications, other alloys such as tin and antimony, tantalum nitride with platinum and gold are used as well.

The thickness of the metal film strongly governs the stability of the resistance. Typically, a metal thickness of 50-250nm is a good compromise between better stability and lower resistance value. For connecting to the circuit, two end caps with connecting leads are pressed on to the two ends of the resistor body.

To obtain the desired resistance a laser beam cuts a spiral slot in the thin metal layer. This is a more modern method as compared with grinding techniques and sandblasting used earlier for trimming the resistance value. Once the final value of the resistance is achieved, several layers of paint are placed on the resistor body, with each layer being baked individually.

Apart from providing a high dielectric strength, the coating protects against ingress of moisture and mechanical stresses. Color code bands on the body mark the resistor value along with the tolerance band. Metal film resistors are available with standard tolerances of 2, 1, 0.5. 0.25 and 0.1%, with the TCR or temperature coefficient of resistance lying between 50 and 100 ppm/K.

Metal film resistors demonstrate good properties for TCR, stability and tolerance. Because these resistors have a low voltage coefficient, they feature high linearity and low noise properties. Therefore, if any of your circuits need low noise, tight tolerance and low temperature coefficient properties, be sure to use metal film resistors. For example, active filters and bridge circuits use metal film resistors.

Metal film resistors show good reliability when operated from 80 percent down to 20 percent of their specified power rating. Although reliability generally increases if the resistor is derated 50 percent, going below 20 percent of the power rating at elevated humidity conditions usually diminishes reliability. Moreover, metal film resistors are more easily damaged by power overloads and voltage surges, as compared to carbon composition or wire-wound resistors.

More Allen Bradley carbon comp resistors now available!

We’ve updated our resistor inventory and added many more NOS Allen Bradley carbon comp resistors. Between the 1W values and 2W values, we now have over 150 different 1W and 2W carbon comp resistors on hand!

Included in the selection are many Military (MIL-SPEC) Allen Bradley carbon comp resistors. The military resistors typically have a tighter tolerance than the standard resistors.

Allen Bradley carbon comp resistors are valued for their consistency and uniformity. The Allen Bradley corporation reference materials have this to say about their hot molded resistors:

Years of accumulated experience have shown that Allen Bradley hot molded resistors are unequaled for uniformity, predictable for performance, appearance, and freedom from catastrophic failure. Allen Bradley resistors are made by an exclusive hot molding process on automatic machines developed, built, and used only by Allen Bradley. There is such complete uniformity from one resistor to the next, million after million, and long term in-circuit performance can be predicted with usable accuracy. When used according to published ratings and recommendations, Allen Bradley hot molded fixed resistors will not open circuit nor exhibit erratic changes of resistance value. They are probably the most reliable of all electronic components.

We get frequent requests for other values so we were thrilled to get more for our stock! Quantities are limited – especially on the MIL-SPEC resistors.

Application information for Allen Bradley resistors

Allen Bradley resistors are one of the best sellers on the West Florida Components web site with good reason.

The following information has been compiled to aid in the everyday selection and application of Allen-Bradley hot molded resistors. The statements should be helpful in evaluating the use of all types of AB hot molded resistors in broad general terms, and are not to be interpreted to be precise or exact.

A comprehensive list is made of the standard normal resistance values in their available tolerance categories, the rated continuous working voltages for all hot molded types, the part numbers, and color codes – all information provided for all values from 1 ohm to 100M ohm, taking into account the available range of values for each type (as of 1985).

Allen Bradley Resistors

Allen Bradley Resistors

1 – Low value resistors exhibit less change due to humidity, temperature and voltage than high value resistors.

2 – Resistance changes due to increase in moisture content are always positive.

3 – Resistance changes due to humidity are temporary, and, in the case of Allen Bradley resistors, are reversible.

4 – Change of resistance which has occurred due to humidity may be essentially eliminated by conditioning the resistor at 100°C or by dry storage.

5 – The effects of humidity may be minimized by operating the resistor with as little as 1/10 rated wattage load.

6 – Resistance change due to load life is permanent and usually ultimately negative.

7 – Resistance change due to load life can be minimized 1% – 2% in many thousands of hours by 50% derating period. The same result can be attained by limiting the maximum operation surface temperature of the resistor underload to 100°C. Permanent resistance changes as a result of storage of temperatures below 100°C are negligible, even for extended time periods.

8 – Resistance change due to soldering is positive and may be permanent if the resistor has excessive moisture present in its body. It can be greatly minimized if resistors are dry at the time of soldering.

9 – The temperature characteristics of Allen Bradley resistors is positive above +80 and below -10.

10 – The temperature characteristics of Allen Bradley resistors is negligible from -10°C to +80°C.

11 – The voltage characteristic (negative) and the temperature characteristic (positive) of Allen Bradley resistors tend to cancel one another in an Allen Bradley resistor in an average operating conditions, where both significant voltage and elevated temperature are present.

12- The heat sink to which a resistor is connected affects its rating. Resistors operated in multiple should be derated unless adequate heatsinks are provided.

13 – The quality and reliability of Allen Bradley resistors is the same for, and independent of, any resistance tolerances shown on the resistor.

14 – Years of accumulated experience have shown that Allen Bradley hot molded resistors are unequaled for uniformity, predictable for performance, appearance, and freedom from catastrophic failure. Allen Bradley resistors are made by an exclusive hot molding process on automatic machines developed, built, and used only by Allen Bradley. There is such complete uniformity from one resistor to the next, million after million, and long term in-circuit performance can be predicted with usable accuracy. When used according to published ratings and recommendations, Allen Bradley hot molded fixed resistors will not open circuit nor exhibit erratic changes of resistance value. They are probably the most reliable of all electronic components.

This information was taken in part from the Allen-Bradley corporation reference book dated 1985.

Opening Up and Tearing Down an IPOD Shuffle

Opening up and tearing down an IPOD Shuffle to see what’s inside…

The 3rd Generation of the IPOD Shuffle is a wonder of technology….1000 songs stored in an aluminum case smaller than a disposable lighter.

Did you ever wonder what electronic components make up the guts of an IPOD Shuffle?

You might be surprised at what goes into the circuitry of the IPOD Shuffle. In descending order by percentage of cost, the main components are:

logic, memory, metals, rechargeable materials, connectors, PCB, crystal, misc, capacitors, transistors, analog, diodes, magnetic, and plastics.

Here’s a partial breakdown by number of electronic components:

Capacitors – 65+
Resistors – 50+
Diodes – 4+

Pretty amazing what goes into equipment that measures only 45.2mm x 17.5mm x 7.8mm when fully assembled! This is possible because the components are extremely small surface mount components.

If you look at the cost breakdown by component family, it’s just as revealing. Naturally, the largest share is for memory in the form of IC’s. Over 70% (about $12.00 worth) is for logic and memory.

Resistors In Series

Electronic components used in electronic circuits to regulate and limit the flow of electric current in the circuit are known as Resistors. Resistors can be connected in two basic configurations – in series and in parallel.

When connected in a series connection, the resistors are connected in a line. The current flows through the resistors one after the other.

When the resistors are connected in such a configuration, they exhibit the following properties:

The current flowing through each of the resistors is the same, that is, I total = I1 + I2 + I3…and so on. This is due to the fact that there is only one path for the current to flow through. The second property that they exhibit is that the total voltage drop across all the resistors connected in series is equal to the sum of the individual voltages of the resistors, that is, V total = V1 + V2 + V3…

Now since, V = IR, so, V total = I R equiv, which comes down to,
I R equiv = I1.R1 + I2.R2 + …, and since I1 = I2 = I3 .., we have
I. R equiv = I. (R1 + R2 + R3..), or R equiv = R1 + R2 + R3..

So the equivalent resistance of these electronic components connected in series is the sum of the individual resistances.

Resistors in series are often used for obtaining specific higher resistance values which are otherwise not available. They are also used in voltage divider circuits. A common application is in household wiring.