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About Resistor In Circuit Board

By on July 28, 2009

Have you come across why some resistors (usually 1/2 watt and below) were soldered directly on the board while to some resistors (usually 1 watt and above) the manufacturers purposely let the leg extended far from the circuit board as seen from the photo below? The reason for it is to avoid interference cause by resistor put too close to the circuit line which located behind the board- that’s what some expert said. So far i have not seen any of this problem yet (interference to the Monitor display) if i put the resistor (greater than 1 watt) just on top of the board instead of purposely leave the longer resistor leg from the circuit board. If you have comment about this please feel free to write your reply-thanks

resistor

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21 Comments

  1. Wes

    July 29, 2009 at 3:22 am

    Electrical and thermal noise: http://en.wikipedia.org/wiki/Resistor
    In precision applications it is often necessary to minimize electronic noise.
    As dissipative elements, even ideal resistors will naturally produce a fluctuating "noise" voltage across their terminals.
    This Johnson–Nyquist noise is a fundamental noise source which depends only upon the temperature and resistance of the resistor, and is predicted by the fluctuation–dissipation theorem.
    For example, the gain in a simple (non-) inverting amplifier is set using a voltage divider.
    Noise considerations dictate that the smallest practical resistance should be used, since the Johnson–Nyquist noise voltage scales with resistance, and any resistor noise in the voltage divider will be impressed upon the amplifier's output.
    In addition, small voltage differentials may appear on the resistors due to thermoelectric effect if their ends are not kept at the same temperature.
    The voltages appear in the junctions of the resistor leads with the circuit board and with the resistor body.
    In applications where thermoelectric effects may become important, care has to be taken (for example) to mount the resistors horizontally to avoid temperature gradients and to mind the air flow over the board.
    Practical resistors frequently exhibit other, "non-fundamental", sources of noise, usually called "excess noise."
    Thick-film and carbon composition resistors generate more noise than other types at low frequencies;
    wire-wound and thin-film resistors, though much more expensive, are often utilized for their better noise characteristics.
    Thin film surface mount resistors typically have lower noise and better thermal stability than thick film surface mount resistors. However, the design engineer must read the data sheets for the family of devices to weigh the various device tradeoffs.

    http://en.wikipedia.org/wiki/Flicker_noise
    Flicker noise is found in carbon composition resistors, where it is referred to as excess noise, since it increases the overall noise level above the thermal noise level, which is present in all resistors.
    In contrast, wire-wound resistors have the least amount of flicker noise.
    Since flicker noise is related to the level of DC, if the current is kept low, thermal noise will be the predominant effect in the resistor, and the type of resistor used will not affect noise levels.

    Large carbon composition resistors mounted to close to the circuit board will increase in thermal noise levels depending on the amount of voltage applied, and where they are located.

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  2. Waleed

    July 29, 2009 at 5:00 am

    no, i never encountered anything like this before. in fact. I found these resisters open and I replaced them with ones that have shorter legs than the original ones and the monitor works just fine.
    but I always wondered why these resisters have high legs. may be, closer to the board will cause more heat and higher legs helps to keep the resister cool and allow more ventilation..I do not know..I am just spectualting.

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  3. John Preher

    July 29, 2009 at 5:13 am

    I always thought these large resistors were mounted off the board because of the amount of heat they dissipate, not sure if that is really a factor though.

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  4. Mark LaPlante

    July 29, 2009 at 12:47 pm

    I understood the reason to be for heat dissapation. The larger watt resistors dissapate more heat.

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  5. beh

    July 29, 2009 at 1:11 pm

    HI JESTINE

    I think one of the reasons that the manufatures put the big resistores
    on top of the boards is couse of thier heat as much as resitors are bigger porpotionally they became warm and warmer and they burn the
    board .ventilation could be another reason for that
    have berlinat day jestin
    beh

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  6. admin

    July 29, 2009 at 7:10 pm

    Hi Wes,

    That was a very good explanation! Thanks a lot!

    Jestine

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  7. admin

    July 29, 2009 at 7:15 pm

    To John, Waleed, Beh And Mark,

    All of your answer were refer to the heat dissipation and that was a very good point too.I hope other readers could pick up this tip from your great post. Thanks guys for sharing and giving the comment.

    Jestine

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  8. Rick B.

    August 1, 2009 at 10:36 am

    It is ALL ABOUT THE HEAT ! The power resistors are mounted in such a way as to allow air to flow freely under, around and over the resistor's body. Also, (and this is important) the long resistor leads pull heat from the resistor body and give heat off to the surrounding air. The resistor lead is copper (plated with tin/lead). Everyone knows copper is an excellent conductor of heat, therefore the long lead in combination with the resistor body both work to keep the resistor from getting too hot. It also reduces the likelihood of PCB discoloration after years of operation. BTW- the copper traces on the PCB dissipate some heat for the resistor also. If you mount the resistor down tight to the PCB, the copper traces on the PCB have to do more work dissipating heat, and the resistor's solder joint(s) take a beating with each thermal cycle (power on and off). Other solder joints near the resistor take a beating too, if the resistor is mounted tight to the PCB. Rick B. Webster, NY USA

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  9. admin

    August 1, 2009 at 9:05 pm

    Hi Rick,

    Wow! Another good point! Thanks for contributing and hope the readers could learn from such explanation by you all.

    Jestine

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  10. electrical upgrades

    August 19, 2009 at 2:08 am

    I think the size is just related to the rate of heat dissipation. Larger the resistor more the heat it dissipates and vice versa.

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  11. mgr

    September 16, 2009 at 12:35 am

    i think it is due to overheat it produces so that can burn
    the board.

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  12. john

    September 22, 2009 at 10:53 am

    Please visit us for more resistor information. We are leader in Power and precision resistors.

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  13. raghava

    October 13, 2009 at 9:03 pm

    Respected Sir,

    I would like to ask a question about the conversion of CRT color monitor 14 inch into TV. by using a portable black and white TV circuit.

    The following resource are available with me.
    1. I have a working condition of computer CRT monitor. -14 inch
    2. Portable black and white TV in working condition - 1 no -14 inch
    3. Portable black and white TV in non working condition - 1 no - 14 inch - circuit is working but picture tube is showing one horizontal line at middle of screen.

    Finally I would like to connect the Portable black and white TV circuit to computer CRT monitor. I want to use the computer CRT monitor as a dual purpose. i.e., one is TV and Computer monitor.

    I hope that my question is clear and looking forward for ur favourable reply.

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  14. admin

    October 14, 2009 at 9:03 pm

    Hi Rag,

    Thanks for the posting but since i have done this conversion before thus i could not give any expertise advice to you-sorry.

    Jestine

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  15. Robel Amanuel

    March 3, 2010 at 4:30 am

    Thank you first of all.
    I am of the comment that the reason why some resistrors are fixed just almost on the board while others are mounted high above the board is due to the difference in the ventilation requirement of the respective resistor.
    but i also have a question here waiting ur comments:
    i found a resistor(resistor no. 11 of a sharp type tape recorder Model no.WF-1100W(BK) serial no. 30905385). This resistor is located near the jack of the tape recorder's speakers. Though its covering (including the color bands) is burnt to the extent that its resistance value can not be determined depending on its color band, i found 48 ohm when i cheked it with digital multimeter. However, i am not certain whether this value is the original or not. so:
    1. can u offer me a means of identifying the original value of this resistor?
    2. if u have a tape recorder of the same type may tell me the value?
    3. thirdly do u believe this resistor can still perform well though it is with out covering?

    thank you in advance
    with all the best wishes
    roble!!

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  16. admin

    March 4, 2010 at 11:07 am

    Hi Robel,

    Thanks for giving that comment. Sorry i do not repair tape recorder thus i do not have any reference for you. As for the value, it can be 47 ohm since sometimes a burnt reistor may have the value intact but the outer layer burnt. Without the covering it should be no problem and if you worry about the covering you can always use a shrink tube to cover it.

    Jestine

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  17. Amir Mukhtar

    February 27, 2011 at 6:15 pm

    Sir Jestine, the above theory is perfect answer. In spite the fact that, The question which you have given above is very thinkable question. But the question also need to be justified why we use resistors in Circuit board. why wont we use other compatible component.
    Actually, Resistor is a device which provide resist facility and protection against over current and voltage. OR Resistors are electrical component that resist the flow of charge in circuits.

    Also, Resistors having a lot of applications in circuit board and installations procedures different to each other with respect to functions and located environment.

    for example, Especially a person who fights against terrorism enemies; like wise, An electronic component that transmits current in direct proportion to the voltage across it. we can use resistors as:

    1. use as fuse to protection.
    2. use as current limitation.
    3. use to set time constant function with capacitor.
    4. use to voltage dividing
    5. use to control feed back .
    6. use to limit overloading .
    7. use to resist voltages.
    8. use in frequency circuit with filters.
    9. use in switching by light dependency (LDR).
    10. use parallel in power supply as discharge element.
    11. use as regulating current with respect to changes (VR).
    12. use as infinite resistivity by rheostat.
    13. etc.

    These above are the common job of resistance. so therefore, there is no hurdle to be fixed justification, resistor as important component of the applications in circuit boards.

    thanks you
    REgards
    Amir mukhtar

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  18. admin

    February 28, 2011 at 4:18 am

    HI Amir,

    Good explanation and the blog readers will appreciate it.

    Jestine

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  19. Bernie Scott

    October 1, 2012 at 6:55 am

    I believe the reason is to help dissapate the heat that the resistor generates. When it is mounted directly touching the board it tends to overheat that area and can also encourage the development of bad solder joints. I have seen 5 watt resistors mounted directly on the board and the board will be badly overheated in that immediate area. The lower the wattage  the longer this will take to occur but it will occur...eventually....

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    • Jestine Yong

      October 1, 2012 at 7:03 am

      Hi Bernie,

      Thanks for the sharing!

      Jestine

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  20. James

    June 15, 2015 at 11:25 am

    I think the main reason for raising the resistor to a height above the PCB is for better heat dissipation (through the exposed leads). This tactic is seen in a lot of rectifiers on the power supply boards too.
    I have read some (e.g., 5A, 10A) rectifier datasheets saying that some of their current ratings are based on having a minimum of 10mm lead length on both sides of the body.

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