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I am currently using 150R series resistors to limit the current to each segment in a 7 segment display. I picked 150R because all the online "led resistor calculators" suggest this value. The power supply is 5v/2.5A.

The datasheet for these displays (Kingbright SC52-11EWA) says the LED's have a forward voltage of 2.0 (2.5 max).

For testing, I've currently just got one segment wired up with a resistor. For some reason, after a few minutes the resistor gets quite warm/hot. The displays themselves do not heat up.

When I use a 330R resistor, it is still slightly warm but a tiny bit cooler than 150R. However, the display is then noticeably dimmer, especially when viewed during the day.

I've never had problems with resistors heating up when lighting LED's.

  1. What am I doing wrong?
  2. What value resistors should I be using?

I've attached a picture of the breadboard, if that's relevant (The display is much brighter than this, the flash just makes it look dim).

test

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  • \$\begingroup\$ Thw reason why it gets warm is obvious: wasted energy = heat. \$\endgroup\$
    – haneefmubarak
    Commented Sep 8, 2013 at 20:19
  • \$\begingroup\$ Why is it "obvious"? I've not has this problem before with powering LED's from 5v. \$\endgroup\$
    – user9993
    Commented Sep 8, 2013 at 20:24
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    \$\begingroup\$ Its obvious because resistors waste off the extra energy by transforming it into heat. SO rather than asking why it is getting hot, it would be more sensible to ask why so much energy is being wasted to the point where you can feel it. \$\endgroup\$
    – haneefmubarak
    Commented Sep 8, 2013 at 20:29
  • \$\begingroup\$ It's obvious because that's the only possible source of energy to make the resistor hot. For a constant-voltage case like this the power converted to heat is inversely proportional to the resistance value. So, a 330 ohm resistor will dissipate half as much heat. Whether it's temperature increases half as much depends on physical parameters that you haven't given us. Did you previously use half-watt resistors or multiplex the display? \$\endgroup\$
    – Joe Hass
    Commented Sep 8, 2013 at 20:31
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    \$\begingroup\$ @ScottSeidman - no, you are mistaken. It will generate a bit over twice as much heat for the same current. But the current will be a less than half as much. In the end it will generate half the heat - the easy way to compare is V^2 / R, since V is nearly constant we just see the 1/R when comparing the two resistors. \$\endgroup\$
    – Chris Stratton
    Commented Sep 9, 2013 at 15:17

2 Answers 2

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This can be easily calculated.

The power supply is 5 V and the LED drops 2 V. That leaves 3 V accross the resistor. 3 V / 150 Ω = 20 mA, which is a typical max current for small LEDs. That means the LED is driven correctly.

Now look at the power dissipation. 20 mA x 3 V = 60 mW. That's well within the capability of what looks to be a "1/4 W" resistor in your picture. Again, everything is fine.

Dig out a datasheet for the resistor and see how hot it is expected to get if you actually were to have it dissipate 1/4 W. That would probably be in the 150-200°C range. Even at 150°C for 250 mW, and assuming 20°C for ambient, you have 130°C / 250 mW = 520°C / W. 60 mW would therefore heat the resistor 31°C, which you can definitely feel. If starting at 20°C, then the resistor would be at 51°C, or 124°F. So it makes perfect sense that it would feel "warm" or almost "hot" to you.

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  • \$\begingroup\$ I dare to suggest to improve the answer by quickly addressing the basic question first, that is: "why does my resistor get warm?" because perhaps the OP is missing this very basic information. Any resistor dissipates energy somehow and most (if not all) of them do so by generating heat. Sometimes that heat is so small that goes unnoticed and sometimes it is very noticeable, but all resistors do warm up to some extent when current passes through them. \$\endgroup\$
    – Dakatine
    Commented Sep 9, 2013 at 12:09
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    \$\begingroup\$ Calculating the thermal output is easy; predicting the temperature rise a bit more tricky, so I thought it worth an experiment. I've had a 1/4 watt, 150 ohm resistor across a 3.3v bench supply for a while now, and I can barely discern warmth from it. I thought that the leads and clip leads might be heat sinking it, so I cut the leads short and inserted it into a breadboard - it's maybe a hair warmer, but still not at a level I'd notice unless I was expecting it. My coworker thinks maybe he can feel it if he insulates the resistor with his finger for a moment. \$\endgroup\$
    – Chris Stratton
    Commented Sep 9, 2013 at 15:48
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try using a higher wattage resistor. if you are currently using a 1/4 Watt 150R, try a 1/2 Watt of the same resistance.

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    \$\begingroup\$ Welcome to EE.SE, Miguel. If you read the other answer - posted five years ago - you will see how to calculate the energy dissipated in the resistor and some guidelines on how to estimate the expected dissipation. The "engineering" aspect of the site means doing design calculations. \$\endgroup\$
    – Transistor
    Commented Aug 24, 2018 at 23:32

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