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Resistor on LED necessary?

Hi,

Of course, everywhere I see a circuit with an LED, there's a current limiting resistor. But I'm wondering if, in my case, it's necessary.

I have an eight digit multiplexed display, using 14 segment LEDs. A PIC selects the digit and turns on its common anode through an NPN transistor (and I also don't have a resistor between the PIC and the transistor's base). The transistor's collector is at +5 with no current limiting resistor. The PIC selects the individual segments within a digit by having its I/O lines tied to the cathodes and setting the output to 0 to turn on the segment. There's no resistor on the cathodes or the anodes.

Everything seems to work great; the LEDs light at a reasonable brightness (though there's some variation in the brightness - I learned too late why I should've bought common cathode LEDs instead of common anode). If I put in current limiting resistors of 220 ohms, the LEDs are dimmer than I'd like (remember that the display is multiplexed, so each digit is off 7/8 of the time, or more if I use PWM to dim the LEDs in a dark room).

Is this a "safe" configuration? Can I get away with no resistors since each LED is off most of the time? What about the PIC - the LEDs might be off most of the time, but some of the PIC's I/O lines are potentially sinking current all the time. Am I going to eventually fry my LEDs, PIC, or both?

Thanks, Bob Alexander

Reply to
realexander
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I would say that is a yes. Most LED's have a max current of 0.020 amps and a forward voltage drop 1.8 volts for a green and 2.1 volts for a red. You are putting at least twice the rated volts on the LED. This will increase the current thru the LED and will eventually burn it out. That's why the resistor is there, it doesn't just limit the current, the resistor is there to be the voltage drop for the excess voltage which is just as important as limiting the current. Even though you are multiplexing the display, not having the resistors there will shorten the working life of the display.

Reply to
Alan the tech

"realexander" schreef in bericht news: snipped-for-privacy@80g2000cwy.googlegroups.com...

You're speaking about something like Luhans superprobe?

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I've one laying around here, continously on for months without problems. Apparently the load does not exceed the 25mA max. as specified in the datasheet.

petrus bitbyter

Reply to
petrus bitbyter

LED's tolerate overcurrent for short periods, but what if your PIC somehow stalls and leaves one ON? You could burn out an LED rather quickly that way.

Reply to
Ancient_Hacker

It's usually considered poor engineering practice* to do this sort of thing in a production design, as there is no guarantee that the LED current will be limited to an acceptable value, especially from unit-to-unit. We generally put drivers in there that are more capable and then limit the current to a predictable range rather than depending on "untested" characteristics of the parts and just hope it doesn't vary out of the acceptable range for brightness or reliability, and hope that the manufacturer doesn't replace the chip with an 'A' version made on a different process with different (and also variable) characteristics.

For a one-off, for use over a modest temperature range, and no particular reliability requirement (eg. a hobbyist project), if the display appears to be a similar brightness to what you see with a conservative DC current passing through it (say 5 or 10mA DC through a single segment), then you're going to be fine. The current will typically halve/double over a wide temperature range, but fortunately it's in the "right" direction (decreasing with increasing temperature)

You don't need resistors on the NPN transistor bases- you're using them as emitter-followers.

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Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Have you tried using a lower value current limiting resistor ? 220 ohms seems a bit high for a multiplexed design. Does the LED datasheet specify a peak current limit ? Typical LEDs allow a much higher current for short peaks compared to their continuous current limit. Depending on the type of LED and the pulse width, perhaps something like 22-47 ohms will work better. You can then also easily measure voltage across resistor, and verify the current.

Reply to
Arlet

Your design can work according to (your) plan, or can fail. To the extent that it is planned, it's your responsibility to see to any necessary design decisions.

In the general case, a LED has a characteristic curve (the I versus V curve that you see on a curve tracer, or more accurately a family of such curves for different temperatures and for diodes from the same manufactured batch). And your drive provisions (power supply and transistors and resistors and such) have a characteristic curve, too, called the load line. Where these two curves cross, is the operating point for the LED. If that operating point is too high, it can waste power or overheat the LED. If it is too low, the lamp will be too dim.

So, the problem is to get the operating point in the 'good enough' region under all conditions of temperature, power supply voltage, and semiconductor characteristics that are expected for your design. Some LED flashlights just connect the LED to a battery; they work, but only because the battery has significant internal resistance (do you know the load line for your battery?). Better LED flashlights, operating close to the high end of acceptable LED current, benefit from resistors or more complex current controls.

You have just come to a complex question of design tradeoffs, requiring lots of the data sheets for your components and significant thought. Welcome to engineering.

Reply to
whit3rd

That multiplexing method is commonly called "dimming".

Reply to
Richard Henry

PICs have some (most?) outputs designed to drive LED loads, and maximum current is specified as 25 mA. The low output is typically about 0.5 V at this current, and the high voltage is about 2.5 V, so if the LED segments drop about 2 volts, you will not exceed the rating of the PIC or the LED.

You can drive the LED segments with up to about 100 mA in a multiplexed display, so the average current is only 100/8 or 12 mA. Using resistors and higher current driver transistors for the digit and segments will give you a brighter and more consistent display. Without resistors the segment current will vary depending on temperature of the PIC's MOSFET outputs as well as the tempco of the LED itself, and also how many segments are on for each digit. What you have is probably OK (safe) for a hobby application but not for any product you want to produce.

Paul

Reply to
Paul E. Schoen

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