LM3914

[meddyliol]

Is it correct to say that an LM3914 chip doesn't need dropper resistors for LED's? Also the 4017. I also want to use a Green LED 10 Bar Display - Bargraph (SL556). Would I need dropper resistors with this?

Thanks

Brian

 

[ronsimpson]

The LM3914 is built to drive LEDs with out resistors. It's output is current regulated.
4017? CD4017 or MC14017 most ICs need resistors. Most parts are voltage output with out real current limiting.

 

[audioguru]

The datasheet of Cmos ICs like the CD4017 from Texas Instruments has graphs that show the typical and minimum output current at different supply voltages and at different output voltages (the voltage that is limited by an LED).

With a 5V supply and a 1.8V red LED the output current is typically 3.5mA. The output transistor dissipates (5V - 1.8V) x 3.5mA= 11.2mW.
With a 9V supply and a 1.8V red LED the output current is typically 18mA. The output transistor dissipates (9V - 1.8V) x 18mA= 129.6mW.
With a 9V supply and a 3.2V white LED the current is typically 17mA. The output transistor dissipates (9V - 3.2V) x 17mA= 98.6mW.

The output transistors have a maximum allowed dissipation of 100mW so if the supply is 9V or more and the LED is 3.2V or less then a current-limiting resistor should be used to share the heat.
The CD4017 has only one output active at a time.

 

[meddyliol]

Thanks for your very helpful replies. The 4017 is a CD4017, I should have mentioned that.

 

[audioguru]

A CD4017 is exactly the same as an MC14017.
A 74HC4017 is similar but is different.

 

[crutschow]

Is it correct to say that an LM3914 chip doesn't need dropper resistors for LED's? Also the 4017. I also want to use a Green LED 10 Bar Display - Bargraph (SL556). Would I need dropper resistors with this?

You normally don't need LED resistors with the LM3914 but its power dissipation can get high if you are operating in the Bar mode at high LED currents with a high supply voltage. In that case you may need to add a resistor in series with the LED power to keep the LM3914 dissipation below its rating.

 

[meddyliol]

Thank you all again for your informative replies.

Brian