Need for current limiting resistor with LED?

[ghall426]

I am using a 4511 BCD to 7 segment display which says it can source 25mA. I need to drive 2 of the 7-segment displays that will display the same number. The typical forward voltage of these displays are 4V (max of 5V) at 10mA.

If I would use a 9V power supply and wire the two displays in parallel, am I safe to not use any current limiting resistors and just hook them directly to the 4511?

Same question if I would use a 5V power supply. Would I take a pretty big hit on the brightness if I did this?

Thanks.

Greg

 

[kchriste]

If I would use a 9V power supply and wire the two displays in parallel, am I safe to not use any current limiting resistors and just hook them directly to the 4511?

Sometimes when running 4000 series CMOS ICs at lower voltages (<6v) you can get away with this. But the 4511 actually has highside bipolar outputs which will not limit the current sufficiently. You definately need the resistors when running on 9V.

The typical forward voltage of these displays are 4V (max of 5V) at 10mA.

This might be a bit of a problem for getting max brightness because the 4511 is rated for a typical output voltage of 4V @ 10ma with a 5V supply. Minimum guarateed output voltage is 3.6V at 10ma with a 5V supply.

 

[freeskier89]

You definitely need the resistors in there for both cases. If you didn't the 4511 (and or your LEDs) would quickly become fried. Really the only case that you don't need current limiting resistors for LEDs is when you hook it up to an IC that has internal resistors such as the LM3914 for example.

 

[ghall426]

You definitely need the resistors in there for both cases. If you didn't the 4511 (and or your LEDs) would quickly become fried. Really the only case that you don't need current limiting resistors for LEDs is when you hook it up to an IC that has internal resistors such as the LM3914 for example.

What value of resistor would I use if my power supply exactly matched the voltage needed for the LEDs? For example, say I wired the 2 displays in series as I orginally stated. That would be a total voltage requirement of 8 volts for the LEDs.

So if could power the circuit with an 8 volt power supply, what size of resistor would I use? Even if I used a 1 ohm resistor, why would there be a voltage drop across it? The 4511 wouldn't even be putting out the full 8V anyway.

Greg

 

[Sceadwian]

If it exactly matched the voltage needed by the LED's you wouldn't require a current limiting resistor at least not for the LED's the 4511 itself, I don't know. But you have to be very sure that there aren't even momentary over voltages, as a little bit of over voltage will result in a LOT of over current for an LED.

 

[RODALCO]

You want to go for a 8 Volts supply, Take off 1.7 volts for the red LED, allowe for about 1.3 volts for the 4511 chip than you have to drop 5 volts across the resistor.

Fron U=IxR and, take LED current as 10 mA a 500 Ohm resistor is needed.
Nearest E12 value is 470 or 560 ohms 1/4 Watt will suffice.

 

[audioguru]

You said it yourself, "the typical voltage is 4V", but what is the actual voltage? 3V? 5V?
If it is 3V then without current-limiting resistors the LEDs and the 4511 blow up. If it is 5V then they won't work unless you increase the supply voltage.