When cascading or stacking lm3914 led drivers how important is the wattage of the resistors? 1/4 watt -1/2 watt-1 watt
It’s always important. If the power rating isn’t high enough, the resistor will be damaged.
I phrased my question wrong–if im using a 9 volt supply what wattage should I use?
Normally a LM3914 would not require resistors to dissipate large amounts of power. But you haven’t provided your circuit or given any details on this hypothetical resistor so you might be doing something very strange.
It depends on how much current flows through the resistor. Nobody can help you without a schematic.
Mr 40m Man
From the LM391x datasheet it appears that you have Vref set to almost 2.0V (using 2k2 + 3k3), and LED current will be around 5.6mA. I would say you can probably built the circuit as shown without LED current limit resistors.
Power ratings of all resistors in the circuit can be 1/4W, no huge current, nothing (should) get hot.
Was there a specific reason for your question, or are you just being cautions ?
Hope that helps.
Regards,
mark
Thank you mark–Im going to use 9 volts rather than 5 volts and I actually already had some 1/2 watt resisitors, and did not understand if 1/2 watt resistors would be too much wattage
given that Vref is so low, I still think you can get away without LED current limit resistors.
-mark
40meterman:
… did not understand if 1/2 watt resistors would be too much wattage
The power rating can’t ever be too much from an electrical view point. The only thing you waste with a too high rating is money and space. A 1/2 W resistor dissipating 1/4 or 1/8 Watt will be happy forever.
1/2 Watt resistors used to be very common, but now that circuits are moving toward lower voltages (5 V, 3.3 V…) 1/4 W has taken over, with 1/8 and 1/10 W showing up now. In the vacuum tube days, they were 2 to 5 Watts. IIRC, 1206 chip resistors are 1/8 W.
1/2W resistors dissipating a 1/4W (or less) is good. 1/4W resistors dissipating a 1/2W (or more) is not good. The rating must be > than the power to be dissipated. A 50% - 100% margin is desirable.40meterman:
and did not understand if 1/2 watt resistors would be too much wattage
Although your case is OK, you should also check the power being dissipated in the LM3914. This will depend on the color of your LEDs. Let be worst case and have them be red. With ~6 mA flowing through each and a Vf about 1.8v, the power dissipated in the LM3914 is;
(9v - 1.8v) x 0.006A x 10LEDs = 432 mW. Add in another (5v x 5ma) 25 mW for power to run the IC and you’ve got about 460 mW max, all 10 LEDs on. That’s comfortably under the 1365 mW spec.