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Basic Electronics » what resistors to use when connecting a new IC

May 31, 2009
by luisgarciaalanis
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So I was out today and I decided to stop by a Radio Shack Store to look for something simple to try out to help me learn more.

I was going through the drawers and foud this interesting single digit digital display:

then I saw it says: Typical Characteristics Fordward Current: 20 mA (amps coming in or it needs) Forward voltage: 1.7V ( voltage coming in or it needs) Peak wavelength: 655nm (don't know what this is)

So I have a battery with 5V the battery does not say how many amps it produces.

I downloaded a led calculator to my iphone and calculated based on 1 LED 1.7V and 20mA and it gave me something like 160 ohm. I then calculated the same for the Maximun ratings and it gave me 100 ohm

So as long as I get something in between I should be alright.

I found 150 ohm resistors, 1/8 watt.... I bought 10 of them since the Display has 10 pins.

when I got home I realised that the V that is coming out of the MCU must likely will not be 5V since the MCU consumes some.

How can I calculate what I really need to buy?

I googled and found this site and they use 460 ohm resistors:

460 its bigger so it resists way more than what I bought. what did I do wrong?

May 31, 2009
by luisgarciaalanis
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the resistors I got are smaller than the ones that came with the nerd kits, I guess I should't have bought 1/8 watt ones. Mmm... 1/4 watt might be better but they had none.

the 7805 data sheet does not mention anything about watts

Wikipedia help:

1W = 1V * 1A

so data sheet says on page 1: Output current = 1.5A V: 5V

so this is 7.5 Amps I don't think I need 7.5 amp ressistors LOL! to big

May 31, 2009
by luisgarciaalanis
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I mean 7.5 Watts :(

June 01, 2009
by mrobbins
(NerdKits Staff)

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Hi there,

I think you are on the right track with the initial calculation: (5-1.7)/0.020 = 165 ohms. But you're right, there's some extra voltage drop due to the microcontroller output pins too, and as I mention in this post, that equivalent resistance is very roughly 30 ohms on any microcontroller pin. So, if you are just driving one side of either LED and the other side is permanently wired to ground or +5, then you could use about ~135 ohms and get the current you expect. Then again, the device will tolerate a wide range of currents, but will vary in brightness. Probably not a problem as long as all the segments match each other!

As far as the power dissipation in the resistor, consider that for the 165 ohm resistor with 20 milliamps, the power dissipation is the product of the voltage across the resistor times the current running through it, P=VI. The voltage is (5-1.7)=3.3 volts, so the power dissipation is (3.3 volts)*(0.020 amps) = 0.066 Watts, or 66 mW, well less than 1/8th watt = 125mW that your resistor is supposed to be able to handle.

Using a bigger resistor like the 460 ohms you mention just leads to less current, so the LED will still light, but not as brightly. For example, we use a 680 ohm resistor in series with the LEDs on the serial PCB included with the kit, and this limits the current to roughly 5mA: (5-1.7)/680 is about 0.005 amps. Hope that helps!


June 01, 2009
by luisgarciaalanis
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Cool!! Thanks for the link!!!

I did a small hack :)


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