# Resistor sizes

Discussion in 'The Projects Forum' started by rhyso, Aug 2, 2010.

1. ### rhyso Thread Starter New Member

Aug 2, 2010
1
0
hello guys i am chasing some info about resistors i have to design a circuit with a seven segment LED display and SN74HC148 8-3 encoder but cant find any info on resistor sizes required to run these off a 6 volt power supply

2. ### tom66 Senior Member

May 9, 2009
2,613
213
It depends on how much current you want. I suggest no more than 20mA for a seven segment display. Luckily there is a formula. If the LEDs drop 2 volts then you can calculate the resistor as so:

$
R = \frac{V_{in}-V_d}{I}
$

$V_{in}$ = 6V, $V_{d}$ = 2V, $I$ = 20mA

For this we calculate 200 ohms. The closest standard E12 value is 220 ohms, so use 220 ohm resistors.

3. ### Wendy Moderator

Mar 24, 2008
20,735
2,499
Actually 200Ω ¼W 5% is also a standard, just like 180Ω and 220Ω

4. ### tom66 Senior Member

May 9, 2009
2,613
213
That's why I said closest standard E12; 200 ohms may well be an E24 value but those are definitely more difficult to get hold of.