# question about the current going into 7 segment !

hello guys, i am setting up the circuit of a CA 7 segment display (1digit) so i was thinking that because it's a CA, i should put a 1K resistor on the common entry so is that right to do ? and is there any need to put a 1uF capacitor for more protection ? 8 resistors of 470ohms each on every segment + 1 resistor of 1K on the commons ?

i will be feeding current from a 12V lead acid battery 3 Amps

If you only use one resistor in the common lead of a 7-segment display, the display brightness will vary depending on how many segments are lit.

I hope you have a 5V regulator in there somewhere. You shouldn't connect 12V to arduino pins, even through an LED and current-limiting resistor.

Splash out on 7 more resistors and hook them to each segment...

and is there any need to put a 1uF capacitor for more protection

No. Protection from what? Capacitors are not used for protection. Put the capacitor where?

westfw:
If you only use one resistor in the common lead of a 7-segment display, the display brightness will vary depending on how many segments are lit.

I hope you have a 5V regulator in there somewhere. You shouldn’t connect 12V to arduino pins, even through an LED and current-limiting resistor.

sure not directly to LCD i used a voltage regulator connected to the shifts
here this is a picture of my circuit

cjdelphi:
Splash out on 7 more resistors and hook them to each segment…

yes of course resistor should be connected to segment but my question was about the common which means 1 resistor in addition
in total will be 9: 8 to segments and 1 on the commons so is that a right thing to do ?

Grumpy_Mike:

and is there any need to put a 1uF capacitor for more protection

No. Protection from what? Capacitors are not used for protection. Put the capacitor where?

firashelou:

cjdelphi:
Splash out on 7 more resistors and hook them to each segment…

yes of course resistor should be connected to segment but my question was about the common which means 1 resistor in addition
in total will be 9: 8 to segments and 1 on the commons so is that a right thing to do ?

it simply means it wont be equal brightness of all the segments (some segments will be dimmer/brighter) and the less lit up the brighter it becomes …

the only concern would be 1 segment consuming more than it should potentially damaging it, but you said 1k resistor ? then no…

ok great so i just have to put the resistors on the segments as usual and leave the commons connected simply to output of the regulator and taking the normal +5V ?

but the reasons i asked this question was : i was thinking that the current will enter the display threw the commons and go out from display threw the segments that’s why this question came to my head

If you use a single resistor on the common pin then the current will be about the same when displaying a '1' as an '8', meaning the segments for the '1' will be 3.5 times brighter than those for the '8'. With a single display that doesn't change very often this may be acceptable.

With several digitis it looks rather scruffy - the eye is relatively insensitive to small variations in brightness (+/- 30% or so), but a factor of 3.5 is obvious.

MarkT: If you use a single resistor on the common pin then the current will be about the same when displaying a '1' as an '8', meaning the segments for the '1' will be 3.5 times brighter than those for the '8'. With a single display that doesn't change very often this may be acceptable.

With several digitis it looks rather scruffy - the eye is relatively insensitive to small variations in brightness (+/- 30% or so), but a factor of 3.5 is obvious.

ok i don't want any problems in brightness but i want to know if it is necessary to do it for the safety of the component or not ? if i connected the commons directly to +5V will that have any kind of problem on the display ?

You can (and should) connect the common anodes directly to power and use a resistor on each segment.

RoyK:
You can (and should) connect the common anodes directly to power and use a resistor on each segment.

ok thanks a lot

You are wrong capacitors offer no component protection.

From where?

noise

Noise will not damage components.

You need decoupling capacitors on each of the chips to ensure that they have a stable power supply and they work as intended but there is no element of component protection.

Grumpy_Mike: You need decoupling capacitors on each of the chips to ensure that they have a stable power supply and they work as intended but there is no element of component protection.

i mean protection from noise

well i put a ceramic 0.1uF capacitor on the + - of the register, 1 cap for each register

the register

??? registers are hardware locations within a processor, do you mean chips?

Grumpy_Mike:

the register

??? registers are hardware locations within a processor, do you mean chips?

yes exactly the chip but i call it register just as short