7447 with resistors

[Overclocked]

(for reference see this page:
I know its Shows the CMOS version of the 7447, but my question is the same regardless)

In the circuit above they have resistors on each output of the 7447 (4511). Can I Just directly attach the 7 segment Display to the output and just use a single resistor on the anode of the Display (the 7447 is a common anode display driver).

It just seems like a waste of resistors to me, so Im trying to save some space. I do have a DIP package with resistors if it cant be done.

 

[fingaz]

I would be inclined to say not, because that way you would not have a separate resistor for each segment. . . with more than one segment lit, the power requirement would increase, and if you have only one resistor for the whole display, I think you would end up with a display that is a)unevenly lit b)doesn't work at all, or c)blows up

To do it right, I would use a resistor for each segment of the display.

 

[ericgibbs]

hi overclocked,

Consider this:

Imagine you have a single LED which is connected to a +5v power rail.

The spec of the LED states that the forward voltage drop across the LED is 1.5V at 10mA.
So you would calculate the resistor to be Rs=(5-1.5)/.01 so Rs=350R [nearest preferred value is 330R]

At these specified values the LED would be 'lit'.

If you now connected another LED of the same type say, across the first LED, if the LED's
are matched, then 5mA would ideally flow in each LED. So they be 'half lit' ie dimmed.

If now 7 LED's were connected in parallel only 0.01/7 mA would flow in each LED.

If you required all 7 LED's to be fully 'lit', you would need a resistor of Rs=(5-1.5)/.07
ie; Rs= 50R [pv 47R]

Problem is:
if only one LED was in series with this Rs value then Is=3.5/50 = 70mA which
most likely would damage the LED and/or the driving device.

In use, a 7 Segment LED driven by semi can have 1 thru 7 LED's ON/OFF. Thats why 7
individual resistors are required. In the above example 7 off 330R.


Hope this answers your post.

Regards

EricG

 

[Dean Huster]

The 7-segment display using a 7444 that E & L instruments makes uses just a single resistor. It works, but the problem is that each digit ends up having a different brightness. The "1" is the brightest and the "8" is the dimmest. You can get away with it for single digit, but a multiple-digit display will not look very nice, making it appear that your LED displays may have been "floor sweepings" from a Poly-Paks order.

On the other hand, having one resistor for each segment can be advantagous because a resistor in this case equals a jumper and makes laying out a PCB for the segments a bit easier. Once you get the layout for one digit, a multiple-digit display is just a replication of the original.

Dean

 

[AllVol]

Overclocked:

You can do it with one resistor.... but I promise you will not be happy with the results.

 

[Hero999]

Why worry when resistors are so cheap anyway?

If you were using a microcontroller you could do it by only having one LED on at any instant and turning them on an off at such a high speed the user can't see it but for your application you really need one resistor for each LED.