Help with datasheet specs

Nightrider7731

I have a Kingbright dual 7-seg led DC56-11EWA (http://www.datasheetarchive.com/datasheet/pdf/85/858408.html) that I want to drive with two Fairchild MM74HC595 (http://www.datasheetarchive.com/datasheet/pdf/91/91254.html).· From what I can tell, the led has a forward voltage of 20mA.· I am using 150 ohm resistors to Qa-Qh. From what I can tell, the 595 has an abs max value for DC output current (Iout) of 35mA per pin.· With those number I thought I was safe.· However, I read another post from a few days ago stating the 595's can only source 15mA per pin.· Is this a correct for this IC?· Do I need to bump the resistors to 200 ohms?

Thanks!

allanlane5

If you look at the datasheet you called out, you find that the light output is specified at 10 mA per segment. Thus 10 mA is the value you can set your resistor for.

The Electrical/Optical characteristics ARE called out at I_forward == 20 mA. Note your V_forward 'Typical' value is 2.5 volts.
(5 - 2.5) / 20 mA == 125 ohms -- not bad. (5 - 2.5) / 150 == 16 mA, still not bad. (5 - 2.5) / 220 == 11.3 mA, still not bad.

So, you can use 220 ohm resistors with this, or 150 ohms, and get good performance. I've looked at the '595 docs, and the amount of total current the chip can use depends on the IC package. The DIP has the most.

But I guess I'm out of experience at this point -- I was unable from the '595 docs to determine for sure if a single '595 could drive 15 mA (or 20 mA) per pin all driven at the same time. Anybody else done this successfully?

Nightrider7731

Thanks for taking the time to review the specs and providing your insight.· Hopefully someone will be able to reply who may have tried this before.· Otherwise I can always fire up one of the LEDs·and leave all segments running to see if something fails.· Or I can play it safe and pop out the 16 150s and pop in some 220s.

kb2hap

I looked at a few datasheets it said the ABSOLUTE MAXIMUM output current rating per pin was 35ma. I wouldn't drive each pin that hard though. probably 20ma is safe though

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
DTQ

Nightrider7731

I wanted to make sure I was reading it right.· I would feel a lot more comfortable if they listed the max for the chip as well as the max per pin like Parallax does for the BS2.· FWIW, I ran both LED full (7 segs+dp) for about 45 minutes with no visible problems.· Since that is longer than my planned application, I should be good to go.

Thanks!

allanlane5

With what resistance? And I assume you ran them off the '595? What was the full part number of the '595 IC?

You now have done it -- let's record the results so we have a more definitive answer for others next time.

Thanks.

Nightrider7731

The specs are in the first post, but I'll state them again for late comers.

1x Kingbright dual 7-seg LED #DC56-11EWA (http://www.datasheetarchive.com/datasheet/pdf/85/858408.html)

2x Fairchild MM74HC595 (http://www.datasheetarchive.com/datasheet/pdf/91/91254.html)

16x150 ohms resistors (yes, they were between the '595s and the DC56)

Your results may vary, but I ran·both with all LEDs for 45 minutes.·

allanlane5

Neat! I assume you are using the DIP package -- I understand the smaller packages may not support the same amount of current.

Nightrider7731

Correct on the DIP.