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Topic: 74hc595 current draw (Read 1 time) previous topic - next topic

winner10920

What causes the 74hc595 to draw the spec'd 70ma? I have a project that is 12v powered and with two of those and the arduino it comes out to 120ma draw at times and 50ma other times, the arduino draws like 35-40ma so they draw between 10-80ma combined, but it varies randomly it seems, moving between the two values kinda quick,
the outputs can all only draw 30ma max since its going to 2.7k resistors so what's the rest being used for?
the reason I ask is I wanna use a cheap and small 7805 powered off 12v but at 120ma its a bit much heat, the 50ma isn't so bad

Docedison

Double check your wiring somethings shorted to ground one or more outputs are shorted to ground.

Doc
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"The solution of every problem is another problem." -Johann Wolfgang von Goethe
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winner10920

None are shorted for sure, its all multimetered out good
and I tried just one output and it does the same thing, just goes back and forth slow enough atleast that the multimeter has time to settle on each usually

winner10920

And I mean it does say supply current 70 ma max and it seems to be between 5 and  40 ma each, so I just wonder when it actually needs all that so I can minimize it, and I don't think its when loading the data because im doing that constantly

Docedison

1. Are any of the chips getting warm.
2. Do you have any inputs not pulled up or down, any at all.
3. Does the power change if you pick it up or touch it.
4. Is it (the board well bypassed a 100 uf on the Vcc line and .1uF caps on Every chip.
5. Can you step this thing trough to find an address that is the high current one or ones
6. Can you repeat it in a regular pattern or is it random in appearance or regularly cycling
7. do you know that the IC's are all good.
8. does the quiescent current change at all when not being driven with serial data
9. is the Vcc stable at all times and if not can you determine the range and it any change in voltage during periods of high current draw.
10. Do you own an O'scope and can you use it well.
... There are a lot of possibilities most of which I have covered here most as you can see are power supply related have you checked it a third time?
When I worked for a living I was a design engineer for a small company in Costa Mesa Ca. I worked there for 20 years and I had a rule that always worked for me
After I finished a design, all modules were checked I had a proof board made a sample run W/O silk or mask just solderplate and I assembled one and verified and many times repaired mistakes. I would then modify my final documentation and board work prepare the paneled art work as I usually made 6 or 8 boards on a panel and then I would go home regardless of the time of day eat and go to bed and the next morning recheck everything again before I told my employer that the project was done. He always asked me If I had "Checked It" and my answer was always yes to the limit of my ability.
The questions I asked above were for PSU, oscillation in any of the chips and pattern dependent issues that might reveal where in the work your board deviates from your design goals and specifications. For Engineering this is a critical evaluation. What you are doing in the above questions is looking for that sore tooth in your design.
Many years ago I was a techie and a good one by the standards of the day... I Never "Applied" for a job after the first one I had. I would every 5 or 6 years get a call and decide that it was a good time to change my neighbors for new ones. One of the things I learned is that there is a pattern that follows all trouble shooting and that is
"Kick it and see which way it jumps". Things that oscillate will usually change when you change feedback with your digital voltmeters... ("fingers" remember I worked with tubes for all my life, from TV's and Radio's to KW class transmitters from 50 mhz to 800 mhz) often times just 'feeling' the PCB or breadboard is enough to alter conditions and ca8use a current change, if the board is quiet (not clocked) and you can make any change in current draw you have a by-passing or layout issue and so on and so forth.
I write too much... it is hard though sometimes to get an idea across. put it through as many tests as you can you may well discover a great deal.
If you own or have access to at least a 20 Mhz BW scope so much the better probe the power and ground rails clock and data lines ir or are any at all of the output voltages from the S.R.'s out of spec, especially low? check it out and I am on california time and by the computer a good part of the day and evening...
So Dude, Let me know.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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