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Why does my 556 Suck? — Parallax Forums

Why does my 556 Suck?

ercoerco Posts: 20,256
in General Discussion
Question for the analog gurus (Tracy, Beau, Gordon et al): My old school 556 timer is sucking current at a prodigious rate. Building a battery powered NE556 IR signal generator, and I'm shocked to learn that the 556 is pulling 25 mA not even driving the IR LED! That will kill my 9V battery right quick. Both timers are oscillating but there's no load. Then I tested a bare 556 chip with no circuitry, just battery connections, and it's still pulling 16 mA not doing anything! Yes, I tested several.

Tested a 555 timer, that only drew 2.7 mA alone so I rebuilt my 556 circuit using two 555's and it's right back up to 28 mA with no IR LED. Can't win! Now I understand why the low-power CMOS 7555 timer is such a big deal.

This makes the case for going digital, but honestly I'm shocked what power hogs these are. 25+ mA idle current is ridiculous, that's about what I need to send through the IR LED.

Where is all that current going?

Comments

  • SapphireSapphire Posts: 496
    When the 555 or 556 toggles its output, there is a brief short from Vcc to Gnd even with no load. So as it oscillates at 38kHz it is constantly drawing power during these on/off transitions.
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2018-01-31 18:45
    Is your NE556 indeed from TI? If so, you could look at the spec sheet for it. The one I found for a gen-u-wine NE556 from TI shows 25mA or even 16mA (at 5V) is indeed pretty high, but the parameter is for non-switching states.

    Have you determined the chip isn't oscillating when open circuit?You might connect up parts for a really long output and see what you get, and use that as a known baseline.

    As for being power hogs, it's long been suggested to put 1uF caps across the power pins for these. They can put out a lot of trash into the power rails.
  • fengfeng Posts: 39
    1 pcs 555 timer = 2.7 mA
    2 pcs 555 timer = 28 mA
    Makes no sense!
  • ercoerco Posts: 20,256
    feng wrote: »
    1 pcs 555 timer = 2.7 mA
    2 pcs 555 timer = 28 mA
    Makes no sense!

    One 555 draws 2.7 mA by itself, only power connections. Two 555s wired together with all timing caps & resistors but no output LED draws 28 mA. Still seems hinky.

    @Gordon: Not TI, you know I use only cheap authentic Chinese Ebay chips :)

    Will try the filter caps you suggest.

    Now I'm really glad I ordered more of these 9V Lipos: https://www.banggood.com/OKcell-9V-800mAh-USB-Rechargeable-Lipo-Battery-for-RC-Toy-p-1022214.html
  • Tracy AllenTracy Allen Posts: 6,664
    Erco, it's against the law. Kirchoff's, that is. Or maybe that blue wire, you know the one, was connected where the red wire should be, just short of magic smoke. My bad. Or maybe the Ebay special multimeter is a bit crazy on the milliamp scale.


  • GordonMcCombGordonMcComb Posts: 3,366
    Erco, The cap won't help reduce the current, just reduce glitch in the power rails. Probably not needed if all you've got is the 555/6 and no digital components, like an MCU. These could be affected by a noisy power line.

    Do an experiment: Connect two 555s together (astable) so that the first one has a 10 second or longer delay, triggering a second 555 with a delay of at least as much. IOW, you want a steady logic level for at least 10 seconds for both stages so you can get a reading. What kind of current load does that create? Seems to me it should be much closer to the ratings in the spec than what you're getting.

    Assuming you're aiming for production quantities, pricewise could you substitute with a really cheap OTP processor, maybe one with a built-in resonator? For a demonstration prototype the higher draw/shorter battery life shouldn't be too terrible -- all you need is the batteries to last long enough for them to approve the design and cut your check!
  • ercoerco Posts: 20,256
    edited 2018-02-01 02:35
    GordonMcComb wrote: »
    For a demonstration prototype the higher draw/shorter battery life shouldn't be too terrible -- all you need is the batteries to last long enough for them to approve the design and cut your check!

    Spoken like the mad genius you are. I'm making ten units for NYTF, will have to live with the non-optimized battery life for now. I have all these cheap Chinese chips and I'm gonna use 'em!
  • DigitalBobDigitalBob Posts: 1,513
    You could look at the original design that was made out of 24 ea. Transistors.. Take a look at the three 5's kit.
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2018-02-01 06:16
    As a point of reference, what's the current draw when the output is oscillating? I wonder if it's close to the 25 mA you're measuring now. If so, the major issue is that you won't have a low standby mode, but if it's for a demo or show, standby periods may be few and far between anyway. IOW you may not be giving up much at all.
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