555 current draw when output is low?

Discussion in 'General Electronics Chat' started by jerseyguy1996, Jun 24, 2011.

  1. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    Looking at my schematic, I see that Vcc, Trigger, Threshold, Discharge and Reset are all at a positive voltage potential (+12 volts in my case) when the output is low. My question is, is current flowing through each of those pins when the output is low or are they just at a positive potential but no current is flowing? I am interested to know how much current is flowing through the IC when the output is low.
     
  2. kkazem

    Active Member

    Jul 23, 2009
    160
    26
    Attached please find the datasheet for the National Semi LM555 Timer. This is the bipolar version and therefore has higher power supply pin current draw in standby than the CMOS versions like TI's TLC555, but it gives you a maximum for current draw.
    Per the datasheet, they give figures for VCC=5V and for VCC=15V; therefore, I'll give you both values here and you can look at the datasheet on your own if you want to. At VCC=5V, I=3 mA typical, and 6 mA maximum (depending on the individual IC, statistically, every IC in a lot will have different readings and this is why they spec in terms of minimum, typical, and maximum. For VCC=15V, I=10 mA typical and 15 mA maximum. This, per NOTE: 2 of the datasheet, is specified when the output is low, as you wanted and it also specifies that when the output is high, the supply current is actually 1 mA lower for VCC=5V. They don't mention that case for VCC=15V, but you could measure a few yourself.
    IMPORTANT: these are under the case where the output pin is unloaded. It would be OK to use a DMM or scope to monitor the pin state without really affecting the supply current, but don't put any load resistors on it if you are going to test for supply currernt in the high state at VCC=12V. Now, if you want to know the approximate supply current when the output is loaded, simply add the current that your load draws by assuming that the output pin is the same as your VCC in the high state and assume it's zero volts in the low state. If you want more accuracy, look at the datsheet curves to see what the output voltage would be versus load and pick the point for your circuit. There may be additional information in the datasheet regarding supply current draw under one-shot versus oscillator modes. That's all I have time for for now. If you have follow-up Q's, please ask and I'll try to get back to you later today.

    Best of luck,
    Kamran Kazem
     
    jerseyguy1996 likes this.
  3. jerseyguy1996

    Thread Starter Active Member

    Feb 2, 2008
    206
    9
    Thanks Kamran for that excellent explanation!
     
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