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NE555: Timing Capacitor leakage current for NE555

Waseem Khayer
Prodigy 75 points
Part Number: NE555

Hello TIers,

I've a 5.5 min (3.06mHz) timer design based on A-Stable NE555 timer where the values of  RA=909k, Rb=47k and C=470uF. The supply voltage is 5V and the output is connected to a microcontroller.

From what I know, a minimum value of threshold current is necessary to trip the threshold comparator. This value is around 0.25µA or maybe even less. So with my timing resistor values mentioned above, I should have no issue to trigger the comparator right?

My other concern is the 470uF electrolytic timing cap which has a leakage current given by this formula: 0.01*C*V,  so roughly for this circuit we're talking about 25uA, therefore would this leakage current of 25uA cause any issue to the operating of the NE555 timer during either charing or discharging process? the values of Ra & Rb will limit my charging current to around 5uA and the discharge to around 28uA so would there be any concern?

I've also looked for a smaller leakage current type electrolytic cap but with no luck as the majority of the suppliers define the leakage using this formula: 0.01*C*V.  MLCC are not an option because of the capacitance loss vs. DC biasing. 

Kindly let me know what you think of it?

Thanks & happy 2017!

Waseem

  • 0
    TI__Guru**** 165206 points

    Waseen, 

    During the time that capacitor voltage is 1/3 VCC + 100mV to 2/3 VCC - 100mV  there will be no comparator input current on pins 2 or 6.

    At the 1/3 VCC threshold a small input current flows out of pin 2.

    At the 2/3 VCC threshold a small input current flows into pin 6.

    There is no data sheet value for the comparator current but I'd expect less than 100nA. So this won't be problem at all.

    If the capacitor really leaked 25uA, then that would certainly stop the oscillator (voltage won't get to 2/3 VCC). There is far too much margin in that leakage formula. 

    I suggest higher voltage capacitors would should have more resistance against leakage at low voltage.

    Better, yet would be to ask capacitor manufacturer for typical leakage data.

  • 0 in reply to Ron Michallick
    Prodigy 75 points

    Thanks Ron!

    So basically what I did already is I spec'ed out the 470uF at 10V because my actual operating voltage will be around 4.5V meaning that I'll gain a smaller leakage current of around 40 to 50% plus that my application is going to be in cold weather usually the highest is 20C. I got some new data from the supplier and you're right the formula gives a very high margin but in reality it might never be grater than 9.4uA. So now it all depends on real testing.

    I believe if I reduced Ra from 909k to 620k then it could help us in giving more margin for the charging current so that the leakage current won't take over, right?

    Again thank you  Ron,

    Waseem

  • 0 in reply to Waseem Khayer
    TI__Guru**** 165206 points

    Waseem,

    620k is very reasonable value for a resistor in a NE555 circuit.

    I do not expect any issues.