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NA555: RA range

Genius 15690 points
Part Number: NA555

Hello,

 

My customer is trying to realize astable timer operation with NA555.

Because they want to realize about 50% duty, they selected 200 ohm for RA (RB=1.5K ~ 3.4Kohm, C=0.1uF).

But some datasheet parameters are specified under RA=2K ~ 100K or 1K ~ 100K condition.

Is there any problem to use RA=200Ohm?

 

Regards,

Oba

  • Hello Oba,

    There isn't a problem with choosing the 200 ohm for RA. As long as the resistors are rated properly for the currents there isn't an issue from a functional stand point regarding the 555.  

    Best Regards,

    Chris Featherstone

  • Oba,

    Thinking about this a bit more, the reason that Ra is larger in the datasheet is because the Ra resistor is tied between the supply and the discharge pin. During discharge an internal transistor is used to discharge the capacitor. The reason to not choose Ra too low is such that there is not too much current flowing into the transistor since one side of Ra is tied to VCC and the other side is effectively tied to ground through the on transistor. The abs max current is rated at +/=225mA so the customer should avoid getting close to this current. What is the customers supply voltage?

    Best Regards,

    Chris Featherstone

  • Hello Chris,

    Thanks for the additional information.

    According to the datasheet, the max VCC is 16V. So even if VCC=16, the current should be 16V/200ohm = 80mA.
    So I think 200 ohm should be the safe resistor value. Do I misunderstand anything?

    Regards,
    Oba

  • Oba,

    Your understanding is correct. This was the only reason I can see for considering the value of Ra. 

    Best Regards,

    Chris Featherstone