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SN74LS123: Rext operation

Toshihiro Watanabe
Genius 10235 points
Part Number: SN74LS123

Hello,

The customer uses SN74LS123 as below circuit. Volume resister is used for Rext.

The customer has two questions. Could you support them?

1) From the datasheet, Rext min value is 5kohm. Customer uses volume resister for Rext, it might be below 5kohm.

   What issue occurs if Rext is below 5kohm?

2) The customer wants to know the VR1 value when SN74LS123 output pulse is desired width.

  Customer measured VR1 resistance by multi-meter. Power supply was off. VR1 position is 10kohm. 

  When customer measured VR1 resistance in above condition, the resistance was around 5kohm.

  Does this mean SN74LS123 internal circuit affects the measurement. I mean internal resistance was connected in parallel at VR1?

Best regards,

Toshihiro Watanabe

  • 0
    TI__Genius 16725 points

    Hello,

    I believe you are viewing this in the wrong way. You need to set the resistor value before you place it into the circuit.

    If you are reading 10kOhms before you place the resistor into the circuit, then that is its resistance. The R/C pin that you are connecting the R pin to is not always in high impedance so measuring that resistance with a multimeter while the resistor is connected in the circuit is not the correct way tot do it.

    If you connect a scope to the output of the device pulse trigger and measure the pulse width, you can figure out what the R value is.

    w= K ×τ= K × R × C, where K is a constant.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    TI__Genius 10235 points

    Hello Karan-san,

    I understand internal circuit is not high impedance hence the customer sees different resistance in measurement.

    Regarding question 1), Is there any concern if RT is less than 5kohm?

    >5kohm is recommended operation and datasheet is characterized. less than 5kohm is exceed it. if RT is less than 5kohm, device is broken or just performance is worse?

    Best regards,

    Toshihiro Watanabe

  • 0 in reply to Toshihiro Watanabe
    TI__Genius 16725 points

    Hi Toshihiro,

    When R is < 5kOhms, there will be a larger current that flows into the device which may cause damage to the device. I recommend keeping the Rext > 5kOhms.

    Thanks!

    -Karan