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LM117: Driving input & output at same voltage

Part Number: LM117

Dear Team,


Kindly provide the clarification on the below issue/concern.

Q1 :

I am applying 20V to input terminal and output terminal of LM 117 IC with same voltage level. What happens to the LM 117 IC?


Q2 :

What happens if i apply 20V to output terminal only for LM 117 IC? No voltage is applied at input terminal.


Note ; It's a typical positive regulator circuit.

  • Hi Kumaresh,

    1. 20V to both input and output is within the device's recommended operating conditions. Nothing should happen to LM117 IC, but without a voltage differential between Vin and Vout, there will be no current flow. 

    2. Applying 20V to the output terminal could be problematic if the input is grounded. LM117 has no reverse current protection and there is risk of damaging the device if voltage is applied at Vout and Vin is grounded. If Vin is floating and there is a very small input capacitor at the input, this should be ok. A large input capacitor could damage the device.

    I hope this is helpful,


  • Hi Erin,

    Thanks for  your answers. It's really helpful.

    for Q1. Will there be any current flow if there is a slight difference in voltage applied?

                 i.e. : 20V applied at input terminal and 19.5V applied at output terminal

    for Q2. Yes the input terminal is floating during the 20V applied at output terminal. What if there is no capacitor at input terminal when we apply 20V at output terminal?

  • Hi Kumaresh,

    1. When the output is forced to a voltage above its setpoint, the channel will close. No current will flow from input to output even though there is a voltage differential. 

    2. With no capacitor at the input terminal and 20V at the output terminal, the reverse current will be very small, but still present. The input will still charge to the output. 

    I hope this is helpful,


  • Hi Erin,

    Thanks for the clarifications. It helped a lot.

    If required, i will visit back after my experiment.