I am using Multisim to simulate a circuit before building it.

The voltage protection circuit with a regulator works fine and the timer circuit works fine as well separately. When I try using the output of the protection circuit as input for the timer Multisim gives me an error.

Circuit Functionality

When an input is supplied to the voltage protection circuit only if the input is equal or greater than 47V should it activate and supply the regulated 14.5V coming from U4, next this input is used as a trigger for the timer as long as this voltage protection output is high the timers LED is active, should the voltage protection circuit go low the timer waits a few seconds and then the output goes low.

Please note, everything as I said works, the voltage protection alone and the timer works as intended but when I try to combine the two circuits is where it fails.

This is the voltage protection circuit:

Voltage Protection Circuit

This is the timer circuit:

Timer Circuit

This is the full circuit when I try to combine Full Circuit

This is Multisim error The Error


2 Answers 2


Input voltage (47 V) in LM675 should not be greater than supply voltage (14.5 V).

  • \$\begingroup\$ hi bravale, i looked at the datasheet again where does it state that? what i understood is it can uphold up to 60V. can you please explain? \$\endgroup\$
    – Abdeen M.
    Mar 12, 2020 at 17:16
  • \$\begingroup\$ LM675 can hold 60 V, but in your circuit the supply voltage is 14.5 volts. If you look specifications, max input voltage is limited from -VEE to VCC. \$\endgroup\$
    – Bravale
    Mar 12, 2020 at 17:27

The 555 Trigger input is dead-shorted to GND.

Update 1: With the short removed, the Trigger input cannot be pulled low by the 675 output because the diodes are pointed the wrong way. The trigger input must go below 4.8 V (Vcc / 3), but D3 prevents any current through R3 and R6.

  • \$\begingroup\$ Hi AnalogKid, are you referring to the connection from R6 to GND? even when removing it the error presists \$\endgroup\$
    – Abdeen M.
    Mar 12, 2020 at 13:04
  • \$\begingroup\$ What is the nature of the error? \$\endgroup\$
    – AnalogKid
    Mar 12, 2020 at 18:17
  • \$\begingroup\$ Hi AnalogKid, thanks for the answer i tried removing D3 and removed the short still the same error. am not quite sure what the nature is. Also as stated by Bravale, the input voltage (47V) shouldn't be greater than supply voltage (14.5V), do you also agree? \$\endgroup\$
    – Abdeen M.
    Mar 12, 2020 at 20:06

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