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LM7800: Design Help

Ray Vincent Aranzaso
Genius 15325 points
Part Number: LM7800

Hi,

Good Day.

I have a question regarding the 78XX series voltage regulators.

In my application, we will be using mechanical relays with about 400 ohms of coil resistance.

The assembly supply voltage will vary from 12 to 30 volts continuously which could be a problem when using the intended 12-volt relay (coil). 

So my question is this, if a 7812 or 7815 regulator is used as a “voltage clamp” for the relay coils, what is the thorough current of the regulator if the supply voltage is less than the regulator’s rating? i.e. 12 supplies when using a 7815.
We do not want to have separate 12V and 24V rated systems - just one design that will work in the 10-30 volt range.
Please advise. Thank you very much.
Best Regards,
Ray Vincent
  • 0
    TI__Expert 4450 points

    Hello Ray,

    Here are the two scenarios:

    1. Using LM7815: When supplying 12V, the device will go into dropout mode having an output of 10V instead of 15V. The output current when supplying 12V will be around 25mA. In order to regulate at 15V the supply voltage has to be at least 17V (Vin = Vout(nom) + Vdo) and the output current will be around 37.5mA.

    2. Using LM7812: When supplying 12V, the device will also go into dropout mode having an output of 10V instead of 12V. The output current when supplying 12V will be around 25mA. In order to regulate at 12V the supply voltage has to be at least 14V (Vin = Vout(nom) + Vdo) and the output current will be around 30mA. 

    Overall, I recommend going with the LM7812 instead of LM7815 because for the other scenarios where supply is greater that 14V it will regulate at 12V not damaging the 12V relay.