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Multiple Linear Regulators in a Circuit

jeffrichley@gmail.com

I have a 24V power supply coming into my circuit and I need a 12V for a fan and 5V to run my logic.  I have 12V and 5V linear regulators that I can use to get the desired voltages.  Would it be better to run the 24V to both or the 24V to 12V and then take the 12V to the 5V?  My reasoning for the 24V -> 12V -> 5V is that the 5V would need to dissipate less power (7V vs 19V), but I didn't know if it is a bad idea to have one regulator pull from another.

 

I appreciate any input you have,

 

Jeff

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  • Jeff -

     

    Running the output of one linear regulator into the input of another linear regulator is possible for low currents, but you must consider the relatively high power dissipation of such a configuration. In most cases you would be better off with a switching regulator with two outputs: one 12V output and one 5V output. If you run the 24V supply into both the 12V and the 5V linear regulators, you must add the quiescent current of the 5V regulator to the output current of that 5V regulator and multiply the result by 19V (24V – 5V) to determine the power dissipation. Then you must determine if that much dissipation is beyond the capability of the regulator or not. Similarly, consider the power dissipation of the 12V regulator. If you run the 12V output into the 5V regulator, you must similarly consider power dissipation in the two regulators. Of course a regulator in a TO-220 package, or even a TO-66 or TO-3 package, can dissipate more power than can the same regulator in smaller packages, but it is generally easier to deal with a switching regulator, which will also give you power efficiency overall. If this is just for hobby use, you might be fine with the linear regulators, but you still need to consider the power dissipation of the regulators.

    Good luck!

Reply

  • Jeff -

     

    Running the output of one linear regulator into the input of another linear regulator is possible for low currents, but you must consider the relatively high power dissipation of such a configuration. In most cases you would be better off with a switching regulator with two outputs: one 12V output and one 5V output. If you run the 24V supply into both the 12V and the 5V linear regulators, you must add the quiescent current of the 5V regulator to the output current of that 5V regulator and multiply the result by 19V (24V – 5V) to determine the power dissipation. Then you must determine if that much dissipation is beyond the capability of the regulator or not. Similarly, consider the power dissipation of the 12V regulator. If you run the 12V output into the 5V regulator, you must similarly consider power dissipation in the two regulators. Of course a regulator in a TO-220 package, or even a TO-66 or TO-3 package, can dissipate more power than can the same regulator in smaller packages, but it is generally easier to deal with a switching regulator, which will also give you power efficiency overall. If this is just for hobby use, you might be fine with the linear regulators, but you still need to consider the power dissipation of the regulators.

    Good luck!

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