A linear regulator in series before a buck regulator

kuksul08

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Is it possible to wire a linear regulator such as say, an LM317, in series before a switching buck regulator (eg. DX 128269, or any such single-mode switching buck regulator). If the linear regulator was put in current mode for 300mA, how would the switching regulator, designed for 3A, respond to a constant current input? And would the linear regulator be "confused" by a switching load, or not be affected?

I understand how these two devices work independently, but if they were connected I don't know what would happen. I have often wondered about this.

In case you are wondering, the idea is to use a linear regulator for small currents since heat generated is minimal, and then bypass it for a "high" mode where efficiency is more important. I may be thinking about this completely wrong too, it's easy for me to miss the obvious solutions sometimes.
 

moderator007

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With a SPDT center off switch and the drivers wired in parallel with the switch breaking a single common from the source. Then each leg on the switch connected to the appropriate common of each driver with the outputs wired parallel to the led. Maybe. Not really sure if the outputs being in parallel would feed back to the other driver which is off causing problems. Just my two cents.
 

AnAppleSnail

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In case you are wondering, the idea is to use a linear regulator for small currents since heat generated is minimal, and then bypass it for a "high" mode where efficiency is more important. I may be thinking about this completely wrong too, it's easy for me to miss the obvious solutions sometimes.

It sounds like you mean "In parallel," not "In series."
 

kuksul08

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No, I mean in series. It's due to a weird wiring setup where a buck driver is located remotely in an enclosure, and the 2-position switch is separate, with only 2 wires between the switch and the enclosure. If I want to add a "low" mode, it would have to go where the switch is located, putting it in series before the buck driver.

Weird, definitely.
 
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Justin Case

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Use a resistor and a switch to drop the voltage sent to the buck driver. The driver will go into direct drive. When you want to go back to normal operation, turn the switch to take the resistor out of the loop. Or connect a variable DC power supply to the buck driver. Run the power supply in voltage-controlled mode. When you need high lumens output, send the appropriate voltage. When you want low output, send low voltage.
 

kuksul08

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Use a resistor and a switch to drop the voltage sent to the buck driver. The driver will go into direct drive. When you want to go back to normal operation, turn the switch to take the resistor out of the loop. Or connect a variable DC power supply to the buck driver. Run the power supply in voltage-controlled mode. When you need high lumens output, send the appropriate voltage. When you want low output, send low voltage.

I considered this.. using an LM317 in a voltage regulator setup. I guess I'll just have to experiment to see exactly what voltage I need to get an appropriate current in the driver.
 

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