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Topic: Voltage regulator or DC/DC converter (Read 2 times) previous topic - next topic

oscarBravo

I'm almost finished a design using an embedded Atmega processor in an environment where I'll have a 24V power supply. I originally designed it with a fairly pricey 24V-5V DC/DC converter, then considered just using a 7805 instead.

My understanding of how a 7805 works is that it draws the same current from its input as it provides to its output, which means (unless I'm mistaken) that it would be running at 20% efficiency and burning the rest off as waste heat. Is this right?

RuggedCircuits

You are right. To a first approximation the 7805 regulator's efficiency is Vout/Vin, or in your case 5/24 = 20.8%.

You also have to be concerned with maximum current draw. The actual power dissipation of the regulator is (Vin-Vout)*Iout = (24-5)*Iout = 19*Iout. This total power dissipation heats up the regulator and if it gets hot enough, it shuts down. My rule of thumb is to keep power dissipation <1W for TO-220 packages. This would limit your current to about 50mA, enough for the Arduino circuitry but not much else.

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The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

CrossRoads

Yes. You'll need a decent heatsink also with the 7805.
Suggest you use a little switching step down regulator such as
http://www.pololu.com/catalog/product/2101
Higher current output parts are available also.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

oscarBravo

Thanks for confirming my suspicions.

I just came across this, which looks like it might be a useful substitute, and still quite reasonably priced.

RuggedCircuits

The LM2575 is not a drop-in substitute -- it is a switching regulator controller, just one component of a switching regulator. You still need to add an inductor, diode, and capacitors around it, and route it properly on a PCB to keep the switching loop tight, etc.

The Pololu part suggested by CrossRoads IS a drop-in solution.

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Beat707: MIDI drum machine / sequencer / groove-box for Arduino

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